Department of Mechanical Engineering

Faculty of Engineering

Chulalongkorn University

2103360

Performance Test on the air conditioning processes

Somsak Chaiyapinunt

Learning outcomes

1. Describe the operation of air conditioning processes.

2. Sketch the air conditioning processes on the psychrometric chart.

3. Evaluate the energy transfer to air.

4. Compare the energy transfer from one energy source to other energy sources (i.e. refrigerant,

electricity or steam).

5. Be able to measure the capacity and power consumption of a small air conditioning unit.

6. Apply critical thinking in making recommendation for a proper equipment to condition the air.

Scenerio

You have just joined the Lisa air conditioning, Ltd. as a young design engineer. The company

produces special type of air conditioning units. The company just received an order to produce special

type of air conditioning units to use in the process in the factory and in buildings in northern part of

Thailand. The special type of air conditioning unit must have the capability of heating, steam

humidifying, cooling with dehumidifying and adiabatic mixing of air stream dependent on the certain

manufacturing process in the factory. The company plans to order the specific equipments or

components (i.e. cooling coil, electric heater, steam heater, condenser and compressor, etc,) to

assemble the air conditioning unit. Therefore, the company asks you as a design engineer to set up a

team and go out to test each equipments and choose them to assemble as the required air conditioning

unit.

Objective

Your team is assigned to evaluate each component of the air conditioning process to see whether they

are suitable to purchase for assemble in the designed unit. Your team is also given the pychrometric

chart to help for evaluating the performance of each component of the air conditioning process.

You are assigned to carry out the performance test on each of the air conditioning process (heating,

steam humidifying, cooling with dehumidifying and adiabatic mixing of air stream) at the supplier’s

site. You will write a technical report to your head of engineering section.

Performance test procedure

Untreated air entering the ducting passes in series through

1) An air measuring intake orifice.

2) A mixing zone. (a place where the air may be mixed with the re-circulated air)

3) A pre-heater: extended fin electric heating element, 0.5 and 1.0 kW, nominally at 220 V.

4) A humidifier supplied with steam from the generator.

5) A cooler/dehumidifier with a precipitate water outlet.

6) A re-heater

7) An axial flow fan with infinitely variable speed control; power factor ~1.0

8) An air measuring duct orifice.

9) A damper which controls the quantity of air discharged to the atmosphere (Any air not discharge is

recirculated and mixes with untreated air in mixing zone).

Test procedure

At the air measuring intake orifice,

Adjust the manometer until they are level. The “zero’ of the manometer should be checked at

intervals during a test, since a change of temperature will affect its setting.

Turn on the axial flow fan.

Run the test apparatus (without turning any equipment except the fan) with a zero recirculation

(open the damper at full opening) for 10-15 minutes, record the data to use as the reference values.

Mass flow rate of the moist air at the inlet orifice can be calculated from

A

Z(1+w )0.0757 Am

kg/s

where, Z is the intake orifice differential pressure in mm H2O.

A is the specific volume of the air at the intake orifice in m3/kgda.

(from the psychrometric chart)

w A is the humidity ratio of the air at the orifice.

At the air measuring duct orifice,

The duct orifice may be calibrated from the intake orifice, provided the damper is positioned for

zero recirculation. In this condition the mass flow rate through both orifices will be the same.

Mass flow rate of the moist air at the duct orifice can be calculated from

Y(1+w )

k E

E

m

where, Y is the duct orifice differential pressure in mm H2O.

E is the specific volume of the air at the duct orifice in m3/kgda.

(from the psychrometric chart)

wE is the humidity ratio of the air at the duct orifice.

Z(1+w )

k=0.0757Y(1+w )

A E

E A

v

v

typically, for the duct orifice, (Y(1+w )

k E

E

m

) Y(1+w )

0.073 E

E

m

to Y(1+w )

0.083 E

E

m

For the heating process

At the pre-heater:

Adjust the heater to heat the air to the designed condition, measure the related parameters.

For the humidification process

Turn on the steam generator (boiler) to produce the steam. When humidification is required, the

rate of steam injector should not exceed that which can be absorbed by the air. If it is found that

mist is seen some distance downstream of the steam distributor, either

i) reduce the heat input to the boiler, or

ii) increase the air flow rate, or

iii) increase the air temperature by switching on more pre-heat.

