1 Refrigeration Technology wu wei-dong Chapter9. Psychrometry and Air Processes

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Refrigeration Technology

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Chapter9. Psychrometry and Air Processes

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Composition and thermal properties of moist air

Adiabatic saturation process and wet bulb temperature

Psychrometric chart Air handing processes Heat and mass transfer between moist air and

solid surface REFERENCES



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9-1.Composition and thermal properties of moist air


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1. Atmospheric air Atmospheric air is not completely dry but a mixture of dry air and wat

er vapor.

In atmospheric air, the content water vapor varies from 0 to 3% by mass.

The processes of air-conditioning and food refrigeration often involve removing water from the air (dehumidifying), and adding water to the air (humidifying).


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2. The thermal parameters of moist air (1) Dry bulb temperature t Dry bulb temperature is the temperature of the air, as measured by an o

rdinary thermometer. The temperature of water vapor is the same as that of the dry air in

moist air. Such a thermometer is called a dry-bulb thermometer in psychrometry,

because its bulb is dry.

(2) Wet bulb temperature tWB: Wet bulb temperature is thermodynamic adiabatic temperature in an adi

abatic saturation process, and measured by a wet bulb thermometer. The wet bulb temperature will be discussed in the next paragraph.


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(3) Dew point temperature tDP: When the unsaturated moist air is cooled at constant vapor pressure or at

constant humidity ratio, to a temperature, the moist air becomes saturated and the condensation of moisture starts, this temperature is called dew point temperature of the moist air.

(4) Relative humidity Ф: Relative humidity is defined as the ratio of the mole fraction of the water vapor

in a given moist air to the mole fraction of water vapor in a saturated moist air at the same temperature and the same atmospheric pressure.

Relative humidity is usually expressed in percentage (%). From the ideal gas relations, relative humidity can be expressed as

---- the mole fraction of the water vapor in moist air;

---- mole fraction of water vapor in a saturated moist air at the same temperature;

satw

w

satw

w

P

P

x

x

,,

satwx ,


wx

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(5) Degree of Saturation μ: Degree of saturation is defined as the ratio of the humidity ratio of moist ai

r w to the humidity ratio of saturated moist air wsat at the same temperature and atmosheric pressure.

(6) Humidity ratio (Moisture Content) w: The humidity ratio is the mass kg of water vapor interspersed in each kg of

dry air. It should be noted that the mass of water refers only to the moisture in actu

al vapor state, and not to any moisture in the liquid state, such as dew, frost, fog or rain.

The humididy ratio, like other several properties to be studied- enthalpy and specific volume-is based on 1kg of dry air.

w

satw

sat PB

PB

w

w

,


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(7) Specific Volume/Moist Volume v: Specific volume of moist air v , m3/kgdry is defined as the total volume of

the moist air (dry air and water vapor mixture) per kg of dry air.

(8) Specific Enthalpy: Specific enthalpy of moist air h (kJ/kgdry) is defined as the total

enthalpy of the dry air and water vapor mixture per kg of dry air. Enthalpy values are always based on some datum plane. Usually the zero value of the dry air is chosen as air at 0 , and the zero ℃

value of the water vapor is the saturated liquid water at 0 .℃


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9-2.Adiabatic saturation process and wet bulb temperature


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1. Adiabatic saturation process An unsturated air, which has dry bulb temperature t1, humidity ratio w1 and entha

lpy h1, flows through a spray of water, as shown in Fig.9-1.


The spray can provide enough surface area so that the air leaves the spray chamber in equilibrium with the water, with respect to both the temperature and the vapor pressue.

In order to perpetuate the process, it is necessary to provide makeup water (w2-w1) to compensate for the amount of water evaporated into the air.

The temperature of the makeup water is the same as that in the sump.

If the device is adiabatic, then the process is called adiabatic saturation process.

Fig.9-1, An adiabatic saturation process

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2.Thermodynamic wet bulb temperature In an adiabatic saturation process, the temperature of the water in the sump

is called the thermodynamic wet bulb temperature tWB of the air , or simply the wet bulb temperature of the air.

