Discharge Lines

Question: My question concerns discharge line trapping. I have been taught that a 6” trap is called for each 10' of vertical rise or combination of traps and check valves.

In going through the Service Application Manual section on piping and the Study Guide, I find the only stipulation is to trap the discharge line if the rise is 10' or more and an additional trap for each additional 30' of rise.

At one point, the statement is made, “Traps should be as sort as fittings will permit.” If this is taken literally, it would mean 2 street and one standard ell coming off a horizontal line to a vertical, or 3 street and 1 standard ell if in the middle of a vertical riser. Neither would produce a 6” trap except in the very large size discharge lines.

So what should the depth of a discharge line trap be?

-John Meyer, Henryville, PA

Answer: The discharge line trap is intended to accumulate oil drained from the discharge riser during shutdown, preventing it from re-entering the compressor. Similarly, the suction line trap drains the riser as well as the evaporator.

Secondly, traps aid the oil to start its climb up both the suction and discharge risers at start-up. The wiping action of the refrigerant gas as it passes through the trap breaks the oil into droplets and sweeps it up the vertical piping.

During normal operation, oil collects in the horizontal suction of the trap and is swept up the piping rise before it can accumulate to a depth that reaches the vertical sections of the trap. Because of this, a properly constructed trap should have as short a horizontal dimension as possible. (See figure.) The short horizontal dimension restricts the capacity of the trap, preventing the hold up of excessive quantities of oil. To limit this dimension, it is best to construct the trap of street ells and a regular ell. The depth of the trap is not as critical as the horizontal dimension, but even if constructed of street ells, a properly applied trap should still have sufficient capacity to catch any oil drained from the riser during the off cycle.

-Joe Farwell, Trane