Measure the related parameters.

For the cooling and dehumidification process

Turn on the air conditioning unit. Satisfactory results are more likely to be achieved if the

following points are noted:

(i) When it is intended to demonstrate de-humidification, the air should be fairly humid (say

>65%) at Station C. If necessary, steam may be injected.

(ii) The cooler has a large surface area on which the condensation takes place. Due to this, an

appreciable time elapses before condensate is discharged from the drain at the same rate as

it is precipitated.

(iii) The change of moisture content of the air is easily determined from the product of the air

mass flow rate and the change of specific humidity. Agreement between the change of

moisture content of the air and the drainage rate will be obtained after a sufficient period

time under the steady condition.

Measure the related parameters.

Note: the power factor of the compressor is 0.9.

For the adiabatic mixing of air streams process

Adjust the damper which controls the quantity of air discharged to the atmosphere (Any air

not discharge is re-circulated and mixed with untreated air in mixing zone).

Measure the related parameters.

At least the following data should be readily obtained:

(a) The condition of the air before and after the various processes (via wet and dry bulb sensors)

(b) The condition of the air stream before and after mixing

(c) The energy transfer rate at each heater, the boiler, fan and refrigeration unit

(d) All air mass flow rates

(e) Pressures and temperatures of refrigerant

(f) Refrigerant mass flow rate

(g) Rate of precipitation at cooler

Post experiment assessment

1) Make an understanding of various air conditioning processes and observe the equipment

clearly.

2) Outline the experiment procedure for each process: heating, steam humidification, cooling

with dehumidification and mixing of two air streams. Concern that what you want from the

experiments.

3) In an observations sheet, specify what data you would like to collect for each experiment.

4) After the experiments, show the processes on the psychrometric chart. All the processes in

this experiments unit may be treated as steady flow processes with insignificant change of

kinetic and potential energy. Thus, for any portion of the unit treated as an open system,

e ie iQ-W = m h - m h

or e iQ-W =m(h -h )

where,

Q is the heat transfer rate ( positive if to the system).

W is the work transfer rate ( electrical or mechanical, positive if from the system).

m is mass flow rate.

eh and ih are the specific enthalpy of the fluids leaving and entering, respectively.

5) Since most of the properties read from the psychrometric chart are based on dry air, the

appropriate conversion from the measured moist air properties to the dry air properties have

to be performed.

6) Discuss the following main point;

- Are your experimental results get along with the theoretical stated.

- The reliability of the experiments.

- It is required by Thai building law to measure the efficiency (COP or EER) of any air

conditioners using in large building. Suppose a building having split type air conditioners

installed, how do you measure the efficiency? What measuring instruments do you really

require?

Report assessment

Write a report to your head engineer.

The report should cover the following:

- Diagram of each process on a psychrometric chart.

- Performance of each process (i.e. the energy transfer to or from the air, energy input to the

equipment, efficiency, etc.).

- COP and EER only apply to the cooling and dehumidification process. Calculate the heat

absorbed by the evaporator based on two ways; the energy absorbed by the air and the energy

absorbed by the refrigerant.

- Discuss and conclude the experiments that have been performed.

- Reliability of the experiments (uncertainty analysis).

- Suppose a building having split type air conditioners installed, how do you measure the

efficiency? What measuring instruments do you really require?

Revised August 9th , 2019.

Appendix

Fresh air consists chiefly of nitrogen and oxygen. The remainder is composed of small

quantities of other gases and vapors, and of these the most important is water vapor. Although the

water vapor content is usually very small or less than 3 % by mass of mixture but it has a considerable

effect on air properties. Therefore air can be assumed as a mixture of dry air and water vapor. A

condition or state of air can be fixed only knowing 3 intensive thermodynamic properties. Total

pressure of the air is the one that can be measured easily by barometer. Fortunately Thailand is

approximately at mean sea level, the standard atmospheric pressure of 101.3 kPa can be used.

Therefore leaves only other 2 intensive properties required. In practice dry bulb temperature is easily

measured by thermometer. Wet bulb temperature is also easily measured by thermometer wetted with

water. The wet bulb temperature may be replaced with relative humidity measured by hygrometer.

