Page 30 - Reciprocating Compressors Operation Maintenance

Reciprocating Compressors and Their Applications 1 7

gas must be taken into the cylinder to compensate for piston ring and
suction valve slip; secondly, because work is performed on this lost
capacity; and finally, because leakages back through the discharge valves
must be recompressed and re-delivered to the discharge system.
There is still another factor: the cooling effect of cylinder jacketing.
Removal of heat by the jacket water would shrink the volume during
compression and would tend to move point F to the left, reducing the
power required. This is a true saving in power expended. Unfortunately,
it is not of any great significance, except in small cylinders handling
rather low density gas through high ratio of compression, where the jack-
et surface is large in proportion to the amount of work performed and
heat generated.
These fluid losses are indicated on the indicator card AOBF, Figure 1-8.
This figure represents a typical actual indicator card such as might be
taken on a machine in the shop or in the field.

BRAKE HORSEPOWER

The actual indicated horsepower is built upon the base of ideal horse-
power and includes the thermodynamic losses in the cylinder. These ther-
modynamic losses (fluid losses) are summed up under the general term
compression efficiency.
The major factor involved in determining the compression efficiency
is the valve loss or pressure drop through the inlet and discharge valves.
These fluid losses are a function of gas density and valve velocity. The
suction and discharge pressures and the molecular weight establish the
density. The valve velocity is fixed by the valve area available in the
selected cylinders and by the piston speed. Valve velocity is normally
stated in feet per minute; it is the ratio of piston area to valve area per
cylinder end, multiplied by feet per minute piston speed.
A better understanding of the losses involved in compression efficien-
cy may be obtained by reference to the indicator diagram, Figure 1-8. If
it were possible to get the gas into and out of the cylinder without fluid
losses, the indicator card ABEH could be realized.
This card may be said to represent the ideal or theoretical horsepower
requirements. But fluid losses are present. Therefore, the actual inlet
pressure in the cylinder is below that at the cylinder inlet flange. Like-
wise, the pressure in the cylinder during delivery interval EH is above
that at the cylinder discharge flange.