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7
LINE SIZING
In sizing refrigerant lines, cost considerations favor keeping
the line size as small as possible. However, suction and
discharge line pressure drops cause loss of compressor
capacity and increased power usage.
Excessive liquid line pressure drops can cause the liquid
refrigerant to flash, resulting in faulty expansion valve
operation. Refrigeration systems are designed so that
friction pressure losses do not exceed a pressure differential
equivalent to a corresponding change in the saturation
boiling temperature.
The primary measure for determining pressure drop is
a change in saturation temperature. Pressure drop in a
refrigerant line causes a reduction in system efficiency.
Correct sizing must be based on minimizing cost and
maximizing efficiency.
Pressure drop calculations are determined as normal
pressure loss associated with a change in saturation
temperature of the refrigerant. Typically, the refrigeration
system will be sized for pressure losses of 2°F differential or
less for each segment of the discharge, suction, and liquid
lines. An HFC refrigerant liquid line is sized for pressure
losses of 1° differential or less.
IMPORTANT: The information above is intended for general
information on refrigerant piping only. Reference specific
AHU and condensing units manuals for specific piping
details.
REFRIGERANT PIPING GUIDELINES
IMPORTANT: The information below is intended for general
information on refrigerant piping only. Reference specific
AHU and condensing units manuals for specific piping
details.
The design of a refrigerant piping system should:
Ensure proper refrigerant feed to evaporators; Provide
practical refrigerant line sizes without excessive pressure
drop; Prevent excessive amounts of lubricating oil from being
trapped in any part of the system; Protect the compressor at
all times from loss of lubricating oil; Prevent liquid refrigerant
or oil slugs from entering the compressor during operating
and idle time; and Maintain a clean and dry system.
REFRIGERANT LINE VELOCITIES
Economics, pressure drop, noise, and oil entrapment
establish feasible design velocities in refrigerant lines. These
are:
Suction line - 700 to 4,000 fpm
Discharge line - 500 to 3,500 fpm
Condenser drain line - 100 fpm or less
Liquid line - 125 to 450 fpm
Higher gas velocities are sometimes found in relatively short
suction lines on comfort air conditioning or other applications
where the operating time is only 2,000 to 4,000 hrs per year
and where the low initial cost of the system may be more
significant than low operating cost.
Industrial or commercial refrigeration applications, where
equipment runs almost continuously, should be designed with
low refrigerant velocities for the most efficient compressor
performance and low equipment operating cost.
The liquid line from the condenser to the receivers should
be sized for 100 fpm or less to ensure positive gravity flow
without incurring a backup of liquid flow. Liquid lines from
the receivers to the evaporator should be sized to maintain
velocities below 300 fpm, thus minimizing or preventing
liquid hammer when solenoids or other electrically operated
valves are used.
Minimum Discharge- Line Velocities
Refrigerant Velocity, fpm
Nominal Pipe
Size, in.
Riser Horizontal
7/8 375 285
1-1/8 430 325
1-3/8 480 360
1-5/8 520 390
2-1/8 600 450
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