Why did the superheat go up as I added refrigerant?

[fourthindiana]

TL;DR Summary

Normally when one adds refrigerant to an air-conditioner, the superheat goes down. A few days ago, I added refrigerant to an air-conditioner, and the superheat went up. Why did the superheat go up when I added refrigerant?

On Friday I worked on a straight air-conditioner (A/C) package unit on the roof of an industrial facility. The straight A/C uses R-22 refrigerant. The straight A/C has a fixed orifice metering device. The compressor of the straight A/C was cold and had condensation on it. The dataplate for the A/C says that the factory charge of refrigerant is 9.1 pounds. When I first checked the evaporator superheat on the A/C, the superheat was 14 degrees. I added a pound of refrigerant to the A/C, and the evaporator superheat increased from 14 degrees to 15 degrees. Then I added another pound of refrigerant to the unit, and the evaporator superheat increased from 15 degrees to 17 degrees.

Here are some statistics of the situation immediately before I added refrigerant to the A/C on Friday:
Suction Pressure: 52 psig
Head pressure: 257 psig
Evaporator Superheat: 14 degrees fahrenheit
Subcooling: Approximately 30 degrees fahrenheit
Outside ambient temperature: 88 degrees fahrenheit
Delta T between return and supply: 16 degrees fahrenheit
Compressor superheat: 17 degrees fahrenheit
Actual compressor amp draw: 10.2 amps
RLA of Compressor: 9.7 amps

After I added a pound of refrigerant, the head pressure increased to 270 psig. The suction pressure stayed at 52 psig. The compressor superheat dropped from 17 degrees to 16 degrees.

Why did the evaporator superheat increase both times that I added refrigerant to the air-conditioner?

 

[AZFIREBALL]

Looks like it is over charged.

 

[PeterDonis]

The dataplate for the A/C says that the factory charge of refrigerant is 9.1 pounds.

Ok. So how much charge did you think was actually in the system before you added refrigerant? If there was already 9.1 pounds or more, there was no reason to add refrigerant.

 

[fourthindiana]

Ok. So how much charge did you think was actually in the system before you added refrigerant? If there was already 9.1 pounds or more, there was no reason to add refrigerant.

I don't definitely know how much charge was actually in the system before I added refrigerant. I thought that there was more than 9.1 pounds in the system before I added refrigerant because the compressor was cold and sweating. I only added refrigerant to the system because my boss instructed me to add refrigerant to the system.

Do you have any thoughts on why the superheat went up as I added refrigerant?

 

[PeterDonis]

I only added refrigerant to the system because my boss instructed me to add refrigerant to the system.

Ah, ok. Otherwise my next question would have been "why did you add refrigerant if you thought there was already at least 9.1 pounds?"

Do you have any thoughts on why the superheat went up as I added refrigerant?

As @AZFIREBALL said, the obvious conclusion is that the system is overcharged. If there's already enough refrigerant charge in the system, adding refrigerant doesn't improve performance; all it does is increase temperatures and pressures and put more stress on the system. Maybe you can think of a way to politely suggest that to your boss.

 

[fourthindiana]

As @AZFIREBALL said, the obvious conclusion is that the system is overcharged. If there's already enough refrigerant charge in the system, adding refrigerant doesn't improve performance; all it does is increase temperatures and pressures and put more stress on the system. Maybe you can think of a way to politely suggest that to your boss.

It was my first week working for him. Tomorrow will be the start of my second work week. I thought it would be presumptuous of me to start arguing with my boss about this.

 

[fourthindiana]

As @AZFIREBALL said, the obvious conclusion is that the system is overcharged. If there's already enough refrigerant charge in the system, adding refrigerant doesn't improve performance; all it does is increase temperatures and pressures and put more stress on the system. Maybe you can think of a way to politely suggest that to your boss.

So you don't know the answer to my question?

 

[PeterDonis]

So you don't know the answer to my question?

Huh? Both of us have given you our best answer.

 

[fourthindiana]

Huh? Both of us have given you our best answer.

Saying that the system is overcharged does not tell me why the superheat went up when I added refrigerant.

