INDIAN INSTITUTE OF TECHNOLOGY KHARAGPUR

Date : Time : 2 hours Full Marks : 30 No. of students : 20

Mid-Semester, Spring 2008 Mechanical Engineering ME2 and DD
Subject No. ME60096 Subject Name: Air conditioning & Ventilation
Note:
1. Use perfect gas model for calculating the required psychrometric properties.
2. Unless otherwise specified, assume the barometric pressure to be 101.3 kPa.
3. Make suitable assumptions, but state them clearly.
Given data:
Molecular weights: Dry air = 28.966 kg/kmol; Water = 18.02 kg/kmol
Universal gas constant = 8.314 kJ/kg.K; Cp of liquid water = 4.18 kJ/kg.K
Specific heats, Cp: Dry air=1.005 kJ/kga.K, Water vapor=1.88 kJ/kgw.K
Stefan_Boltzmann Constant,  = 5.669 X 10-8 W/m2.K4
Unless otherwise specified, Specific heat of moist air, cpm (kJ/kga.K):
cpm = 1.005+1.88w, where w is humidity ratio of air (kg water/kg air)
Antoine’s equation for saturation pressure (psat) of water:
 3985 
ln(p sat )  16.54   ; p sat in kPa & T in K
 T  39.00 
Latent heat of vaporization of water,hfg in terms of temperature (0  t  40oC):
hfg  2500.95  2.37267 t kJ / kgw

Humidity ratio(ws) and enthalpy (hs) of saturated air (5  t  30oC):

w s  4.914  t(0.0102  0.0243714 t ) grams of water / kga

hs  12.4312  t(1.00455  0.0633071 t ) kJ / kga

Carrier Equation:

1.8(p t  p,v )(DBT  WBT )

p v  p,v 
2800  1.3(1.8DBT  32)
,
where pv, pv and pt are vapour pressure, saturated vapour pressure at WBT and barometric
pressure, respectively. DBT and WBT are in oC and units for pressures should be consistent.
------------------------------------------------------------------------------------------------------------
1. Atmospheric air at 42C (DBT) and 28C (WBT) is in contact with a water pond of 100 m2
surface area. The surface of the pond is at 30C and the convective heat transfer coefficient
between the pond surface and surrounding air is 23 W/m2.K. Assuming a value of 1.0 for the

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Lewis number, find the direction and magnitude of heat transfer between the pond surface and the
atmospheric air. Also find the evaporation rate per hour. (5+1=6)
2. An unshielded sling type psychrometer shows a dry bulb temperature of 32C and a wet bulb
temperature of 21C. The surrounding surfaces are at an average temperature of 36C. The
sensing bulb surfaces of both dry bulb and wet bulb thermometers have an emissivity of 0.9. The
convective heat transfer coefficient between surrounding air and the sensing bulbs of the dry bulb
and wet bulb thermometers are 33 W/m2.K and 23 W/m2.K, respectively. Assuming the humid
specific heat of moist air to be 1.03 kJ/kga.K and the Lewis number for air-water mixture to be 0.9,
find the actual air dry bulb temperature and humidity ratio. Neglect radiation between dry and wet
bulbs. (3+5 = 8)
3a. Prove that when moist air flows over a wetted surface (maintained at a constant temperature),
the exit condition lies on the straight line joining the inlet state of air and saturated air at the wetted
surface temperature on a psychrometric chart. Assume the specific heat and the convective heat
transfer coefficient to be constant, and a value of 1.0 for Lewis number. (4)
3b. Air at 25C (DBT) and 60% relative humidity flows over a cooling and dehumidifying coil that
has an apparatus dew point temperature (ADP) of 7C. If the coil has a bypass factor 0.1, find the
exit air temperature and relative humidity. (4)
4. A large auditorium is designed for a seating capacity of 800 people. The auditorium has a
sensible cooling load of 400 kW and a room sensible heat factor (RSHF) of 0.8. The design inside
and outside conditions are 27C (DBT) and 50% relative humidity, and 42C (DBT) and 28C
(WBT), respectively. Fresh outdoor air at a rate of 9 litres/s/person (measured at outdoor
conditions) should be supplied to the building for ventilation purposes. For proper distribution of air
inside the auditorium, the supply air flow rate to the auditorium should be 30 kg/s (dry air basis).
The air conditioning system uses a cooling and dehumidifying coil that has an apparatus dew point
temperature (ADP) of 7C. The dry bulb temperature of air at the exit of the cooling and
dehumidifying coil is 11C. a) Draw the schematic of the proposed system that can satisfy the
given requirements, b) Show the cycle on psychrometric chart, c) Find the bypass factor, cooling
capacity and sensible heat factor of the cooling and dehumidifying coil. (2+2+4 = 8)

End of the paper

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 INDIAN INSTITUTE OF TECHNOLOGY KHARAGPUR
Date : Time : 3 hours Full Marks : 50 No. of students : 20
End-Semester, Spring 2008 Mechanical Engineering ME2 and DD
Subject No. ME60096 Subject Name: Air conditioning & Ventilation
Given data:
Use perfect gas model for calculating the required psychrometric properties.
Unless otherwise specified, assume the barometric pressure to be 101 kPa.
Molecular weights: Dry air = 28.966 kg/kmol; Water = 18.02 kg/kmol
Universal gas constant = 8.314 kJ/kg.K; Cp of liquid water = 4.18 kJ/kg.K
Specific heats, Cp: Dry air=1.005 kJ/kga.K, Water vapor=1.88 kJ/kgw.K
Stefan-Boltzmann Constant,  = 5.669 X 10-8 W/m2.K4
1 met = 58.2 W/m2
Solar angles: (l: latitude angle, h: hour angle, : declination, : Tilt angle)
Altitude angle,   sin 1 ( cos l. cos h. cos   sin l. sin  )

 cos l. sin   cos  . cos h. sin l

solar azimuth angle,   cos 1  
 cos  
angle of incidence,  cos 1 (sin  . cos   cos  . cos  . sin )
wall solar azimuth angle,   180    
surface azimuth angle,  is ' ' if it is to the east of south
Solar Radiation data for 40oN, June 21st:
 0.205 
direct normal radiation, I DN  1085. exp  
 sin  
 1  cos  
diffuse radiation, I d  0.134 I DN  
 2 
 1  cos  
reflected radiation, I r   g I DN (0.134  sin  ) 
 2 
In the above equations, g is the ground reflectivity. IDN, Id and Ir are in W/m2.
Antoine’s equation for saturation pressure (psat) of water:
 3985 
ln( psat )  16.54   ; psat in kPa & T in K
 T  39.00 
Carrier Equation:
1.8( pt  pv, )( DBT  WBT )
pv  pv, 
2800  1.3(1.8DBT  32)

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 ,
where pv, pv and pt are vapour pressure, saturated vapour pressure at WBT and barometric
pressure, respectively. DBT and WBT are in oC and units for pressures should be consistent.
0.425 0.725
Surface area of a human body (m2): ADu  0.202 m h
, m in kg & h in m.
1a) Derive an expression for sol-air temperature and explain its physical significance. Find the sol-
air temperature of a horizontal roof that is subjected to an incident solar radiation of 800 W/m2.
The absorptivity of the surface for solar radiation is equal to 0.9, outside air temperature is 35C,
long wave radiation from the surface to the surroundings is 72 W/m2, and the convective heat
transfer coefficient between the surface and surrounding air is 23.3 W/m2.K. (2+1=3)
1b) The horizontal roof of an air conditioned building is made of 15 cm thick concrete (k=1.73
W/m.K). The roof has an effective surface area of 100 m2, a decrement factor of 0.48 and a time
lag of 5 hours. The inside and outside surface heat transfer coefficients of the roof are 6.3 W/m2.K
and 23.3 W/m2.K, respectively. The air conditioned space is maintained at 25C. The following
table shows the values of sol-air temperatures for a particular day between 8 A.M. to 7 P.M.
Time Tsol-air, C Time Tsol-air, C Time Tsol-air, C
8 A.M. 35.1 12 Noon 53.2 4 P.M. 45.5
9 A.M. 41.2 1 P.M. 54.3 5 P.M. 40.0
10 A.M. 46.2 2 P.M. 52.9 6 P.M. 33.6
11 A.M. 50.7 3 P.M. 50.3 7 P.M. 26.8

If the mean sol-air temperature for the day is 33.3C, find a) Peak cooling load on the building due
to the roof and time of occurrence of this peak load, b) Peak CLTD value, and c) Inner surface
temperature of the roof at peak load. (3+1+2=6)
2a) Explain briefly the concepts of Solar Heat Gain Factor (SHGF) and Cooling Load Factor (CLF).
(2+1=3)
2b) Find the heat transfer rate through a south facing, unshaded, vertical window of height 1.2 m
and width 1.5 m at solar noon on June 21st (declination = 23.5) of a building located at 40oN. The
window made of standard DSA glass has a transmittivity of 0.8 and an absorptivity of 0.12 for solar
radiation and the surrounding ground has a reflectivity of 0.2. The transmittivity and absorptivity
may be considered to be same for direct and diffuse radiations. Both the inside and outside
surface heat transfer coefficients of the window are 3.66 W/m2.K and inside and outside
temperatures are same and are equal to 24C. (5)
2c) Neglecting the conduction resistance of the glass, find the window glass temperature at solar
noon. (2)

