Basic Principals of Air conditioning

By Dr. Esam Mejbil Abed

REPUBLIC OF IRAQMinistry of Higher Education & Scientific Research

University of Babylon

College of EngineeringMechanical Engineering Department

Definition

Air conditioning is the controlling of air temp., its humidity, itspurity and its flow rate at a certain place to produce a comfort zone

Ventilation is refreshing the air in a crowded place by feeding

the place with a mount of air at a certain time and draw the same a

mount of air to obtain an healthy environment empty of dust , gases

and bad smell.

Importance of air

The human need to 1.2 kg. of water , 2.7 kg. of food and 16 kg. of

air per day . The healthy requirements is :

Moving the air by 2.5 L/s

A third of the feeding air must be fresh air and the 2 thirds is

return air after purifying

the entire air in the air conditioned areas must be moved at a

velocity between 0.15 to 0.25 M/S . This velocity doesn’t cause a

noise

Human’s comfort

The human needs to a comfort medium and we can do that by the next 5 properties treatment :

1. Dry bulb temp.(d.b.t) : It is needed to cool or heat the air before flowing

2. The humidity ratio (H) : The air must be humidified or dehumidified before flowing.

3. The air movement : the flow rate of air and its distribution must be changed to be homogeneous at the conditioned areas to make everyone feel the same conditions (the static air cause a temp.diff. between the breath level and the ceiling level about 8-16 c0 .)

4. Air purifying : By using filters to prevent dust and killing the bacteria

5. Ventilation : It is required to use fresh air to provide the needed oxygen for the conditioned areas

AIR PROPERTIES

1. DRY BULB TEMP.(d.b.t):It is the temp. which the thermometer can

measure

2. WET BULB TEMP.(w.b.t): If a moist wick is placed over a

thermometer bulb, the evaporation of moisture from the wick will

lower the thermometer reading .This temp. is known as the w.b.t.

3. DEW POINT (d.p): is the temp.below which water vapor in the air

start to condense.

4. HUMIDITY (H): Is the presence of moisture or water vapor in the

air.the amount of humidity in the air affects the rate of evaporation of

perspiration from the body.Dry air causes rapid evaporation.

5. RELATIVE HUMIDITY(rh): Is a term used to express the amount of

moisture in a given sample of air. It is stated in a percentage

PSYCHROMETRIC CHART

Dry bulb temp.10 20 40 50300

20

30

40

50

80

70

60

5

100

90

30

25

20

10

15

2

4

6

12

10

8

16

14

18

20

22

26

24

28

30

EXAMPLE : ( 1 )

Moist air exists at 40 oc dry- bulb temperature and 20

oc thermodynamic wet-

bulb temperature , and 101.325 kpa pressure .

Determine the humidity ratio, enthalpy , dew-point temperature, relative

humidity, and volume.

PSYCHROMETRIC CHARTAPPLICATION

Dry bulb temp.10 20 40 50300

20

30

40

50

80

70

60

5

100

90

30

25

20

10

15

2

4

6

12

10

8

16

14

18

20

22

26

24

28

30

PSYCHROMETRIC CHARTAPPLICATION

Dew point = 7oc

Dry bulb temp.10 20 40 50300

20

30

40

50

80

70

60

5

100

90

30

25

20

10

15

2

4

6

12

10

8

16

14

18

20

22

26

24

28

30

PSYCHROMETRIC CHARTAPPLICATION

1

32

Process 12 represents cooling and

dehumidification

Process 13 dehumidify-Process 32 cooling

Cooling capacity = mair ( h1 – h2 )

Alcohol on ones handevaporates and cools

Sensible Heat

Heat which causes a change in temperature of a substance

Sensible Heat

Heat which causes a change in temperature of a substance is called sensible heat.

If a substance is heated and the temperature rises as the heat is added, the increase in heat is called

sensible heat.

Likewise, heat may be removed from a substance. If the temperature falls, the heat removed is, again,

sensible heat.

Sensible heat can be measured in degree of temperature.

Latent HeatHeat energy absorbed in the process of changing form of a substance without a change in temperature or pressure. Heat energy that cannot be measured with a thermometer.

32o

Latent HeatHeat which brings a change in state with no

change in temperature.

When we think about ‘change of state’ conditions we are considering Solids to Liquids and Liquids

to Vapours.

Latent or Sensible?• What changed from previous slide

• What type of heat was involved

32o

Latent Heat

32o 32o

Saturated Vapor

Vapor and liquid are in contact with one another.

Subcooling

Cooling the refrigerant below its condensing temperature. A refrigerant must be in a complete

liquid state before it can be subcooled.

SuperheatTo raise the temperature of a vapor beyond its

boiling point. No liquid can remain when a refrigerant is superheated.

Pressure / Temp. Charts

Boiling points at atmospheric pressure (14.7PSIA)

R-11 = + 75o

R-123 = + 82o

R-12 = - 21o

R-134a = - 15o

R-22 = - 41o

Vapor Pressure 80°F(Saturated state)

R-11 R-22R-134a

1.5 PSIG 86.4 PSIG 143.6 PSIG

Saturation Point: - The boiling temperature of a

substance at a given pressure.

Saturation: - A mixture of liquid and vapour at its

saturation temperature and pressure.

Superheat: - Vapour at a temperature above its

saturation temperature, measured in degrees of

superheat.

Subcooled: - Liquid below its saturation temperature,

measured in degrees of subcooling.

Terminology

Vapor Compression

Evaporator

Condenser

Expansion ValveCompressorMotor

Cooling Tower

Vapor Compression

Evaporator

Condenser

Expansion ValveCompressorMotor

Cooling Tower

OptimumChilled Water Circuit for wax pack. unit

Evaporator

Condenser

Expansion ValveCompressorMotor

PREPARED BY: ENG. MOSTAFA FADEL

CHILLED WATER

STORAGE TANK

WAX PAC. UNIT90M3/Hr

Hot

water

tank

OVER FLOW

CHILLER 165 T.O.R

90M3/Hr

What is Refrigeration?

The low pressure, low temperature vapour is then sucked into the compressor where it is

compressed into high pressure, high temperature vapour.

Vapor Compression

Evaporator

Condenser

Expansion ValveCompressorMotor

Cooling Tower

More Terminology

Evaporator split: - The temperature difference betweenthe evaporator saturation temperature and the leavingprocess temperature.

Evaporator range: - The temperature difference betweenthe entering and leaving process temperature.

Condenser split: - The temperature difference betweenthe condenser saturation temperature and the leavingcondenser water temperature.

Condenser range: - The temperature difference betweenthe entering and leaving condenser temperature.

System analysis

Look at the following exercises and see if you can analyze the system operation

R22 System operating logs analysis

Look at the following operating logs of a R22 water chiller and evaluate the system operation

based on the actual readings compared to design conditions.

Cooling load calculations

1. Sensible heat : causes increase in temperature.

• Heat transfers from walls ,windows, roofs,and floors according to the temp. Difference between outdoor and indoor

• Heat from outside walls which supposed to the sun.

• Heat from interior source like lights, motors, heaters, and other heating equipments.

Cooling load calculations

1. Latent heat : causes increase in humidity

• Water vapor from humans breath .

• Humidity from air leakages from outside .

• Humidity from process inside the building .

COOLING LOAD (Q) = Q SENSIBLE + Q LATENT KW

Cooling load calculations

1 TON OF REFREGIRATION(T.O.R) = 3.5 KW

THE END

THANK YOU FOR YOUR TIME.