Thermodynamics

and
Refrigeration
Efren A. Dela Cruz
Introduction
• The word thermodynamics was derived from Greek
words “Therme” means heat and “Dynamis” means
Force (strength or power). Thermodynamics is the
science that deals with the study of energy and
entropy; it deals with the properties of
matter/substance related to heat and work; it deals
with processes and cycles on the transformation of
energy of all kinds, from one form to another.
• Refrigeration deals with processes of cooling; it deals
with the processes and cycles of different methods of
refrigeration; analysis of Carnot and vapor
compression refrigeration systems; refrigerant and
their properties; and the application of psychometrics
in air conditioning.
Thermodynamics System
• Open System
• Closed System
• Isolated System
Laws of Thermodynamics
• Zeroth law - when two bodies have equality of temperature with a
third body, they in turn have equality of temperature with each
other.
• First Law – also known as the Conservation of Energy principle,
states that energy can neither be created not destroyed, it can
only change forms.
• Second Law - deals with the quality of energy (energy
degradation). There are two classical statements of this law:
• Kelvin-Planck statement: It is impossible to construct a device that will
operate in a cycle and produce no effect other than the raising of a
weight and the exchange of heat with a single reservoir.
• Clausius statement: It is impossible to construct a device that operates
in a cycle and produces no effect other than the transfer of heat from a
cooler-body to a hotter body.
• Third law – states that the entropy of a perfect crystal is zero at
the absolute zero of temperature.
Properties of Thermodynamics
substance/ system.
• Extensive
• Intensive
• Specific
Phase and state of
Thermodynamic substance.
• State of Substance - condition of a substance as
described by certain observable macroscopic
parameter called Properties.
• Phase- quantity of matter that is homogeneous
throughout(solid, liquid, gas)
• Change of Phase
• Solid to Liquid - Melting
• Liquid to Solid - Solidification (Freezing)
• Liquid to Gas - Evaporation
• Gas to liquid - Condensation
• Solid to Gas - Sublimation
• Gas to Solid - Deposition
Processes and Cycle
• Process- Simply the change of state
Types of processes
• Isothermal - constant temperature.
• Isobaric - constant pressure
• Isometric - constant volume
• Isentropic - constant entropy
- Reversible Adiabatic
• Isenthalpic - constant enthalpy
• Polytropic – PVn = C

• Cycle - a series of processes one after the other such

that the initial and final states are the same.
Properties and Property Relations
• W= mg
• ρ= m/V
• v = V/m = 1/(m/V) = 1/ρ
• γ = W/V
• Q = AV
• ṁ = Q/v = AV/v = Avρ
• ṁin = ṁout
Properties and Property Relations
• SG = RD = W/Wwater = γ V/ γwater Vwater = γ / γwater
• = ρ.g/ ρwaterg = ρ / ρwater
= vwater/ v
• For Gasses
• SG = MW/MWa
Properties and Property Relations
Temperature
Measures of the degree of hotness or coldness of a body.
Always use absolute temp. in the analysis of
thermodynamics.

• For absolute temp.

T = 459.69 + oF in oR
= 273.15 + oC in oK

• Conversion of oF & oC
oF = 1.8 oC + 32
oC = (oF- 32)/1.8
Properties and Property Relations
Pressure
• For solids
P = F/A (Stress)
• For liquids and gasses
• a) absolute press. is greater than atmospheric
• P = Patm + Pg
• b) Absolute press. is less than atmospheric
• P= Patm - Pg (vacuum)
• 1atm =101.325 kPa
• =14.7 Psi
• = 760 mm Hg
• = 29.92 in Hg
Conservation of Energy
For a steady-state closed system
• Q =E + W
• Q =U + W
Steady Flow Energy Equation(Q + Ein =E + W + Eout)

• Energy Entering System = Energy Leaving System

• P1 + K1 + Wf1 + U1 + Q = P2 + K2 + Wf2 + U2 + W
• Q = P + K + Wf + U + W
• P1 + K1 + H1 + Q = P2 + K2 + H2 + W
• Q = P + K + H + W
Ideal Gas
Boyle’s Law
• If the temperature of a given quantity of gas is held
constant, the volume of the gas varies inversely
with the absolute pressure during a change of
state.
•V1 or V=C
• P P
•
• PV = C or P1V1 = P2V2
Ideal Gas
Charles’s Law
• If the pressure on a particular quantity of gas is
held constant, then, with any change of state, the
volume will vary directly as the absolute
temperature.
•VT or V = CT
•
•V=C or V1 = V2
•T T1 T2
Ideal Gas
Gay-Lussac’s Law
• If the volume of a particular quantity of gas is held
constant, then, with any change of state, the
pressure will vary directly as the absolute
temperature.
•PT or P = CT