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2d) Find the heat transfer rate if the window is converted into an inset window by providing an
external overhang of depth 0.3 m. For the shaded portion of the window, consider only the diffuse
and reflected solar radiations. (3)
3a) Discuss briefly the concept of adaptive thermal comfort and how the concept is useful in
tropical countries such as India? (4)
3b) Neglecting heat transfer due to respiration, and clothing effects, state whether a 1.7 m tall
person weighing 60 kg and doing a light activity (M = 1.8 met) can be at neutral equilibrium or not
when his skin temperature is 33C. The space conditions are: air temperature = surrounding
temperature = 29C, air velocity,V = 0.4 m/s, relative humidity of air = 40%. The convective heat
transfer coefficient between the skin and air can be estimated by the equation: hc (W/m2.K) = 13.5
V0.6, where V is in m/s. Assume the skin to behave as a blackbody and make any other suitable
assumptions, but state them clearly. It is given that for sustained activity, 50% of the total skin area
can remain wet, but not more than that. (4)
3c) With neat sketches explain how the locations of external openings have to be decided if the
aim is to provide as much natural ventilation as possible by utilizing the wind and stack effects.
Assume the building to be non-air conditioned. (3)
4) With neat sketches, discuss briefly the working principle, advantages, disadvantages and
applications of any 2 of the following systems: (2 X 4 = 8)
i. Variable Air Volume Systems
ii. Air-water systems
iii. Room air conditioners
5a) Shown below is the duct layout of an air conditioning system. The fan used in the system
develops a Fan Total Pressure of 120 Pa at design conditions. Using Equal Friction Method find
the required diameters of the duct runs and the amount of dampering required at each of the
outlets, 1,2 and 3.

1
1.5 m3/s

2
3.0 m3/s

3
1.5 m3/s

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Use the following equation for estimating the friction pressure drop: (4)
.
1.852
 p f  0.022243 Q air
 
 L  D 4.973 ; (pf/L) in Pa/m; L and D are in m and Qair is in m3/s
For estimating momentum pressure drops, take the equivalent lengths as: upstream-to-
downstream = 3m, upstream-to-branch = 12m, elbow = 3m, fan outlet = 2m and air outlet in
conditioned space = 3m.
5b) Assuming the duct characteristics to remain same, what is the required FTP, if the air flow rate
to each zone is reduced by 50%? (2)
5c) If the fan operates at 900 RPM and consumes 1200 W at design conditions, what will be the
required fan speed and power consumption when the airflow rate is reduced by 50%? What are
the assumptions made? (1+1+1=3)
End of the paper

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 INDIAN INSTITUTE OF TECHNOLOGY KHARAGPUR

Date : Time : 2 hours Full Marks : 30 No. of students : 15

Mid-Semester, Spring 2009 Mechanical Engineering ME2,DD & UG
Subject No. ME60096 Subject Name: Air conditioning & Ventilation
Note:
4. Use perfect gas model for calculating the required psychrometric properties.
5. Unless otherwise specified, assume the barometric pressure to be 101.3 kPa.
6. Make suitable assumptions, but state them clearly.
Given data:
Molecular weights: Dry air = 28.966 kg/kmol; Water = 18.02 kg/kmol
Universal gas constant = 8.314 kJ/kg.K; Cp of liquid water = 4.18 kJ/kg.K
Specific heats, Cp: Dry air=1.005 kJ/kga.K, Water vapor=1.88 kJ/kgw.K
Stefan_Boltzmann Constant,  = 5.669 X 10-8 W/m2.K4
1 met = 58.15 W/m2, 1 clo = 0.155 m2.K/W
1 Ton of Refrigeration (TR) = 3.517 kW
Antoine’s equation for saturation pressure (psat) of water:
 3985 
ln( psat )  16.54   ; psat in kPa & T in K
 T  39.00 
Latent heat of vaporization of water,hfg in terms of temperature (0  t  40oC):
h fg  2500.95  2.37267 t kJ / kgw

Carrier Equation:
1.8( pt  pv, )( DBT  WBT )
pv  p ,

2800  1.3(1.8DBT  32)

v

,
where pv, pv and pt are vapour pressure, saturated vapour pressure at WBT and barometric
pressure, respectively. DBT and WBT are in oC and units for pressures should be consistent.
Equations for Predicted Mean Vote (PMV) & Percent People Dissatisfied (PPD):
PMV   0.303exp(0.036M )  0.028 L

where M is the metabolic rate and L is the thermal load on the body(difference between internal
heat generation and heat loss to the actual environment)
PPD  100  95 exp  (0.03353 PMV 4  0.2179 PMV 2 ) 

------------------------------------------------------------------------------------------------------------
1. Find the direction and magnitude of total heat transfer (sensible + latent) when air at 40oC and
27% RH flows over a wetted surface that is maintained at a constant temperature of 24oC. The

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wetted surface has a surface area of 0.25 m2 and the convective heat transfer coefficient between
the wet surface and the air flowing over it is 60 W/m2.K. Show the process undergone by air on
psychrometric chart. Make suitable assumptions and state them clearly. (3+1 = 4 marks)
2. Shown below is a hybrid air conditioning system that uses a desiccant bed for dehumidification
of outdoor (OD) air followed by sensible cooling in a heat exchanger (HX) and cooling and
humidification in an evaporative cooler. The cool air from the evaporative cooler is supplied to the
conditioned space at a flow rate of 0.4 kg/s. The outdoor air is at 42oC (DBT) and 28oC (WBT).
The dry and wet bulb temperatures of air at the exit of the heat exchanger are 45oC and 20.8oC,
respectively. The evaporative cooler has an efficiency of 0.9, while the desiccant bed has an
efficiency (defined as the ratio of reduction in humidity ratio of air as it flows through the desiccant
bed to the humidity ratio at the inlet of the bed) of 0.7. Find a) the sensible and latent cooling
capacities of the above system (in kW) if it maintains a conditioned space at 27oC (DBT) and 65%
RH, and b) heat transfer rate in the heat exchanger (HX). Is it possible to maintain the required
conditions in the conditioned space by using only evaporative cooler? (4+1+1= 6 marks)

Exhaust Conditioned
space

27C (DBT)

Desiccant bed Evaporative Supply

l

OD air
HX
42C (DBT)

 t s  tc 
  
3. A condensation resistance factor  t a  tc  is a parameter that is used in connection with
condensation of water vapour on building walls. In the above expression ts and tc are the
temperatures of the hot and cold surfaces of the wall and ta is the dry bulb temperature of air on
the hot side of the wall. The inside surface temperature of a cold storage wall is to be maintained
at 15oC when the outside ambient air is at 35oC (DBT) and 40% RH. For this cold storage wall,
find the minimum value of condensation resistance factor and the minimum wall thickness (in mm)
required to prevent condensation. The effective thermal conductivity of the wall is 0.6 W/m.K and
the design heat transfer coefficient between the outer surface of the wall and surroundings is 23
W/m2.K. Supposing the cold storage is constructed using the minimum insulation thickness

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calculated from the above data, do you expect condensation to take place when, other conditions
remaining same, a) Ambient RH increases to 45%, and b) Surface heat transfer coefficient drops
to 15 W/m2.K due to change in wind velocity. (4 + 2 = 6 marks)
4. Netaji auditorium has a seating capacity of 900. The design inside conditions are: 27oC (DBT),
humidity ratio = 0.015 kgw/kga, mean radiant temperature = 30oC. The convective heat transfer
coefficient and the average radiative heat transfer coefficient between the occupants and
surroundings are both equal to 4.7 W/m2.K. To calculate the evaporative heat transfer rate from
the human body, a Lewis number of 0.85 may be assumed with a skin wettedness factor of 0.1.
Assume that the clothes worn do not offer any resistance to evaporative heat transfer from skin
and all the sweat generated from the body evaporates from the skin. For the occupants assume
an average body area of 1.7 m2, activity level of 1.2 met, clothing resistance of 0.5 clo and clothing
area factor of 1.15. The average skin temperature is 33.8oC. The average respiration rate of the
occupants may be taken as 0.17 grams of air/s, and the temperature and humidity ratio of air that
is breathed out are 35oC and 0.032 kgw/kga, respectively. From the above data, a) find how many
people inside the auditorium are likely to be dissatisfied with the prevailing thermal environment,
and b) find the required cooling capacity of the air conditioning plant in TR when the total external
cooling load (excluding ventilation) on the conditioned space is 30 kW. For the purpose of
ventilation, outside air at a flow rate of 8 litres/person/s (measured at outside conditions) is
supplied to the auditorium. The outside air is at 35oC (DBT) and 50% RH. The internal load on the
auditorium is due to the occupants only. (6+2 = 8 marks)
5. Answer briefly any 2 questions: (2 X 3 = 6 marks)
a) Explain the concept of adaptive thermal comfort and state how it is different from the normal
thermal comfort standards developed based on heat balance.
b) Explain why people may experience sick building syndrome in modern air conditioned
buildings, even though the buildings are maintained at conditions that provide thermal
comfort to the occupants.
c) With suitable equations show the process followed by air on a psychrometric chart when air
is humidified by: a) spraying water, and b) by adding saturated steam.
d) Discuss the concept of effective sensible heat factor (ESHF). How is it useful in air
conditioning calculations?
End of the paper

