Industrial Refrigeration II - Systems, Vessels, and Arrangements
Matching the system to the load
Systems questions test whether you can name the arrangement and explain why it is used. Direct expansion feeds only the refrigerant that will boil off. Liquid overfeed (recirculated) systems pump several pounds of liquid for every pound vaporized, which wets the coil fully, improves heat transfer, gives the pump a liquid head, and returns surplus liquid to a low-pressure receiver. Flooded and thermosyphon evaporators depend on natural circulation. Two-stage and booster systems split compression: a low-stage machine discharges into an intercooler or desuperheater, and a high-stage machine finishes the job, sometimes with an economizer or side load tapped at intermediate pressure.
Vessels to know: high-pressure receiver, low-pressure receiver (surge drum), intercooler, oil separator and oil pots, and purge units for non-condensables. Oil migrates with hot discharge gas, is knocked out by separators, and must be drained from low points without dumping refrigerant. Hot-gas defrost and evaporative condensers with proper water treatment round out the picture.
Traps: confusing the duty of the high-pressure receiver with that of the low-pressure receiver; assuming a bigger recirculation rate always helps (it can cavitate the pump and add heat); forgetting that non-condensables raise head pressure and belong in a purger, never vented as ammonia to atmosphere.
Rule of thumb: ammonia liquid overfeed ratios commonly run about 3:1 to 4:1. If a coil starves or the pump loses its seal, check surge-drum level and the pump strainer before adjusting the expansion valve.