ME2121-2

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ME2121-2 PERFORMANCE EVALUATION OF

AIR-CONDITIONERS

(E1-03-01)

2014/2015

Department of Mechanical Engineering

National University of Singapore

AS A SAFETY MEASURE, WEARING OF SHOES DURING EXPERIMENT IS

MANDATORY. WEARING OF SHORTS OF ANY KIND (E.G. BERMUDAS, MINI

SHORTS) IS ALSO PROHIBITED AND STUDENTS ARE REQUIRED TO WEAR

LONG PANTS.

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TABLE OF CONTENTS

LIST OF FIGURES (i)

NOMENCLATURE (ii)

INTRODUCTION 1

DESCRIPTION OF EQUIPMENT 1

THEORY OF OPERATION 1

PROCEDURE 2

ERROR ANALYSIS 5

LIST OF FIGURES

Figure I Basic configuration of an air-conditioner 4

Figure 2 The refrigeration cycle 4

Figure 3 Psychrometric chart 5

Figure 4 P-h diagram for R-22 refrigerant 6

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NOMENCLATURE

•

am air flow rate (kg dry air/s)

T temperature (°C)

ω absolute humidity of air (kg moisture/kg dry air)

Cpa specific heat of air = 1.02 kJ/kg dry air K

hfg latent heat of water = 2465 kJ/kg

v specific volume (m3/kg)

subscripts:

ai air inlet

ao air outlet

1

INTRODUCTION

PURPOSE

Air conditioners are widely used in tropical countries like Singapore where the controlling of humidity

and temperature is required for human comfort and for storing special equipment such as computers.

This laboratory manual outlines an experimental procedure for the performance study of a room air

conditioner.

SCOPE

The performance study is carried out to examine the refrigeration cycle of the air conditioner and to

evaluate the cooling capacity and power consumption under different operating conditions.

DESCRIPTION OF EQUIPMENT

The basic configuration of the air conditioner unit is shown in Figure 1. It consists essentially of a

compressor, a condenser, an evaporator and two capillary tubes connecting the condenser and the

evaporator. A propeller fan blows outdoor air across the condenser coil and a centrifugal fan induces

the room air to flow across the evaporator coil, and supply the air into the room. Both fans are driven

by one electric motor. The flow rate of air supplied into the room can be regulated by adjusting the

speed of the fan motor. The supply air flow rates for high and low fan speeds are indicated on the unit.

A thermostat, which will make or break the electric circuit of the compressor motor, is employed in the

unit to control the room air temperature. Saturated temperature and pressure readings of the refrigerant

both before and after the condenser and evaporator are given via dial gauges. The required temperature

data at the different locations can be read off by turning the knob of a digital thermometer unit.

THEORY OF OPERATION

(a) Refrigeration cycle

In the air conditioner, the room air is cooled and dehumidified at the evaporator, where the refrigerant

R-22 is allowed to evaporate, thus, creating a cooling effect. An amount of heat equivalent to the heat

absorbed by the evaporator and the work supplied to the compressor is rejected at the condenser to the

atmosphere. The process undergone by the refrigerant is as follows (Figure 2):

High pressure liquid refrigerant flows from the condenser to the evaporator through the capillary tubes.

The pressure of the refrigerant reduces as it flows through the capillary tubes due to friction and

acceleration. The low pressure refrigerant then evaporates in the evaporator providing the required

cooling effect. The vapour refrigerant thus generated is sucked by the compressor where the vapour

pressure is raised and superheated vapour is delivered to the condenser. The vapour is air cooled at the

condenser and liquefied (condensation) for the beginning of the next cycle.

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(b) Cooling and dehumidification of air

The saturation temperature of the refrigerant in the evaporator is below the dew point of the room air.

When room air is forced to flow across the evaporator coil, heat (sensible & latent) is transferred from

the air to the refrigerant which then evaporates. This process reduces the air temperature (cooling) and

humidity (dehumidification).

The heat absorbed by the evaporator can be calculated as follows:

Sensible heat transfer Qs = maCpa(Tai - Tao)

and Latent heat transfer Q1 = mahfg(ωai-ωao)

The sum Qs + Q1 is known as the capacity of the air conditioner, Qt

Qt = ma (hai-hao)

The performance of an air conditioner is measured by the coefficient of performance (COP) expressed

as InputPower

CapacityCoolingCOP =

When the capacity is expressed in Btu/h and the power in Watt, this ratio is called Energy Efficient Ratio (EER).

