MACH NUMBER is the ratio of the actual velocity at a given point to the velocity of sound in the same gas at the conditions existing at this point. These are known as local conditions. MECHANICAL EFFICIENCY is the ratio, expressed in percent, of the Thermodynamic Work Requirement in the cylinder to the actual shaft horsepower. NORMAL AIR is the term used for average atmospheric air at sea level in a temperate zone where it contains some moisture. It is defined in the ASME Test Code For Displacement Compressors as being at 14.696 psiA, 68°F, 36% RH, and
weighing 0.075 lb/cu ft. The K value is 1.395. PERFECT INTERCOOLING is obtained when the gas is cooled to first stage inlet temperature following each stage of compression. PERFECT GAS (See Ideal Gas). PISTON DISPLACEMENT of a reciprocating compressor cylinder is the net volume displaced by the piston at rated machine speed, generally expressed in cfm. For single-acting cylinders it is the displacement of the compressing end only.
For double-acting cylinders it is the total of both ends. For multistage compressors, the displacement of the first stage only is commonly stated as that of the entire machine. POLYTROPIC HEAD is an expression used for dynamic compressors to denote the foot-pounds of work required per pound of gas. PRESSURE is force per unit area. (See Absolute, Discharge, Gauge, Inlet, Saturated Vapor, Vapor). A PROCESS occurs whenever the system undergoes either a change in state or an energy transfer at a steady state (See State). A REVERSIBLE PROCESS is an ideal process that may be stopped and made to retrace its steps and restore to the systems or surroundings all work and heat previously removed. It is frictionless. An IRREVERSIBLE PROCESS is one in which a portion of the original system energy is dissipated and cannot be returned to the system through its own operation. The system and/or surroundings cannot be returned to their original
state. An ADIABATIC PROCESS is one during which there is no change in temperature. An ISENTROPIC PROCESS is one wherein the entropy remains constant. An ISOTHERMAL PROCESS is one in which there is no change in temperature. A POLYTROPIC PROCESS is one in which changes in gas characteristics during compression are considered. PSYCHROMETRY deals with the properties of air-water vapor mixtures in the atmosphere. RATIO OF SPECIFIC HEATS is the ratio of the specific heat at constant pressure to the specific heat constant volume. It may vary considerably with pressure and temperature. | 