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 INDIAN INSTITUTE OF TECHNOLOGY KHARAGPUR

Date : Time : 3 hours Full Marks : 50 No. of students : 15

End-Semester, Spring 2009 Mechanical Engineering ME2,DD & UG
Subject No. ME60096 Subject Name: Air conditioning & Ventilation
Given data
Use perfect gas model for calculating the required psychrometric properties.
Unless otherwise specified, assume the barometric pressure to be 101 kPa.
Molecular weights: Dry air = 28.966 kg/kmol; Water = 18.02 kg/kmol
Universal gas constant = 8.314 kJ/kg.K; Cp of liquid water = 4.18 kJ/kg.K
Specific heats, Cp: Dry air=1.005 kJ/kga.K, Water vapor=1.88 kJ/kgw.K
Antoine’s equation for saturation pressure of water (psat):
 3985 
ln( psat )  16.54   
 T  39.00  , psat in kPa and T in K

Solar angles:(l: latitude, h: hour angle, : declination, : Tilt angle, : wall azimuth angle)
Altitude angle,   sin 1 ( cos l. cos h. cos   sin l. sin  )

 cos l. sin   cos  . cos h. sin l

solar azimuth angle( from north),   cos1  
 cos  
angle of incidence,  cos1 (sin  . cos   cos  . cos  . sin )
wall solar azimuth angle,   180    
Solar Radiation data for June 21st:
 0.205   1  cos    1  cos  
I DN 1085. exp   , I d  0.134I DN  , I r   g I DN (0.134  sin  ) 
 sin    2   2 

IDN, Id and Ir are direct normal, diffuse & reflected radiation in W/m2, g is ground reflectance.
Velocity distribution for an isothermal, free jet through a circular opening:
7.41 Vo A o
V (x, r ) 
2
 
x 1  57.5 r 2  
2
  x   , Vo and Ao are velocity and area of opening at supply air outlet.

Answer Question No.1 and any 5 questions from the remaining

1a) From basic definitions and using ideal gas equations, show that:
 RaT   Pt   0.622W 
i) v     0.622  W  ; ii )     
 0.622Pt   0.622Psat   0.622  W 

10

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where v is specific volume of moist air, W is humidity ratio, T is DBT, Psat and Pt are saturated
vapour pressure and total pressure, respectively, Ra is gas constant of dry air and  is relative
humidity. (2+2 = 4 marks)
1b) Using basic principles of psychrometry, explain why moist air is dehumidified when it is
compressed and then stored in the storage tank of air compressor? How does this method of
dehumidification compare with other methods of dehumidification? (3 marks)
1c) An air conditioned room is maintained at 26oC (DBT) and 50% relative humidity. The room has
a room sensible heat factor (RSHF) of 0.65. If air is supplied to the room at 12oC (DBT), what
should be the relative humidity of supply air so that it can take care of the room sensible and latent
cooling loads? (3 marks)
st
2a) Find the total solar radiation flux incident on June 21 , on a west facing, vertical wall of a
building at a time when the horizontal projection of sun’s rays is normal to the west facing vertical
wall. The building is located at 42oN and the declination for June 21st is 23.47o. Assume a ground
reflectance of 0.2. (5 marks)
2b) Find the clock time at which the sun rises and sets at Kharagpur (23oN latitude & 80oE
longitude) on 1st of May (declination = 14.51o, equation of time = 2 minutes and 50 seconds). The
clocks in India are set for a standard meridian of 82.5oE. (3 marks)
3a) Derive an expression for sol-air temperature considering both shortwave (solar) and longwave
radiations. Explain how the wind speed affects sol-air temperature? (3 marks)
3b) The sol-air temperature for the horizontal roof of an air conditioned building is given by the

equation: t sol  air ( C )  42  23.3 * cos(15  192) , where  is the solar time in hours measured from the
o

midnight (i.e.,  = 0 hours at 12’0 clock, midnight). The roof has an area of 84 m2 and is made up
of 150 mm thick concrete (k=1.73 W/m.K) with 6mm thick plaster (k=8.65 W/m.K) on both sides
of the roof. The internal and external surface conductance values for the roof are 8.3 W/m2.K and
23.3 W/m2.K, respectively. The roof has a time lag of 5.2 hours and a decrement factor of 0.6. If
the air conditioned space is maintained at a dry bulb temperature of 26oC, find the minimum and
maximum cooling loads on the building due to the roof and the corresponding solar times.
(5 marks)
4a) What are the factors that cause infiltration in buildings? Explain a practical method that can be
used for measuring infiltration rate in buildings. (1+2 = 3 marks)
4b) In a naturally ventilated building, the air flow rates due to wind and stack effects are given by
Qwind ( m 3 / s )  0.55 * A *Vwind and Qstack ( m 3 / s )  0.6 * A * 2 g * hNPL (Ti  To ) / Ti
the equations: , where A is the
area of opening in m2, Vwind is wind speed in m/s, hNPL is the difference in heights between the
opening and the neutral pressure level in m, Ti and To are the inside and outside air temperatures

11

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in K and g is acceleration due to gravity in m2/s. From the following input data find the area of the
opening required so that the air flow rate due to natural ventilation takes care of the entire internal
heat generation rate (Qint) of the building. Wind speed = 25 kmph, hNPL = 1.5 m, Qint =3 kW, Ti =
33oC and To= 29oC. Assume an average air density of 1.2 kg/m3 and a cp value of 1.02 kJ/kg.K.
(5 marks)
5a) Derive Borda-Carnot equation, and then using this equation, obtain an expression for pressure
loss due to sudden contraction. (2+2 = 4 marks)
5b) In a sudden enlargement in an air conditioning duct, the cross-sectional area increases from 1
m2 to 2.2 m2. If the available static pressures in the upstream and downstream of the fitting (i.e.,
the sudden enlargement) are 0.5 inches of H2O column and 0.7 inches of H2O column,
respectively, what is the maximum possible air flow rate through the fitting? The densities of water
and air may be taken as 1000 kg/m3 and 1.2 kg/m3, respectively. (4 marks)
6a) Define the terms Effective Draft Temperature (EDT), Air Distribution Performance Index
(ADPI) and Space Diffusion Effectiveness Factor (SDEF). (3 marks)
6b) An isothermal, free jet enters the room through a circular outlet. What should be the velocity of
air at the supply outlet, if it is required to provide a supply air flow rate of 90 litres/s with a throw of
12 m? Also find the entrainment ratio at throw. (3+2 =5 marks)
7a) Find the minimum life cycle cost of a ducting system by optimizing the duct diameter. Use the
data given below:
a) Thickness of the duct material : 1.5 mm
b) Density of the duct material : 8000 kg/m3
c) Cost of the ducting material : Rs. 12/- per kg
d) Volumetric flow rate of air : 2.0 m3/s
e) Density of air : 1.2 kg/m3
f) Friction coefficient, f : 0.02
g) Number of operating hours : 12000 hours
h) Cost of electricity : Rs. 5 per kWh
i) Efficiency of fan : 60 %
j) Total length of duct : 150 m
Assume the cost of fan, fan efficiency, density of air and friction coefficient to remain constant
independent of the duct diameter. (5 marks)
o
7b) A fan-duct system is designed such that when the air temperature is 20 C, the mass flow rate
is 3 kg/s when the fan speed is 25 rps and the fan motor requires 1.2 kW. Now the operating air
temperature is changed to 5oC, and the fan speed is changed so that the same mass flow rate of
air prevails. What are the revised fan speed and power requirement? (3 marks)
End of the paper

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Indian Institute of Technology Kharagpur, Department of Mechanical Engineering
ME60096 (Air conditioning & Ventilation) – Mid Semester – 2010
Duration: 2 hours Total marks : 45
Given data: Barometric Pressure, pt = 101 kPa, Universal gas constant: 8.314 kJ/kmol.K
Molecular weights (kg/kmol): Dry air = 28.966; Water = 18.02
Cp(kJ/kg.K): Dry air=1.005, moist air (avg.): 1.0216; Water vapor=1.88; Liquid water = 4.18
Latent heat of vaporization of water, hfg(t) = 2501 2.368*t; hfg in (kJ/kg) and t in oC
1 met = 58.2 W/m2, 1 clo = 0.155 m2.K/W; fcl = (Acl/AD) = 1+0.3*Icl, Icl in clo
Du-Bois area, AD = 0.202m
0.425 0.725
h ; m in kg, h in m, AD in m2,  = 5.678 X 10-8 W/m2.K4

a) Antoine’s equation for saturation pressure of water (psat)

 3985 
ln( psat )  16.54    , psat in kPa and T in Kelvin
 T  39.00 

1.8 pt (DBT  WBT )

b) Apjohn Equation: pv  p 'v  , DBT & WBT in oC
2700
pv is the vapour pressure, p’v is the saturated vapour pressure at WBT and pt is the barometric
pressure (pressure units should be consistent)

c) Saturation humidity ratio (kgw/kga), Wsat(t) = 0.00364+0.000375*t, (Validity: 0.1  t  10oC)

Wsat(t) = 0.000042+0.00073*t, (Validity: 11  t  21oC)

Part-A is compulsory. Answer any 2 questions from Part-B

Part-A
A1. Using suitable equations and the assumption of ideal gas behavior, explain the procedure for
generating the psychrometric chart for a given barometric pressure. (5 marks)

