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Understanding Superheat and Subcooling

(The content of this post is intended for consideration by trained service personnel only)

Superheat and subcooling are the terms used to describe two of a heat pump system’s operating characteristics. We, in the service business, generally rely on these numbers to evaluate system performance as well diagnose system problems. The values essentially provide us with information about what’s going on inside the evaporator and condenser coils. And depending on the metering device used in the system, one or the other number is the value used to determine optimum system charge.

If you wanted to define the words non-mathematically, superheat is the increase in temperature of the refrigerant vapor in the evaporator before it exits the coil, and subcooling is the decrease in temperature of the refrigerant liquid in the condenser before it exits the coil. The diagram below offers a visual illustration.

The two numbers are actually calculated temperature values, using simple arithmetic with saturated temperatures and tubing temperatures.

Normal operation always results in some percentage of the evaporator coil filled 100% with vapor and some percentage of the condenser coil filled 100% with liquid. Since the vapor starts out at the same saturated suction temperature, the vapor will take in heat or warm up, before it exits the evaporator coil. Likewise, the liquid starts out at the same saturated condensing temperature, so it will give up heat or cool down before exiting the condenser coil.

So to calculate the superheat, subtract the saturated suction temperature from the suction line temperature. Which in the diagram is 50 – 40 = 10 F superheat.

To calculate subcooling, subtract the liquid line temperature from the saturated condensing temperature: 110 – 100 = 10 F subcooling.

You can get more explanation of heat pump charging details in the refrigerant system video for rent:

Troubleshooting Heat Pump Refrigerant Systems

 
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Posted by on July 13, 2011 in Superheat/subcooling

 

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Charging TXV Systems

(The content of this post is intended for consideration by trained service personnel only)

Somewhere in a previous post there is some discussion and pics relative to charging heat pumps in the cool cycle. The method of charging depends on the metering device feeding the evaporator coil. Fixed orifice systems have to be charged by the superheat method, TXV systems by the subcooling method. The video below illustrates charging by the subcooling method…

You can get more explanation of heat pump charging details in the refrigerant system video for rent:

Troubleshooting Heat Pump Refrigerant Systems

 

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Replacing A Reversing Valve

(The content of this post is intended for consideration by trained service personnel only)

Replacing reversing valves is one of the more taxing repairs to heat pumps. I’m sure every tech has his preferred method for doing it, which probably depends on his personal level of confidence, skills and dexterity with a torch. The primary objective (for me) is getting the new valve in, without destroying the internal heat sensitive components. The valve body can only withstand temperatures approaching 250 F degrees…which ain’t much considering the 1400 F or so necessary to get a good braze joint.

And you have to consider in most cases, the valve is in a position where you can’t just “sweat” it out. There has to be some “slack” available and usually isn’t. Even if you have the special torch attachment that can heat all three tubing connections simultaneously, you still need room to pull the valve away from the tubing. Even if you manage that, you got to consider brazing in the new one without burning it up.

So, I usually study the “geometry” of the valve and try to cut the tubing in places I can easily access on the re-braze…Then, once you get the “assembly” out, you can heat the connections in whatever manner you prefer and remove the old valve.

Then, you only have to braze in the new valve one tube at a time, allowing you to use wet rags or whatever, to keep the valve body cool.

Once you complete that step, using swaged ends or couplings allows you to reconnect the cut joints.

 
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Posted by on May 12, 2011 in Reversing Valve

 

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R-410A System Pressures

(The content of this post is intended for consideration by trained service personnel only)

Heat pump system pressures are indirect measurements of saturated temperatures…the pressures are simply the result of the particular refrigerant used in the system. If you know what the saturated temperatures are supposed to be, you can easily determine the pressures.

Lets assume we’re connected to the R-22 heat pump system below running in the cool cycle, measuring pressures, tubing temps and calculating superheat and subcooling.