•P=C or P1 = P2
•T T 1 T2
Ideal Gas
Equation of State or Characteristics Equation of a Perfect
Gas (Ideal Gas)
• Combining Boyle’s Charles’s and Gay-Lussac’s laws,
• P1V1 = P2V2 = C, constant
• T1 T2
• PV = mR
• T
• PV = mRT Pv = RT
Actual Gasses
• Pv = ZRT
• R = Ř/M, Ř = 8.314 kJ/kmol K
Ideal Gas
Internal Energy of an Ideal Gas
• Joule’s law states that “the change of internal energy
of an ideal gas is a function of only the temperature
change.” Therefore, U is given by the formula,
• U = mcv (T2-T1)
• whether the volume remains constant or not.
Enthalpy of an Ideal Gas
• The change of enthalpy of an ideal gas is given by the
formula,
• H = mcp (T2-T1)
• whether the pressure remains constant or not.
Ideal Gas
Relation Between cp and cv
• Ratio of Specific Heats
• k = cp/cv > 1
• From h = u + pv and pv = RT
• dh = du + RdT
• cpdT = cvdT + RdT
• cp = cv + R
• cv = R/k-1
• cp = kR/k-1
Power Cycles
Carnot Cycle (Engine)
• QA = T1(S2 – S1) = TH ΔS
• QR = T3(S4 – S3) = -TL ΔS = TL ΔS
• W = QA - QR = (TH - TL) ΔS

•η =
𝑾 𝑻𝑯 − 𝑻𝑳
𝑸𝑨
= 𝑻𝑯
Power Cycles
• Vapor Power Cycle (Rankine Cycle) 𝑾𝒏𝒆𝒕 𝑾𝑻 − 𝑾𝑷
η= =
𝑸𝑨 𝑸𝑨
Sample Questions
• What is the power in kW produced by a 100 HP
engine?
a. 74.6 b. 80.5 c. 150 d. 125

• Refers to the temperature at which molecular

motion ceases, according to the kinetic theory of
heat.
a. critical point b. triple point c. absolute zero
d. absolute temperature
Sample Questions
• In a two phase system, 30% moisture means:
a. 70% liquid and 30% vapor b. 70% vapor and 30%
liquid
c. 100% vapor and 30% liquid d. 100% liquid and 30%
vapor

• Kinetic energy is the energy of the body due to its:

a. elevation b. temperature c. motion d. pressure
Sample Questions
• Power is work done per:
a. 33,000 ft-lb b. 2450 kg c. unit time d. work times distance

• To create the Kelvin and Rankine absolute temperature

scales, two states were assigned selected
temperature. The two states were:
a. ice point and steam point at atmospheric pressure
b. ice point and steam point at absolute zero pressure
c. saturated liquid point and saturated vapor point at
atmospheric pressure
d. Ice point and critical point at atmospheric pressure
Sample Questions
• Combustion occurs as the piston is at the top of
cylinder thus the process is isometric, if the
temperature increases from 127oC to1327oC during the
combustion process. Determine the final pressure in
kPaa if the initial pressure is 200 kPaa.
a. 2000 b. 1600 c. 800 d. 1300

• Water is flowing with a flow rate of 0.002 m3/s. What is

the average velocity at an outlet where the area is 4
cm2?
a. 50 m/s b. 20 m/s c. 10 m/s d. 5 m/s
Sample Questions
• Area under T-s diagram is defined as:
a. work b. heat c. change in internal energy
d. none of the above

• An inventor proposed to operate an engine between

two layers of the ocean. One at 90oF and the other
at 40oF. The maximum possible efficiency of the
engine is nearest to
a. 10.1% b. 9.1% c. 8.1% d. 7.!%
Sample Questions
• A closed stationary system consists of 2 kg mass.
During a certain process, 20 kJ of work is done on
the system and the internal energy increased by 40
kJ/kg.
a. System will gain 120 kJ of heat from surrounding
b. System will not gain heat
c. System will absorb 60 kJ of heat from the surrounding
d. System will transfer 60 kJ of heat to the surrounding

• Thermodynamics cycle used in vapor power plants

a. Erricson b. Brayton c. Rankine d. Carnot
Sample Questions
• The state of a pure substance is defined completely if
we define
a. pressure and temperature b. two independent intensive
properties
c. two intensive properties d. any two properties

• An isentropic process is used to approximate an actual

process. For such a process
a. the temperature change must not be large
b. both work and heat transfer is zero
c. no work must occur
d. the heat transfer is zero
Sample Questions
• The amount of transferred heat required to change
the temperature of unit weight of substance at one-
degree unit of temperature.
a. Sensible heat b. Heat of fusion c. Latent heat
b. d. Specific heat

• A gas turbine works on the principle of

a. Carnot Cycle b. Bell-Coleman Cycle
c. Rankine Cycle d. Brayton Cycle
Sample Questions
• What is the theoretical lift of a pump handling
water at atmospheric pressure?
a. 33.9 ft b. 40 ft c. 24 ft d. 26.9 ft

