ME2121-2

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ME2121-2 PERFORMANCE EVALUATION OF AIR-CONDITIONERS

2013/2014

Department of Mechanical Engineering National University of Singapore

AS A SAFETY MEASURE, WEARING OF SHOE DURING EXPERIMENTS IS MANDATORY WEARING ANY KIND OF SHORTS (e.g BERMUDAS, MINI SHORTS) ARE PROHIBITED.

YOU ARE REQUIRED TO WEAR LONG PANTS DURING THE EXPERIMENTS.

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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 Photograph of the air-conditioner unit 4

Figure 2 The schematic diagram of a vapour compression

refrigeration cycle 4

Figure 3 Psychrometric chart 8

Figure 4 P-h diagram for R-22 refrigerant 9

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NOMENCLATURE

ma 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

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INTRODUCTION

PURPOSE Air conditioners are widely used in tropical countries like Singapore where controlling of relative humidity and temperature is required for human comfort and for storing expensive data centre equipment, servers etc. This laboratory manual outlines an experimental procedure for the performance evaluation of a room air conditioner. SCOPE The performance study is carried out to examine the vapour compression refrigeration cycle of the air conditioner and to evaluate the cooling capacity, power consumption and the Coefficient of Performance (COP) under different operating conditions.

DESCRIPTION OF EQUIPMENT

Figure 1 shows photograph of the room air conditioner used in this experiments. It consists essentially of an evaporator, a compressor, a condenser, and a capillary tube 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 conditioned 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 can be read from dial gauges. The refrigerant temperature data at different locations can be read by turning the knob of a digital thermometer unit at the required location.

THEORY OF OPERATION

(a) The vapour compression 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. The sum of the heat absorbed by the refrigerant in the evaporator and the work supplied to the compressor is rejected at the condenser to the atmosphere. The various processes the refrigerant undergoes in the air conditioner working on the vapour compression refrigeration cycle are as follows (Figure 2): High pressure liquid refrigerant enters the evaporator from the condenser through the capillary tube. The pressure of the refrigerant gets reduced as it flows through the capillary tube due to friction and acceleration. The low pressure refrigerant-vapour mixture (about 80% liquid and 20% vapour) evaporates in the evaporator providing the required cooling effect to the air blowing across the evaporator. The slightly superheated refrigerant vapour emerging from the evaporator is sucked by the compressor and is compressed to a very high pressure and temperature. The highly superheated refrigerant vapour enters the condenser and is air cooled and undergoes condensation. The liquid refrigerant emerging from the condenser is at a sub-cooled liquid state and the cycle is repeated.

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(b) Cooling and dehumidification of air The saturation temperature of the refrigerant in the evaporator is well below the dew point of the room air . When the room air is blown across the evaporator coil, it gets cooled and dehumidified by transferring the sensible and latent heat to the refrigerant flowing through the evaporator tubes. Hence, the two processes undergone by the air in the evaporator are: cooling and dehumidification. The cooling process contributes sensible heat, whereas, dehumidification process contributes latent heat resulting from the condensation of the water vapour in the air.

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

Sensible heat transfer, Qs = ma Cpa (Tai - Tao) (1)

and Latent heat transfer, Q1 = ma hfg (ωai-ωao) (2)

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

The Cooling Capacity, Qt = ma(hai-hao) (3)

The performance of an air conditioner is expressed in terms of its coefficient of performance (COP) defined as:

InputPowerCompressor

CapacityCoolingCOP (4)

When the capacity is expressed in Btu/h and the power in Watt, this ratio is called Energy Efficiency Ratio (EER), (Note that I Btu = 1.055 kJ).

)(

)/(

WattInputPowerCompressor

hrBtuCapacityCoolingEER (5)

PROCEDURE (a) Instrumentation and readings The instrumentation consists of 4 pressure gauges, 2 humidity sensors, 5 digital thermometers, and a digital wattmeter. The saturation temperature of the refrigerant is indicated on the pressure gauge. Identify the measuring points where the following readings are to be taken: (i) Condenser and evaporator pressures (ii) Condensing and evaporating temperatures

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(iii) Refrigerant temperatures T1 to T5 (5 different locations) (iv) Dry bulb and relative humidities 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. (b) Steps Before the commencement of the experiments, the students are required to explain the working principle of a vapour compression refrigeration system to the supervisor/lab assistant. Then, follow the procedure given below: (i) Start the air conditioner with "High" fan speed setting (ii) After ensuring the air temperature leaving the air conditioner has reached a steady state value, take the readings (i), (ii), (iii) and (iv) in item (a) of the procedure. (iii) * Repeat the procedure for "Low" fan speed setting

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

COMPUTATION

For the two different fan speeds (high and low), determine

(i) the cooling capacity of the air conditioner (ii) the coefficient of performance (COP) and EER of the air conditioner given by equations (4) and (5), respectively (iii) the fraction of the cooling capacity used to dehumidify the air. The air volume flow rates across the evaporator coil at high and low fan speeds are 0.165m3/s and 0.118m3/s, respectively measured after the fan. (iv) * Estimate the percentage error in the calculated quantity in (i) and (ii) caused by the error in temperature measurement. You can neglect the error in the air mass flow rate. The state of the air can be fixed on the Psychrometric chart given in Figure 3 with the dry bulb temperature and relative humidity reading at the air inlet and outlet. The properties of moist air at the inlet and outlet can be read from the psychrometric chart. e.g. For air at 32C dry bulb and 26C wet bulb, v = 0.89 m3/kg dry air, W= 0.0188 kg water/kg dry air, h = 80.3 kJ/kg

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DIAGRAM AND SKETCHES

(i) Draw a neat schematic diagram of the vapour compression refrigeration cycle. Indicate the points where the temperature and pressure measurements are 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 P-h chart shows the absolute pressure, whereas, the pressure reading from the experiments is gauge pressure). (iii)* If the air conditioner is run for 12 hours in a day at (a) "Low" (b) "High" position for 30 days, estimate the total heat rejection by the air conditioner. Discuss whether it is possible to recover part of the energy rejected to the atmosphere. (Any suggestions may be explained with the aid of sketches).

* for formal report only.

Figure 1. Photograph of the air-conditioner unit (left – front view; right – side view)

Evaporator

Condenser

Warm air in

Cool air out

Heat rejected

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Figure 2. The schematic diagram of a vapour compression refrigeration cycle

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

Work

(capillary tube)

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Therefore, the relative error in the sensible heat is: Similarly, the error in the latent heat transferred to the refrigerant can be established as follows: The latent heat transferred to the refrigerant, The corresponding errors in the cooling capacity, COP can be expressed as follows: where, W is the power input to the compressor

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

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