Refrigeration and Air Conditioning
B.Sc. Mechanical Engineering- 5th Year
Part two: - Air Conditioning
Introduction:
 Air conditioning may be required in buildings which have a high heat gain and as a result
a high internal temperature.
 The heat gain may be from solar radiation and/or internal gains such as people, lights and
business machines.
 The diagram below shows some typical heat gains in a room.
Cooling Load in a Building:
 If the inside temperature of a space rises to about 25oC then air conditioning will
probably be necessary to maintain comfort levels.
 This internal temperature (around 25oC) may change depending on some variables such
as:
 Type of building
 Location of building
 Duration of high internal temperature
 Expected comfort conditions.
 Degree of air movement
 Percentage saturation.
 In some buildings it may be possible to maintain a comfortable environment with
mechanical ventilation but the air change rate will tend to be high (above about 8 air
changes per hour) which can in itself cause air distribution problems. Since air conditioning
is both expensive to install and maintain. It can be reduced by careful building design and
by utilising methods such as:
 Window blinds or shading methods
 Heat reflecting glass
Prepared by: Dr.Mohan, Assistant Professor, Dept. of Mechanical Engg. Dilla University
Refrigeration and Air Conditioning
B.Sc. Mechanical Engineering- 5th Year

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Open able windows
Higher ceilings
Smaller windows on south facing direction
 Alternative lighting schemes.
 In dryer regions humidification is required through most of the year whereas in tropical air
conditioning one of the main features of the system is the ability to remove moisture from
warm moist air.
 Dampers are used in air conditioning central plant systems to control the amount of air in
each duct.
 It is common to have 20% fresh air and 80% recirculated air to buildings.
 In buildings with high occupancy the fresh air quantity should be calculated this may
require a higher percentage of fresh air (i.e. more than 20%)
 A removable fibreglass dust filter is positioned in the fresh air intake duct.
 The secondary filter, after the mix point, is used to remove fine dust particles or other
contaminant picked up in the rooms and recirculated back into the plant. A removable bag
filter is generally used for this where a series of woven fibre bags are secured to a
framework which can be slid out of the ductwork or air handling unit (A.H.U.) for
replacement.
 If air conditioning is the only answer to adequate comfort in a building then the main choice
of system can be considered.
 Full comfort air conditioning can be used to provide cool air (approx. 13oC to 18oC) for
cooling and warm air (approx. 28oC to 36oC) for heating and the air is cleaned by filters,
dehumidified to remove moisture or humidified to add moisture.
 Air conditioning systems fall into three main categories, and are detailed in the following
pages;
1. Central plant systems.
2. Room air conditioning units.
3. Fan coil units.
4. VRF System
1. Central plant systems have one central source of conditioned air which is distributed in
a network of ductwork.
2. Room air conditioning units are self-contained package units which can be positioned
in each room to provide cool air in summer or warm air in winter.
3. Fan coil units are room units and incorporate heat exchangers piped with chilled water
and a fan to provide cool air.
2. ROOM AIR CONDITIONER
 These units use refrigerant to transfer cooling effect into rooms.
 Room air conditioning units fall into two main categories:
1. Split type with indoor and outdoor unit
2. Window/wall unit swith evaporator at inside and condenser at the outside face.
2.1 Split Air Conditioner
 Split air conditioners have two main parts, the outdoor unit is the section which
generates the cold refrigerant gas and the indoor unit uses this cold refrigerant to cool
the air in a space.
Prepared by: Dr.Mohan, Assistant Professor, Dept. of Mechanical Engg. Dilla University
Refrigeration and Air Conditioning
B.Sc. Mechanical Engineering- 5th Year
 The outdoor unit uses a compressor and air cooled condenser to provide cold refrigerant
to a cooling coil in the indoor unit.
 A fan then blows air across the cooling coil and into the room.
 The indoor unit can either be ceiling mounted (cassette unit), floor mounted or duct type.
 The drawing below shows a ceiling mounted (cassette unit).
Ceiling 4 way Cassette Unit Indoor Unit:
Prepared by: Dr.Mohan, Assistant Professor, Dept. of Mechanical Engg. Dilla University
Refrigeration and Air Conditioning
B.Sc. Mechanical Engineering- 5th Year
Outdoor Unit:
Different Type of Split Units:
2.2 WINDOW / WALL UNITS:
 Window or wall units are more compact than split units since all the plant items are
contained in one box.
 Window units are installed into an appropriate hole in the window and supported from a
metal frame.
 Wall units like the one shown below are built into an external wall and contain all the
necessary items of equipment to provide cool air in summer and some may even provide
heating in winter.
Prepared by: Dr.Mohan, Assistant Professor, Dept. of Mechanical Engg. Dilla University
Refrigeration and Air Conditioning
B.Sc. Mechanical Engineering- 5th Year
FUNDAMENTAL PROPERTIES OF MOIST AIR:
Properties of Moist Air:
 In most of the air‐conditioning system the working substance is atmospheric air.
 Therefore, it is extremely important to study the properties of atmospheric air and also the
different processes required for a proper design of an air-conditioning system
 Atmospheric air consists of many gases, water vapour and the pollutants. The
concentrations vary from place to place.
 Also it decreases with the altitude, being almost dry air at an altitude of 10 km
 The composition of p dry part in atmospheric air is listed below:
Components Mol. Mass Vol % Mass %
Prepared by: Dr.Mohan, Assistant Professor, Dept. of Mechanical Engg. Dilla University
Refrigeration and Air Conditioning
B.Sc. Mechanical Engineering- 5th Year
 For use in an air‐conditioning system, the pollutant is removed from the atmospheric air.
 The residue of atmospheric p air after the removal of pollutants is known as the moist air,
as it contains the various gases including the water vapour.
 The molecular weight of the dry air is 28.966 and the characteristic gas constant is 287.036
J/kg‐K.
 The molecular weight of the water vapour is 18.05 and the characteristic gas constant is
461.52 J/kg‐K
 Air at a given temperature and pressure can exist in two conditions: unsaturated and
saturated.
 At a given state, if the moisture content in dry air is less than the maximum permissible
moisture corresponding to the given temperature and pressure, the air is known as
unsaturated air.
 That means, it is the moisture content which decides the state of air.
Gibbs – Dalton Law
 According to this law, the total pressure exerted by a mixture of non‐reacting ideal gases
is equal to the sum of partial pressure exerted by the constituent gases, i.e.
Pt = P1 + P2 + P3 + ………..
 where Pt is the total pressure
P1 = n1GT/V; G is the universal gas constant
P2 = n2GT/V
P3 = n3GT/V
 For moist air, it becomes
Pt = Pa + Pv = Pa + Pw
 Where Pt is the total pressure, pa is the partial pressure of dry air, Pw or Pv is the partial
pressure of water vapour.
 It is difficult to find the exact property of the moist air, however it is noticed that up to 3
atm. pressure, moist air behaves as an ideal gas.
Amagat Law of Partial Volumes:
 According to this law, each constituent is assumed to occupy volume corresponding to the
given total pressure and the temperature. The total volume of the gas is then equal to the
summation of individual volumes, i.e.
V = V1 + V2 + V3 + ………..
 Where V is the volume of the mixture
V1 = n1GT/p; G is the universal gas constant
V2 = n2GT/p,
V3 = n3GT/p.
Prepared by: Dr.Mohan, Assistant Professor, Dept. of Mechanical Engg. Dilla University