1.0 Introduction

Air conditioning is the conditioning of air within a defined space, usually a residence or a

place of business. Conditioning usually involves heating or cooling, humidifying or

dehumidifying, and filtering or cleaning air and controlling its moisture level:

conditioning it to provide maximum indoor comfort. An air conditioner transfers heat

from the inside of a building, where it is not wanted, to the outside. Refrigerant in the

system absorbs the excess heat and is pumped through a closed system of piping to an

outside coil. A fan blows outside air over the hot coil, transferring heat from the

refrigerant to the outdoor air. It is because the heat is removed from the indoor air, the

indoor area is cooled. Air-conditioning is a system giving automatic control of the

required environmental conditions, by method of heating, cooling, humidification,

dehumidification, cleaning and movement of air in the building, vehicles and others.

These control conditions may be desirable to maintain the health and comfort of the

occupants or to meet the requirements of industrial process irrespective of the external

climatic conditions. According to the experiment, heating process was carried out with

using of preheater in the system. The cooling process was also being examined to

investigate the temperature change of air at the outlet without using a preheater. The

heating and cooling process, water vapor present in the air due to humidify effect was

noted whenever there was a change in speed in the blower. For every change in the air

pressure, the refrigerant temperature at each step of the cycle was noted through the

experiment.

2.0 Objective

The objective of this experiment to study the heating and cooling effects in an air-

conditioning system by controlling the speed of the blower and also to determine the

heating power of the heater and the cooling power of the cooling coil in the evaporator.

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3.0 Theory

The air conditioning unit consists of an air system and a refrigeration system. The fluid flowing in the refrigeration system was refrigerant 134A whereas the fluid in the air system was air alone itself. The refrigeration system consists of a compressor, condenser, expansion valve and an evaporator. All of these engineering devices are able to operate for a long period of time under steady condition. A compressor takes low temperature, low pressure fluid and compresses it. Upon compression, the temperature and pressure of the fluid increases. Expansion valve takes high temperature, high pressure fluid and extracts energy from it so that the fluid leaves in lower temperature, lower pressure state. Evaporators and condensers are special cases of heat exchangers. Instead of heat transfer changing the temperature of the fluid, it changes the state of the fluid (quality) which usually lies in the liquid-vapor mixture region. The evaporator adds heat into the fluid where the temperature remains constant and the enthalpy increases. On the other hand, a condenser extracts heat from the fluid, keeping the temperature constant but decreasing the enthalpy.

The sketch shows how the refrigeration system works

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The air system in the air conditioning unit consists of a blower, pre-heater, re-heater and humidifier. These components are arranged in sequential order to heat or cool, humidify or dehumidify, clean and purify and recirculate the air to the conditioned space. The blower draws in the air across the orifice form the environment. The pre-heater is essential due to the possibility of the entering air temperature being below 0℃ and could possibly freeze the refrigerant in the cooling coil. Reheater helps in controlling the humidity. It warms up the air as warm air has the capacity to hold water vapor more than cold air. The simplified diagram below helps in understanding the concept of conditioning air. When the blower draws in warm air, a pre-heater is not necessary. Hence, the warm air is allowed to pass through the evaporator. The cooling coil in the evaporator takes in heat from the warm air, thus it creates a cooling effect and the temperature of warm air drops. The cooled air is passed through a reheater to rise the temperature of air a little to increase its ability to absorb water vapor from the water tube as it passes through the humidifier.

When the blower draws in cold air, it must be bought to a minimal temperature where it doesn’t affect the coolant in the evaporator and the temperature is increased by few degrees. However, as air reaches the reheater, it is heated to higher temperature and more energy is used in warming a cold air than increasing the temperature of warm air. Hence, warmer the air, the more water vaporizes from the humidifier which was carried by the air through the outlet.

Therefore, a heating process in an air conditioning is the one where the air is conditioned using the preheater and reheater switched on and a cooling process is one where the air being drawn by the blower is unaffected by a conditioning element. It simply passes by the evaporator, releases heat and creates the cooling effect and ejects form the other end of the tube.

The heating power of the heater and the cooling power of the coil were determined using the psychometric chart which was used for tracking the changes of state of the air in air conditioning systems. The psychometric chart shows the following constants as lines or curve,

tp : relative air humidity in %

t : temperature in ℃

h : enthalpy in kJ/kg

x : Absolutely humidity in kg/kg

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Graph shows the psychometric chart

4.0 Apparatus

Pressure gauge

Manometer

Compressor

Evaporator

Condenser and Blower

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5.0 Experimental Procedures

Cooling process

1. The solenoid valve with receiver was selected.2. Solenoid valve selector was set to expansion valve (SV3).3. The condensing unit was switched on.4. The blowers was switched on and regulated at a lower speed.5. The temperature and relative humidity at the inlet (AT1, AH1) and outlet (AT2,

AH2) of the cooling coil was reached once steady state was reached. A steady state condition was reached when the digital readings for AT1 AND AH1 had stopped fluctuating.

