Page 219 - Reciprocating Compressors Operation Maintenance

Operation and Maintenance of Reciprocating Compressors 2O5

TABLE 3-1 l
POWER CONSUMPTION OF COMPRESSOR AT
DIFFERENT LOAD CONDITIONS

A
Percent
Of Time
At Each B C D E
Control Control Capacity, Brake Motor Kilowatts
Step Step cfm Horsepower Efficiency Required*
( AxCx.746^
2-Step Control
1 D J
100% 70 570 151 90.4% 87.2
0% _JQ _JQ 18.9 65.0% _M
100 570 93.7
3-Step Control
100% 55 448 151 90.4% 68.5
50% 30 122 83.1 89.0% 20.9
0% Jl _Q 18.9 65.0% JL2
100 570 92.7
5-Step Control
100% 13 106 151 90.4% 16.2
75% 60 366 118 90.4% 58.4
50% 21 86 83.1 89.0% 14.6
25% 6 12 49.8 82.0% 2.7
0% 0 __Q 18.9 65.0% _Q
100 570 91.9
*Kilowatts required = bhp x. 746/motor efficiency

even pressures and still automatically match the output of the compressor
to process needs.

Principle of Operation

An unloading device (see Figure 3-61), is fitted to each suction valve.
At partial loads, this device does not allow the suction valve to close
when the piston is in its bottom dead-center position, but delays this clos-
ing in a controlled way. Thus, a certain amount of gas, which is stepless-
ly adjustable, is allowed to return towards the suction manifold before
the compression starts.