Going back to basics

I want to thank Howard McKew for his "Back to Basics" columns, specifically Test 206, (January 2006, pages 16 and 17) in Engineered Systems. They are type of features that an old O&M engineer such as myself enjoys the most. Keep those types of articles coming.

I have a question, however: in the double duct system downstream of the supply fans, there is a component called FMS-H and FMS-C. What do they stand for? I am looking forward to more O&M types of basic operational articles.

Al Cohen
Chief Engineer (Ret.)
University of California at Los Angeles

McKew responds:

I am glad you enjoy "Back to Basics." As this has grown in recent years from being just a multiple choice test with the addition of designer, commissioning, and TAB "Helpful Hints," the challenge is to keep the information format simple and also accurate. Maybe I didn't make the FMS clear enough. The FMS-H is the flow measuring station for the hot fan system, FMS-C is the flow measuring station for the cold fan system, and FMS-R is for the return air.

Thanks again for the comments and check out our new blog at http://blog.esmagazine.com.

Potential piping problems

I was appalled to hear a major manufacturer's representative recently suggest installing a liquid injection/hot gas device in a condensing unit of a split system in order to save the expense of piping the hot gas to the evaporator. I was concerned enough to send you this note because you have an advertiser for such a device in your publication. After 42 years in the business, I do have an opinion on the proper way to pipe gas into an air conditioning system.

There are several methods for utilizing low load, hot gas bypass on thermostatic valve-controlled equipment. As far as comfort cooling is concerned, there is only one way to install the injection point for hot gas, and that is into the entering side of the evaporator.

Anyone who thinks they can utilize one of the methods of liquid injection into the suction line is only fooling themselves or has no idea of what they are doing. Hot gas/liquid injection into the suction piping will allow the compressor to properly operate, but that is not where the problem lies. Thermostatic expansion valves lose control at reduced load and start to hunt, to the extreme of two positioning. When they get this far out of control, they can allow massive slugs of liquid to return to the compressor, which is a very bad thing.

When a thermostatic expansion valve is forced to operate near its closed position, in order to try and maintain proper superheat because the load is low, the velocity of the gas moving through the evaporator coil and suction piping is reduced. This requires a much longer time before the expansion valve bulb on the outlet of the coil can see the results of any new valve position. This can allow the thermostatic expansion valve to put a lot of liquid into an evaporator before the bulb realizes it and screams: "shut down." It gets worse as the system gets larger.

The low velocity in the evaporator coil and suction piping can also prevent proper oil return to the compressor. Injecting the hot gas into the inlet of an evaporator allows a constant, high velocity of gas flow to be maintained, regardless of what the thermostatic expansion valve is doing. The thermostatic expansion valve's sensing bulb can respond quickly to any new valve position.

You now have the volume of vapor needed for the compressor. You have higher velocity for proper oil return. The suction pressure is maintained and doesn't drop too low causing a frozen coil. The thermostatic expansion valve provides the de-superheating required for the introduction of hot gas without an extra component. And, you have the coil velocity needed to allow the thermostatic expansion valve to properly control superheat.

There are systems out there that don't maintain high velocity in the complete suction side of the system and they usually have problems. Always strive to maintain full load velocity in the entire suction side of a system. End of story. No compromise.

Douglas Summerfield
Airtech Mechanical Corp.
Commerce City, CO