In order to control the air condition, you must find out the condition of air you have and the

condition you are desired. From the equipment provided, including the psychrometric chart, find the

main properties of ambient air. Change the air condition by applying suitable psychrometric

processes: heating, humidification with steam, cooling with dehumidification and mixing of two

airstreams. Are the heat added to or subtracted from the air equal to the electricity supply or heat

added to refrigerant. How about the moisture content and relative humidity? Are they reasonable?

Brief review of air conditioning process

Air conditioning processes include heating in order to increase temperature where in the

moisture content (humidity ratio) of the air remains constant, humidification with steam in order to

increase the moisture content of the air with some increase in temperature, cooling normally will be

with dehumidification, evaporative cooling, adiabatic mixing of two or more airstream, and cooling-

tower processes. Humidification is a process that results in an increase in the amount of moisture in

the air. Dehumidification, on the other hand, results in a decrease in the moisture content of the air.

These processes are used to condition the air in building in order to provide comfortable surrounding

for the occupants, and maintain the environmental control necessary for a variety of commercial and

industrial processes.

The following topics are some common processes in air conditioning.

1. Heating

Heating process is used to increase the dry bulb temperature of air without adding or

subtracting moisture. In other words, the humidity ratio (w) of the air remains unchanged during the

process, and the process can therefore be represented by a horizontal line on psychrometric chart.

Heating Coil

Q t2, w2, 2t1, w1, 1

1 2

w

t

Fig. A.1 Heating

Where t is the temperature in oC

w is the humidity ratio

is the relative humidity in %

2. Steam Humidification

Problems associate with low relative humidity resulting from heating can be eliminated by

humidifying the heated air. This is accomplished by passing the air through a steam humidifying

section. The path for a typical humidification process in psychrometric chart slopes up to the right,

indicating that the humidity ratio is highly increased during the process. There is also a small increase

in dry bulb temperature.

Steam

Injection

t3, w3, 3t2, w2, 2

3

2

w

t

Fig. A.2 Steam Humidification

3. Cooling with Dehumidification

Normally the temperature of the refrigerant in the air conditioner evaporator or cooling coil is

much lower than the entering air dew point. Dehumidification will be automatically accomplished by

cooling. Therefore air leaving the evaporator should be saturated (100% Relative humidity). However

in actual there are few percentages of the air passing through the evaporator with unsaturated

condition. This is the reason that the leaving air appears unsaturated (Relative humidity less than

100%) while condensate ( wm ) is clearly seen.

Cooling Coil

Q t2, w2, 2t1, w1, 1

1

2

w

t

Fig. A.3 Cooling with Dehumidification

4. Adiabatic Mixing of Air Streams

The mixing is usually achieved by simply merging two air streams into a single stream. The

air streams are assumed to have different dry bulb temperatures and humidity ratios. The heat transfer

,w wm h

,w ws t p= x x

to the surroundings during the process is small, so that the process is termed adiabatic mixing. The

mixture air stream leaves with a dry bulb temperature and a humidity ratio that are between

corresponding values for the two entering air streams. When two air streams at two different states

(states 1 and 2) are mixed, adiabatically, the state of the mixture (state 3) will lie on the straight line

connecting state 1 and 2 on the psychrometric chart.

1

2

3

ma1, t1, w1, h1, 1

ma2, t2, w2, h2, 2

ma3, t3, w3, h3, 3

2

1

w

t

3w2

w3

w1

t1 t3 t2

h2

h3

h1

Fig. A.4 Adiabatic Mixing of Air Streams

Certain definitions

Humidity ratio, W is the ratio of the mass of water vapor to the mass of dry air.

w w da=M M

Relative humidity, is the ratio of the mole fraction of water vapor, xw in a given moist air sample

to the mole fraction, xws in an air sample saturated at the same temperature and pressure.

Wet-bulb temperature is the temperature of a thermometer with a wetted wick over which air flows at

a specific velocity.

Or it is the temperature which water, by evaporating into moist air at dry-bulb temperature and

humidity ratio can bring air to saturation adiabatically at same temperature, while total pressure is

constant.

Coefficient of Performance, COP is the ratio of the useful refrigeration effect to the net energy

supplied from the external sources.

Energy Efficiency Ratio, EER is the capacity in Btuh divided by input in watts.

Correction to the observed wet bulb temperature

The wet bulb temperature given on the psychrometric chart is the sling wet bulb temperature. The

sling wet bulb temperature is the wet bulb temperature obtained from the wet bulb thermometer

placed in an air stream moving at 3.5 m/s or more.