A2. A car’s interiors are kept at 23oC (DBT) and 40 % (relative humidity), while the outside
conditions are: 4oC (DBT) and 90 % (relative humidity). The windshield of the car is made up of 6
mm thick, Pyrex glass (k = 1.01 W/m.K). If the internal and external heat transfer coefficients
between the inner and outer surfaces of the windshield and surrounding air are 8 W/m2.K and 60
W/m2.K, respectively, find whether any condensation of water vapour takes place on the
windshield. If condensation takes place, state on which side (inner or outer) of windshield it
occurs. Does the speed of car influence the formation of condensation in any manner? (5 marks)

A3. In a mixing chamber 1 kg/s of air stream 1 at 30oC (DBT) and 100% (relative humidity) mixes
with 1 kg/s of air stream 2 at 25oC (DBT) and 60% (relative humidity). Assuming the mixing
process to be adiabatic, find the condition of air at the exit of mixing chamber. Show the process
on psychrometric chart. (5 marks)

A4. In a humidifier air is humidified by bringing it in contact with a spray of hot water at 80oC. If the
air enters the humidifier at 40oC (DBT) and 30% (relative humidity), and its humidity ratio
increases by 5 grams/kg of dry air, what is the dry bulb temperature of air at the exit of the
humidifier? Show the process on psychrometric chart. (5 marks)

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A5. Based on the requirement of a neutral condition, explain briefly whether the air dry bulb
temperature in a room should be increased, decreased or kept constant when:

a) Activity level increases; b) Air velocity increases; c) Surrounding temperature increases;

d) Clothing resistance decreases; and e) Moisture content in room increases (5 marks)

Part-B

B1. A psychrometer measures dry bulb and wet bulb temperatures of air in a room as 27oC and
18oC, respectively. The mean radiant temperature of the room is 30oC. The convective heat
transfer coefficients between the dry bulb and air, and between the wet bulb and air are both equal
to 8 W/m2.K. Both the dry and wet bulb sensors have emissivity values of 0.95. The Lewis number
for air may be taken as 0.90. The dry bulb sensor is unshielded, while a perfect radiation shield is
provided for the sensor of the wet bulb only. From this data, find a) The true dry bulb temperature
of air, and b) Humidity ratio of air, c) What will be the temperature indicated by the wet bulb
thermometer, if the wet bulb is also unshielded? Make reasonable assumptions and state and
justify them clearly. (10 marks)

B2. A winter air conditioning system is to be designed for a sensible heat loss from the building of
40 kW and a latent heat loss from the building of 5 kW. The required inside conditions are 22oC
(dry bulb) and 40 % (relative humidity). The outside conditions are 8oC (dry bulb) and 100 %
(relative humidity). Air is to be supplied to the room at a dry bulb temperature of 37oC. The supply
air should consist of 10 % of outdoor air (by mass) for ventilation. The system consists of a pre-
heater, steam humidifier and a re-heater. The dry bulb temperature of air increases by 10oC in the
pre-heater. In the humidifier, dry saturated steam is added at a temperature of 120oC (hg=2706
kJ/kg). Draw a schematic of the system and show the processes on psychrometric chart. Find a)
Heat input to the pre-heater; b) Steam consumption in kilograms per hour; and c) Heat input to the
re-heater. Verify the overall energy balance for the system (10 marks)

B3. A room is maintained at 25oC (dry bulb) and 60 % (relative humidity). The average height and
weight of the occupants inside the room are 1.7 m and 60 kg, respectively. The average activity
level of the occupants is 1.5 met and clothing resistance (Icl) is 0.5 clo. The convective and
radiative heat transfer coefficients between the occupants and the air and surroundings are 8.1
W/m2.K and 4.7 W/m2.K, respectively. a) What should be the mean radiant temperature of the
room so that the skin temperature of the occupants is 33oC, and the total evaporative heat loss
from the skin is 40 % of the total heat loss from skin? b) What is the mean clothing temperature?
Use the following equations for estimating heat losses due to respiration. (10 marks)

Qresp,sensible (in W) = 0.0014*M*AD(34-ta); Qresp,latent (in W) = 0.0173*M*AD(5.87-pv)

where M is metabolic rate in W/m2; AD is Du-Bois area in m2; ta is dry bulb temperature of air in oC,
and pv is water vapour pressure in air in kPa.

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 INDIAN INSTITUTE OF TECHNOLOGY KHARAGPUR
Date : Time : 3 hours `Full Marks : 50 No. of students : 21
End-Semester, Spring 2010 Mechanical Engineering ME2 & 5DD
Subject No. ME60096 Subject Name: Air conditioning & Ventilation
----------------------------------------------------------------------------------------------------------------------------------
Answer all questions

Given data
 3985 
saturation pressure of water: ln( psat )  16.54   ; psat in kPa and T in K
 T  39.00 
Solar Radiation data for June 21st:
 0.205   1  cos    1  cos  
I DN 1085. exp   , I d  0.134I DN  , I r   g I DN (0.134  sin  ) 
 sin    2   2 
IDN, Id and Ir are direct normal, diffuse & reflected radiation in W/m2, g is ground reflectance. is
the tilt angle and  is the altitude angle.
altitude angle,   cos(l ).cos(h ).cos( )  sin( l ).sin( ) ; where l, h and  are the latitude, hour angle and
declination, respectively.
Velocity distribution for an isothermal, free jet through a circular opening:
7.41 Vo Ao
V ( x, r )  ; Vo and Ao are velocity and area of opening at supply air outlet
 
2
x 1  57.5 r 2 
2

 x 
Frictional pressure drop and equivalent diameters of ducts:
.
 p 
1.852
0.022243 Q air .
Frictional pressure drop through ducts:  f   ; Δpf in Pa; D & L in m; Qair in m3 /s
 L  D 4.973
.
1.3(a.b)0.625
Equivalent diameter of a rectangular duct (for same Qair and pf L ): Deq 
(a  b)0.25
======================================================================
1) An air conditioned classroom measuring 20m X 12m X 8m has a seating capacity of 200
students. On an average each student generates 16 cm3 of CO2 per second. The CO2
concentration of outdoor air is 35 ppm (parts per million). The air conditioned system re-circulates
75 percent of supply air, while the remaining 25 percent is the outdoor air. The space air
distribution system is such that 20 percent of the air supplied to the classroom by passes the
occupied zone. If the maximum permissible concentration of CO2 inside the classroom is 1000
ppm, find the amount of outdoor air required in terms of a) litres/second/student and b) air
changes per hour (ACH). (5)

2) To measure the natural ventilation rate in a building of internal volume 1200 m3, a tracer gas is
injected into the building at a constant rate of 18 cm3/s. Find the ventilation rate, if at steady state
the concentration measured in the building shows a value of 0.6 cm3/m3 of air. What is the time
required for the tracer gas concentration inside the building to reach a value of 99 percent of the
steady-state value? Assume that tracer gas concentration before injection is 0 and there are no
tracer gas sources or sinks inside the building. (2+3 = 5)

3) Find the heat flux through a thin, opaque wall (overall heat transfer coefficient of the wall, U=1.2
W/m2.K) when the total solar radiation incident on the surface of the wall is 800 W/m2. The wall
has an absorptivity of 0.8. The external heat transfer coefficient is 23 W/m2.K, the outer and inner
dry bulb temperatures are 37oC and 25oC, respectively. b) Under what assumption, the answer
obtained is correct? c) Account for total energy balance for the wall. (3+1+1=5)

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4) Find the maximum solar radiation incident on the flat roof of a building on June 21st. The roof
measures 21 m by 12 m, and the building is located at 22°20'N and 87°25'E. Also find the total
number of sunshine hours for this location on June 21st . Declination for June 21st is 23.5o.
(4+1 = 5)
5) A duct run made of a circular duct consists of a sudden contraction, followed by 20 m of straight
duct and then a sudden expansion. If the flow rate of air through the duct run is 0.9 m3/s, find the
total pressure drop from section 1 to 2. Assume an air density of 1.2 kg/m3. Use the following
1 2
equation for contraction coefficient:
Contraction coefficient (Cc): L
Cc  0.61  0.17 Ar  0.34 Ar  0.56 Ar
2 3

Where Ar = A2/A1. D2 D1 D2
(5)
L = 20 m, D1=0.3 m, D2 = 0.75 m 1.2 m3/s
Figure for Problem 3a
6) A two-branch duct system of rectangular 10 m 12 m
cross section is shown below. The fittings 6m
have the following equivalent length of the
straight duct; upstream to branch: 8 m,
elbow: 4 m and outlets: 8m. There is
Fan
negligible pressure loss in the straight 8m
through section (upstream to downstream) of 6m
the branch. The designer selects a fan that
2 m3/s
provides 40 Pa fan total pressure at a flow
rate of 3.2 m3/s. Taking an aspect ratio of 4 Figure for Problem 3b
everywhere, select the dimensions of the duct
system. (5)

7) A centrifugal fan, provides 2.4 m3/s at a fan total pressure of 50 Pa and, consumes 160 W
when the speed is 3000 RPM and air temperature is 21oC. What will be the flow rate, fan total
pressure and power consumption when the changes to 4500 RPM and the temperature is 27oC?
What is the fan efficiency? (1+1+2+1 = 5)

8) Write short notes on ANY 1 of the following with neatly drawn diagrams, wherever necessary:
(5)
a) Evaporative cooling systems for hot and humid climates
b) Multi-zone, single duct, constant volume, all-air systems
c) Radiative cooling systems

