2 psychrometric processes

8/10/2019 2 Psychrometric Processes

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Psychrometric processes

Introduction:

In the design and analysis of air conditioning plants, the

processes being performed on air. Once identified, the

processes can be analyzed by applying the laws of

conservation of mass and energy. All these processescan be plotted easily on a psychrometric chart. This is

the changes taking place in important properties such as

processes that air undergoes in a typical air conditioning

plant are discussed below.


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Fig. 1: Sensible cooling process O-A on psychrometric chart


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Fig.2.:Sensibleheatingprocessonapsychrometric chart


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Example:-

heats 0.1 m3/s of air from 15oC and 80% RH to 55oC? The

barometric pressure is 101.325 kPa.


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C):-c) Cooling and dehumidification (Process O

-

it in contact with a cold surface as shown in Fig. 3, some

of the water vapor in the air condenses and leaves the airstream as liquid, as a result both the temperature and

humidity ratio of air decreases as shown.

The heat and mass transfer rates can be expressed in

conservation of mass and conservation of energy

equations as given below:


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Fig.3 : Cooling and dehumidification (Process O-C)


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By applying mass balance for the water:

By applying energy balance:

from the above two equations, the load on the cooling coil,Q is given by:

the 2 term on the RHS of the above e uation is normall small

compared to the other terms, so it can be neglected.


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It can be observed that the coolin and de-humidification

process involves both latent and sensible heat transfer

processes, hence, the total, latent and sensible heattransfer rates (Qt, Ql and Qs) can be written as


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A lower value of SHF, say 0.6, implies a high latent heat load

such as that occurs in a humid climate.From Fig. 3, it can be seen that the slope of the process line

Fromthe

definition

of

SHF,


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Fromtheaboveequations,wecanwritetheslopeas:

Thuswecanseethattheslopeofthecoolinganddehumidification

lineis

purely

afunction

of

the

sensible

heat

factor,

SHF.

Hence,

we

candrawthecoolinganddehumidificationlineonpsychrometric

c ar e n a s a ean e are nown. nsomes an ar

psychrometric charts,aprotractorwithdifferentvaluesofSHFis

andinparalleltothegivenSHFlinefromtheprotractorasshownin

Fig.4.


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Fi . 4: A s chrometric chart with rotractor for SHF lines


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By pass factor:

In Fig.3, the temperature Ts is the effective surface

tem erature of the coolin coil, and is known as a aratus

dew-point (ADP) temperature. In an ideal situation, when

all the air comes in perfect contact with the cooling coilsur ace, en e ex empera ure o a r w e same as

ADP of the coil. However, in actual case the exit

apparatus dew-point temperature due to boundary layerdevelopment as air f lows over the cooling coil surface and

also due to temperature variation along the fins etc.

Hence, we can define a by-pass factor (BPF) as:


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It can be easily seen that, higher the by-pass factor

and the cooling coil temperature. When BPF is 1.0, all the

air b - asses the coil and there will not be an coolin or

de-humidification. In practice, the by-pass factor can be

increased by increasing the number of rows in a coolingco or y ecreas ng e a r ve oc y or y re uc ng e n

pitch.

,

is given by:


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Example:

coolingcoil

at

25oC

and

50%

RH

and

leaves

the

cooling

coil

at

11oC

and90%RH.Theapparatusdewpointofthecoolingcoilis7oC.

Find:

a)The

required

cooling

capacity

of

the

coil.

ens e ea ac or or eprocess.

c)Bypassfactorofthecoolingcoil.

.

condensatewater

to

leave

the

coil

at

ADP

(hw=

29.26

kJ/kg)


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Heating and Humidification (Process O-D):

During winter it is essential to heat and humidify the room

.

heating the air and then adding water vapour to the air

stream through steam nozzles as shown in the fig.6.

Mass balance of water vapor for the control volume

, . .

Where m a is the mass flow rate of dry air.


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Fig. 6:- Heating and Humidification Process .


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From ener balance:

WhereQ h is the heat supplied through the heating coil .

Hw is the enthalpy of steam.

mass transfer, we can define a sensible heat factor for

the rocess in a wa similar to that of a coolant anddehumidification process.


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e) Cooling & humidification (Process O-E):

, ,

temperature drops and its humidity increases. This

rocess is shown in Fi . 7.

This can be achieved by spraying cool water in the air

stream. The temperature of water should be lower thanthe dry-bulb temperature of air but higher than its dew-

DPT w O .


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Fig. 7: Cooling and humidification process


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Itcanbeseenthatduringthisprocessthereissensible

ea

rans er

rom

a r

o

wa er

an

a en

ea

rans er

fromwatertoair.Hence,thetotalheattransferdepends

.

Ifthetemperatureofthewatersprayedisequaltothe

,

willbezeroasthesensibleheattransferfromairtowater

.

Ifthe

water

temperature

is

greater

than

WBT,

then

there

.

temperatureislessthanWBT,thenthenetheattransfer

.

Theprocessofcoolingandhumidificationisencountered

,

coolingtowers

etc.


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F):-humidification (Process O-Heating and de

This process can be achieved by using a hygroscopic

,

the moisture. If this process is thermally isolated, then

the enthalpy of air remains constant, as a result thetemperature of air increases as its moisture content

decreases as shown in Fig. 8.

In general, the absorption of water by the hygroscopic

released during this process, which is transferred to air

and the enthalpy of air increases.


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Mixingofairstreams:

Mixing of air streams at different states is commonly

encountered in many processes, including in air

conditioning. Depending upon the state of the individual

streams, the mixing process can take place with orw ou con ensa on o mo s ure.

.

mixing of two moist air streams during which nocondensation of moisture takes place.

As shown in the figure, when two air streams at state

points 1 and 2 mix, the resulting mixture condition 3 can

e o a ne rom mass an energy a ance.


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g. . x ng o wo a r s reams w ou con ensa on


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From the mass balance of dry air and water vapor:

From the above equations, it can be observed that

the final enthalpy and humidity ratio of mixture are

weighted averages of inlet enthalpies and humidity

ratios


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ii) Mixing with condensation:

As shown in Fi . 10, when ver cold and dr air mixes

with warm air at high relative humidity, the resulting

mixture condition may lie in the two-phase region, as aresu ere w e con ensa on o wa er vapor an some

amount of water will leave the system as liquid water. Due

,

wil l be less than that at point 4. Corresponding to this willbe an increase in temperature of air due to the release of

latent heat of condensation.


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Fig. 10. Mixing of two air streams with condensation


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2 psychrometric processes

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