Proper Charging with Sub-cooling Revisited

Sub-cooling seems to be a great mystery to a lot of service techs in the field yet, any unit with a TXV type metering device, should be charged by sub-cooling instead of super-heat. Since a TXV has a pre-set super-heat (or an adjustable super-heat) setting, it will always try to maintain that super-heat even if the unit is over or under charged. This is why charging by sub-cooling is important for units equipped with TXV’s.

Let’s review the process of sub-cooling first, and then see how to charge by sub-cooling. The compressor puts out HOT HIGH PRESSURE VAPOR as part of the compression process. This vapor enters the condenser coil, which performs the function its name states, to turn the vapor into a liquid. As the vapor travels through the condenser, heat is removed and the vapor starts to turn to liquid. At some point in the coil, the vapor has been turned to all liquid. This is called saturated liquid and corresponds to the temperature and pressure you read on your gauges. This usually occurs before all the passes in the coil have been used. These extra passes through the coil take the saturated liquid and removes even more heat, or, sub-cools the refrigerant. This is important, since a metering device must have a continuous column of liquid to operate properly. By sub-cooling the liquid before it leaves the condenser, we “guarantee” that the liquid will remain liquid all the way to the TXV. If there is insufficient sub-cooling, the liquid can turn back to a gas (flash gas) and the metering device will not properly meter the refrigerant into the coil.

Now that we understand how sub-cooling is supposed to work, we can now proceed with how to charge by sub-cooling. As stated earlier, when your gauge set is attached to the unit, the “head” or liquid pressure you read is the saturated liquid temperature/pressure. Knowing this, and the manufacturer’s recommended sub-cooling, you can properly charge the unit. You need to strap a thermometer or thermocouple to the liquid line outside the condenser so you can read its temperature and then add or remove charge to get the desired sub-cooling for that unit.

Let’s look at an example. If you had a “head pressure” of 198 PSIG and looked at a temperature/pressure chart, you would see that the saturated temperature is 100 degrees. If the unit required 10 degrees of subcooling, the thermocouple attached to the liquid line should read 90 degrees (10 degrees “colder” than saturated temperature). Let’s say the thermocouple reads 101 degrees. Do I add or remove refrigerant to get the 90 degrees temperature I need? The key to answering this is to remember that the closer to ZERO or temperatures above the saturated temperature, indicates an undercharge. Think of it this way, since the unit is short of refrigerant, the discharge temperature of the compressor increases because there isn’t sufficient cooling of the compressor. Since the discharge temperature is higher, it will require more of the condenser coil to remove the heat of compression, which causes little or no subcooling. So, we need to add refrigerant to cool the compressor to lower the discharge temperature to get our desired subcooling.

Conversely, if the liquid line were 80 degrees, this would be 20 degrees of subcooling, which is an overcharge. The compressor is cold, the discharge temperature is down, the condenser does not have to remove as much heat, so the refrigerant reaches saturated temperature quicker leaving more coil passes for subcooilng. In order to get the 10 degrees desired sub-cooling, we would need to recover refrigerant to raise the compressor discharge temperature to require more of the condenser to be used to reach saturated temperature leaving us with the proper amount of passes to sub-cool the saturated liquid.

If you keep this in mind, properly charging a unit by sub-cooling will become very easy.


About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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4 Responses to Proper Charging with Sub-cooling Revisited

  1. rjp says:

    Excellent summary of sub-cooling. Now if we could get everyone working on units with txvs to charge by it. I think some feel they need fancy digital gauges to calculate sub-cooling when a simple temperature probe would allow accurate charging.

    • Thank you! — It is good to know when people appreciate an effort.
      I agree — digital gauges are nice but super-heat and sub-cooling calculation have been being done even before I started in this trade 48+ years ago. Problem really is that even if they have the fancy gauges– wil they take the time to actually check it?

  2. I find problems are often compounded. If a drier is not known to be restricted, the last tech adds refrigerant, overcharging the unit. The drier and liquid line are in a very difficult position to get temperature probes on. The sight glass, just after the drier, is still flashing, which you can see from a small hole in the non-removeable panel. This happened to me recently. I’m guessing subcooling and superheat often don’t get measured because of the hassle to attach probes. The fancy gauges have big clamps that do not get in some places. Panels and piping configurations make taking good measurements very time consuming.

    • Kevin
      very true but a ‘good tech’ will take the time to do it right the first time. I know I’m preaching. Other cluse are just in the pressures. With a high side restriction both head and suction pressures are going to show something. As long as the suction line is acessable somewhere, a restriction would show as high super-heat. Even with a txv, super-heat can help diagnose a problem (see posts on TXV’s).

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