An adiabatic saturation process is a constant wet bulb temperature process, the wet bulb temperatures of air at the inlet and the outlet are same, and they are equal to tWB.

That is 21 WBWB tt


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3.Measurement of wet bulb temperature A simple wet bulb thermometer used to measure the wet bulb temperature i

s based on the principle of the adiabatic saturation process. It is an ordinary thermometer being wrapt with a cloth sleeve of wool or fla

nnel, around its bulb. The cloth sleeve should be clean and free of oil and thoroughly wet with cl

ean fresh water. The water in the cloth sleeve evaporates as the air flows at a high velocity

(≥2.5m/sec). The evaporation, which takes the heat from the thermometer bulb, lowers t

he temperature of the bulb. The thermometer indicates approximately the wet bulb temperature tWB.

sec/5.2 m


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The difference between the dry-bulb and wet-bulb temperatures is called the wet-bulb depression.

If the air is saturated, evaporation cannot take place, and the wet-bulb temperature is the same as the dry-bulb temperature; and the wet-bulb depression equals zero.

sec/5.2 m


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9-3. Psychrometric chart


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A psychrometric chart graphically represents the thermodynamic properties of moist air, as shown in Fig.9-2.

It is very useful in presenting the air conditioning processes.


The psychrometric chart is bounded by two perpendicular axes and a curved line:

1) The horizontal ordinate axis represents the dry bulb temperature line t , in ; ℃

2) The vertical ordinate axis represents the humidity ratio line w , in kgw/kgdry.air

3) The curved line shows the saturated air, it is corresponding to the relative humidity Ф=100% .

Fig.9-2, Psychrometric Chart (B=101.3 kPa)

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The psychrometric chart incorporates seven parameters and properties. They are dry bulb temperature t , relative humidity Ф , wet bulb

temperature tWB, dew point temperature tDP , specific volume v, humidity ratio w and enthalpy h.

①Dry-bulb temperature t is shown along the bottom axis of the psychrometric chart.

The vertical lines extending upward from this axis are constant-temperature lines.

②Relative humidity lines Ф are shown on the chart as curved lines that move upward to the left in 10% increments.

The line representing saturated air ( Ф= 100% ) is the uppermost curved line on the chart.

And the line of Ф = 0% is a horizontal ordinate axis itself.


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③ Wet-bulb temperature tWB : On the chart, the constant wet-bulb lines slope a little upward to the left, and the wet bulb temperature is read following a constant wet-bulb line from the state-point to the saturation line.

④ Dew point temperature tDP : This temperature is read by following a horizontal line from the state-point to the saturation line.

⑤ Specific volume v: It is shown from the constant-volume lines slanting upward to the left.


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⑥ Humidity ratio w: it is indicated along the right-hand axis of the chart.

⑦ Enthalpy h: It is read from where the constant enthalpy line crosses the diagonal scale above the saturation curve. The constant enthalpy lines, being slanted lines, are almost coincidental as the constant wet-bulb temperature lines.

Only two properties are needed to characterize the moist air because the point of intersection of any two properties lines defines the state-point of air on a psychrometric chart.

Once this point is located on the chart, the other air properties can be read directly.


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9-4. Air handing processes


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1. The specifications for human comfort

relative humidity:

air flow rate:

%60%40

sec/25.0 m


CtC 00 2722

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2. Main air handing processes and their variations in properties

(1) Sensible cooling along a cooling coil, or sensible heating along a heating coil


The sensible cooling happens when the air is cooled without altering the specific humidity.

The process is shown on the psychrometic chart by line 1a in Fig.9-4.

During this process, the relative humidity of the air will increase.

The sensible cooling can only take place under the condition when the temperature of the cooling coil is not below the dew point temperature of the air being processed.

Fig.9-4, Main air handing processes and their variations in properties

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The sensible heating is similar to sensible cooling, but with the dry bulb temperature increasing (line 1b in Fig.9-4).

It should be noted that there should be no water within the heating system because the evaporation of the water will increase the specific humidity of the air.

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(2) Adiabatic humidification and dehumidification using a humidifier or chemical dehumidifier



The adiabatic humidification occurs when water vapor, of which temperature is near the wet bulb temperature of the moist air, is added to the air (line 2a in Fig.9-4).