So, we’re looking at a head pressure of 225 psi, a suction pressure of 75 psi, liquid line temp @ 100F and suction line temp @ 55F. If we convert the head and suction pressures to saturated temperatures, the results are a condensing temp of 110F in the outdoor (condenser) coil and a boiling temp of 45F degrees for the liquid refrigerant in the indoor (evaporator) coil, giving us 10F degrees of subcooling, and 10F degrees superheat.

Now, suppose for the sake of discussion, we recover all the 22 refrigerant from the system, replace the oil, replace the 22 TXV with a 410A TXV, re-charge the system with 410A refrigerant, and start the equipment back up…Well, guess what? The system temperatures would be the same values (or at least near the same values). The only thing different would be the head and suction pressures.

We couldn’t actually convert a 22 system to 410A that simply…but you could have a 22 system and 410A system side by side, operating under the same indoor and outdoor conditions and see about the same operating temperatures. The gist of the post is to illustrate the fact that heat pumps (mechanical refrigeration systems) are designed to produce or generate, temperatures. The subsequent system pressures are simply the result of the saturated pressure-temperature relationships for a particular refrigerant at a particular temperature…

You can get a full explanation and illustrations of heat pump operating pressures and temperatures in the refrigerant system video for rent:

Troubleshooting Heat Pump Refrigerant Systems

 
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Posted by on May 3, 2011 in R410A Pressures

 

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Troubleshooting TXV Systems

(The content of this post is intended for consideration by trained service personnel only)

This short video illustrates the symptoms of some of the problems associated with TXV systems…can you diagnosis the numbers?

You can see a more in depth explanation of TXV operation and illustrated failures in the “Troubleshooting Heat Pump Refrigerant Systems” and “Troubleshooting TXV’s” rental videos:

Troubleshooting Heat Pump Refrigerant Systems

Troubleshooting TXV’s

 

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Superheat…or Subcooling?

(The content of this post is intended for consideration by trained service personnel only)

So, here’s the question: How do you charge fixed orifice and txv systems? Same way? No. Fixed orifice systems can only be charged via superheat. TXV systems have to be charged by the subcooling method. The explanations for the answers are related to how TXV’s do what they do and how fixed orifice devices do what they do. Fixed orifice devices deliver a rate of refrigerant flow dependent on the pressure differential across the orifice. TXV’s are active devices that “look” at the suction line temperature and evaporator temperature and maintain a fairly consistent superheat value…Following are illustrations of both. (You can “click” the pics for a slightly larger view)

Fixed Orifice System…R-22

I started a service call on this system knowing there’s a small leak and that the system needed topping off…

An early reading of suction pressure and superheat…

Then I got carried away…”0″ deg superheat.

Removed some refrigerant, and got a number that was about right for the particular indoor and outdoor conditions.

Charging fixed metering systems by superheat is a delicate process, requiring an indoor wetbulb temperature, outdoor drybulb temperature and some “tool” for determining the required superheat for the given system and conditions.

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TXV System…R-410A

This one is a new system start-up.

Early reading…subcooling top right value.

After adding a few ounces of 410A…

You can see a complete explanation of superheat and subcooling in the “Troubleshooting Heat Pump Refrigerant Systems” rental video:

Troubleshooting Heat Pump Refrigerant Systems

 
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Posted by on April 21, 2011 in Charging Heat Pumps

 

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Overcharged System

(The content of this post is intended for consideration by trained service personnel only)

I was called on this unit, found some electrical problems and noticed the compressor sounded not quite right. On checking the compressor amp draw, it was running right at RLA. I messed with it a while and finally connected the gauges and temp sensors. What you’re seeing are the system pressures, saturated temps and liquid line temp. The net result is 50 degrees subcooling.

You can see an explanation and demonstrations of all the common refrigerant system failures in the “Troubleshooting Heat Pump Refrigerant Systems” rental video.

Troubleshooting Heat Pump Refrigerant Systems

(The content of this post is intended for consideration by trained service personnel only)

 
 
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