REFRIGERATION AND AIR

CONDITIONING

LECTURE 3: Air Conditioning System

SAMUEL KELBESA (MSc)

DEPARTMENT OF MECHANICAL ENGINEERING
OFFICE: SE 13:F1.01
PHONE NO: +251912709680
E-MAIL ADDRESS: samuelkelbesa@dbu.edu.et
 Introduction
qThe air conditioning is the branch of engineering science which deals with
the study of air, i.e. supplying and maintaining desirable internal
atmospheric conditions for human comfort, irrespective of external
conditions.
q human comfort according to American Society of Heating, Refrigeration
and Air conditioning Engineers (ASHRAE) defined as the condition of
mind which express satisfaction with thermal environment.
q Air conditioning is not only for human comfort, but also for industrial
purpose, food processing, storage of food and other materials.
 Cont.
q There are several factors which affect the comfort of air conditioning
systems:
1. Temperature of the air
2. Humidity of the air
3. Purity of the air
4. Motion of the air

NB: human being feel comfortable at 21oC DBT and 56% RH.
Recall:
Classification of Air conditioning system:
1. Based on major function:
a. Comfort AC and
b. Industrial AC
2. Based on season of the year:
a. Summer AC
b. Winter AC and
c. Year round AC
3. Based on equipment arrangement:
a. Central AC
b. Unitary AC and
c. Combination of the two
 1A. Comfort Air Condition
qThe air is brought to the required dry bulb temperature and relative humidity for
human health, comfort and efficiency.
NB: if sufficient data of the required condition is not given, the it is assumed to be
210C dry bulb temperature and 50% relative humidity.
The sensible heat factor is generally kept as following:
a. For residence = 0.9
b. For reasturant or busy office = 0.8
c. Auditorium or cinema hall = 0.7
d. Ball room dance hall = 0.6
NB: we adopt the comfort AC for homes, offices, shops, restaurants, theatres,
hospitals, schools, etc…
 1B. Industrial Air Conditioning System
q it is important system of AC these days in which the inside dry bulb
temperature and relative humidity of the air is kept constant for proper
working of the machines and for the proper research and
manufacturing processes.
qSome of the sophisticated electronic and other machines need a
particular dry bulb temperature and relative humidity.
qSometimes also these machines require a particular method of
pychrometric processes.
qThis types of AC system is used in text miils, paper mills, machine-
part manufacturing plants, tool rooms, photo-processing plant,….
 Cont.
Example 2
Following data refers to an air conditioning system designed for an industrial
process for hot and wet climate.
Outside condition = 30 0C DBT and 75% RH
Required inside conditions = 200C and 60% RH
The required condition is achieved by cooling and dehumidifying and then by
heating. If 20 m3 of air is absorbed by power plant every minute, find,
1. Capacity of cooling coil in TR
2. Capacity of heating coil in kW
3. Amount of water removed per hour and
4. By pass factor of heating coil if its surface temperature is 35 0C
2A. Winter Air Conditioning System
2B. Summer Air Conditioning System
2C. Year Round Air Conditioning System
 Space Air Distribution
q conditioned point of typical HVAC system are location of AC system
within thermodynamic processes.
q the most important location of AC are:
1. Design or room condition
2. Outdoor condition
3. Mixing condition
4. Supply air condition
 Cont.
q generally conditioned air introduces room or conditioned space in
three ways:
a. Outdoor air: 100% fresh air (ventilation system)
b. Mixing air: combination of fresh and recirculating air
c. Return air: the air returning from the room at the absence of outdoor
air.

NB: Every conditioned point represented by two properties of air.

 Cont.
Sensible heat: amount of heat changed due to change in temperature of
the body or the system.
We can calculate sensible heat by using the following formula:
Qs = ሶ Cp Δ t
Where
ሶ − = ∗
V – air flow rate (m3/sec) and v – specific volume (m3/kg)
 Cont.
Latent heat: is the amount of heat related to change in phase of the
system by identifying the difference in humidity ratio.
We can calculate the latent heat by using the following formula:
= ሶ (W2 - W1)hpfg

