Service Hotline - 01/08/2001: Charge It and Wet Systems

ANSWER:

By Paul Reed DuPont Wilmington, DE

Systems with sight glasses can be charged with refrigerant until the sight glass shows only a few small bubbles. Continuing to add more refrigerant will typically overcharge the system without ever seeing an elimination of the bubbles in the sight glass. This is caused by a number of things, but primarily the new refrigerants have lower liquid densities than CFCs. Therefore, a small amount of bubbling may occur, since there is less liquid weight above the sight glass for a given height of liquid in the liquid line.

If a lot of bubbling occurs when you suspect the charge is correct, then look at whether you can increase subcooling. Lack of subcooling will also cause flashing in the liquid line.

In addition, check the suction superheat, as well as subcooling. If both are similar to expected values for the old refrigerants, do not continue to charge.

If a system has a receiver, charge the receiver to its normal operating level. This will automatically take care of the density differences between old and new refrigerant.

If you do know the refrigerant charge size, then it is best to start with about 75% of that weight if charging any new refrigerant. Ideal charge sizes vary somewhat based on several factors, but generally they fall into the 80%-to-90% of CFC charge weight range.

Also always remember to remove liquid from the refrigerant cylinder. You can flash this liquid in the manifold gauges if you need to add vapor. This eliminates any composition change.

This is a lot to remember, but a lot of this is true for any refrigerant, not just the new ones. Retrofit guidelines for DuPont refrigerants can be found at www.suva.com.

Regarding your question on charging with fixed metering devices, I assume you are talking about orifice plates or capillary tubes. Use the sight glass if there is one. Of course, many small hermetic systems do not have a sight glass. If there is no charge listed anywhere on the device, my best recommendation is to contact the equipment manufacturer and determine the best charge estimate. It is difficult to estimate the correct charge without some visual indication.

Wet Systems

From Michael Dennis Catonsville, MD

Could someone give me an explanation of what a freeze tap is? How is it used? Also, could you explain to me how an evaporator becomes oil logged? Is there a way to determine if this has occurred if the system is a sealed one? Can this problem be prevented?

ANSWER:

By Daniel Kramer, P.E. Patent Attorney and former chief engineer for Kramer-Trenton

When you have a very wet system, the vacuum pump removes the air and refrigerant first, then it pumps only water. The water tends to contaminate the oil and could, in time, damage the vacuum pump. To keep much of the water from getting into the vacuum and to speed up moisture removal, it is desirable to install a cold trap teed off from the line from the system to the vacuum pump. The cold trap can be any kind of vessel, such as a small cylinder or even a small drier with one end capped.

Take a vacuum-insulated ice bucket and lay in a couple of inches of dry ice (-110°F). Then pour in a couple of inches of acetone (-140°F). Yes, ethyl alcohol (-179°F) can be used, but it should be at least 95.5%. This can be obtained in some liquor stores as pure distilled spirits. Pure methyl alcohol (-144°) will also work. Then simply immerse the lower part of the drier into the freeze mixture. Replenish the dry ice as necessary.

The extreme cold will attract and condense the moisture from the system while the vacuum pump is working.

With respect to your oil-logged evaporator, I think that you are observing some problems and attributing it to oil logging. Is the evaporator starved (warm suction)? Is it flooding old (cold or frosted suction)? It is best to observe what is happening and report those finding in detail for assistance.

In large low-temperature systems, evaporators become oil logged sufficiently to rob the compressor of enough oil to cause trip-out when the evaporator is starved by incorrect TXV adjustment or shortage of charge.

Publication date: 01/08/2001