 

[PeterDonis]

Saying that the system is overcharged does not tell me why the superheat went up when I added refrigerant.

The superheat goes up when you add refrigerant because the system is overcharged. The superheat going up when you add refrigerant is telling you that the system is overcharged.

 

[fourthindiana]

The superheat goes up when you add refrigerant because the system is overcharged. The superheat going up when you add refrigerant is telling you that the system is overcharged.

My apologies. I did not know that you were saying that the superheat went up because the system is overcharged. I thought that your saying that the system is overcharged was just an incidental fact you were telling me, not the cause of the high superheat. I'm fortunate to have you helping me.

Ut oh. The system must be severely overcharged then because I added 2 pounds, and then my co-worker added another 1 pound & 13 oz. I'm not sure what to do about this. If I tell my boss about this, my boss might blame me for overcharging the system, even though my boss told me to add refrigerant. Sure, it is my boss' fault, but my boss does not have to be fair with me. My boss owns the company, which makes him a dictator. If I tell him about how the system is severely overcharged, my boss might blame me for overcharging the system (even though he told me to add refrigerant) or he might take it like I am questioning his competence and be offended that I'm questioning his competence and fire me.

Anyway, how does the system being overcharged cause the superheat to go up when I add refrigerant? In my thread asking how high charge level causes high subcooling, you told me that low charge level causes low superheat because the refrigerant gets subcooled less in the condenser which makes the refrigerant enter the evaporator at a warmer temperature, which leads to more superheat.

 

[AZFIREBALL]

It has to do with the balance of refrigerant within the system; between the low and high side.

Low superheat with high sub-cooling generally indicates over charge.
Too much liquid refrigerant in both sides.

High superheat with low condenser sub-cooling generally indicates an under charge.
Not enough liquid on either side.

 

[PeterDonis]

The system must be severely overcharged then because I added 2 pounds, and then my co-worker added another 1 pound & 13 oz. I'm not sure what to do about this.

Actually, of the indicators you gave, the head pressure going up when you added a pound of refrigerant, to me, is better evidence of overcharging than the superheat. If head pressure is going up under constant load, that's an indication that the extra charge is not doing you any good as far as performance is concerned.

Also, you said the outside temperature was 88 F, but how humid was it? What kind of load was the system under, comparatively? Relatively high or relatively low?

Also, the suction pressure you give is 52 psig, which corresponds to a saturation temperature of 28 F. That seems a bit low. Is it possible that there is some freezing on the evaporator coils?

how does the system being overcharged cause the superheat to go up when I add refrigerant? In my thread asking how high charge level causes high subcooling, you told me that low charge level causes low superheat because the refrigerant gets subcooled less in the condenser which makes the refrigerant enter the evaporator at a warmer temperature, which leads to more superheat.

I think you mean "high" instead of "low" where I bolded it in the quote above.

It's actually not as simple as "in an overcharged system, adding charge will cause superheat to go up". A better way of putting it might be that the system is losing the ability to control superheat. Under certain conditions, an overcharged system can actually have the superheat drop to zero or below, i.e., liquid refrigerant can be coming out of the evaporator into the suction line. If it gets into the compressor, it can severely damage it (this is called liquid slugging).

I know it's hard in a new job where you don't know what the boss is like (and you have at least one discouraging indication, that he told you to add refrigerant to a system that didn't need it). I would expect the system's performance to be at least slightly degraded under these conditions. Is there a way you can measure that and give that as a reason to take another look?

 

[AZFIREBALL]

BTW - PeterDonis
If "Nature cannot be fooled." -- Richard Feynman
Then, why are there transgender people?

 

[fourthindiana]

Also, you said the outside temperature was 88 F, but how humid was it? What kind of load was the system under, comparatively? Relatively high or relatively low?

Based on the fact that I remember it being uncomfortably hot for me at the time, I think that it was fairly humid at the time. I cannot give you an exact number though. I know that part of what the air conditioner cools is a break room that is approximately 1,000 square feet. There was somewhere between 10 to 20 people in the breakroom at the time that I took these measurements. I don't know whether or not the A/C cools more than just the breakroom.

