The sizing and arrangement of suction and discharge piping for the halocarbons must ensure proper entrainment of oil, to return this to the compressor. Pipes for these gases usually have a higher velocity at the expense of a greater pressure drop than those for ammonia. Pipe sizes may only be increased in runs where the oil will be assisted by gravity to flow in the same direction as the gas.

Horizontal pipes should slope slightly downwards in the direction of flow, where this can be arranged. If a suction or discharge line has to rise, the size may be decreased to make the gas move faster. In the case of a lift of more than 5 m, a trap should be formed at the bottom to collect any oil which falls back when the plant stops.

Suction and discharge risers (Figure 11.1) will normally be sized for full compressor capacity, and velocities will be too low if capacity reduction is


PIPING FOR OIL RETURNOperated. In such installations, double risers are required, the smaller to take the minimum capacity and the two together to carry the full flow. Traps at the bottom and goosenecks at the top complete the arrangement. At part capac­ity, any oil which is not carried up the main riser will fall back and eventually block the trap at the bottom, leaving the smaller pipe to carry the reduced flow, with its quota of oil. When the system switches back to full capacity, the slug of oil in this trap will be blown clear again.

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