TITLE.PM5

REFRIGERATION CYCLES 733

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\M-therm\Th14-2.pm5

2. When the vapour is superheated after compression. If the compression of the
   vapour is continued after it has become dry, the vapour will be superheated, its effect on T-s
   diagram is shown in Fig. 14.11. The vapour enters the compressor at condition ‘2’ and is com-
   pressed to ‘3’ where it is superheated to temperature Tsup. Then it enters the condenser. Here
   firstly superheated vapour cools to temperature T 1 (represented by line 3-3′) and then it condenses
   at constant temperature along the line 3′-4 ; the remaining of the cycle ; however is the same as
   before.

Liquid

line

dg f

s (Entropy)

Saturation

line

Rn

T 2

a

b

12

e

Evaporation

Compression

W

Expansion

T (Temp.)

T 1 4

3

3 ′

Condensation

Tsup.

Fig. 14.11. T-s diagram.
Now, Work done = Area ‘2-3-3′-4-b-2’
and Heat extracted/absorbed = Area ‘2-1-g-f-2’

∴ C.O.P. =

Heat extracted

Work done

Area - - - -

Area - - - - -

=

′

=

−

−

‘’

‘’

21 2

233 4 2

21

32

gf

b

hh

hh

...[14.10 (c)]

In this case h 3 = h 3 ′ + cp (Tsup. – Tsat.) and h 3 ′ = total heat of dry and saturated vapour at the
point ‘3′’.

3. When the vapour is wet after compression. Refer Fig. 14.12.
   Work done by the compressor = Area ‘2-3-4-b-2’
   Heat extracted = Area ‘2-1-g-f-2’

∴ C.O.P. =

Heat extracted

Work done

Area - - - -

Area ----

==−

−

‘’

‘’

21 2

234 2

21

32

gf

b

hh

hh

...[14.10(d)]

Note. If the vapour is not superheated after compression, the operation is called ‘WET COMPRESSION’
and if the vapour is superheated at the end of compression, it is known as ‘DRY COMPRESSION’. Dry compression,
in actual practice is always preferred as it gives higher volumetric efficiency and mechanical efficiency and there
are less chances of compressor damage.