Also, the suction pressure you give is 52 psig, which corresponds to a saturation temperature of 28 F. That seems a bit low. Is it possible that there is some freezing on the evaporator coils?

Yes, it is possible. I looked for freezing on the outside of the evaporator coils, and I did not see any freezing. But it's still possible that there was freezing that I did not see.

I think you mean "high" instead of "low" where I bolded it in the quote above.

You are correct.

It's actually not as simple as "in an overcharged system, adding charge will cause superheat to go up". A better way of putting it might be that the system is losing the ability to control superheat. Under certain conditions, an overcharged system can actually have the superheat drop to zero or below, i.e., liquid refrigerant can be coming out of the evaporator into the suction line. If it gets into the compressor, it can severely damage it (this is called liquid slugging).

I don't understand what you mean when you say that the system is losing the ability to control superheat. No air-conditioner has any components that control superheat unless the a/c has a TXV.

In my thread asking how high charge level causes high subcooling, you told me that low charge level causes high superheat because the refrigerant gets subcooled less in the condenser which makes the refrigerant enter the evaporator at a warmer temperature, which leads to more superheat. So if low charge level causes high superheat, how can high charge level cause superheat to increase?

I know it's hard in a new job where you don't know what the boss is like (and you have at least one discouraging indication, that he told you to add refrigerant to a system that didn't need it). I would expect the system's performance to be at least slightly degraded under these conditions. Is there a way you can measure that and give that as a reason to take another look?

Not really. This straight A/C is at a factory. I work for an HVAC company, not the factory itself. The factory is a long way from my HVAC company.

 

[Nik_2213]

I'm not familiar with HVAC: Is there a test port where you could connect a pressure gauge ?

 

[PeterDonis]

If "Nature cannot be fooled." -- Richard Feynman
Then, why are there transgender people?

Off topic.

 

[PeterDonis]

I don't understand what you mean when you say that the system is losing the ability to control superheat. No air-conditioner has any components that control superheat unless the a/c has a TXV.

Just that the behavior of superheat becomes less predictable with changes in load. See below.

if low charge level causes high superheat, how can high charge level cause superheat to increase?

We should first clarify what "low charge" and "high charge" mean. An orifice tube system will have what is called a critical charge; "low charge" typically means that the charge is below critical, which is not desirable since performance will be bad. This is the range we were discussing before: in this range adding charge decreases superheat, and decreasing superheat means decreasing evap out air temperature, which is what the system is supposed to be doing, cooling the air. So adding charge up to the critical charge basically means more cooling. In fact, if charge is low enough there will be no cooling at all--evap out air will be at or even higher than ambient. (In the previous discussion we saw that this is because with a low enough charge the refrigerant never even condenses at all; the compressor can't compress it enough to get the saturation temperature above ambient.)

Above critical charge there is a range of charges over which superheat and head pressure remain roughly constant as you add charge, and that's the range you want to be in for best performance; typically the design charge (9.1 pounds for the system you describe) is towards the higher end of that range, the idea being to give some reserve to account for leakage while still keeping the charge above critical and keeping performance basically the same. Without having the manufacturer's test data for the system to see how the 9.1 pounds design charge was determined, I would guess that critical charge for this system is perhaps 7.5 pounds, with 1.6 pounds of reserve charge designed in. But that's just a guess.

"High charge" or "overcharged" typically means the charge is above the design charge, which is not desirable: above the design charge, head pressure will typically start to rise and the wear on the compressor increases for no compensating gain in performance. That's why I said the increase in head pressure looked like a better indicator of overcharge.

