Air Conditioning Liquid-Line Sizing

Liquid-line sizing presents less of a problem than suction line sizing for the following reasons:

• The smaller liquid-line piping is much cheaper than suction-line piping.
• Compressor lubricating oil and fluorinated hydrocarbon refrigerants, such as R-22 in the liquid state, mix well enough that, in normal comfort air-conditioning uses, positive oil return is not a problem.
• Vertical risers, traps, and low velocities do not interfere with oil return in liquid lines.

Although liquid-line sizing offers more latitude than suction-line sizing, high-pressure drops should be avoided to prevent flash-gas formation in the liquid line. Flash gas interferes with expansion-valve operation. It also causes liquid-distribution problems where more than one evaporator coil is being used. Where applications requirements are such that flash gas is unavoidable, there are methods of making allowance for it. Liquid refrigerant pumps and separation tanks can be used. (See ASHRAE Guide and Data Book for details.) The acceptable pressure drop depends on the amount of subcooling the condenser unit offers and the inherent losses resulting from liquid lift, if present. It is advisable to have the liquid slightly subcooled when it reaches the expansion valve. This helps avoid flash-gas formation and provides stable operation of the expansion valve.

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