Refrigeration

The Professor: Frosted Compressor Head

September 3, 2012
TZAK 1
Fig. 1. Frost coming back to an air-cooled compressor. Evaporator has more than nine degrees of superheat. Compressor has more than 25 degrees of total superheat. (Photo courtesy Ferris State University)

Many service technicians believe that frost on a suction line or on the compressor’s head itself indicates there is liquid refrigerant coming back to the compressor. This simply is not true.

All frost means is that the suction line or compressor is below freezing, and the moisture in the air has reached its dew point temperature and condensed. This condensed moisture has then froze to ice because of the temperature being below 32° F.

Fig. 1 shows frost coming back to an air-cooled compressor. In this photo, the evaporators had over 9° of superheat and the compressor has over 25° of total superheat. The amperage draw is also normal when compared to nameplate amperage.

Compressor superheat, or sometimes referred to as total superheat, is assurance that there is no liquid refrigerant present at the compressor and that the saturated vapor in the evaporator has gained 25° of sensible heat before reaching the compressor. The condensing unit was a low temperature application running –10° box temperatures. With –10° box temperatures, the evaporating temperatures averaged about –24°. With –24° evaporating temperatures and the system having 25° of compressor (total) superheat, the compressor return gas temperatures was about 1°. (Equation 1.) Dew (condensed water vapor) will freeze at this temperature (1°) and become frost on the lines and compressor’s head.

Equation #1

Evaporator temperature -24°F + Compressor superheat 25°F
= Compressor in Temperature 1°F

It is important for service technicians to understand the difference between suction gas-cooled and air-cooled compressors. In an air-cooled compressor, the suction return gas does not pass over the windings of the compressor. The return gas simply enters the compressor through the suction service valve on the side of the compressor. This gas enters the suction valve and cylinders right away without seeing any other heat source. If there is any liquid (refrigerant or oil) entrained in this suction gas, the valves and/or pistons/rods themselves can be seriously damaged.

This is not the case for refrigerant gas-cooled compressors. Liquid refrigerant coming back to the compressor must first pass around or through the motor windings. There is a good chance that the windings will be producing enough heat to vaporize any liquid refrigerant before it is sucked up through the suction cavities to the valve structures. Any refrigerant must travel in close proximity to the motor windings before it flows uphill and enters the valve structures and cylinders of the compressor.

The only sure way a service technician can tell if liquid refrigerant is coming back to the compressor (floodback) is to measure the compressor superheat at the compressor. Do not rely on a frost pattern on the compressor because it is not a reliable method. This can be accomplished by taking the evaporating pressure with a gauge set and converting it to a temperature with a pressure/temperature chart. Next, with a thermometer or thermistor, measure the compressor in temperature on the suction line about 6 inches from the compressor. The compressor in temperature should always be warmer than the evaporating temperature. If it is at the same temperature or colder, liquid refrigerant is probably present at the compressor. To figure compressor superheat, subtract the evaporating temperature from the compressor in temperature. The equation is: Compressor in Temperature minus Evaporating Temperature equals Compressor Superheat.

Publication date: 9/3/2012
 

Did you enjoy this article? Click here to subscribe to The NEWS Magazine

Recent Articles by John Tomczyk

You must login or register in order to post a comment.

Multimedia

Videos

Image Galleries

2014 MCAA Annual Convention

Scenes from the 2014 MCAA Annual Convention in Scottsdale, Ariz.

Podcasts

Kyle Gargaro, editor-in-chief of The NEWS, hosted the 2014 ACCA CEO Forum. At the event, six well-known, highly respected company executives, Gary Michel, Ingersoll Rand/Trane; Chris Nelson, Carrier Corp.; Chris Peel, Rheem Mfg. Co.; Rod Rushing, Johnson Controls; Brent Schroeder, Emerson Climate Technologies; and Doug Young, Lennox; provided individual industry outlooks and fielded questions directly from attending contractors. Listen to the entire event on the NEWSMakers podcast. Posted on April 14.

More Podcasts

THE MAGAZINE

ACHRNEWS

NEWS 04-14-14 cover

2014 April 14

Check out the weekly edition of The NEWS today!

Table Of Contents Subscribe

SERVICE CALLS POLL

Which statement on service calls best applies to your business?
View Results Poll Archive

HVACR INDUSTRY STORE

plumbing-hvac.gif
2014 National Plumbing & HVAC Estimator

Every plumbing and HVAC estimator can use the cost estimates in this practical manual!

More Products

Clear Seas Research

 

Clear Seas ResearchWith access to over one million professionals and more than 60 industry-specific publications, Clear Seas Research offers relevant insights from those who know your industry best. Let us customize a market research solution that exceeds your marketing goals.

DON'T MISS A THING

Magazine image
 
Register today for complete access to ACHRNews.com. Get full access to the latest features, Extra Edition, and more.

STAY CONNECTED

facebook icontwitter iconyoutube iconLinkedIn i con