qMixing condition: is the function of the outdoor condition, the return

air condition and the qualities of the outdoor air and return air and its
depend on the ratio of return air condition and outdoor air condition.
NB: mixing condition can be plotted after connecting the state of design
condition and outdoor air condition by straight line.
 Important Factor For AC Processes
q beside the important terms and processes related to psychometric
chart, there is some important factor must be taken under
consideration for AC system.
These important factors are:
1. Room sensible heat factor
2. Grand sensible heat factor
3. Effective room sensible heat factor and
4. By pass factor
 1. Room Sensible Heat Factor (RSHF)
Is the ratio of room sensible heat to room total heat.
Mathematically,
RSHF = RSH/RTH
RTH = RSH + RLH
Where,
RSH – room sensible heat
RLH – room latent heat
RTH – room total heat
NB: RSHF can be plotted by using protractor on chart by using sensible heat
factor scale, on the right hand side of the chart and parallel line is drawn from
design state point to saturation.
 Grand Sensible Heat Factor (GSHF)
Is the ratio of total sensible heat (room sensible heat and out air sensible
heat) to total grand heat.
GSHF = TSH/GTH
TSH = RSH + OASH
TLH = RLH + OALH
GTH = TSH + TLH
NB: GSHF can be plotted from the mixing condition to the saturation
line.
Cont..
 By Pass Factor
Is part of the total air through the coil which fail to come contact with
the surface coil.
We can calculate by pass factor by using the following formula:
BPF = (Ts – ADP)/(Tm – ADP)
 Effective Room Sensible Heat Factor
(ERSHF)
Is the ratio of the effective room sensible heat to effective room total
heat.
ERSHF = ERSH/ERTH
ERSH = RSH + OASH*BPF
ERLH = RLH + OALH*BPF

NB: ERSHF can be plotted from the room condition to ADP (apparatus
dew point temperature).
Cont..
 Cooling Load Estimation
the total heat required to be removed from the space in order to bring at
the desired temperature by the air conditioning and refrigeration
equipment is known as cooling load.
The purpose of load estimation is to determine the size of the air
conditioning and refrigeration equipment that is required to maintain
inside design conditions during period of maximum outside
temperature.
The design load is based on inside and outside design conditions and its
air conditioning and refrigerating capacity to produce and maintain
satisfactory inside conditions.
 Components of a cooling load
The two main components of
a cooling load imposed on air
conditioning plants operating
during hot weather as follows
1. Sensible heat gain
2. Latent heat gain
 Sensible heat gain
ü the heat flowing into the building by conduction through exterior walls, floors,
ceilings, doors, and windows due to temperature difference
ü the heat received from solar radiations (it consists of the heat transmitted directly
through glass of windows, ventilators or doors and the heat absorbed by the walls
and roofs exposed to solar radiation and later on transfer to the room by
conduction)
ü the heat conducted through the interior partition from rooms in the same building
which are not conditioned.
üThe heat given off by light, motors, machineries, cooking operations, industrial
processes,…
üThe heat liberated by occupant
üThe heat carried by the outside air which leaks in through the crack in the doors,
windows and through frequent opening (infiltration)
ü the heat gain from fan work
üThe heat gained through the wall of duct carrying conditioned air through
unconditioned space in the building
 Latent heat gain
When there is an addition of water vapor to the air of enclosed space, a
gain latent heat is said to occur. This latent heat removed during the
process of summer air conditioning.
The latent heat gain may occur due to any one or all of the following
sources
- The heat gain due to moisture in the outside air entering by infiltration
- The heat gain due to condensation of moisture from occupant
- The heat gain due to the condensation of moisture from any process
such as cooking foods which takes place in conditioned space
- The heat gain due to moisture passing directly into the conditioned
space through permeable walls or partitions from outside or from
adjoining regions where the water vapor pressure is higher.
 Example
An AC system is designed for a restaurant when the following data is available:
Total heat flow through the walls, roof and floor = 6.2kW
Solar heat gain through glass = 2kW
Equipment sensible heat gain = 2.9kW
Equipment latent heat = 0.7kW
Total infiltration air = 400m3/hr
Outdoor condition = 35 0C DBT and 360C WBT
Inside design condition = 27 0C DBT and 55% RH
Minimum temperature of the air supplied to the room = 17 0C
 Cont..
Total amount of fresh air supplied = 1600m3/hr
Seating chair for dinning = 50
Employee serving the meals =5
Sensible heat gain per person = 58W
Latent heat gain per sitting person = 44W
Latent heat gain per employee = 76W
Sensible heat added from meals = 0.17kW
Latent heat added from meals = 0.3kW
Motor power connected to the fan = 7.6kW
 Cont…
If the fan is situated before the conditioner, then find the following:
a. Amount of the air delivered to the room in m3/hr
b. Percentage of recirculated air
c. Refrigeration load in the coil in tonnes of refrigeration and
d. Dew point temperature of the cooling coil and by-pass factor.
 Cont..
Solution
Step 1: listing out given data:
 Cont..
Step 2: drawing the flow diagram for the AC system:
Continued,..

Step 3: placing the AC processes in psychometric chart: