Temperature vs Time
Time, t (s)
Temperature, T (degree celcius)
LAB REPORT:
AIR CONDITIONING
NAME: KAVENESH GUNASEGARAN
ID: 015619
DATE: 29/10/2015
LAB INSTRUCTOR: MR. ARUN PRABHAKAR
LECTURER: DR. YOUSIF ABDALLA ABAKR
SUMMARY.
The aim of this experiment is to understand the air conditioning process and the indicators of moisture and heat content of the environmental air. Besides that, through this experiment, students also obtained the knowledge to calculate the required heat exchanges that will achieve the required air condition.
The objective is to experiment with air heating and compare air condition with the real air condition measured using wet and dry bulb thermometers. Besides, the student also carried out experiment with air cooling and compares expected condensate collection with experimental observations.
The air conditioner used in this system is the PA Hilton model. In this model, it has two main systems, which are the air cycle and the refrigerant cycle.
For the air cycle, water initially is boiled using a boiler. Then the steam from the boiler is then sucked by a fan. Then the steam passes the evaporator where it acts as a cooling coil, causing heat from the steam is extracted. Since heat is extracted, this causes the steam to undergo condensation process. As a result, condensate is produced and is channeled out of the system by a drain. After the evaporator, there is a secondary heater that is used to ensure that the temperature of the steam is ideal. However, during the experiment, the secondary heater is used to ensure that mist is not formed. Before the air from the evaporator leaves the system, it passes through an orifice. The function of an orifice is to calculate the differential pressure between inside and outside of the air conditioning unit.
For the refrigerant cycle, the refrigerant used is the R134a. The refrigerant cycle has four main processes that it undergoes. Firstly, the refrigerant with high pressure passes the condenser where it undergoes condensation. This occurs because the refrigerant has a much higher temperature than the surrounding of the condenser, so heat is extracted. Then, the refrigerant passes through the expansion valve where the pressure is reduced. After the expansion valve, the refrigerant then passes through the evaporator, where the temperature of the surrounding is much greater than the temperature of the refrigerant. Thus, this causes the refrigerant to undergo evaporation process as heat is extracted from the surrounding. As the refrigerant leaves the evaporator, it has low pressure and is in the superheated state. The refrigerant then passes the compressor where the pressure is increased, causing the saturation pressure to increase and the cycle continues.
When analysing the calculations for this experiment, there's an important quantity that must be considered. That quantity is known as uncertainty. Uncertainty is caused by the coarseness of the measuring tool. Thus, this causes non-accurate results to be obtained. Besides, uncertainty is also defined as the parameter that determines the dispersion of a measured quantity. For this experiment, the uncertainty is calculated using a method called Kline-McClintock Method.
The formula used to measure the uncertainty in mass flow is;
ϵma=1.5942Δz νD2ϵΔz2+1.5942ΔzνD32ϵνD2
Besides, the uncertainty in sensible heating, ϵHB-C is calculated using;
ϵHB-C=hB-hC2ϵma2+ma2ϵhB2+ma2ϵhC2+ hw2ϵmw2
Furthermore, the psychometric chart also plays an important role in air conditioning, especially in the refrigeration unit. The psychometric chart enables us to determine quantities such as enthalpy, specific volume, relative humidity, absolute humidity and even the dew point temperature of the refrigerant. All of these quantities can be determined by just knowing the dry and wet bulb temperatures. On the other hand, the pressure vs enthalpy graph also plays a significant role in this experiment as the enthalpy and specific volume of refrigerant is determined from this graph.
THEORY.
Air conditioning is a process mainly concerned with controlling temperature and humidity. It is a complex electronic circuitry that is used to for controlling the comfort of living organisms. Besides, an air conditioning unit is required to maintain people's steady core body temperatures of 37 degrees Celsius.
In air conditioning, the refrigeration unit plays an important role in reducing the temperature of air that is entering the air conditioning unit. The refrigeration unit consist of 4 main components, namely the condenser, expansion valve, evaporator and compressor. Each of these components play an essential role in ensuring the temperature of the air entering the air conditioning unit reduces. Another important point about the refrigeration unit is that the fluid that circulates inside the refrigeration unit is known as the refrigerant. The refrigerant has a low saturation temperature over a range of pressure. The most common refrigerant is the R134a.