(Note that I Btu = 1.055 kJ).

PROCEDURE

(a) Instrumentation and readings to be taken:

The instrumentation consists of four pressure gauges, five temperature sensors (refrigerant Side), two

humidity/temperature sensors (air side) and a wattmeter. The saturation temperature of the refrigerant is

also indicated on the pressure gauge dial1.

Identify the following measuring points where the following readings are to be taken:

(i) Condenser and evaporator pressures

(ii) Condensing and evaporating temperatures

(iii) Refrigerant temperatures entering and leaving the compressor and condenser

(iv) Temperature (Dry Bulb) and Relative Humidity values of air before and after the evaporator

coil.

For each measurement (temperature, pressure, power input) note down carefully the accuracy of the

readings e.g. for thermocouple reading ∆ T = ± 1 ° C.

1 This may also be obtained from R-22 saturation properties table.

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(b) Steps

Follow the procedure given below:

(i) Start the air conditioner with "High" fan speed setting

(ii) After the air temperature leaving the air conditioner has reached a steady value, take the

readings (i), (ii), (iii) and (iv) in item (a) of the procedure.

(iii) Switch off the unit at the end of the experiment

COMPUTATION

At the fan speed used, determine

(i) the cooling capacity of the air conditioner

(ii) the coefficient of performance (COP is defined as the ratio of useful cooling effect to work

input by compressor) and the energy efficiency ratio (EER is defined here as cooling effect in Btu/h to

power in Watt). Note that 1 W = 3.412 Btu/h.

(iii) the fraction of the capacity that goes to dehumidify the air. The outlet air volume flow rates at

high and low fan speeds are measured to be 7.7 m3/min and 6.8m3/min, respectively.

The properties of moist air can be determined from the psychrometric chart given in Figure 3. e.g. For

air at 32°C dry bulb and 62.5% RH , v = 0.89 m3/kg dry air, ω= 0.0188 kg water/kg dry air, h = 80.3

kJ/kg

DIAGRAM AND SKETCHES

(i) Draw a neat schematic diagram of the refrigeration cycle. Indicate the points where the

temperature and pressure measurements were made

(ii) Indicate the measured pressures and temperatures on a P-h Chart for refrigerant R-22 when the

circulation fan was running at high speed.

(Note: The chart shows the absolute pressure, and not the gauge pressure that you read).

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Figure 1. Actual air-conditioner standalone unit (left – front view; right – side view)

Figure 2. The schematic of a vapour compression refrigeration cycle

Work

(capillary tubes)

Evaporator

Condenser

Warm air in

Cool air out

Heat rejected

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ERROR ANALYSIS

The sensible heat transferred to the refrigerant,

The relative error in the sensible heat can be expressed as:

Therefore, the absolute error:

Where, x = Tai - Tao and

Therefore, the relative error in the sensible heat is:

)(aoaipa

as

TTCmQ −=•

22

22

∆+

∆+

∆

=

ƥ

•

x

x

C

C

m

m

Q

Q

pa

pa

a

a

s

s

22

2

∆+

∆+

∆=

ƥ

•

x

x

C

C

m

m

Q

Q

pa

pa

a

a

s

s

( ) ( )22

aoaiTTx ∆+∆=∆

222

22

)()(

−

∆+∆+

∆+

∆=

ƥ

•

aoai

aoai

pa

pa

a

a

s

s

TT

TT

C

C

m

m

Q

Q

−

∆+∆+

∆+

∆

=∆

•

•

2

222

2

)(

)()(

aoai

aoai

pa

pa

a

a

s

s

TT

TT

C

C

m

m

Q

Q

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Similarly, the error in the latent heat transferred to the refrigerant can be established as follows:

The latent heat transferred to the refrigerant,

Therefore, the corresponding errors in the cooling capacity, COP can be expressed as follows:

where, W is the power input to the compressor

The relative error in the cooling capacity and COP can be expressed as:

)(aoaifg

al

hmQ ωω −=•

2

222

2

)(

)()(

aoai

aoai

fg

fg

a

a

l

l

h

h

m

m

Q

Q

ωω

ωω

−

∆+∆+

∆+

∆=

ƥ

•

lstQQQ +=

22

∆+

∆=

∆

W

W

Q

Q

COP

COP

t

t

W

QCOP

t=

( ) ( )22

lst QQQ ∆+∆=∆

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