Industrial Refrigeration I - Fundamentals of the Refrigeration Cycle
Heat, temperature, and the pressure–temperature relationship
Industrial refrigeration rests on a few ideas that exam questions probe relentlessly. Heat is energy in transit; temperature is its intensity. Removing heat from a load requires a refrigerant that boils colder than the load, and the pressure–temperature (P–T) relationship is the most useful tool an operator owns. For any refrigerant, saturation pressure fixes saturation temperature: raise the pressure and the boiling point rises; lower it and the boiling point falls. Subcooled liquid and superheated vapor sit off the saturation line, which is why a P–T chart alone cannot give you a superheat value unless you also know the actual line temperature.
Traps: gauge versus absolute pressure (add about 14.7 psi at sea level); confusing evaporator outlet superheat with compressor suction superheat (suction superheat includes pickup in the suction line); calling sensible heat "latent." Expect questions on ammonia (R-717): high latent heat per pound, efficient, detectable by odor well below harmful concentrations, toxic, mildly flammable, and incompatible with copper and brass.
Worked example: compression ratio is discharge absolute divided by suction absolute. Suction 20 psig and discharge 150 psig give 34.7 psia ÷ 164.7 psia ≈ 4.7:1. Using gauge pressures (150 ÷ 20 = 7.5) is the classic wrong answer. Single-stage ammonia machines are generally held near or below roughly 4:1 to protect discharge temperature and oil.