At lower velocities, particularly if the relative humidity is low, the measured wet bulb temperature

will be higher than the sling wet bulb temperature.

evap

net

QCOP

W

The air velocity in the Hilton Recirculating Air Conditioning Unit is usually between 1 and 2 m/s. The

measured wet bulb temperatures at the velocity between 1and 2 m/s are usually known as “screen”

value.

Some correction has to make to the “screen” value. It is necessary to convert the observed wet bulb

temperature to the corresponding “sling” temperature before entering the state point on the

psychrometric chart.

The graph provided in Fig. 5 may be used to make this conversion.

Example

Observed wet bulb temperature (screen) is 20ºC (at 1 to 2 m/s).

Observed dry bulb temperature is 25ºC.

From Fig. 5, tscreen – tsling = 0.46

tsling = 20 – 0.46

= 19.54 ºC

The state point on the psychrometric chart is located at the intersection of 25ºC db and 19.54 ºC wb

(sling).

Additional notes

The specific enthalpy obtained from the psychrometric chart has unit of kJ/kg of dry air. The

mass flow rate obtained from the measured value of the pressure drop at the orifice plate is the mass

flow rate of the moist air.

The mass flow rate of the dry air can be calculated from the following relation:

(1 w)dam=m

where m is the mass flow rate of the most air.

dam is the mass flow rate of the dry air.

w is the humidity ratio.

Of the three processes in this experiment (heating, steam humidification and cooling with

dehumidification) the mass flow rate of the moist air at different location in the apparatus may be

different but the mass flow rate of the dry air at different location in the apparatus will be the same.

The value of equipment efficiency is usually used as the equipment performance index. The

efficiency of the equipment is usually defined as the ratio of the capability of the equipment to

perform the task to the energy input, i.e. energy absorbed by the fluid to the energy input, etc.

The Coefficient of Performance (COP) and Energy Efficiency Ratio (EER) are used only for the

air conditioning unit (cooling and dehumidification process only). The reason is that the compressor

does not directly perform the cooling effect at the evaporator but it only raises the temperature of the

refrigerant vapor and sends it to the condenser. Therefore the power input to the compressor cannot be

directly used to define the efficiency of the evaporator (as previously stated in the definition of the

equipment efficiency).

The heat absorbed by the evaporator can be determined in two different ways. It can be

calculated from the heat balance of the air passing through the evaporator and the water condensed

from the evaporator. It can also be calculated directly from the heat balance on the refrigerant flow

through the evaporator. The heat absorbed calculated from both methods should be compared for

verification.

The compressor of the Hilton Recirculating Air Conditioning Unit A770 is of hermetic type

using R12 as the refrigerant. Compressor speed is around 2700 to 3000 rev/min, according to load.

Swept volume is 21 cm3/rev. Power factor is typically equal to 0.9.

The typical iso-thermal line used in ph diagram of a specified refrigerant. (can be used to find the

refrigeration effect on the evaporator).

https://www.jsrae.or.jp/jsrae/stady/Eng 20saikuru.htm Accessed on November 13, 2018

6. REFERENCES

1. WILLIAM Z. BLACK & JAMES G HARTLEY, Thermodynamics., 2nd ed., Harper Collins

Publishers.

2. YUNUS A. CENGEL & MICHEAL A. BOLES, Thermodynamics: An Engineering Approach,

McGRAW- HILL Book Company.

3. ASHRAE Handbook, Fundamentals-2001.

4. P.A. Hilton Ltd., Experimental Operating and Maintenance manual, Recirculating Air

Conditioning Unit A770, A770/M/E/2, May 1987, Hampshire, SO20 6PX, England.

Revised August 9th , 2019.

Figure 1. Apparatus for the air conditioning process. [4]

Figure 2. Control panel. [4]

Figure 3. ASHRAE Pyschrometric Chart [3]

Figure 4. Pressure-Enthalpy Diagram for Refrigerant 12

http://mechanical-fet.weebly.com/uploads/2/7/9/3/27933099/__r12_p-h_diagram_high_quality.png

accessed July 23, 2020.

Figure 5. Relationship between “Sling” and “Screen” wet bulb temperatures. [4]

0.5

0

1.0

1.5