• The absolute zero temperature in Celsius scale.

a. 100 b. 0 c. -273 d. 273

• Torque is a force that tend to produce

a. tension b. compression c. rotation
d. bending
Sample Questions
• A mass of 5 kg of saturated vapor at 200 kPa is heated
at constant pressure until the temperature reaches
300oC. Calculate the work done by the steam during
this process. At 200 kPa saturated vg= 0.8857 m3/kg, at
200 Kpa and 300oC v= 1316.2 L/kg
a. 430.5 kJ b. 540.3 kJ c. 504.3 kJ d. 403.5 kJ

• The process that has no heat transfer

a. Isothermal b. Isometric c. Isentropic d. Adiabatic
Sample Questions
• The boiling point of water in an open container at sea
level is 100oC. If the pressure on the open container is
decreased such as going up to the top of a mountain,
the boiling point will be
a. increased b. decreased c. the same d. none of
the above

• A rigid container is heated by the sun. There is no shaft

work associated with the container. From the first law
of thermodynamics. You determine the heat added to
be
a. equal to the work b. equal to the change in internal
energy
c. equal to zero d. equal to unity
Sample Questions
• A steam turbine exhausts at 6.9 kPa into a condenser.
10,000 kg/hr of steam is delivered on inlet at 1.38 MPa
and 220oC. The turbine delivers 746 kW. Neglecting
heat loss, calculate the quality of steam entering the
condenser. Steam properties; at 1.38 MPa and 220oC
h=2058 kJ/kg; at 6.9 kPa saturated hf= 162.2 kJ/kg, hfg=
2410 kJ/kg
a. 67.5% b. 70.5% c. 80.6% d. 99.6%

• A device used to keep moisture from passing through

the system
a. humidifier b. trap c. evaporator d. dehydrator
Sample Questions
• A steam throttling calorimeter receives steam at 830
kPa and discharges it at 124 kPa. If its thermometer
reads 116oC, what is the moisture content of the
entering steam. Steam properties; at 0.83 MPa hf=
726.6 kJ/kg, hfg= 2041.6 kJ/kg; at 124 kPa and 116oC h=
2704.7 kJ/kg
a. 1.4% b. 3.1% c. 0.4% d. 2.5%

• An adiabatic process is characterized by which of the

following?
a. temperature change is zero b. heat transfer is zero
c. work is zero d. absolute zero temperature
Sample Questions
• As heat is removed from a substance it gets colder.
When no more heat can be removed and the
temperature cannot be lowered any further, we
have reached
a. perfect zero b. absolute zero c. double zero
d. cold zero

• A 100% efficient pump can draw a suction of.

a. 34 ft H2O b. 14.7 Psi c. 29.92 in Hg d. all (a-c)
Sample Questions
• If the temperature of the medium is 0oC, what will
be the temperature if it is doubled?
a. 0oC b. 524oR c. 273oC d. 460oF

• A closed system experiences a reversible process

where heat rejection is the only energy transferred.
The entropy change must be
a. zero b. positive c. negative d. equal to
heat transferred
Sample Questions
• If a centrifugal pump tends to vibrate, it might be
caused by:
a. misalignment b. worn bearings
c. clogged or damaged impeller d. Any of the above

• A manometer measures:
a. temperature b. pressure c. mass d. volume
Sample Questions
• A pump is used to increase the pressure of the water
entering the boiler of steam power cycle. Which
statement is true concerning the pump?
a. The pump produces work b. The pump has no effect on
the cycle
c. The enthalpy of water leaving the pump is lower than the
enthalpy of water entering
d. The enthalpy of water leaving the pump is higher than the
enthalpy of water entering

• A pyrometer measures:
a. temperature b. pressure c. volume d. mass
Sample Questions
• The steam power cycle is modeled by ideal cycle
known as the:
a. Otto Cycle b. Brayton Cycle c. Rankine Cycle
d. Diesel Cycle

• An open system first law should be utilized for all

except:
a. turbine b. pump
c. piston cylinder with no intake/exhaust valve
d. boiler
Sample Questions
• An inventor claims to have built an engine which will
revolutionize the automotive industry. Which of the
following should be the best test to determine if the
inventors claim is true?
a. Conservation of mass b. First law of thermodynamics
c. Zeroth law of thermodynamics d. Second law of thermodynamics

• In an ideal cycle, liquid leaves the condenser and is

expanded in such a manner that the enthalpy of the liquid is
equal to the enthalpy of the resulting saturated mixture.
This type of expansion is known as:
a. throttling process b. isothermal process
c. adiabatic process d. isochoric
Sample Questions
• A type of process where the pressure remains
constant
a. Isothermal b. Isochoric c. Isobaric d. Adiabatic

• A pump discharges into a 3-m per side cubical tank.