6. Reading for the pressure differential was recorded from the manometer.7. All four refrigerant temperatures were recorded by turning the nob each time (TT1,

TT2, TT3, and TT4) followed by analogue readings from the pressure gauges (P1, P2).

8. The blower speed regulator was rotated again to gradually increase the speed and step 5,6,7 and 8 were repeated 3 more times each time the blower speed was increased to acquire a total of 5 sets of readings of temperature, pressure and humidity while keeping the initial setting unchanged. However, once the blower speed was increased, the system took 15 minutes to reach a stable state.

9. All the switches were closed once the readings were acquired.

Heating process

1. The solenoid valve with receiver was selected.2. Solenoid valve selector was kept on expansion valve (SV3).3. The condensing unit was switched on.4. The preheater was switch on.5. The blower was switched on and the lowest speed was selected.6. The temperature and relative humidity at the inlet (AT1, AH1) and (AT2, AH2)

of the preheater were recorded once the process was stabilized in approximately 15 minutes.

7. The blower regulator speed was changed gradually (increasing it) and step 6 was repeated.

8. Step 7 was repeated 2 more times to acquire 5 sets of reading in total, while waiting 15 minutes after each regulation to stabilize the system.

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6.0 Results

Cooling process

Manometer Air temperature

℃

Refrigerant temperature℃ Relative humidity %

Pressure gauge (psi)

∆P, N/m2 AT1 AT2 TT1 TT2 TT3 TT4 AH1 AH2 P1 P250 28.7 15.1 11.4 38.1 39.2 8.7 50.7 81.2 138.0 35.5100 29.0 15.8 13.2 39.1 40.0 9.8 48.5 89.9 142.2 41.2150 29.2 16.6 13.4 38.9 39.7 9.9 45.7 92.2 142.2 34.1200 29.3 16.2 13.7 39.1 39.9 10.4 45.0 93.0 142.2 34.1250 29.3 16.4 13.9 39.2 40.5 10.6 45.1 93.6 142.2 34.1

∆P, N/m2 ∆h, kJ/kg v, m/s m, kg H, kJ50 23 5.472 0.065 1.5100 25 7.74 0.092 2.3150 15 9.48 0.113 1.7200 18 10.94 0.131 2.36250 18 12.23 0.1461 2.36

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7.0 Calculation

Simple calculation

When AT1 = 28.7 ℃ and AH1 = 50.7, the h1 = 59 kJ/kg

When AT2 = 15.1 ℃ and AH12= 81.2, the h2 = 36 kJ/kg

To calculate the velocity of the air across the orifice

v¿0.598√ 2∆ Pρ

The density of air at 1 atom pressure is equal to 1.1945 kg/m3

v = 0.598√ 2(50)1.1945

= 5.472 m/s

When m= ρ V, V is the volumetric flow rate of air, V = v. A

Diameter of the air duct = 12cm = 0.12m

Air duct cross sectional area, A = π x (0.006)2 ≈ 0.01 m2

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h1 =

h2 =

m= 1.1945 × 5.472 × 0.01 = 0.065 kg/s

The enthalpy, H = m× (∆h), H = 0.065× (59-36) = 1.5 kJ

Heating process

Manometer

Air temperature

℃

Relative humidity %

∆P, N/m2 AT1 AT2 AH1 AH250 26.6 22.3 50.5 73.5

100 26.4 22.2 50.6 70.4150 26.2 22.5 50.9 67.8200 26.1 22.8 51.2 66.7250 26.1 22.9 51.7 68.9

∆P, N/m2 ∆h, kJ/kg v, m/s m, kg H, kJ50 3 5.5 0.065 0.195100 3 7.77 0.092 0.276150 3 9.52 0.113 0.34200 3 11.0 0.130 0.4250 3 12.29 0.146 0.438