For a brief idea of how a refrigeration unit works is as the following. Initially the refrigerant vapour enters the condenser with high pressure and temperature greater than the temperature of the surrounding in the condenser. Due to the difference in temperature, this causes the vapour to lose thermal energy to the surrounding. As a result, the refrigerant vapour undergoes condensation by which it converts into liquid. At this point, the refrigerant liquid is in the sub-cooled liquid phase. As the refrigerant leaves the condenser, the liquid still in high pressure and has high saturation temperature but there's a drop in enthalpy due to loss in thermal energy.
Since the liquid refrigerant is in high pressure, thus it can still be expended. Therefore, it undergoes a constriction called expansion valve (or throttle). As the refrigerant passes through this constriction, the pressure immediately drops, causing the saturation temperature of the sub-cooled liquid to also drop. However, since the constriction is short, well insulated and no work done, thus it can be said that the enthalpy of the liquid remains constant.
As the sub-cooled refrigerant has passed the expansion valve, the refrigerant now has a lower pressure and low saturation temperature. Now the refrigerant approaches the evaporator. The temperature of the evaporator is much larger than the temperature of the refrigerant. Thus, this causes heat transfer to occur again. This time, the refrigerant absorbs thermal energy from the surrounding. As a result, this causes the sub-cooled refrigerant to evaporate into superheated vapour. As the refrigerant leaves the evaporator, although it is superheated state, the refrigerant still has low pressure and saturation temperature. Its temperature is still lower than the evaporator.
After passing the evaporator, the cycle needs to repeat again. Thus, the low pressure superheated refrigerant now passes the compressor where the pressure of the refrigerant is increased. As the pressure is increased, the saturation temperature also increases. Then, the cycle repeats again, where the refrigerant now passes through the condenser.
The component that connects the refrigeration unit and the air conditioning unit is the evaporator. This is because when the refrigerant enters the evaporator, the temperature of the evaporator is greater than the refrigerant. Thus, as atmospheric air is sucked into the air conditioning unit and when it passes the evaporator, heat is extracted from the atmospheric air and is absorbed by the refrigerant. As a result, the refrigerant undergoes evaporation due to absorption of heat and the atmospheric air undergoes condensation due to the extraction of heat. The figure below will provide a rough idea on how this process occurs.
Figure 1: Schematic diagram of the air conditioning unit used in the experiment.
APPARATUS.
Figure 2: Show the type of air conditioning unit that was used for this experiment.
For this air conditioning experiment, a PA Hilton model was used as the air conditioning unit. This particular model works with humid air and also R134a (tetrafluoroethane) refrigerant. This air conditioning unit is made up of multiple parts. Among the main part of this PA Hilton model are shown in the figure below:
Figure 3: Figure shows all the main components of the PA Hilton model.
Based on Figure 2, the names of all the components are given in the figure below.
Figure 4: Figure shows the labeling of all the component of the PA Hilton model based on Figure 2.
RESULTS.
Table 1: Stabilisation of the System.
Stabilisation of the system
Expected Time (min)
Actual Time (min)
TdC ( )
TwC( )
5
5
27.0
26.0
10
10
29.0
27.0
15
15
31.5
28.5
20
20
32.5
28.5
25
25
31.0
28.5
30
30
31.0
29.5
35
35
31.0
28.5
40
40
31.0
28.5
45
45
31.0
28.5
50
50
31.0
29.0
55
55
31.5
29.0
60
60
31.0
29.0
Table 2: General parameter of experiment.
Atmospheric Pressure
Patm (bar)
1
Pre-heated heat input
Qp (W)
2 K
Water boiler heat input
Qb (W)
5 K
Table 3: Calculation of Mass flow of air in a system.
Orifice Differential Pressure
z (mm)
4.8
Uncertainty in z (mm)
0.05
Wet Bulb Temperature of the air at D
TwD (˚C)
29.0
Uncertainty in TwD (˚C)
0.25
Dry Bulb Temperature of the air at D
TdD (˚C)
31.0
Uncertainty in TdD (˚C)
0.25
Specific Volume of the air at station D
vD (m3/kg)
0.895
Uncertainty in vD (m3/kg)
Air Mass Flow Rate
ṁa (kg/s)
3.961
Uncertainty in ṁa (kg/s)
Table 4: Calculation of water loss and enthalpy decrease through air conditioner.
