Refrigerant Blends

From John West

Chester, VA

My question involves units using blends and the proper recharge. Say you determine that you have a small leak and must pump the system down and repair the leak.

How would you determine whether to add back as a vapor or to remove the entire charge and weigh in as a liquid? I have heard at a few seminars that as long as too much has not leaked out, it would be OK to add back as a vapor. How much is too much?

Answer:

By Paul Reed

DuPont

Wilmington, DE

Small leaks of the refrigerant blends sold by DuPont can be topped off the same as for any other refrigerant you have used in the past. It is not necessary to remove the entire charge under most circumstances.

The only exception might be for a system that has remained idle for a long period of time and most of the refrigerant has leaked away. In this case, I would suggest that the remaining charge be removed, the leak repaired, and a fresh charge be placed in the unit. This will eliminate any capacity loss that might occur if the old remaining charge is left in.

You also asked, “How would you determine whether to add back as a vapor or to remove the entire charge and weigh in as a liquid?” In that case, we recommend that all our refrigerant blends should be removed from the shipping cylinder as a liquid to eliminate any composition change in the cylinder. Then if vapor is needed, the refrigerant can be flashed through the manifold gauges or a throttling valve.

We do not recommend removing vapor from our cylinders, since this can change the composition of the remaining refrigerant in the cylinder, affecting a later charge.

Normal refrigerant practices continue to be to repair the leak and top off the charge. There is no need to change this practice.

Freezing Time

From Larry Schlussler

Sun Frost

Arcata, CA

We manufacture refrigerator/ freezers for vaccine storage in the developing world. These units are powered by photovoltaics (solar panels). With these systems, efficiency is of prime importance.

Ice is made in half-liter plastic containers in the freezer section. The water in these containers often subcools to near 20?F before it freezes. This process decreases the efficiency of the cooling system and increases the freezing time.

Is there an additive we could put in the water to prevent subcooling?

Answer:

By Dan Kramer P.E.

Specialist Grade Member of RSES and Professional Engineer

My research suggests that the purest water is most resistant to freezing. In fact, there is a suggestion that quiescent demineralized water could be supercooled down well below 0?F before freezing occurs.

Therefore, as a first try, I would suggest adding about 1 weight percent sodium chloride (table salt) to your water. This amount should lower the actual freezing point only about 1?, but it might reduce the subcooling effect.

I also believe agitation will foster nucleation, so shaking the containers as they approach 32? might help.

Leaving the containers open and dropping an ice cube into the about-to-freeze water should start the freezing process.

Sharp metallic edges should foster the beginning of nucleation. Toward this end, I would cut up and put in your water some copper or stainless steel mesh fabric (pot cleaners?).

My son’s Handbook of Chemistry and Physics has a table that describes ice as having a hexagonal crystalline form. Aluminum oxide crystals, which are insoluble in water, are also described as having a hexagonal form.

The insoluble crystals would not have the effect of lowering the freezing point of the water, but might help to induce nucleation. On this track, you might contact a crystallo-grapher (such as at a large university near you) for a suggestion of an insoluble crystal that has a structure exactly matching the structure of ice.

New Refrigerants

From Josh Hendry

Huntsville, TX

A while ago, Dan Kramer noted that all azeotropes are in the 500 series of refrigerants and zeotropes are 400. I found that very helpful in keeping up with all of today’s new refrigerants. But I found one problem, and that is with R-410A. It is said to be an azeotrope, but it is not a 500 series refrigerant.

Answer:

By Dan Kramer P.E.

Specialist Grade Member of RSES and Professional Engineer

Here is a case in which ASHRAE and a refrigerant manufacturer have a difference of opinion.

ASHRAE has listed R-410A as a zeotrope (nonazeotrope) simply because there are very small differences between the bubble and dewpoints. Typically these differences are in the region of 0.21?F at 110? and 0.13? at -30?.

You can see that these differences are so small that, for all practical purposes, R-410A is an azeotrope, even though, by the ASHRAE definition, it is not.

Publication date: 08/06/2001