Refrigeration Piping Charging Residential AirConditioning R

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Liquid subcooling is the only method of overcoming the liquid line pressure losses in
order to guarantee liquid at the expansion device in the evaporator. If the subcooling is
insufficient, flashing will occur within the liquid line and degrade the efficiency of the
system. Friction pressure drops in the liquid line are caused by accessories such as
solenoid valves, filter driers, and hand valves, as well as by the piping and fittings
between the receiver outlet and the refrigerant feed device at the evaporator. Liquid
line risers are also a source of pressure loss. The loss due to a riser is approximately
0.556 psi per foot of liquid lift. The total loss is the sum of all friction losses plus the
pressure loss from liquid risers. Refrigeration systems that have no liquid risers and
have the evaporator below the condenser and/or receiver benefit from a gain in
pressure due to liquid weight. They can thus tolerate larger friction losses without
flashing. When flashing occurs, the overall efficiency is reduced and the system may
malfunction. The only way to reduce the effect of pressure loses and friction is by
subcooling the refrigerant.
Suction Lines
Suction lines are more critical than liquid and discharge line from a design and
construction standpoint. They should be designed to:
• Provide a minimum pressure drop at full load.
• Return oil from the evaporator to the compressor under minimum load conditions.
• Prevent oil from draining from an active evaporator into an idle one.
A pressure drop in the suction line reduces a system’s capacity by forcing the
compressor to operate at a lower suction pressure, in order to maintain a desired
evaporating temperature in the coil. As the suction pressure is decreased, each pound
of refrigerant returning to the compressor occupies a greater volume, and the weight
of the refrigerant pumped by the compressor decreases. For example, a typical low
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