TITLE.PM5

REFRIGERATION CYCLES 739

dharm

\M-therm\Th14-2.pm5

14.3.9. Volumetric efficiency

A compressor which is theoretically perfect would have neither clearance nor losses of any
type and would pump on each stroke a quantity of refrigerant equal to piston displacement. No
actual compressor is able to do this, since it is impossible to construct a compressor without
clearance or one that will have no wire drawing through the suction and discharge valves, no
superheating of the suction gases upon contact with the cylinder walls, or no leakage of gas past
the piston or the valves. All these factors effect the volume of gas pumped or the capacity of the
compressor, some of them affect the H.P. requirements per tonne of refrigeration developed.
‘Volumetric efficiency’ is defined as the ratio of actual volume of gas drawn into the
compressor (at evaporator temperature and pressure) on each stroke to the piston displacement. If
the effect of clearance alone is considered, the resulting expression may be termed clearance volu-
metric efficiency. The expression used for grouping into one constant all the factors affecting
efficiency may be termed total volumetric efficiency.
Clearance volumetric efficiency. ‘Clearance volume’ is the volume of space between
the end of the cylinder and the piston when the latter is in dead centre position. The clearance
volume is expressed as a percentage of piston displacement. In Fig. 14.20 the piston displacement
is shown as 4′-1.

p (Pressure)

v (Volume)

3 Discharge 2

Expansion

4 ′ 41 Suction

Compression

pd

ps

Clearance

volume

Swept volume or

Piston displacement

Fig. 14.20

During the suction stroke 4′-1, the vapour filled in clearance space at a discharge pressure

pd expands along 3-4 and the suction valve opens only when pressure has dropped to suction

pressure ps, therefore actual volume sucked will be (v 1 – v 4 ) while the swept volume is (v 1 – v 4 ′).

The ratio of actual volume of vapour sucked to the piston displacement is defined as clearance

volumetric efficiency.

Thus,

Clearance volumetric efficiency, ηcv =

vv

vv

vv

vv

14

14

14

13

−

−′

= −

− (Q v^4 ′ = v^3 )

Considering polytropic expansion process 3-4, we have

pvs 43 n=p vd n