Gas Compression and Flow Dynamics NGT 150 Reciprocating Gas Compressors Chapter 4 Calculations “This product was funded by a grant awarded by the U.S. Department of Labor’s Employment and Training Administration. The product was created by the grantee and does not necessarily reflect the official position of the U.S. Department of Labor. The Department of Labor makes no guarantees, warranties, or assurances of any kind, express or implied, with respect to such information, including any information on linked sites and including, but not limited to, accuracy of the information or its completeness, timeliness, usefulness, adequacy, continued availability, or ownership.” Unless otherwise specified, this work by ShaleNET U.S. is licensed under a Creative Commons Attribution 4.0 International License.

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

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

Double-acting cylinder

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

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.

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

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.

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.

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)