9) Using the assumption of similarity in velocity profiles and conservation of x-momentum, find
how the mid-plane velocity of air for an isothermal, plane, rectangular, free-jet varies with the
distance measured from the outlet. Compare this with a circular free-jet and comment on the
applicability of rectangular and circular jets. (3+1+1=5)
10) What should be the diameter of a circular outlet, which provides an entrainment ratio of 60 at
throw, when the flow rate at the outlet is 0.12 m3/s? (5)

End of the paper

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 Indian Institute of Technology Kharagpur
Department of Mechanical Engineering
ME60096 (Air conditioning & Ventilation) – Mid Semester – 2011
Duration: 2 hours Total marks : 60
Given data

a) Barometric Pressure, pt = 101 kPa, Universal gas constant: 8.314 kJ/kmol.K

b) Molecular weights (kg/kmol): Dry air = 28.966; Water = 18.02
c) Cp,avg (kJ/kg.K): Dry air = 1.005, moist air : 1.0216; Water vapor = 1.88;
d) Cp,avg (kJ/kg.K): Liquid water = 4.18
e) Latent heat of vaporization of water at temperature t,
hfg(t) = 2501 2.368t; hfg in (kJ/kg) and t in oC

f) Antoine’s equation for saturation pressure of water (psat)

 3985 
ln( psat )  16.54    , psat in kPa and T in Kelvin
 T  39.00 

g) Apjohn Equation:
1.8 pt (DBT  WBT )
pv  p 'v  , DBT & WBT in oC
2700
pv is the vapour pressure, p’v is the saturated vapour pressure at WBT and pt is the barometric
pressure (pressure units should be consistent)

Make suitable assumptions, wherever necessary, and state the assumptions clearly

1. In a spray type cooling tower, the temperature of water is reduced by 4oC by exchanging heat
with an air stream flowing through the cooling tower. The flow rate of water is 20 kg/s and its
average temperature is 32oC. The average dry bulb and wet bulb temperatures of air are 42oC
and 28oC, respectively. The water is sprayed in the cooling tower using spray nozzles and energy
exchange takes place between air and water droplets (assumed to be spherical and of uniform
diameter) only. The convective heat transfer coefficient between air and the water droplets is 240
W/m2.K. Neglecting heat transfer between the surface of the cooling tower body and
surroundings, find a) the required diameter of the water droplets to accomplish the given task.
b) Amount of make-up water required to account for loss of water due to evaporation (in kg/s).
Assume an average density of liquid water to be 995 kg/m3.
(12+4 = 16 marks)

2. An air conditioning system has to be selected for a building that has a maximum occupancy of
1000 people. The building has a sensible cooling load of 300 kW and a latent cooling load of 100
kW. The design inside conditions are 26oC (DBT) and 50% (relative humidity), while the design
outside conditions are 42oC (DBT) and 28oC (WBT). For ventilation outside air at the rate of 8
liters per second per person (measured at outside conditions) is required. A cooling and
dehumidification coil with a bypass factor of 0.15 is to be chosen. From the data given find: a)
Required supply air flow rate in kg/s, and b) Sensible and latent cooling capacities of the cooling &
dehumidification coil in kW. The humidity ratio (wsat) and enthalpy (hsat) of saturated moist air in

17

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terms of temperature t (4 < t < 14oC) can be obtained using the following equations (t is in oC for
both the equations):

wsat = 2.97 X 103 + 4.78 X 104 t (kgw/kgda)

hsat = 7.37 + 2.22 t (kJ/kgda)
(12+4 = 16 marks)

3. Find the outdoor air required (in liters per second) for an air conditioned building such that the
concentration of CO2 inside the building does not exceed 1000 parts per million (ppm). The
building has occupancy of 1000 persons with an average CO2 generation rate of 10
cm3/second/person. The air conditioning system is designed for a recirculation of 80% of the
room air. The air distribution system is such that 20 % of the air supplied to the building bypasses
the occupied zone. The CO2 concentration of outdoor air is 350 ppm. The recirculation and
bypass are in terms of volumetric flow rates of air. (10 marks)

4. An experiment is conducted in a building to measure the outside air flow rate into the building
due to natural ventilation. In the experiment Sulfur Hexafluoride (SF6) is injected into the building
for a certain amount of time so that the concentration of SF6 at the end of injection is 20 ppm. Now
the injection is stopped and the decrease in concentration is measured as a function of time.
Measurements show that after 1 hour the concentration of CO2 drops to 2 ppm. Assuming well
mixed conditions inside the building and concentration of SF6 in the OD air to be 0 and no other
sources of SF6 inside the building, find the outdoor air flow rate into the building in litres per
second if the internal volume of the building is 4800 m3.
(8 marks)

5. Write a short note on any one of the following:

(10 marks)

a) Adaptive thermal comfort; its meaning and relevance

b) Evaporative cooling systems for hot and humid conditions
c) Practical measurement of psychrometric properties; instruments required and precautions
to be taken

End of the paper

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 INDIAN INSTITUTE OF TECHNOLOGY KHARAGPUR
Date : Time : 3 hours Full Marks : 50 No. of students : 10
End-Semester, Spring 2011 Mechanical Engineering ME2 & 5DD
Subject No. ME60096 Subject Name: Air conditioning & Ventilation
----------------------------------------------------------------------------------------------------------------------------------

Answer all questions

1a. An evaporative cooler can cool incoming air at 27oC (dry bulb) and 15oC (wet bulb) to a final
dry bulb temperature of 18oC. Assuming the air and water flow rates to remain constant, to what
final temperature the evaporative cooler can cool air if the dry bulb and wet bulb temperatures of
air at the inlet to the cooler change to 35oC (dry bulb) and 21oC (wet bulb)? Justify your answer
with suitable equations and assumptions. (4)
o
1b. Air at 40 C (dry bulb) and 50 % relative humidity is in contact with water which is at a
temperature of 32oC. Find the magnitude and direction of total heat transfer rate between air and
water per unit area of the water surface. Take the convective heat transfer coefficient between air
and water surface as 23 W/m2.K, and the Lewis number as 0.85. (4)
2. An air conditioned building has
design sensible and latent loads of 60 Figure for Problem 2
kW and 12 kW, respectively. The 60 kW (sensible)
design inside and outside conditions exhaust return
12 kW (latent)
o Bldg. o
are: 25 C and 60 % relative humidity, 25 C,
60 % RH
and 33oC (dry bulb) and 24oC (wet
bulb), respectively. For ventilation
outside air at a mass flow rate of 1.2
kg/s is mixed with the return air as
OD air
shown in the figure. The air 33oC (dbt),
1 2 3
conditioning system consists of two 24oC (wbt)
cooling coils, Coil 1 and Coil 2,
respectively. Coil 1 with a sensible heat C il 1 C il 2
factor of 0.6 is used to cool and dehumidify the outside air only, while Coil 2 handles only sensible
load (sensible heat factor = 1.0). If the temperature of air supplied to the conditioned space is
12oC, find the latent and sensible cooling capacities of Coil 1 and 2. Show the complete process
on psychrometric chart. (6+2 = 8)
3a. To reduce cooling load, the exposed roof of a building is evaporatively cooled by spraying
water. Write the relevant governing equation(s), boundary and initial conditions required for
estimating the cooling load due to this evaporatively cooled roof. (3)
3b. Derive Borda-Carnot equation and, using this equation derive an expression for dynamic loss
for air flow through a sudden contraction. (6)
3c. Explain the basis of fan laws and state how they are useful in practice. (3)
4. An air-water system shown in the figure given below is used to maintain the conditioned space
of a building at 25oC (dry bulb) and 60 % (relative humidity).The building has a sensible load of
300 kW and a latent load of 80 kW. For ventilation purpose, 20 % of the supply air (by mass) is
outside air, which is at 42oC (dry bulb) and 28oC (wet bulb). The cooling coil used in the primary
air system (Coil 1) handles the entire latent load on the building and has a coil ADP of 7oC and a
bypass factor of 0.15. In the chilled water system, chilled water at 7oC is supplied to the fan coil
unit (FCU) kept inside the building. After extracting the required amount of sensible heat, the
chilled water leaves the FCU at 16oC. Find a) Required sensible and latent cooling capacities of
coil 1, b) Chilled water flow rate through FCU, and c) Effective surface temperature of the fan coil
unit such that no condensation takes place in FCU. (5+2+1 = 8)

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 300 kW (sensible)
Primary air Coil 1 80 kW (latent)
25oC,
Chilled water 60% RH

Figure for Problem 4 FCU

5a. It is required to design suitable supply air outlets for a movie theatre so that a uniform
distribution is obtained in the conditioned space. The air outlets, circular in shape have to be
located on the opposite walls separated by a distance of 16 m. The total supply air flow rate is 12
m3/s. If the maximum allowable velocity at the supply air outlet is 6 m/s, find the size and number
of outlets required. Also find the entrainment ratio at the end of throw (Vmax = 0.25 m/s). State the
assumptions made while arriving at the above answers? (5+2 = 7)

5b. A dual duct, all air system is used to air condition a building which has a peak sensible cooling
load of 150 kW and a peak sensible heating load of 60 kW. The room-to-supply air temperature
difference is 12 K for cooling and 28 K for heating. The maximum allowable velocity of air in both
cold and hot air ducts is 9 m/s. The index run of the cold air duct has an equivalent length of 180
m, while the index run of hot air duct has an equivalent length of 75 m. Assuming uniform
diameter throughout the index runs, find a) Minimum diameters of cold and hot air ducts, b) Using
Equal Friction Method, find the maximum power rating of the electric motor that drives the fan in
kW, assuming an overall fan-motor efficiency of 0.6. Take the density of air as 1.15 kg/m3 and the
specific heat to be 1.02 kJ/kg.K. (5+2 = 7)

