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PSYCHROMETRICS 455

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Equation (10.15) indicates that the enthalpy of an air-water vapour mixture is equal to the
enthalpy of saturated air at the same wet bulb temperature, less small correction term (W 2 s – W 1 )
hf 2. This correction term is called enthalpy deviation.

h 1 = hair (^) (1) + W 1 hvapour (^) (1) ...(10.16)
h 2 s = hair (^) (2) + W 2 s hvapour (^) (2) ...(10.17)
or hair (^) (1) + W 1 hg 1 + (W 2 s – W 1 )hf 2 = hair (2) + W 2 shg 2
cptdb 1 + W 1 hg 1 + (W 2 s – W 1 )hf 2 = cptdb 2 + W 2 shg 2 ...(10.18)
After arranging, we get
W 1 =
ct t W h h
hh
p db db s g f
g f
()() 21 22
12
−+ 2 −
−
or W 1 =
ct t Wh
hh
p db db s g
g f
() 21 2
12
−+ 2
− ...[10.18(a)]
Note. The wet bulb temperature is not a property of moist air as it is influenced by heat and mass transfer
rates. Thus in psychrometric equations and psychrometric charts where the wet bulb temperature appears, it is
always the thermodynamic wet bulb temperature that is considered.

10.4. Psychrometers

A psychrometer is a device which is used for measuring dry bulb and wet bulb tempera-

tures simultaneously.

The psychrometers may be classified as follows :

1. Laboratory psychrometer


2. Sling psychrometer


3. Aspirating psychrometer


4. Continuous recording psychrometer.
   The description of a sling psychrometer is given below :
   Refer Fig. 10.3. The sling psychrometer consists of two thermometers mounted on a base
   plate. The one with the ‘sock’ is wet-bulb thermometer ; the other is dry-bulb. The wet bulb exists

Instrument is rotated about

2 to 3 times per second

until reading attains

constant values.

Handle is firmly

grasped and

thermometers

are swung.

Wet bulb

Sock

Dry bulb

Fig. 10.3. Sling psychrometer.

below the dry-bulb. This is done purposely so that ‘sock’ can be dipped in water without wetting the

dry-bulb. The handle of the frame helps for rotating the psychrometer to produce necessary air

motion. As the psychrometer is rotated it provides necessary air velocity over the thermometer.

Fast movement of air past the ‘sock’ is necessary to bring the air at temperature tdb always in

immediate contact with the wet ‘sock’. The temperature spread between dry bulb and wet bulb

readings depends upon the amount of moisture in the air. Dry air, or air that has low moisture