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.

Evaporator

Condenser

Compressor Expansion Device

Heat absorbed

Heat rejected

Cin

Eout

Cout

Ein

T4

1

Cin

Cin

Cin

T1

1

Cin

Cin

Cin

T2

1

Cin

Cin

Cin

T3

1

Cin

Cin

Cin

T5

1

Cin

Cin

Cin

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.