Given data: Unless otherwise specified, use the following data

Barometric pressure,pt = 101 kPa, Latent heat of vaporization of water at 0oC = 2501 kJ/kg,
Molecular Weights: Dry air: 28.97 kg/kmol, water: 18.03 kg/kmol.
Specific heats: Dry air: 1.005 kJ/kga.K, moist air: 1.0216 kJ/kga.K, water vapour: 1.88 kJ/kgw.K
 3985 
Saturation pressure of water: ln( psat )  16.54    ; psat in kPa; T in K
 T  39 

1.8 pt (dbt  wbt )

Water vapour pressure of moist air: pv  psat (wbt )  ;
2700
dbt and wbt in oC, pressure units to be consistent

Velocity distribution for isothermal, free jets through a circular opening:

7.41 Vo Ao
V ( x, r )  ; Vo and A o are velocity and area of supply air outlet .
 
2
 2
x 1  57.5 r 2 
 x 
Frictional pressure drop for air flow through a circular duct:
.
 p 
1.852
0.022243 Qair .
Frictional pressure drop through ducts:  f   ; Δpf in Pa; D & L in m; Qair in m3 /s
 L  D 4.973

End of the paper

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 INDIAN INSTITUTE OF TECHNOLOGY KHARAGPUR
Date : Time : 2 hours Full Marks : 60 No. of students : 35
Mid-Semester, Spring 2012 Mechanical Engineering ME2 & 5DD
Subject No. ME60096 Subject Name: Air conditioning &
Ventilation

Given data: Molecular weights: Dry air = 28.966 kg/kmol; Water = 18.02 kg/kmol
Specific heat, Cp: Dry air=1.005 kJ/kga.K, Water vapor=1.88 kJ/kg.K, moist air (avg)=1.0216
kJ/kga.K
Specific heat of Liquid water = 4.18 kJ/kg.K, Universal gas constant = 8.314 kJ/kmol.K
Latent heat of vaporization of water = 2501 kJ/kg, Barometric Pressure, pt = 101 kPa, 1TR =
3.517 kW
a) Antoine’s equation for saturation pressure of water (psat)
 3985 
ln(p sat )  16.54    , psat in kPa and T in Kelvin
 T  39 .00 
1.8 p t (DBT  WBT )
b) Apjohn Equation: p v  p'v  , DBT & WBT in oC
2700
pv is the vapour pressure, p’v is the saturated vapour pressure at WBT and pt is the barometric
pressure (pressure units should be consistent)
c) Saturation humidity ratio (kgw/kga), Wsat(t) = 0.00359+0.00028*t, (Validity: 10  t  0oC)
Wsat(t) = 0.00364+0.000375*t, (Validity: 0.1  t  10oC)
Wsat(t) = 0.000042+0.00073*t, (Validity: 10 < t  21oC)
d) Enthalpy (kJ/kgw) of superheated steam at temperature t (oC) = 2501+1.88*t
e) Du-Bois area of human body, AD = 0.202m0.425h0.725; m in kg, h in m, AD in m2
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------

Important Note: Either psychrometric equations or psychrometric chart can be used for
performing calculations. If psychrometric chart is used then the state points and processes have to
be clearly marked on the chart, and the chart should be submitted along with the answer paper
Answer all questions

1a) Prove that on the psychrometric chart the exit state of moist air in contact with a cooling and
dehumidification coil lies on the straight line joining the inlet state of air to the saturation condition
corresponding to the apparatus dew point (ADP) of the cooling coil. State the assumptions made
while arriving at this conclusion. (6)

1b) An air conditioning system for a hospital uses 100% outdoor air, which is at 42oC (DBT) and
28oC (WBT). The sensible and latent loads on the conditioned space maintained at 26oC (DBT)
and 50% relative humidity are 53 kW and 25 kW, respectively. Air is supplied to the conditioned
space at DBT of 12oC. If chilled water is used as coolant in the cooling and dehumidifying coil, find

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whether a conventional system or a system with reheating is required for this purpose. Justify your
answer with suitable reasoning. (9)

2a) Air at 26oC (DBT) and 50% relative humidity flows through a humidifier in which superheated
steam at 150oC is added such that the humidity ratio of air increases by 10 grams/kg of dry air.
Find the dry bulb temperature of air at the exit of the humidifier. (6)

2b) In one element of a counterflow type cooling tower, the temperature of 12 kg/s of water
decreases from 35oC to 33oC as it exchanges energy with 16 kg/s of air. The condition of air at
the inlet to the element is 42oC (DBT) and 28oC (WBT). If the convective heat transfer coefficient
between water and air is 480 W/m2.K, find the required heat transfer area of the element. What is
the dry bulb temperature of air at the exit of the element? (9)

3) A human being with a body mass of 60 kg and height of 1.71 m is in a conditioned space that is
at 26oC (DBT) and 50% relative humidity with an average air velocity (Vair) of 0.25 m/s. The
activity level of the person is 1.5 met (1 met = 58.15 W/m2).The average skin temperature is 34oC
while the surrounding surface temperature is 29oC. The radiative heat transfer coefficient between
the human being and surrounding surfaces is 4.7 W/m2K. The convective heat transfer coefficient
(hc in W/m2K) is given as a function of surrounding air velocity (Vair in m/s) as: hc = 8.3Vair0.6. The
heat transfer resistance offered by the clothes worn by the human being is 0.5 clo (1 clo = 0.155
m2K/W) and the surface area ratio of the person with and without clothes is 1.15. The clothes do
not offer any resistance to evaporation from skin. The respiration rate of the human being is 0.2
grams/s and the temperature and humidity ratio of air that is breathed out are 35oC and 0.032
kgw/kga, respectively. Find a) Average clothing temperature, b) whether the human being finds
the thermal environment of the conditioned space comfortable or not. Studies show that human
beings may not be comfortable if the skin wettedness ratio is greater than 0.10. c) If the person is
not comfortable suggest a simple and practical method which can make the surroundings more
comfortable. Justify your answer briefly. Assume a Lewis number of 0.9 and a Lewis Ratio (LR) of
16.5 K/kPa. (5+5+5 = 15)

4) An air conditioned building has a total cooling load of 500 kW with a room sensible heat factor
of 0.75. The inside and outside design conditions are: 26oC (DBT)/50% relative humidity and 42oC
(DBT)/28oC (WBT), respectively. The cooling and dehumidification coil has bypass factor of 0.1.
For ventilation purposes, outdoor air at a flow rate of 5 m3/s (measured at room conditions) is
supplied to the building. Find a) The required supply air conditions (flow rate, DBT and humidity
ratio) and b) Required sensible and latent cooling capacities of the air conditioning system in Tons
of Refrigeration (TR). (6+9 = 15)

End of the paper

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 INDIAN INSTITUTE OF TECHNOLOGY KHARAGPUR
Date : Time : 3 hours Full Marks : 60 No. of students : 35
End-Semester, Spring 2012 Mechanical Engineering M.Tech & DD
Subject No. ME60096 Subject Name: Air conditioning & Ventilation
----------------------------------------------------------------------------------------------------------------------------------

Answer all questions

The attached psychrometric chart may be used, if required

1. The dry and wet bulb thermometers of a psychrometer indicate 27oC and 18oC, respectively.
The convective heat transfer coefficient between the thermometer bulbs and surrounding air is 4.7
W/m2.K. If the mean radiant temperature of the surroundings is 30oC, and emissivity of both the
bulbs is 0.9, find a) the true dry bulb temperature of air, and b) humidity ratio of air. Assume that
the sensing bulbs of the dry and wet bulb thermometers do not have radiation shields. However
the sensing bulbs of dry and wet bulb thermometers do not see each other. (Stefan Boltzmann
constant,  = 5.678x10-8 W/m2.K4). (2+4 = 6)

2. Shown here is a hybrid air conditioning

system that uses a desiccant wheel (D) for r
dehumidification of outdoor (o) air followed by
sensible cooling in heat exchanger (HX) using
room return air (r), and cooling and Fig. for 2 S
humidification in an evaporative cooler (E). D
The cool air from the evaporative cooler is
supplied to the conditioned space (S) at a E
flow rate of 0.5 kg/s. The outdoor air (o) is at o H
42oC (DBT) and 28oC (WBT). The heat
exchanger effectiveness is such that the wet bulb temperature at the exit of heat exchanger is 18oC. The
evaporative cooler (E) has an efficiency of 0.9, while the desiccant wheel (D) has an efficiency (defined as
the ratio of reduction in humidity ratio of air as it flows through the desiccant bed to the humidity ratio at the
inlet of the bed) of 0.60. Find a) the sensible and latent cooling capacities of the above system (in kW) if it
maintains a conditioned space at 27oC (DBT) and 60% RH, and b) heat transfer rate in the heat
exchanger, HX in kW.

