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HVAC School - For Techs, By Techs

Jun 16, 2020

In this short podcast episode, Bryan explains what suction line traps and inverted traps are. He also covers the purposes they serve.

It's a bit hard to find literature on suction line traps, so it's always best to read the manual and follow the manufacturer's guidelines. We traditionally use P-traps on suction lines to hold oil and let it go up the walls of the refrigerant piping. You need enough velocity to lift oil (mineral or alkylbenzene) up the riser. We know that POE carries much easier with refrigerants than mineral oil; it is very miscible with common refrigerants. That's why it's especially important to get all of the mineral oil out of retrofit systems.

In refrigeration, we have lower temperatures, pressures, and densities; that combination adversely impacts oil carry. Oil logging is a bigger concern even with POE oil. So, P-trapping with POE oil is a more common practice in refrigeration than it is in air conditioning.

In air conditioning, we can make a case for the inverted trap: in an air handler that's higher than the condenser, we want the suction line to go above the air handler and then go down into the evaporator coil. When the system goes off, there is still refrigerant in the evaporator coil, so refrigerant will condense into a liquid. We don't want that liquid to rush down the suction line and into the compressor upon startup, so we use an inverted trap to prevent flooded starts from happening. However, we can use hard shutoff TXVs and other strategies to prevent liquid refrigerant migration. Unfortunately, inverted traps can also keep mineral oil stuck in the evaporator coil.


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