1. Gas Compression and Flow Dynamics NGT 150
Reciprocating Gas Compressors
Chapter 4
Calculations
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2. Cylinder has full charge of gas at suction pressure
Pressure in cylinder has become greater than pressure in discharge line
Pressure in cylinder drops, closing the discharge valve
Pressure in cylinder drops below suction pressure, a new charge of gas begins to enter cylinder

3. Piston Displacement
The volume displaced in cubic feet per minute as the piston moves from position 1 to position 3
Single-acting cylinder

4. Double-acting cylinder

5. The ratio generally fall between 5.0 and 2.5
Compression Ratio
The proportion of the absolute discharge pressure to the absolute suction pressure.
The ratio generally fall between 5.0 and 2.5
Because of the volumetric and temperature efficiency of the compressor
If a ratio of greater than 5.0 is needed then two or more stages are used.
Two determine ratio required for two stages take the square root of 7.8

6. Clearance Volume
Is the space in cubic inches remaining in the compressor cylinder at the end of the discharge stroke.
As the piston moves in the cylinder it must never reach the end of the cylinder or it will damage itself and the cylinder head.
The space provided for this protection plus the space that exists around the valves make up the clearance volume.

7. Is usually expressed as a percent of piston displacement.
Clearance Volume
Is usually expressed as a percent of piston displacement.
See page 56

8. Clearance Volume
For double acting cylinders – it is the total piston displacement for both the head end and the crank end.
Often the clearance is assumed to be equal between the head end and the crank end even the crank end displacement is less than the head end due to the volume of the piston rod.
For small diameter cylinders the crank end must be calculated.

9. Volumetric Efficiency (p. 58)
Is the ratio of the volume of gas actually drawn into the cylinder (corrected to suction temperature and pressure) to the volume of piston displacement (DSA).
Depends on the amount of gas that flows into the cylinder during the suction stroke.
Since clearance volume gas must expand to suction pressure before gas can enter the cylinder – the clearance affects volumetric efficiency.

10. Volumetric Efficiency (p. 58)
Principal reasons that the cylinder may not deliver the piston displacement capacity are:
High clearance and high ratios
Heating of the gas during admission to the cylinder
Leakage past valves and piston rings
Re-expansion of the gas trapped in the clearance volume space from the previous stroke
Re-expansion has the greatest effect on volumetric efficiency. (see page 58 right column)