Wet bulb Temperature of the air at B
TwB (°c)
34.0
Uncertainty in TwB (°c)
0.25
Dry bulb Temperature of the air at B
TdB (°c)
49.5
Uncertainty in TdB (°c)
0.25
Wet bulb Temperature of the air at C
TwC (°c)
29.0
Uncertainty in TwC (°c)
0.25
Dry bulb Temperature of the air at C
TdC (°c)
31.5
Uncertainty in TdC (°c)
0.25
Specific humidity at B from chart
ωB (kg/kg)
0.027
Uncertainty in ωB (kg/kg)
Specific humidity at C from chart
ωC (kg/kg)
0.0245
Uncertainty in ωC (kg/kg)
Mass of water lost
ṁw (kg)
-0.000292
Uncertainty in ṁw (kg)
Enthalpy of air at B from chart
hB (J/kg)
120000
Uncertainty in hB (J/kg)
Enthalpy of air at C from the chart
hC (J/kg)
95000
Uncertainty in hC (J/kg)
Sensible heating of air across air-conditioner
HB-C (J)
Uncertainty in HB-C (J)
Table 5: Calculation of heat gain by air conditioner refrigeration cycle.
superheated refrigerant R134a vapour leaving evaporator
Temperature, t1 (°c)
23.0
Uncertainty in t1 (°c)
0.5
Pressure, P1 (kN/m2)
375
Uncertainty in P1 (KN/m2)
12.5
Enthalpy from chart, h1 (J/kg)
317000
Uncertainty in h1 (J/kg)
R134a after compressor
Temperature, t2 (°c)
75.0
Uncertainty in t2 (°c)
0.5
Pressure, P2 (kN/m2)
1400
Uncertainty in P2 (KN/m2)
25
Enthalpy from chart, h2 (J/kg)
353000
Uncertainty in h2 (J/kg)
R134a before expansion valve, at the high pressure
Temperature, t3 (°c)
50.0
Uncertainty in t3 (°c)
0.5
Pressure, P2
(kN/m2)
1350
Uncertainty in P3 (KN/m2)
Enthalpy from chart, h3 (J/kg)
170000
Uncertainty in h3 (J/kg)
R134a mass flow rate
ṁr (g/s)
18
Uncertainty in ṁr (g/s)
0.5
Mass of condensate collected
mc (g)
1288
Uncertainty in ṁc (g)
Condensate collection time
t (s)
3745.2
Unfndnscertainty in t (s)
Specific volume leaving evaporator
v1 (m3/kg)
Uncertainty in v1 (m3/kg)
Volumetric efficiency of compressor
ηvol
Compressor swept volume, Vswept
Graph 1: Based on Table 1, stabilization of the system, a graph of Temperature vs Time is plotted.
DISCUSSION.
Based on my knowledge in air conditioning, the process associated with the removal of heat from the air is condensation process that occur in the evaporator. This is mainly because when the refrigerant enters the evaporator, it is in the low pressure and low saturation temperature subcooled state. Since the temperature of the evaporator is much greater, this causes the refrigerant to undergo evaporation by absorbing heat from the surrounding. In other words, the surrounding in the evaporator loses thermal energy. Thus, it can be said that the air passing through the evaporator also loses thermal heat energy and undergoes condensation. As a result, the temperature of air would be much cooler when it leaves the evaporator.
Besides, when the experiment is carried out, it was obvious that one of the most important process was occurring in the evaporator. It is said as such because as air from the boiler passes the evaporator, the air consists of temperature greater than the evaporator. This is because at that particular moment, the low pressure sub-cooled refrigerant is passing through the evaporator. Due to the difference in temperature between the sub-cooled refrigerant and the surrounding of the evaporator, the refrigerant undergoes evaporation where it changes state to become superheated. So at that instant, the refrigerant is actually absorbing heat from the surrounding in the evaporator. Thus, heat from the air that is passing the evaporator is also absorbed, resulting in a much lower temperature. As temperature decreases, the humidity of air also decreases. This statement is proven in Table 4, where the specific humidity of air at position B is greater than specific humidity of air at position C. The figure below shows an idea of how the process of condensation of the boiled air undergoes condensation in the air conditioning unit.