(6+2 = 8)

3. Find the total solar radiation flux incident on June 21st, on a west facing, vertical wall of a
building at a time when the horizontal projection of sun’s rays is normal to the west facing vertical
wall. The building is located at 32oN and the declination for June 21st is 23.5o. Assume a ground
reflectance of 0.2. (8)

4. A building has a U-value of 0.8 W/m2.K and a total exposed surface area of 400 m2. The building is
subjected to an external load (only sensible) of 3 kW and an internal load of 1.8 kW (only sensible). If the
required internal temperature is 25oC, using the concept of balanced outdoor temperature, state whether a
cooling or a heating system is required when the ambient air temperature is 8oC. How the results will
change, if the U-value of the building is reduced to 0.6 W/m2.K? What conclusions can be drawn from this
example regarding the effect of insulation on cooling and heating periods of buildings? (3+1+2 = 6)

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5. An air conditioned building located in Kharagpur (latitude,l = 22.3o N) has an opaque flat roof with an

area of 72 m2. The roof is made out of a material that has a U-value of 2.4 W/m2.K and negligible thermal

capacity (negligible thermal lag and decrement). The internal and external surface heat transfer coefficients

for the roof are 8.3 W/m2.K and 23.3 W/m2.K, respectively. The design inside and outside dry bulb

temperatures are 25oC and 37oC, respectively. The external surface of the roof has a solar absorptivity of 0.9

(same for direct and diffused radiation). The effect of longwave radiation between the roof and sky is

equivalent to a reduction of 3 K in the sol-air temperature. Using the given data and ASHRAE clear sky

model a) estimate the cooling load on the building due to the roof at solar noon on June 21st (declination, =

23.5o), b) the internal and external surface temperatures of the roof. (6+4 = 10)

6. In a naturally ventilated building, the air flow rates due to wind and stack effects are given by the

equations: Qwind (m 3 / s )  0.55 * A *Vwind and Qstack (m 3 / s )  0.6 * A * 2 g * hNPL (Ti  To ) / Ti , where A is the area of

opening in m2, Vwind is wind speed in m/s, hNPL is the difference in heights between the opening and the

neutral pressure level in m, Ti and To are the inside and outside air temperatures in K and g is acceleration

due to gravity in m/s2. From the following input data find the area of the opening required so that the air

flow rate due to natural ventilation takes care of the entire internal sensible heat generation rate (Qint) of the

building. Wind speed = 12 km/h, hNPL = 1.5 m, Qint = 1.8 kW, Ti = 305 K and To= 300 K. Assume an

average air density of 1.2 kg/m3 and a cp value of 1.02 kJ/kg.K. (6)

7. In a straight, 30m long, horizontal duct of uniform cross-section, air flows at a rate of 1.6 m3/s. If the

velocity of air through the duct is 8 m/s, find the required fan power when, a) a circular duct is used, and b)

a rectangular duct of aspect ratio 1:4 is used. Take the efficiency of the fan to be 0.7. If a GI sheet of 0.5

mm thick with a density of 8000 kg/m3 is used to construct the duct, how many kilograms of sheet metal is

required for circular and rectangular cross sections? Use the following equations: (6+2 = 8)

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Pf 0.022243Q1.852 1.3( a.b)0.625
 ; D  , where Q is the flow rate in m3/s, Pf/L is the frictional
( a  b)0.25
eq
L Deq4.973
pressure drop per unit length (Pa/m), Deq is the equivalent diameter of the rectangular duct of
sides a and b.
8. In an air conditioning system, 1.8 m3/s of air (density = 1.2 kg/m3) flows through a straight
circular duct that has a cross sectional area of 0.25 m2 for a length of 8 m, followed by a sudden
expansion to an area of 0.6 m2. The total length of the straight duct is 18 m. Find a) the total
pressure drop (frictional + dynamic) for the duct system, and b) Static Regain factor for the
expansion. Use the equation given in Problem 6, for estimating the frictional pressure drop and
Borda-Carnot equation for estimating the dynamic loss due to sudden expansion. Assume
dynamic loss coefficients of 0.03 and 1.0 for duct entry and exit. (6+2 = 8)

Given data: Barometric pressure,pt = 101 kPa, Latent heat of vaporization of water at 0oC = 2501
kJ/kg, Molecular Weights: Dry air: 28.97 kg/kmol, water: 18.03 kg/kmol.
Specific heats: Dry air: 1.005 kJ/kga.K, moist air: 1.0216 kJ/kga.K, water vapour: 1.88 kJ/kgw.K
.
, , psat in kPa and T is in K
.
, , dbt and wbt in oC

Solar angles:(l: latitude, h: hour angle, : declination, : Tilt angle, : wall azimuth angle)
Altitude angle,   sin 1 ( cos l. cos h. cos   sin l. sin  )
 cos l. sin   cos  . cos h. sin l 
solar azimuth angle( from north),   cos 1  
 cos  
angle of incidence,  cos (sin  . cos   cos  . cos  . sin )
1

wall solar azimuth angle,   180    

ASHRAE Clear sky model for Solar Radiation data for June 21st:
 0.205   1  cos    1  cos  
I DN 1085. exp   , I d  0.134I DN  , I r   g I DN (0.134  sin  ) 
 sin    2   2 
IDN, Id and Ir are direct normal, diffuse & reflected radiations in W/m2, g is ground reflectance.

End of the paper

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 INDIAN INSTITUTE OF TECHNOLOGY KHARAGPUR
Date : Time : 2 hours Full Marks : 60 No. of students : 20
Mid-Semester, Spring 2013 Mechanical Engineering UG/PG/DD
Subject No. ME60096 Subject Name: Air conditioning &
Ventilation

Given data: Molecular weights: Dry air = 28.966 kg/kmol; Water = 18.02 kg/kmol
Specific heat, Cp: Dry air=1.005 kJ/kga.K, Water vapor=1.88 kJ/kg.K, moist air (avg)=1.0216
kJ/kga.K
Specific heat of Liquid water = 4.18 kJ/kg.K, Universal gas constant = 8.314 kJ/kmol.K
Latent heat of vaporization of water = 2501 kJ/kg, Barometric Pressure, pt = 101 kPa, 1TR =
3.517 kW
a) Antoine’s equation for saturation pressure of water (psat)
 3985 
ln(p sat )  16.54    , psat in kPa and T in Kelvin
 T  39 .00 
1.8 p t (DBT  WBT )
b) Apjohn Equation: p v  p' v  , DBT & WBT in oC
2700
pv is the vapour pressure, p’v is the saturated vapour pressure at WBT and pt is the barometric
pressure (pressure units should be consistent)
c) Saturation humidity ratio (kgw/kga), Wsat(t) = 0.00359+0.00028*t, (Validity: 10  t  0oC)
Wsat(t) = 0.00364+0.000375*t, (Validity: 0.1  t  10oC)
Wsat(t) = 0.000042+0.00073*t, (Validity: 10 < t  21oC)
d) Enthalpy of superheated steam hv (kJ/kgw) = 2501+1.88*t, where t is temperature of water in oC
e) Latent heat of vaporization of water, hfg (kJ/kgw) = 25012.36*t, where t is temperature of water in oC
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Answer all questions

Make suitable assumptions/approximations, state and justify them clearly

1) Ambient air at 42oC (DBT) and 28oC (WBT) is

2.5 atm.
compressed adiabatically to a pressure of 2.5 1 atm.,
o
st
atm. in the 1 stage of a 2-stage air compressor. 42 C, 28oC
Intercooler
The compressed air after 1st stage is cooled in an
intercooler placed between the 1st and 2nd stage compressors by using ambient air as shown in
the figure. Find whether there is any possibility of condensation of water vapour in the intercooler?
(6)

2) Measurements on a room air conditioner show that the dry and wet bulb temperatures of the air
supplied to the room are 15oC and 12oC, respectively, while the supply air flow rate is 0.24m3/s. If
the air conditioner maintains the room at 26oC (dry bulb) and 50% (relative humidity) and
consumes 2.0 kW of electrical power, find the sensible, latent, total cooling capacities and COP of

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the air conditioner. Assume 100 % recirculation of room air without any outdoor air for ventilation.
(10)

3) A winter air conditioning system is to be designed to provide for a building sensible and latent
heat losses of 80 kW and 10 kW, respectively. The required inside conditions are 22oC (dry bulb)
and 40 % (relative humidity). The outside conditions are 10oC (dry bulb) and 100 % (relative
humidity). Air is to be supplied to the room at a dry bulb temperature of 40oC. The supply air
should consist of 10 % of outdoor air (by mass) for ventilation. The system consists of a pre-
heater, a steam humidifier and a re-heater. Outdoor air is first preheated in the pre-heater before
mixing it with the re-circulated room air. The dry bulb temperature of air increases by 16oC in the
pre-heater. In the humidifier, dry saturated steam is added at a temperature of 120oC. Find a)
Mass flow rate of air supplied to the building, b) Heat input to the pre-heater; c) Steam
consumption in kilograms per hour; and d) Heat input to the re-heater. Verify the overall energy
balance for the system. Represent the complete process on a psychrometric chart.
(12)

4) Air is cooled and dehumidified in a parallel flow type, chilled water spray washer by bringing it in
contact with 40 kg/s of chilled water sprayed at a temperature of 8oC. The chilled water
exchanges heat and mass with air which enters the spray washer at a mass flow rate of 50 kg/s,
inlet dry bulb temperature of 26oC and a relative humidity of 50 % (dew point ≈ 14.75oC).
Assuming the interfacial area for heat and mass transfer between water spray and air to be
infinite, find the maximum possible cooling capacity of this spray washer.
(10)