The flow rate is 300 liters per minute and the fluid
has specific gravity of 1.2. what is the mass flow
rate in kg/s?
a. 4 b. 5 c. 6 d. 7
Sample Questions
• A type of process where the temperature remains
constant
a. Isothermal b. Isochoric c. Isobaric d. Adiabatic

• A pump discharges into a 3-m per side cubical tank.

The flow rate is 300 liters per minute and the fluid
has specific gravity of 1.2. how long will it takes to
fill the tank?
a. 45 min b. 60 min c. 75 min d. 90 min
Sample Questions
• A Carnot engine produces 25 kW while operating
between temperature limits of 10000K and 3000K. What
is the heat supplied?
a. 45.7 kW b. 10.7 kW c. 35.7 kW d. 55.7 kW

• What is the heat rejected?

a. 45.7 kW b. 10.7 kW c. 35.7 kW d. 20.7 kW

• A type of process where the volume remains constant

a. Isothermal b. Isochoric c. Isobaric d. Isometric
Sample Questions
• A closed rigid container has a Volume of 1 m3 and holds air at
344.8 kPa and 273 K. Heat is added until the temperature is
6000K. (Rair = 0.287 kJ/kg K), (kair = 1.4). What is the mass of air?
a. 4.4 kg b. 5.4 kg c. 3.3 kg d. 2.3 kg

• What is the final pressure?

a. 745.8 kPa b. 757.8 kPa c. 775.8 kPa d. 547.8 kPa

• What is the change in enthalpy?

a. 1947.7 kJ b. 1647.7 kJ c. 1445.6 kJ d. 1274.7 kJ

• What is value of heat added on the system?

a. 1147.1 kJ b. 1032.6 kJ c. 1471.1 kJ d. 1005.1 kJ
Sample Questions
• A scuba tank contains 1.5 kg of air with initial temperature of
15oC. The tank is left near an engine exhaust line, and the tank's
pressure doubles. What is the final temperature?
a. 303oC b. 303oK c. 303oR d. 303oF

• What is the change in enthalpy?

a. 433.9 kJ b. 543.7 kJ c. 276.7 kJ d. 124.7 kJ

• What is the change in internal energy?

a. 303.4 kJ b. 308 kJ c. 310 kJ d. 318 kJ

• What is value of heat added on the system?

a. 303.4 kJ b. 308 kJ c. 310 kJ d. 318 kJ
End of Topic

Thank you.
Refrigeration
• Branch of science that deals with the process of
reducing or maintaining the temperature of the
surrounding.
• Artificial cooling either by the application of ice
and other cool body or by utilizing the latent heat of
vaporization.
• Types of Refrigeration
• Natural Refrigeration
• Mechanical Refrigeration
Methods of Refrigeration
• Ice Refrigeration
• Vapor compression refrigeration
• Ice cycle refrigeration
• Steam jet refrigeration
• Absorption refrigeration
• Cryogenic or very low temperature refrigeration
Unit of Refrigerating Capacity
The standard unit of refrigeration is ton of
refrigeration or simply ton denoted by the symbol TR
• One ton of refrigeration (TR) is the cooling rate
required to freeze one ton of water at 32˚ F into ice
at the same temperature in one day or 24 hours.
• TR = 1 TON / day ( 2000 lbs/ TON) ( 144 BTU /
lbs ) ( 1 day / 24 hrs )
• TR = 12000 BTU / hr
• TR = 12000 BTU / hr ( 1 hr / 60 min )
= 200 BTU / min (ENG)
Unit of Refrigerating Capacity
Where: Latent heat of fusion
• 144 Btu / lb
• 355 KJ / kg
• 80 kCal / kg
And ,
• 1 Btu = 1. 055 KJ
• TR = 211 KJ / min
• = 211 KJ / min ( 1min/ 60 sec ) = 3.52 KW
• = 200 Btu / min ( 0. 252 kCal / Btu ) = 50 .4 kCal / min
Note: Always used absolute temperature in computation
of refrigeration loads.
Carnot Refrigeration (Reversed
Carnot) Cycle
T-S DIAGRAM
PROCESSES WHICH CONSTITUTE THE CYCLE
T 1-2 Adiabatic compression
3 2 2-3 Isothermal rejection of heat
3-4 Adiabatic expansion
4-1 Isothermal addition of heat
4 1
USEFUL REF’N

S
Carnot Refrigeration (Reversed
Carnot) Cycle
COEFFICIENT OF PERFORMANCE (COPR)
• Index of performance
USEFUL REFRIGERATION
• COPR = -----------------------------------------
NET WORK
Carnot Refrigeration (Reversed
Carnot) Cycle
• QA = T1(S1 – S4) = TL ΔS
• QR = T2(S2 – S3) = TH ΔS
• W = QR - QA = (TH - TL) ΔS

