What Is Liquid Subcooling?

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1 VA Lake City Florida Rob, Though I still haven t received the information from the contractor for this project, I have done some preliminary work on it. One thing of concern to me is something that you may recall that I brought up in a recent telephone conversation with you the potential loss of liquid subcooling due to the vertical rise. I estimate that the total vertical rise will be about 35, and before getting into the specifics, allow me to use this letter to you as a bit of a forum to explain the process (I might be able to use part of this in a tutorial at some point. ) Your indulgence is appreciated. What Is Liquid Subcooling? The terms superheat and subcooling seem to be two of the most misunderstood terms in our industry. Why this is - is beyond me given they are really quite simple concepts. I will leave superheat to another discussion, but subcooling very simply is the removal of enough heat from a liquid that its temperature is lowered. That s it! Water in a glass is a subcooled liquid. Why Care About Subcooling In A Refrigeration System? There are really two different discussions that the answer to this question might go into, but for this dialogue I will dwell on only one part maintaining a solid column of liquid at the metering device. Page 1 of 10

2 Let s consider what happens at the metering device in a refrigeration system as liquid refrigerant is passed through it. The first thing that happens is that the high pressure liquid is reduced in pressure as it passes across the orifice of the metering device (typically thermostatic expansion valves are the most common metering devices used in our applications, although this statement is still true in a fixed orifice system as well). How does an evaporator coil remove heat from the surrounding air? It s really quite simple: The consequence of changing the liquid refrigerant to a vapor termed the latent heat of evaporation is how. By lowering the pressure at the metering device, this allows the liquid to exist in a fine mist form, although it is still a liquid. As heat passes across this coil, this mist can very readily boil (evaporate, hence the term evaporator) and this action of the refrigerant changing phase from a liquid to a vapor produces the net refrigeration effect. That said, if there is not a solid column of liquid at the inlet of the metering device (that is to say that it is 100% liquid) then obviously the capacity of the evaporator coil will be reduced. Page 2 of 10

3 Subcooling can be lost either of two ways: By an increase in the temperature of the liquid, or a decrease in the pressure of the liquid. If subcooling in a refrigeration system is lowered to the point that there is none, any additional pressure loss or temperature increase will cause the liquid to flash, or quickly boil because the liquid cannot exist at that condition. This then will create partial liquid/partial vapor at the inlet of the metering device, and the capacity of the evaporator coil will diminish. Vertical Lift And Its Consequences The phenomena that I am referring to has only to do with gravity; it is independent of line size as it is strictly the weight of the column of liquid at the bottom of a vertical section of tubing. Page 3 of 10

4 Though this doesn t follow exactly the same laws of physics in a static environment, the results are actually very similar, and the following applies: The illustration above shows the effect in a static environment and while velocity will change the results in a dynamic one, this can be used as a reasonable model to quantify how much additional pressure a vertical column of liquid in a refrigeration system s liquid line will exert. The chart on the next page summarizes this. Page 4 of 10

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6 Where Does The Subcooling Come From And How Much Does A Typical Refrigeration System Have? Subcooling in a refrigeration system is a result of the refrigerant passing through the condenser. As the high-pressure discharge vapor is passed through the condenser s tubes, it is changed from a vapor to a liquid. As this liquid progresses through the bottom of condenser s coil, the additional heat being removed results in a lowering of the temperature of the liquid thus, subcooling. Normally speaking, the condenser will produce from 5 F. to 8 F. of subcooling at the exit point of the condenser (unless a specific subcooling circuit is added). What isn t often considered, however, is what happens to the liquid after it leaves the condenser. As it passes through the various devices and vessels, both its pressure is reduced and sometimes its temperature is increased. An increase in the liquid temperature is very common when, for example, the liquid line is routed through a warm area such as a ceiling (it is particularly worse in the winter, not the summer, as the building s heat tends to be trapped in the ceiling areas). Subcooling Requirements For This Project Based on an anticipated 35 liquid lift for our application here, as you can see from the chart, I am suggesting that we need to have at least F. of liquid subcooling to offset for the loss in pressure that will occur as a result of the vertical lift. It would be impractical to think that the condenser can produce this amount of subcooling, or for that matter, that the subcooling would be maintained all the way to the evaporator inlets. Increasing Liquid Subcooling So then, how do we go about increasing the subcooling in the liquid line? Without a doubt, mechanical subcooling (using a refrigeration system to refrigerate the refrigerant ) is by far the overall best, although it is also quite expensive. Page 6 of 10

7 Although I am not usually an advocate of the use of liquid-to-suction heat exchangers, they will provide additional subcooling, and that is my recommendation for this project. A liquid-to-suction heat exchanger is a device that allows the cold suction line to cool the liquid by passing each through a tube-in-shell heat exchanger, as shown in the excerpt from one of the manufacturer s submittal sheet, and following that, my own conceptual illustration: The ill effect of using these is that we have to account for the pressure loss in the suction line passing through the heat exchanger, and the fact that the heat lost from the liquid is added to the suction vapor. In our situation, neither of these are a problem because of the sizing of the ParaLoop refrigeration system and the fact that the capacity control valves include a de-superheating TEV which will act to cool the suction vapor adequately to offset for heat added by the heat exchangers. Since our application is one in which each docking station can be operated independent of the others (or in any combination), the best design would be to incorporate one of these per branch line in other words, one per docking station. Page 7 of 10

8 My reservation in suggesting this, however, is because their smallest ones are vastly too large for our requirements and would unduly heat the suction vapor. The other concern is the space requirements, particularly when it comes to the seven docking stations that are to be located on the island wall. What I am, therefore, suggesting is that we include one of these heat exchangers per main circuit (each circuit that is, each loop controls seven docking stations). The sizing of this is something that I want to consult the manufacturers on. They should also be able to tell me what they anticipate for the refrigerant line loss and how much heat will be added to the suction vapor. I have included a conceptual illustration on the following page showing how I visualize these may be installed. Page 8 of 10

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10 Please give this some thought and let me know how to proceed, and thanks again for taking time to read through this. Best regards, Frank L. Smith Applications Engineer Maynard-Fixturcraft, Inc. Nashville, Tennessee Direct Phone: (615) Direct Fax: (615) Page 10 of 10