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Why do I need refrigerant traps in my piping?

Why do I need refrigerant traps in my piping?
Product line:
All Air Cooled Cooling Units
Data Center Cooling

Q: Why do you need to trap every 20 feet of height?

A:  A discharge line trap has important functions both during the compressor on period and during its off period. During the on period, the trap acts as a turbulator to entrain oil droplets and carry them efficiently up the elevated discharge line. Therefore, traps would be most useful in unloading or multi-compressor systems to better ensure proper oil circulation during low system loads.  Also, you are required to change the size of the pipe after the “P” trap. 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 foot of the riser.


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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