QA
• COPR =
𝑻𝑳
=
𝑾𝑵𝒆𝒕 𝑻𝑯 −𝑻𝑳
Carnor Heat Pump
• QA = T1(S1 – S4) = TL ΔS
• QR = T2(S2 – S3) = TH ΔS
• W = QR - QA = (TH - TL) ΔS

• Performance Factor or COP of Heat Pump

QR
• COPH = PF=
𝑻𝑯
=
𝑾𝑵𝒆𝒕 𝑻𝑯 −𝑻𝑳
Therefore
• COPH = COPR + 1 and
• COPR = COPH – 1
 Standard Vapor Compression
Cycle
The most widely used refrigeration cycle in the
practice in this cycle vapor compressed, then
condensed to a liquid, followed by a pressure drop,
so that the fluid can evaporate at a low pressure
FLOW DIAGRAM QR
3 2

E.V.

4 1
QA
 Standard Vapor Compression
Cycle
PROCESSES CONSTITUTING THE STANDARD VAPOR
COMPRESSION CYCLE
• 1-2 Reversible adiabatic compression of refrigerant from
saturated vapor to condenser pressure
• 2-3 Reversible isobaric rejection of heat, causing
condensation of the refrigerant
• 3-4 Irreversible isenthalpic expansion from saturated
liquid to evaporator pressure
• 4-1 Reversible addition of heat at constant pressure
causing evaporation to saturated vapor.
 Standard Vapor Compression
Cycle
SYSTEM ANALYSIS
• QA = QL = RE = TR = mR ( h1 – h4 )
• QR = mR ( h2 – h3 ) = QwA = mw Cpw ( twout – twin )

RE RE QA
• COPR = -------------- = ------------------- = -------------------
Wnet Wc Wc

mR ( h1 – h4 ) h1 – h4
= ------------------------- = ----------------------
mR (h2 – h1 ) h2 – h1
Heat Exchanger
Two functions of heat exchange in the system
• To ensure that no liquids enter the compressor
• To sub cooled the liquid from the condenser to prevent
bubbles of vapor from impending the flow of refrigerant
through the expansion valve
For refrigerating effect
• RE = h6 – h5 = h1 – h3
• = m ( h6 – h5 ) = m ( h1 – h3 )
Compressors
Heart of the vapor compression refrigeration system
Types of Compressors
• reciprocating – back and forth piston
• screw – rotary motion – positive displacement
• vane - rotary motion – positive displacement
• centrifugal - rotary motion – positive displacement
operates by virtue of centrifugal force
Compressors
Volumetric Efficiency
• Basis for predicting performance of reciprocating
compressors
Two Methods
• actual volumetric efficiency
• clearance volumetric
Compressors
Actual Volumetric Efficiency
• ηva = Volume Flow Rate Entering The Compressor (m3/s) x 100
• Displacement Rate of Compressor (m3/s)
•
• = V1 ; V1 = mr v1
• Vd
•
Displacement Rate; Vd ( Piston Displacement )
- Displacement swept through by piston in their suction stroke per
unit time

• Vd = πD2LN/4 = ALN
Compressors
Actual Volumetric Efficiency
Where:
• A = cross section area of cylinder
• D = diameter of the bore of cylinder
• L = length of stroke
• N = no. of cycle compression per unit time
• n = angular speed
For single acting
• N = n (no. of cylinder)
For double acting
• N = 2n (no. of cylinder)
Compressors
• Clearance Volumetric Efficiency
• ηvc = Volume Gas Drawn In The Cylinder
vd
v1 = vsuc
= 100% - vc v1 - 1 x 100% vc vdis
v3 – vc vc Where;
vsuc = specific volume of vapor entering
= 100% - C v1 - 1 x 100% vdis = specific volume of vapor leaving
vc
Psychrometry and Air-conditioning
Air Conditioning
• The simultaneous control of temperature, humidity, air
movement, and quality of air in the space.
• Air conditioning includes entire heating operation as well
as the regulation of velocity, thermal radiation and the
gravity of air including removal of foreign particles and
vapors
TWO TYPES OF AIR CONDITIONING UNIT
• Comfort air conditioning
• Industrial air conditioning
Psychrometry and Air-conditioning
FOUR FUNCTION OF AIR CONDITIONING
• Control of temperature
• Control of humidity
• Control or air circulation or movement
• Maintain the desired condition of products
BASIC PROCESSES IN AIR CONDITIONING
• Sensible heating
• Sensible cooling
• Humidifying
• Dehumidifying
• Heating and Humidifying
• Heating and Dehumidifying
• Cooling and Humidifying
• Cooling and Dehumidifying
Psychrometry and Air-conditioning
PSYCHROMETRY- is the study of the mixture of air and water
vapor
• PSYCHROMETRIC PROPERTIES – properties of moist air
• PSYCHROMETRIC CHART – is the graphical representation of the
thermodynamics properties of moist air
• Moist air – is a binary mixtures dry air and cooler vapor
• Dry air – none condensing component of the mixture, mainly the
nitrogen and oxygen
• Vapor – is the considerable component of mixture, the water vapor
or stem which may exist in a saturated or super heated state
• Pt – total mixture pressure
• Pa – partial pressure exerted by dry air
• Ps – partial pressure exerted by vapor or steam
• Pt = Pa + Ps
Psychrometry and Air-conditioning
• Saturated air – the vapor in the air is saturated
• Unsaturated air – air containing superheated vapor
• Latent heat – heat does not affect the temperature of the
substance but damage its state
• Sensible heat – is the heat absorbed or given by a substance
that changes its temperature
• Total heat – sum of latent heat and sensible heat
• (Humidity ratio, Moisture Content, Mixing ratio)
or W - is the ratio of the mass of water vapor to the mass of dry air
0.622 Ps kg of water vapor
• W = ---------------- = -------------------------
P t – Ps kg of dry air
Psychrometry and Air-conditioning
RELATIVE HUMIDITY (Ф) – is the ratio of the partial pressure of water vapor in the
air to the saturation pressure corresponding to the temperature of the air
Ps
RH = ----------------- or Ps = RH Pd = Ф Pd
Pd
• Where: Pd – saturation pressure corresponding to the dry bulb temperature (@ steam table)
• USING CARRIERS EQUATION
• Ps = Pw – [ Pb ( 6.66 x 10 –4 ) ( tdb – twb )

