In refrigeration systems, suction line pressure drop directly reduces the vapor density entering the compressor suction cylinders. Each 1°F drop in saturation temperature reduces compressor cooling capacity by approximately 1.5% to 2.0% while simultaneously increasing electrical power consumption (kW/ton) by 1.5%. A 2°F (1.1°C) limit balances copper piping material costs against life-cycle operating efficiency.

ASHRAE mandates a minimum gas velocity of 1,000 to 1,500 fpm (5.1 to 7.6 m/s) in vertical suction risers to drag lubricating oil upward against gravity. Crucially, this velocity must be maintained at the minimum part-load operating condition (e.g. 25% or 50% compressor unload). If velocity drops below 1,000 fpm, oil droplets fall back down the riser, accumulating in the evaporator and starving the compressor crankcase of lubrication.

Double suction risers are required when an unloading or variable-speed compressor has a wide operating turndown ratio (greater than 2:1) such that a single pipe sized for acceptable pressure drop at 100% capacity cannot sustain 1,000 fpm at minimum capacity. A smaller base riser handles low-load oil return, while a larger parallel riser with an oil trap only flows at higher loads.

Liquid flashing occurs when the cumulative pressure drop from line friction, valves, filters, and vertical elevation lift exceeds the liquid subcooling provided by the condenser. Once pressure drops below bubble point pressure, vapor bubbles form in the liquid line, choking the thermostatic expansion valve (TXV) orifice and causing severe refrigeration hunting.

Ammonia reacts vigorously with copper and copper-bearing alloys (brass and bronze) in the presence of trace moisture, forming soluble cuprammonium complexes that rapidly corrode and pit pipe walls. Industrial ammonia systems must be constructed strictly using seamless carbon steel (Schedule 40 or 80 per ASME B36.10M) or stainless steel.