A humidifier performs this function by supplying the water vapor.

During the adiabatic humidification process along the constant wet bulb temperature line, the specific humidity of air will increase.

Reduction in dry bulb temperature will happen as the evaporated water will absorb heat .

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The adiabatic dehumidification is a reverse process of the humidification, as shown by line 2b in Fig.9-4.

This dehumidification is usually achieved by use of chemical agents, like absorbents or adsorbents.

In this process, the specific humidity decreases along the constant wet bulb temperature line, whilst the dry bulb temperature will increase, as a result of chemical reaction

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(3) Cooling with change in specific humidity using conventional air conditioners



When the temperature of evaporator is lower than the dew point temperature of the entering air, some water vapor in the air will condense and drain off.

The dry bulb temperature and the specific humidity of the air will reduce.

This process can be shown by line 3 on the psychrometric chart in Fig.9-4.

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9-5. Heat and mass transfer between moist air and solid

surface


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There are a lot of heat exchangers in refrigeration and air conditioning systems.

Moist air makes heat transfer and/or mass transfer with the solid surface of the heat exchangers.

If the solid surface is dry during the process, there is heat transfer only. However, if the solid surface is wetted, there are both heat transfer and

mass transfer.


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1. Sensible heat transfer between moist air and solid surface

The sensible heat transfer rate dqsensible, KW, from the wetted surface to the moist air is

)( assensible ttdAdq


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2. Mass transfer between moist air and solid surface

The mass transfer rate of water vapor dm, kgw/s, from the wetted surface to the moist air is

)( as wwdADdm


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3. Latent heat transfer between moist air and solid surface

The latent heat transfer rate dqlatent , KW, from the wetted surface to the moist air is

fgaslatent hwwdADdq )(


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4. Lewis number and Lewis factor Lewis number is defined as

The Lewis number LeN is a property of a mixture. For moist air, Lewis number LeN has values not far from 1.

In order to relate the heat transfer process, a new nondimensional, Lewis factor is introduced and is defined as

Lewis factor Lef is not a property of moist air, it is variable with the conditions of heat and mass transfer.

The value of Lewis factor Lef is considered in the region of 0.5-1.3.

k

kCpScLe D

N

Pr/

DCpLe

airmoistf


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5. Total heat transfer between moist air and solid surface

The total heat transfer rate, sensible and latent heat transfer, from the wetted surface to the moist air .

Applying the Lewis factor Lef =1.0

fgasaslatentsensibletotal hwwdADttdAdqdqdq )()(


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The Eq. above can be written as

For dry air, specific heat

For moist air, specific heat

For the most cases of air conditioning

Therefore

fgasairmoist

astotal hwwdACp

ttdAdq

)()(

)/(003.1 KkgkJCp airdry

vaporerheatedairdryairmoist CpwCpCp sup

)/(0.2sup KkgkJCp vaporerheated airdrykgkgw /)014.0007.0(

)/(02.1 KkgkJCp airmoist )(thwhtCp fgairmoist

)( asairmoist

total hhCp

dAdq


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Therefore the concept of enthalpy potential is a very useful one in quantifying the total heat transfer in those processes where there is direct contact between air and water.

If ta> ts, the sensible heat is from the moist air to the solid surface;

If wa >ws , the mass and latent heat is from the moist air to the solid surface;

If ha >hs or ta, wa >ts, wa , the total heat is from the moist air to the solid surface.


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REFERENCES

1. Stoecker W.F., Jones J.W., Refrigeration and air conditioning, 2nd Edition, McGraw-Hill Book Company, New York, USA, 1982

2. ASHRAE, ASHRAE Handbook-Fundamental, New York, USA, 2005

3. Eckert E.R.G., Drake R.M., Analysis of heat and mass transfer, McGraw-Hill Ltd, New York, USA, 1972

4. Kloppers J.C., Kroger D.G., The Lewis factor and its influence on the performance prediction of wet-cooling towers, International Journal of Thermal Science, (44),879-884, 2005


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1 Refrigeration Technology wu wei-dong Chapter9. Psychrometry and Air Processes