• Where:
• Pb – barometric pressure
• tdb – dry bulb temperature
• twb – wet bulb temperature
• Pw – saturated pressure corresponding to wet bulb temperature
Psychrometry and Air-conditioning
• WET BULB DEPRESSION – is the difference between the
reading of the dry bulb and wet bulb temperature
• DEW POINT TEMPERATURE – (tdb), is the saturation
temperature corresponding to the actual partial pressure of
the stem in air or the temperature at which condensation of
the moisture begin when the air is cooled at constant pressure
• DEGREE OF SATURATION (U) – is the ratio of the air humidity
ratio (SH or W) to the humidity ratio of saturated air ( SHs or
Ws ) at the same temperature
Ф (Pt – Pd)
U = ----------------------
(Pt – Ps)
Psychrometry and Air-conditioning
• ENTHALPY (h) of a mixture of dry air and water vapor is the sum of
the enthalpy of the dry air and the enthalpy of the water vapor.
h = Cpt + Whg (kJ/kg)
• Where:
• Cp = specific heat of dry air at constant pressure
= 1.0062 KJ/kg K = 1 kJ / kg K
• hg = enthalpy of saturated steam at temperature t ( steam table )
( by approximation )
• hg = 2501 + 1. 863 tdb ; kJ / kg
• SPECIFIC VOLUME OF AIR
Ra T Ra T
Va = --------- = ---------- ; m3 / kg
Pa P t – Ps
Psychrometry and Air-conditioning
MIXING AIR; M3
m3 = m1 + m2
m3h3 = m1h1 + m2h2 ; m1h1 + m2h2
h3 = ---------------------
m3
m3 W3 = m1 W1 + m2 W2 ;
m1W1 + m2W2
W3 = ----------------------
m3
m3 tdb3 = m1tdb1 + m2 tdb2 ;
m1tdb1 + m2tdb2
tdb3 = ------------------------
m3
m3 tdp3 = m1tdb1 + m2 tdp2 ;
m1tdp1 + m2tdp2
tdp3 = ------------------------
m3
 BOILING TEMPERATURE AND COLOR CODE OF SOME
REFRIFERANTS
REFRIGERANT REFRIGERANT NAME BOILING POINT CYLINDER
NUMBER COLOR
CODE

R-11 TRICHLOROMONOFLUROMETHANE 74.7°F ORANGE

R-12 DICHLORODIFLUOROMETANE -21.6°F WHITE
R-22 MONOCHLOROTRIFLUOROMETHANE -41.4°F GREEN
R-500 REFRIGERANT 152A/12 -28°F YELLOW
R-502 REFRIGERANT 22/115 -50.1°F ORCHID
R-17 AMMONIA -28.0°F SILVER
R-134a TETRAFLUOROETHANE -14.90°F L-BLUE
BOILING TEMPERATURE AND COLOR CODE OF SOME
REFRIFERANTS

https://energy-models.com/refrigeration
Ref and Aircon Problems
• A vapor compression refrigeration system has a 30-kW
motor driving compressor with inlet enthalpy of 345 kJ/kg
and discharges with an enthalpy of 398 kJ/kg. Saturated
liquid enters the expansion valve at 238.5 kJ/kg enthalpy.
Determine the capacity of the unit in Tons of refrigeration.
a. 14.7 b. 17.1 c. 11.7 d. 16.6