Figure 5: Describes the cooling process of the air from the boiler.
From the figure above, it can be seen clearly that the evaporator acts as the cooling coil where it absorbs heat from the air that is passing through it. As a result, cool air is produced. However, there is a heating coil after cooling coil to ensure that the cool air that is produced is not too cool and meets the required conditions.
Since the air is cooled and dehumidified while passing past the evaporator, the rate of heat and moisture removal can be determined. However, there are a few important assumptions that need to be taken into account. One of the main assumptions is considering air moving in the air conditioning unit is a steady-flow process. Besides that, it is also assumed that the kinetic energy and the potential energy term is zero. The conservation of energy and mass also must be obeyed.
As stated, the energy gained by the refrigeration cycle can be obtained. The most essential principle that must be considered is the principle of conservation of energy. There are mainly three forms of energy that is generated within the refrigeration cycle, which is heat, work and enthalpy (in the form of Steady Flow Energy Equation). However, when analyzing these three forms of energy, there is another important quantity that needs to be considered. That quantity is the mass of flow in the system. From Table 1, the mass flow rate of air is calculated using the formula;
ṁa= zvD
Where; ṁa= mass flow rate of air
z = orifice differential pressure
vD= specific volume of air at D
Thus, after substituting the values,
ma= 0.1167 kgs
From this mass flow rate, other energy such as sensible heating of air across the air conditioner can be calculated. This sensible heating of air is energy lost by the air during the condensation process as it passes evaporator. It is calculated by considering the energy balance within the air conditioning unit. The energy balance on the cooling is determined through the formula;
HB-C=ṁahB-hC+ṁwhw
Where; HB-C= sensible heating of air
ṁa= mass flow rate of air
ṁw= mass of water lost
hw= enthalpy of condensate water
The value for hw is obtained for the steam table. The value of hw is given as 96.4 kJ/kg. This value is obtained by assuming the temperature of water is 23 degree Celsius. By substituting all the values, the HB-C value is given as:
HB-C = 2889.35 J
Furthermore, the heat gain by the refrigeration cycle can also be calculated based on data collected in Table 5. From this table, it can be seen that the highest temperature recorded is when the refrigerant leaves the compressor. This result can be justified by saying that the refrigerant has low pressure when it passes the evaporator. Since, temperature is proportional to pressure, it is fair to say that as the pressure is low, the temperature is also relatively low. After passing the compressor, the pressure of the refrigerant immediately increases, which causes the temperature to increases too. However, for calculating the heat gain in the refrigeration, an important point must be highlighted again. That is when refrigerant passes through the expansion valve, the pressure drops. Not only that, by analyzing the expansion valve, it can be seen that there is no work in the valve. Besides, the expansion valve is also thermally insulated well, so the heat transfer is also zero. Thus, by applying the Steady Flow Energy Equation:
Q + W = ṁ (hafter - hbefore)
Since it is proven that both Q and W is zero, thus;
ṁ (hafter - hbefore) = 0
hafter = hbefore
Thus, to calculate the heat gain by the refrigeration unit, again the Steady Flow Energy Equation is applied. This time, the section considered is the section before and after the evaporator. This is because the refrigerant only gained thermal energy when it undergoes evaporation in the evaporator. However, when calculating the heat gained, work done is considered as zero. Thus;
Q + W = ṁ (hafter - hbefore)
Since W=0;
Q = ṁ (h1 - h3)
Where;
ṁ= mass flow rate of refrigerant
h1= enthalpy when leaving evaporator
h3= enthalpy before expansion valve
Thus, after substituting the values, the heat gained obtained is;
Qgained= 2646 J
From the experiment, it is assumed that the air conditioning system obeys the principle of conservation of energy. From this principle, it can be said energy cannot be created or destroyed, as it can only be transformed from one form to another. Thus, by applying this essential law, it is true that the energy lost from the air in the air cycle is equal to the energy gained by the refrigerant in the refrigeration cycle. Therefore, the value for HB-C must me equal to the value for Qgained. However, there values are not the same. There are many causes that may result in the difference in energy values. There are possibilities of heat loss to the surrounding. This may occur if the air duct that transports the air in the air conditioning unit is made out of material of that has high thermal conductivity. Therefore, this enables heat transfer to occur and causes heat to escape. This may affect the results of the experiment. Not only that, there are many other errors that may have contributed to the difference in energy as stated later on in this discussion.