5) A building has sensible and latent cooling loads of 100 kW and 25 kW, respectively. The
conditioned space of the building is to be maintained at 26oC (DBT) and 18oC (WBT) by using a
cooling and dehumidification coil that has a bypass factor of 0.1. Outdoor air at a flow rate of 1200
litres/s (measured at outdoor conditions) is to be supplied to the building to meet the ventilation
requirements. The design outdoor conditions are: 42oC (DBT) and 28oC (WBT). Find a) Supply air
flow rate in kg/s, b) Sensible and latent cooling capacity of the cooling & dehumidification coil, and
c) Supply air temperature and humidity ratio. (12)

6) Using suitable control volume equations explain how the size of a counter-flow type cooling
tower can be calculated once the air and water flow rates, inlet conditions of air and inlet and
outlet temperatures of water are specified. State clearly all the assumptions made while arriving at
the equations. Explain the terms range, approach and NTU (Number of Transfer Units) with
reference to the cooling tower. (10)
End of the paper

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 INDIAN INSTITUTE OF TECHNOLOGY KHARAGPUR
Department of Mechanical Engineering
Date : Time : 3 hours Full Marks : 80 No. of students : 20
End-Semester, Spring 2013 Mechanical Engineering
UG/PG/DD
Subject No. ME60096 Subject Name: Air conditioning
& Ventilation

Given data: Molecular weights: Dry air = 28.966 kg/kmol; Water = 18.02 kg/kmol
Specific heat, Cp: Dry air=1.0 kJ/kga.K, Water vapor=1.88 kJ/kg.K, moist air (avg)=1.02 kJ/kga.K
Specific heat of Liquid water = 4.18 kJ/kg.K, Universal gas constant = 8.314 kJ/kmol.K
Latent heat of vaporization of water = 2501 kJ/kg, Barometric Pressure, pt = 101 kPa, 1TR =
3.517 kW
 3985 
Saturation pressure of water, psat: ln( psat )  16.54   ; psat in kPa & T in K
 T  39.00 
Humidity ratio of saturated air, wsat = 0.0191+0.00156*DBT, where wsat is in kgw/kga, 24 ≤
DBT ≤ 36oC

Answer all questions. Make suitable assumptions wherever necessary and state them
clearly

1a) Starting from the fundamental principles, derive the heat balance equation for a human being
who is at thermally neutral condition. Include all the possible modes of energy transfer in the heat
balance equation. State what other conditions have to be satisfied for this human being to be
thermally comfortable. From the thermal comfort equation derived above, list all the relevant
environmental, personal and physical parameters on which the thermal comfort of the human
being depends? Of these, which parameter(s) can be controlled easily by the air conditioning
engineer? (6+3+1=10)

1b) State and describe briefly the methods used in practice to provide acceptable indoor air
quality in occupied spaces. (6)
 t t 
2a) A condensation resistance factor    s c  is a parameter that is used in connection with
 t a  tc 
condensation of water vapour on building walls. In the above expression ts and tc are the
temperatures of the hot and cold surfaces of the wall and ta is the dry bulb temperature of air on
the hot side of the wall. The inside surface temperature of a cold storage wall is to be maintained
at 15oC when the outside ambient air is at 35oC (DBT) and 40% RH. For this cold storage wall,
find the minimum value of condensation resistance factor and the minimum wall thickness (in mm)
required to prevent condensation. The effective thermal conductivity of the wall is 0.9 W/m.K and
the design heat transfer coefficient between the outer surface of the wall and surroundings is 23
W/m2.K. (6)
2b) At a particular instance, the thin, flat roof of a building of area 120 m2 is exposed to a total
solar radiation flux of 900 W/m2, of which 75% is direct and 25% is diffuse radiation. The

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absorptance of the surface to both direct and diffuse radiation is 0.9. The ambient dry and wet bulb
temperatures and humidity ratio at this instance are 35oC, 23.8oC and 0.014 kgw/kga,
respectively, while the inside air temperature is 28oC (dry bulb). The external and internal heat
transfer coefficients are 23 W/m2.K and 6 W/m2.K, respectively. The thermal capacity and heat
transfer resistance of the roof and the long-wave radiation from the roof may be assumed to be
negligible. From this data calculate the rate at which heat is transferred to the inside air, when:
a) The roof is completely exposed without any insulation
b) An insulation with an R-value of 0.5 m2.K/W and negligible thermal capacity is added
c) The roof is not insulated but is shaded such that it receives only diffuse radiation
d) The roof is not insulated but water is sprayed on the exposed roof such that evaporation takes place
over the entire roof without any run-off of water (2X3 + 4 = 10)

3a) Explain briefly why for houses located in northern hemisphere it is preferable to locate the
windows on south facing walls. A glass window of 3 m2 area is subjected to a total solar radiation
flux of 600 W/m2 at a particular instance of time. The average transmittance and absorptance of
glass for solar radiation are 0.8 and 0.12, respectively. At steady state, the glass rejects 60% of
the absorbed radiation to the outside. The window has a U-value of 5.9 W/m2.K, a shading
coefficient of 0.8 and a CLF value of 0.83. Find the cooling load on the building due to this window,
when the inside and outside dry bulb temperatures are 26oC and 42oC, respectively. (2+4 = 6)
3b) Find the sol-air temperature of a horizontal roof that is subjected to an incident solar radiation
of 900 W/m2. The absorptance of the surface for solar radiation is equal to 0.9, outside air
temperature is 33C, long wave radiation from the roof to the surroundings is 90 W/m2, and the
convective heat transfer coefficient between the surface and surrounding air is 23 W/m2.K. (4)
3c) The horizontal roof of an air conditioned building is made of 15 cm thick concrete (k=1.73
W/m.K). The roof has an effective surface area of 120 m2, a decrement factor of 0.48 and a time
lag of 5 hours. The inside and outside surface heat transfer coefficients of the roof are 8 W/m2.K
and 23 W/m2.K, respectively. The air conditioned space is maintained at 25C. The following table
shows the calculated values of sol-air temperatures for a particular day between 8 A.M. to 7 P.M.
Time Tsol-air, C Time Tsol-air, C Time Tsol-air, C
8 A.M. 35.1 12 Noon 53.2 4 P.M. 45.5
9 A.M. 41.2 1 P.M. 54.3 5 P.M. 40.0
10 A.M. 46.2 2 P.M. 52.9 6 P.M. 33.6
11 A.M. 50.7 3 P.M. 50.3 7 P.M. 26.8

If the mean sol-air temperature for the day is 33.3C, find the peak cooling load on the building
due to the roof, time of occurrence of this peak load and the inner surface temperature of the roof
at peak load. (6)
4a) A large function hall has to be maintained at 26oC (DBT) and 0.01 kgw/kga (humidity ratio)
when the outdoor conditions are 35oC (DBT) and 0.016 kgw/kga (humidity ratio). The total external
load on the function hall consists of 16 kW (sensible load) and 8 kW (latent load). The occupancy
is 200 with sensible and latent heat gains of 60 W and 40 W per occupant, respectively. The total
lighting and other sensible internal equipment load is 5 kW. Ventilation is required at the rate of 8
liters/s/occupant (air density = 1.16 kga/m3). Estimate the required cooling capacity of the air
conditioning plant in TR and the coil sensible heat factor. Based on the value of coil sensible heat

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factor, State what practical problems may be encountered with this design and a possible solution
to the problem. (7+1+2 = 10)

4b) The required amount of air for the above system is to be supplied to the air conditioned space
at a supply temperature of 12oC and density of 1.2 kg/m3 from an air handling room that is located
15 m away. A uniform duct of square cross-section is used for supplying this air to the conditioned
space. The duct is to be designed using equal friction method with a frictional pressure drop of 1.2
Pa/m. The total dynamic loss from the fan outlet to the supply air outlet is equal to an equivalent
straight duct length of 60 m. Find a) The required size of the duct and velocity of air through the
duct, b) The required fan power input assuming a fan efficiency of 0.7. The frictional pressure drop
(Pf/L in Pa/m) in terms of the volumetric air flow rate (Q in m3/s) and diameter (deq), and
equivalent diameter (deq) of a rectangular duct of sides a and b (in m) are given by: (5+1 = 6)

∆ . .
. .
. ; .

5a) An air-water system shown in the figure given below is used to maintain the conditioned space
of a building at 26oC (dry bulb) and 0.01 kgw/kga (humidity ratio).The building has a sensible load
of 300 kW and a latent load of 80 kW. For ventilation purpose, 10 % of the supply air (by mass) is
outside air, which is at 35oC (dry bulb) and 0.016 kgw/kga (humidity ratio). The cooling coil used
in the primary air system (Coil 1) handles the entire latent load on the building and has a coil ADP
of 7oC and a bypass factor of 0.1. In the chilled water line, the chilled water temperature rises by
7K due to heat transfer as it flows through the fan coil unit (FCU) kept inside the building. Find a)
Required sensible and latent cooling capacities of Coil 1, and b) Required chilled water flow rate
through FCU. (6+2 = 8)
300 kW (sensible)
Primary air Coil 1 80 kW (latent)
26oC,
Chilled water 0.01 kg/kg

Figure for Problem FCU

5b) What is the maximum allowable airflow rate through a circular opening of diameter 3 cm, if the
throw is not to exceed 6 m? What is the total flow rate of air (supplied + entrained) at a distance of
3 m from this opening? The velocity distribution for isothermal, free air jets through a circular
opening is given by the equation:
.
,
.

where Vo and Ao are the air velocity at the outlet and area of the outlet, respectively. (3+5 = 8)
End of the paper

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