• R-12 enters the evaporator at 3oC (hg=352.76kJ/kg ;

hf=202.78kJ/kg). Determine the amount of flash vapor that
may be formed in kg/s if the refrigeration load is 400 kW and
condensing temperature is 37oC (hf=235.503 kJ/kg).
a. 0.734 b. 0.744 c. 0.477 d. 0.374
Ref and Aircon Problems
• In an air-water vapor mixture, the temperature which is
the measure of the total heat of the mixture
a. dew point b. dry bulb c. sum of dry bulb and wet bulb
d. wet bulb

• In the refrigeration system the heat absorbed in the

evaporator per pound of refrigerant passing through
a. equals the increase in enthalpy
b. equals the increase in volume
c. equals the change in temperature
d. equals the change in entropy
Ref and Aircon Problems
• A reversed carnot cycle uses R-12 as the working
fluid. Refrigerant enters the condenser as saturated
vapor at 30oC and leaves as saturated liquid. The
evaporator temperature is at constant -10oC.
Determine the COP
a. 6.75 b. 7.56 c. 6.57 d. 7.65

• The refrigeration system has a refrigerating

capacity of 220 kJ/kg. The heat required to remove
is 630 kJ/hr. Calculate the mass circulated per hr.
a. 3.18 kg/hr b. 10 kg/hr c. 8 kg/hr d. 2.8 kg/hr
Ref and Aircon Problems
• If the wet bulb and dry bulb reading of air temperature
are identical
a. the thermometers are in error b. the air is saturated
c. the relative humidity is zero d. the mixture is completely dry

• Heat is supplied to 20 lbm of ice at 32oF at the rate of

160 BTU/s. If the heat of fusion is 144 BTU/lb, how long
will it take to convert ice to water at 50oF
(Cpw=1BTU/lboR).
a. 25.45s b. 20.25s c. 22.05s d. 25.20s
Ref and Aircon Problems
• On a cold night the temperature was reported to be
same in Fahrenheit and Celsius reading at
a. 33oC b. -40oC c. -33oC d. 40oC

• Consider an air-water vapor mixture similar to the

atmosphere. If the dry bulb temperature equals the
dew point temperature, the relative humidity is
a. 0% b. 25% c. 75% d. 100%
Ref and Aircon Problems
• Sensible heat
a. can be measured/computed with a thermometer
b. cannot be measured
c. increases with cold d. depends on the volume

• What power is required to drive a refrigeration

system with a COP of 5 to remove a heat equivalent
of 100 tons
a. 80 Hp b. 94.4 Hp c. 105.2 Hp d. 30 Hp
Ref and Aircon Problems
• Atmospheric pressure air enters to a cooling unit at
27oC (Psat=3.5kPa) and has a specific humidity of 0.009
kg of vapor per kg of dry air. Determine the relative
humidity.
a. 40.6% b. 43.1% c. 41.3% d. 46%

• As heat is removed from a substance it gets colder.

When no more heat can be removed and the
temperature cannot be lowered any further, we have
reached
a. perfect zero b. absolute zero c. double zero d. cold zero
Ref and Aircon Problems
• In an ideal refrigeration cycle, liquid leaves the
condenser and is expanded in such a manner that the
enthalpy of the liquid is equal to the enthalpy of the
resulting saturated mixture. This type of expansion is
known as:
a. throttling process b. isothermal process
c. adiabatic process d. isochoric

• The lower horizontal line of the refrigeration cycle

plotted on the pressure enthalpy diagram represents
a. compression of refrigerant vapor
b. evaporation of liquid refrigerant
c. condensation of refrigerant vapor
d. metering of liquid refrigerant
Ref and Aircon Problems
• 10 kg of ice at 0oC are added to 100 kg of liquid at
40oC with specific heat of 4 kJ/kg K. Calculate the
temperature of the liquid just as the ice melts. The
heat of fusion of ice is320 kJ/kg
a. 26oC b. 28oC c. 30oC d. 32oC

• In which part of vapor compression system there is

a change in pressure and temperature
a. Evaporator b. condenser c. compressor d. dryer
Ref and Aircon Problems
• What refrigerant is most commonly used for air
conditioning of passenger air craft
a. ammonia b. R-11 c. CO2 d. air

• The amount of heat required to change the

temperature of one unit weight of substance into
one degree unit of temperature
a. specific heat b. sensible heat c. latent heat d. heat
Ref and Aircon Problems
• Air at a 4m by 4m by 4m room has a relative humidity of
80%. The pressure in the room is 120kPa and
temperature of 35oC (Psat= 5.628kPa). What is the mass
of vapor in air on the room? (Rv=0.4615kJ/kg K)
a. 2.03kg b. 1.50kg c. 0.80kg d. 4.80kg

