Lee Zu Jian A0101835N Lab Group 2N1 20/09/2013 FORMAL REPORT FOR ME2121-2 PERFORMANCE EVALUATION OF AIR-CONDITIONERS
SEMESTER 3 2013/2014
DEPARTMENT OF MECHANICAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE
Objective To examine the vapour compression refrigeration cycle of the air conditioner and evaluate the cooling capacity, power consumption and the Coefficient of Performance (COP) under different fan speeds.
Results Using the readings recorded in the raw data sheet as well as the psychrometric chart, In the high fan experiment, Flow rate = 7.7m3/min For air inlet, Ti = (24.3 ± 0.1) oC Relative Humidity = (49.4 ± 0.1)% Using psychrometric chart, Enthalpy, h = 49.0 kJ/kg of dry air Specific volume, v = 0.856 m3/kg of dry air Humidity ratio, w = 0.0095 kg moisture /kg of dry air Wet bulb temperature = 17.4 oC
For air outlet, To = (13.4 ± 0.1) oC Relative Humidity = (85.2 ± 0.1)% Using psychrometric chart, Enthalpy, h = 35.0 kJ/kg of dry air Specific volume, v = 0.823 m3/kg of dry air Humidity ratio, w = 0.0085 kg moisture /kg of dry air Wet bulb temperature = 12.3 oC
In the low fan experiment, Flow rate = 6.8m3/min For air inlet, Ti = (25.8 ± 0.1) oC Relative Humidity = (45.4 ± 0.1)% Using psychrometric chart, Enthalpy, h = 50.5 kJ/kg of dry air Specific volume, v = 0.860 m3/kg of dry air Humidity ratio, w = 0.0095 kg moisture /kg of dry air Wet bulb temperature = 17.8 oC
For air outlet, To = (13.8 ± 0.1) oC Relative Humidity = (85.2 ± 0.1)% Using psychrometric chart, Enthalpy, h = 36.0 kJ/kg of dry air Specific volume, v = 0.824 m3/kg of dry air Humidity ratio, w = 0.0085 kg moisture /kg of dry air Wet bulb temperature = 12.5 oC
Discussion/ Analysis of Results
QN1. Schematic Diagram of Refrigeration Cycle Temperature readings were taken at the points indicated by the squares and pressure readings were taken at points as shown by the circles.
Eout
Ein
Evaporator
T1
T5
Cin
Cin
Heat absorbed
Cin
Cin
Cin
1
Cin
1 Expansion Device
Compressor
T2
T4
Cin
T3
Cin
Cin
Cin
Cin
1
Cin
Cin
Cin
Condenser
Cin
1
Heat rejected
Cin
Cout
1
Calculations QN2. (i) As there are 2 methods to calculate the cooling capacity of the air conditioner, both methods will be used and the average of the results obtained by each method will be taken.
First Method For high fan experiment,
=2118 W
For low fan experiment,
=2023 W
Second Method For high fan experiment,
=2183 W
For low fan experiment,
=1994 W
Taking the average of both methods,
Cooling capacity at high fan = 0.5(2118 + 2183) = 2151 W Cooling capacity at low fan = 0.5(2023 + 1994) = 2009 W
(ii)
The coefficient of performance is defined as:
The Energy Efficiency Ratio is the same, except that the capacity is expressed in Btu/h instead.
For high fan,
For low fan,
(iii)
The fraction of the capacity that goes to dehumidify the air is as follows:
For high fan,
17.9%
For low fan,
16.9%
(iv)
Error Analysis
The sensible heat transferred to the refrigerant, The relative error in the sensible heat can be expressed as:
since only the error caused by temperature measurement is taken into account. Therefore, the absolute error:
where
and
Similarly, the error in the latent heat transferred to the refrigerant can be established as follows: The latent heat transferred to the refrigerant,
The error in the cooling capacity and COP can be expressed as:
where W is the power input to the compressor.
For high fan experiment,
Error in the cooling capacity:
Error in COP:
For low fan experiment,
Error in the cooling capacity:
Error in COP:
From the error calculations above, it can clearly be seen that most of the error is contributed by the error in the latent heat. This is most likely because the absolute humidity of air is not directly measurable in this experiment and relies on the psychrometric chart to obtain an estimate of the value, which is subject to human error. Furthermore, the change in absolute humidity is very small, which amplifies the significance of the error as well.
Diagram and sketches QN3. At high fan speed, Absolute pressure readings = Gauge pressure + 1 bar Pressure at evaporator inlet Pressure at evaporator outlet Pressure at condenser inlet Pressure at condenser outlet
QN4. Total heat rejection can be estimated to be the total energy of the hot exhaust air minus the initial energy of the air into the condenser.
For high fan, Total heat rejected
kJ kJ
For low fan, Total heat rejected
kJ kJ
The amount of heat rejected is very large. However, it may be possible to recover part of the energy rejected to the atmosphere. This is because the thermodynamic process in the compressor requires an increase in both temperature and pressure, therefore if the exhaust heat can somehow be transferred back to the compressor, some of the heat may possibly be converted to work, thus increasing the efficiency of the system. Conclusion Through this experiment, I have learnt about the workings of the air conditioner and its refrigeration cycle and evaluated its performance by calculating the cooling capacity, power consumption, the resulting Coefficient of Performance and Energy Efficiency Ratio. Also, the operation of the air conditioner rejects an extremely large amount of thermal energy into the atmosphere and we should look for ways to make this energy useful.
