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, 1
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Transcript
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.