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Why do I need refrigerant traps in my piping?

Published date: 18 July 2020

Why are refrigerant traps required?
Product line:
All Air Cooled Cooling Units
Data Center Cooling
Q: Why are traps important every 20 feet of height?

A:  A discharge line trap plays an important function both during the compressor on and off cycles. During cycles, the trap collects oil to be entrained in the discharge gas and to be carried by the velocity of the refrigerant flow.  Traps are utilized in discharge risers and at the base of the riser. It is customary to reduce the riser by one pipe size to maintain velocities over 1000 ft/min. Figure 1 shows the proper method for reducing the pipe diameter for discharge risers. This approach will prevent oil from being trapped in the horizontal portion of the pipe.

figure 1



During the off period, the traps catch and retain oil residing on the pipe walls that would otherwise possibly drain back to the compressor head, causing damage on startup. At the very least, it is advised to always install a single trap at the base of the riser if the riser will be less than 20 feet.


Q: Is it ok if I undersize, or oversize my discharge line since I have already piped the copper?

A: Discharge gas lines (often referred to as hot gas lines) allow refrigerant to flow from the discharge of the compressor to the inlet of the condenser. Undersizing discharge lines will reduce compressor capacity and increase compressor work. Over sizing discharge lines increases the initial cost of the project and can reduce the refrigerant gas velocity to a point where oil is not returned to the compressor. Undersized discharge lines reduce system capacity. Oil movement in discharge lines is further complicated by risers, where gravity is working against oil return.  See figure 2 for a typical Discharge Line installation


figure 2


Q: Is it ok to add a discharge muffler to my air conditioner?

A: Schneider Electric does not require a discharge line muffler nor do we recommend one.  However, the customer owns the equipment and may do as they please.  Warranty issues will be based upon Platform Engineering and, or the District Service Manager discretion.   Nonetheless, should a discharge muffler be installed it is recommended to be placed on the discharge line to minimize pulsations (that cause vibration). Oil is easily trapped in a discharge muffler, so it should be placed in the horizontal or downflow portion of the piping, as close to the compressor as possible. 

Q: Why are we putting in Inverted Traps on the liquid and discharge lines?

A: Whenever a condenser is located above the compressor, an inverted trap or check valve should be installed at the condenser inlet to prevent liquid refrigerant from flowing backwards into the compressor during off cycles.  Schneider Electric does this on both lines to ensure oil is trapped during the off cycles as well.  Routing the discharge line so it rises to the top of the condenser, then drops to the level of the condenser inlet, creating an inverted trap is also acceptable.

Q: Please explain the need for S-Traps?

Schneider Electric requires S-Traps for risers longer than 25 ft should have S traps every 20 ft. S-Traps are also used to help promote oil return.

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