Refrigeration Piping Charging Residential AirConditioning R

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evaporator.
Movement of the oil through suction lines is dependent on both the mass and velocity
of the suction vapor. As the mass or density decreases, higher velocities are required
to force the oil along.
Nominal minimum velocities of 700 fpm in horizontal suction lines and 1500 fpm in
vertical suction lines have been recommended and used successfully for many years
as suction line sizing design standards. Use of the one nominal velocity provided a
simple and convenient means of checking velocities. However, tests have shown that
in vertical risers the oil tends to crawl up the inner surface of the tubing, and the larger
the tubing, the greater velocity required in the center of the tubing to maintain tube
surface velocities that will carry the oil. The exact velocity required in the vertical line
is dependent on both the evaporating temperature and the line size, and under varying
conditions, the specific velocity required might be either greater or less than 1500 fpm.
An HFC refrigerant, however, is designed for 1500 fpm or greater.
Always pitch vapor lines in the direction of flow, 1/2 inch per ten-foot of suction line.
“P” traps for uphill oil return should be used after the first 6- foot and every 12-foot
thereafter. It is good practice to use an inverted trap just before entering the
compressor.
Double Risers
On systems equipped with capacity control compressors, or where tandem or multiple
compressors are used with one or more compressor cycled off for capacity control, a
single suction line riser may result in either unacceptably high or low gas velocities. A
line properly sized for light load conditions may have too high a pressure drop at
maximum load, and if the line is sized based on full load condition, then velocities may
not be adequate conditions to move oil through the tubing at light load. On air
conditioning applications where somewhat higher pressure drops at maximum load
conditions can be tolerated without any major penalty in overall system performance,
it is usually preferable to accept the additional pressure drop imposed by a single
vertical riser. However, on medium or low temperature applications where pressure
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