Similarly, it also can be seen that the air conditioning system also obeys the conservation of mass principle. This principle states that the mass enters a system will be equal to the mass leaving the system. Figure below shows an idea on how conservation of mass occurs.
Figure 6: Figure shows the flow of air in the air cycle in an air conditioning unit.
Thus, by only concentrating in the evaporator (cooling coil) section;
ṁ2= ṁ3 + ṁ4
By using the principle of conservation of mass, the total mass entering the system is equal to the total mass leaving the system. Therefore, by using this principle, it can be said that the mass of water loss by air must be equal to the mass of condensate. The mass of water loss by air is the amount of water that is converted from steam to water and mass of condense is the amount of water obtained from the condensation process. The mass flow rate of water loss in air is obtained by;
ṁw= ṁa (wB- wC)
Where;
ṁa= mass flow rate of air
wB= specific humidity at B
wC= specific humidity at C
Thus, by substituting all the values, the mass flow rate of water loss in air during the condensation process is;
ṁw= -0.000292 kg/s (negative because of water loss)
ṁw= 0.000292 kg/s
Now calculating mass flow rate of condensate is calculated by;
ṁa= mass of condensatetime
ṁa= 1.2283745.2
ṁw= 0.000343 kg/s
By comparing both mass flow rates, it can be seen that the mass flow rate of condensate is greater than the mass flow rate of water loss in air. Therefore, it can be said that the mass of condensate will be greater than the mass of water loss.
The results obtained from the experiment may vary from the actual theoretical result. This may be caused by errors such as human error. There are many examples of human error. One of the main human errors that could have occurred is parallax error. This error is due to the position of the eye not being perpendicular to the measuring scale. This may cause wrong results being tabulated. Besides that, another type of human error is the timing error. This occurs when a person that is in charge of recording time using a stopwatch might not have stopped the time at the exact moment where he is supposed to. Thus, as a result the result obtained is not accurate and although the difference in time might be a bit, but it might cause a significant difference in the final result. To overcome this, the person in charge should be more focus and should concentrate in what they do.
Other than human error, there are other errors that may affect the result of the experiment such as the error while taking temperature for wet bulb temperatures. This is because for a wet bulb thermometer, there's an additional muslin sock that acts a water reservoir. Assuming during the experiment there is a formation of residue on the sock, this may cause minimal contact between the air that is flowing and the thermometer. Thus, the temperature will be affected. Unfortunately, if this occurs, there's nothing much that can be done.
CONCLUSION.
From this experiment, there are many conclusions that can be made. Firstly, from this experiment, it clearly shows the importance of the psychometric chart. This is said as such because by using this chart, just by knowing two variables (dry and wet bulb temperatures), other property of the fluid such as specific volume, specific humidity and many more can be determined. Not only that, the usage of the pressure vs enthalpy graph is also essential in this experiment as the state of the refrigerant can be determined after a particular process.
Besides that, through this experiment, we have a better understanding about measurement uncertainties and how to calculate it. As stated above, the method used to calculate uncertainties is the Kline-McClintock Method. By carrying out experiment, it is also observed that air conditioning cycle does obeys the principle of conservation of energy. However, due to errors and heat lost to surrounding, the heat lost by air through conduction is not equal to the heat gain by the refrigerant through evaporation. It is observed that the heat lost is much greater than the heat gained.
Other than conservation by energy, air conditioning cycle also obeys the conservation in mass. The experimental data shows that the mass flow rate of condensate is greater than mass flow rate of heat lost by air. Again, the difference in value is assumed to be caused by error and heat lost to surrounding during the process.
APPENDIX
