Podcasts: Feb. 24, 2014

A: Electronic expansion valves (EXVs) use the same principles as any other throttling device or thermostatic expansion valve. It’s designed to inject an appropriate amount of refrigerant liquid into the evaporator to ensure the heat load is being absorbed. At the same time it ensures there is a positive superheat to protect a compressor and make sure the superheat isn’t too small to prevent liquid flood back to the compressor. An electronic expansion valve is different from a piston or fixed orifice, as an EXV offers numerous advantages over other devices. In many newer systems, there is a high mass flow. It could be a variable-speed compressor, and the EXV allows you to properly match the liquid injection into that evaporator for high degrees of mass flows. It also offers a positive shutoff during the off cycle, preventing liquid migration into the evaporator or compressor sump.

Q: What should you consider when selecting an EXV?

A: You want to look at the range of the load. What is the total load affecting that valve? Many think a 3-ton valve is sized for a 3-ton system, but many times a 3-ton valve will control a load that ranges from 1 ½–3 tons. The type of refrigerant is also important. R-410A has a higher maximum working pressure, so you need a valve with a higher working pressure. If it is a CO₂ transcritical, you need to select the valve appropriately.

Jeff Staub
Application engineering manager, Danfoss
www.danfoss.com

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Publication date: 2/24/2014