• What is the heat transfer in the glass surface area of

0.7 m2 having an inside temperature of 25oC and 13oC
outside temperature. The thickness of the glass is
0.007m and its thermal conductivity is1.8 W/m K
a. 5.6kW b. 3.6kW c. 6.2kW d. 2.16kW
Ref and Aircon Problems
• When air is saturated the wet bulb depression is
a. zero b. unity c. 50 d. 100

• The amount of heat absorbed by one ton of H2O as

it changes from solid to liquid state at 32oF is
equivalent to
a. 288000BTU b. 120000BTU c. 388000BTU d. 488000BTU
e. 188000BTU
Ref and Aircon Problems
• The power of a carnot refrigeration system in
maintaining a low temperature region at 238.9 K is 1.1
kW per ton. The coefficient of performance is
a. 3.0 b. 3.2 c. 4.2 d. 3.5 e. 4.5

• The heat rejected is

a. 5.26kW b. 4.62kW c. 4.26kW d. 5.62kW
e. 5.52kW

• The temperature of heat rejection is

a. 314.6 K b. 312.6 K c. 331.6 K d. 313.6 K
e. 300 K
Ref and Aircon Problems
• How much heat in calorie is produced in 5 min by
an electric iron which draws 5 amp from a 220
volts line. There are approximately 2.239 cal/joule
a. 738870 b. 738880 c. 739000 d. 739880

• The boiling point of ammonia at atmospheric

pressure is:
a. -28oC b. 28oC c. 28oF d. -28oF
Ref and Aircon Problems
• Determine the time in seconds will it take to raise temp
of 136 kg of water from 30 to 80oC by means of a 3 kW
immersion heater when the heat losses are 10%.
a. 10500s b. 10000s c. 10447s d. 10545s
e. 11000s

• A refrigeration system has a volumetric flow rate of 7

L/s at the suction of compressor. If the volumetric
efficiency of the compressor efficiency is 0.8, estimate
the volume displacement rate in L/s
a. 5.6 b. 6.0 c. 6.5 d. 8.75
Ref and Aircon Problems
• How much air in kg/s is needed to evaporate 100 kg of
moisture in 6 hr if the drying air temperature is 43oC
and the ambient temperature is 30oC. Assume the
latent heat of vaporization of H2O is 2500kJ/kg and the
specific heat of air is 1 kJ/kg.
a. 0.89 b. 1.28 c. 7.69 d. 192.3

• Consider 1.0 cm wall made of polyethylene board

(k=0.026W/mK) which is exposed to still air
(h=9.37W/m2K) on the inside and the other surface
exposed to 24km/hr wind (h=34W/m2K). Determine the
overall coefficient of heat transfer in W/m2K.
a. 0.0075 b. 0.052 c. 1.92 d. 5.73
Ref and Aircon Problems
• The tank of an air compressor has a volume of 0.2 m3
and is filled with dry air (R=287J/kgK) at a temperature
of35oC. If the absolute pressure of the tank is 7.5 bars,
what is the mass of air in the tank?
a. 1.0 kg b. 1.7 kg c. 1.5 kg d. 1.2 kg

• The vertical line of refrigeration cycle plotted in the

pressure enthalpy diagram represents
a. compression of refrigerant vapor
b. evaporation of liquid refrigerant
c. condensation of refrigerant vapor
d. metering/expansion of liquid refrigerant
Ref and Aircon Problems
• A refrigeration system on the reversed carnot cycle has
a minimum and maximum temperature of -25oC and
72oC respectively. If the heat rejected in the condenser
is 6000 kJ/min. Determine the required power.
a. 16.868kJ/min b. 168.68kJ/min
c. 1,686.8kJ/min d. 168,680kJ/min

• A house hold refrigerator with COP of 1.8 removes heat

from refrigerated space at a rate of 90kJ/min.
Determine the power consumed by the refrigerator
a. 0.96 kW b. 0.83 kW c. 1.5 kW d. 0.56 kW
Ref and Aircon Problems
• An air conditioner removes heat steadily from a
house at a rate of 50 kJ/min, while drawing electric
power at a rate of 6 kW. Determine the rate of heat
discharge to the outside air.
a. 410 kJ/min b. 220 kJ/min c. 510 kJ/min d. 120 kJ/min

• A carnot refrigerator operates in a room with

temperature of 25oC. The refrigerator consumes
500 W of power and had a COP of 4.5. Determine
the temperature of the refrigerated space.
a. -29.2oC b. -39.2oC c. -19.2oC d. -9.2oC
Ref and Aircon Problems
• A refrigerator receives 6000 kJ/min of heat when
operating between temperature limits of minus
15oC and 38oC. If the coefficient of performance is
60% of a carnot refrigerator operating at the same
temperature limits, find the required power input of
the refrigerator.
a. 30.5 kW b. 34.2 kW c. 3.52 kW d. 35.2 kW
Ref and Aircon Problems
End of Topic

Thank you.