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Sample calculation

When AT1 = 26.6 ℃ and AH1 = 50.5, the h1 = 55 kJ/kg

When AT2 = 22.3 ℃ and AH12= 73.5, the h2 = 52 kJ/kg

The density of air at 1 atom pressure is equal to 1.184 kg/m3

v = 0.598√ 2(50)1.184

= 5.5 m/s

When m= ρ V, V is the volumetric flow rate of air, V = v. A

Diameter of the air duct = 12cm = 0.12m

Air duct cross sectional area, A = π x (0.006)2 ≈ 0.01 m2

m= 1.184 × 5.5 ×0.01 = 0.065 kg/s

The enthalpy, H = m× (∆h), H = 0.065 × (55−52) = 0.195 kJ

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h1 = 55

h2 = 52h

8.0 Discussion

The experiment was done to study the heating and cooling effects in an air

conditioning system by controlling the speed of the blower and also to determine the

heating power of the heater and the cooling power of the cooling coil in the evaporator.

From the results are obtained, in cooling process at 50 N/m2 of air pressure, the air

temperature started to be in the inlet 28.7 ℃ and then it decreased after the air passed

through the cooling coil in the evaporator to be 15.1 ℃ which is the outlet temperature.

In heating process at 50 N/m2 of air pressure, the air temperature started to be in the inlet

26.6 ℃ and then it increased after the air passed through the preheater and then it

decreased when the air is allowed to pass the evaporator, The evaporator is acting as the

heat sink takes away heat from the hot air and cools the air temperature to be 22.3 ℃

which is the outlet temperature. So, generally, the air temperature decreases after it goes

through the process of the air conditioning system. Also, the pressure was decreased after

it exits from the expansion valve which helps the evaporator to change the situation of the

working fluid to vapor.

In cooling process, at 50 N/m2 of air pressure, the air humidity was to be 0.507 which is

in the inlet before getting to the evaporator, and then it increased to be 0.812 which is in

the outlet as the air passes the humidifier and holds water vapor along with it. In heating

process, at 50 N/m2 of air pressure, the air humidity was to be 0.505 which is in the inlet

before getting to the evaporator, and then it increased to be 0.735 which is in the outlet as

the air passes the humidifier and holds water vapor along with it. So, generally, the

humidity is increased in the reheat process which it can control and adjust the humidity.

From the results are obtained, as long as the air pressure increases the velocity will

increase as well as the mass flow rate will increase. In heating process when the air

pressure increases the enthalpy will increase gradually.

The enthalpy in the heating process must be more than in cooling process. So, the more

enthalpy means more water vapor which leads more humidity. There are some errors

occurred due to some mistakes happened during the experiment, and the possible errors

might be like a sufficient time was not allowed for a steady state condition to be achieved

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before recording the readings and that affected the results, the readings of the humidity

were taken from the humidity selector gauge were not accurate due to the environmental

impacts which play the major role in changing the temperature continuously so that will

make the readings inaccurate,

The experiment can be improved, for example, a sufficient time must be allowed for a

steady state condition to be achieved before recording the readings, the room should be

insulated avoiding the environmental impacts. So, the readings of the experiment can be

very accurate.

9.0 Conclusion

In conclusion, cooling process is carried out by passing the air over the coil. These coils

are cooled by the refrigerant passing through them and are also called evaporator coils. In

certain cases the coil is also cooled by the some gas passing inside it. Heating process of

the air is important when the air conditioner is used as the heat pump to heat the air. In

the heat pump the air is heated by passing it over the heating coil that carry the high

temperature refrigerant. In some cases the heating of air is also done to suit different

industrial and comfort air-conditioning applications where large air conditioning systems

are used. In heating process the humidity was noted during the experiment. The

experiment is achieved with errors due to mistakes happened during the experiment. The

experiment can be considering as acceptable.

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10. References

1. Frank Kreith (Editor), Shan K. Wang (December 6, 1999). Air Conditioning and

Refrigeration Engineering. London: CRC Press. p33-60.

2. Boles, Dr. Michael A., and Dr. Yunus A. Cengel, Thermodynamics, An Engineering

Approach, McGraw-Hill Inc., New York, 1994.

3. Dewitt, David P., and Frank P. Incropera, Introduction to Heat Transfer, Third Edition,

John Wiley & Sons, New York, 1985, pp. 351-360, 757.

4. Jan F. Kreider. Handbook of heating, ventilation, and air conditioning. CRC press.

5. MATHEWS, D. 2013, Using Your Air Conditioner To Control Humidity. Available:

http://blog.ccacac.com/2013/02/05/humidity/. last accessed on 30 April 2015.

6. theMechy . (2012). Air conditioning system. Available:

http://infomechy.blogspot.com/2012/11/lab-report-sand-testing-preparation-gfn_4.html.

Last accessed 30th Apr 2015.

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