What happens to superheat in an overcharged condition is less predictable from simple rules of thumb; you need to look at the specific numbers for a particular case. Let's do that for your case,, using the properties of R-22 refrigerant to infer other key numbers. We'll use the table of properties here:

https://wcec.ucdavis.edu/wp-content/uploads/2012/08/DuPont-R22-thermo_prop.pdf
Here are the numbers you gave for before you added a pound of refrigerant:

Suction Pressure: 52 psig
Head pressure: 257 psig
Evaporator Superheat: 14 degrees fahrenheit
Subcooling: Approximately 30 degrees fahrenheit
Compressor superheat: 17 degrees fahrenheit

Now, let's look up the suction and head pressures in the table to get the saturation temperature; I get

28 F for 52 psig
119 F for 257 psig

That means the evaporator temperature would have been 28 + 14 = 42 F, the compressor out temperature would have been 119 + 17 = 136 F, and the condenser out temperature would have been 119 - 30 = 89 F.

And here are the numbers you gave for after you added the first pound of refrigerant:

Suction Pressure: 52 psig
Head pressure: 270 psig
Evaporator Superheat: 15 degrees fahrenheit
Subcooling: Not given, but assumed to be similar to above
Compressor superheat: 16 degrees fahrenheit

Looking up the new head pressure gives

123 F for 270 psig

So now the evaporator temperature would have been 28 + 15 = 43 F, the compressor out temperature would have been 123 + 16 = 139 F, and the condenser out temperature would have been 123 - 30 = 93 F.

Do you see what happened? Because of the head pressure increase, the high side saturation temperature went up by 4 degrees. But the suction pressure stayed the same, so the evaporator saturation temperature stayed the same; but now the refrigerant coming into the orifice tube was 4 degrees warmer, which translated into higher evaporator out superheat.

Another way to look at it is that, with the liquid coming into the orifice tube 4 degrees warmer, after expansion the mixed saturated liquid/vapor inside the evaporator would have started further along in the phase change, i.e., higher quality. That means less heat transfer during the phase change and more for superheat.

 

[fourthindiana]

Just that the behavior of superheat becomes less predictable with changes in load. See below.
We should first clarify what "low charge" and "high charge" mean. An orifice tube system will have what is called a critical charge; "low charge" typically means that the charge is below critical, which is not desirable since performance will be bad. This is the range we were discussing before: in this range adding charge decreases superheat, and decreasing superheat means decreasing evap out air temperature, which is what the system is supposed to be doing, cooling the air. So adding charge up to the critical charge basically means more cooling. In fact, if charge is low enough there will be no cooling at all--evap out air will be at or even higher than ambient. (In the previous discussion we saw that this is because with a low enough charge the refrigerant never even condenses at all; the compressor can't compress it enough to get the saturation temperature above ambient.)

Above critical charge there is a range of charges over which superheat and head pressure remain roughly constant as you add charge, and that's the range you want to be in for best performance; typically the design charge (9.1 pounds for the system you describe) is towards the higher end of that range, the idea being to give some reserve to account for leakage while still keeping the charge above critical and keeping performance basically the same. Without having the manufacturer's test data for the system to see how the 9.1 pounds design charge was determined, I would guess that critical charge for this system is perhaps 7.5 pounds, with 1.6 pounds of reserve charge designed in. But that's just a guess.

"High charge" or "overcharged" typically means the charge is above the design charge, which is not desirable: above the design charge, head pressure will typically start to rise and the wear on the compressor increases for no compensating gain in performance. That's why I said the increase in head pressure looked like a better indicator of overcharge.

What happens to superheat in an overcharged condition is less predictable from simple rules of thumb; you need to look at the specific numbers for a particular case. Let's do that for your case,, using the properties of R-22 refrigerant to infer other key numbers. We'll use the table of properties here:

https://wcec.ucdavis.edu/wp-content/uploads/2012/08/DuPont-R22-thermo_prop.pdf
Here are the numbers you gave for before you added a pound of refrigerant:

Suction Pressure: 52 psig
Head pressure: 257 psig
Evaporator Superheat: 14 degrees fahrenheit
Subcooling: Approximately 30 degrees fahrenheit
Compressor superheat: 17 degrees fahrenheit

Now, let's look up the suction and head pressures in the table to get the saturation temperature; I get

28 F for 52 psig
119 F for 257 psig

That means the evaporator temperature would have been 28 + 14 = 42 F, the compressor out temperature would have been 119 + 17 = 136 F, and the condenser out temperature would have been 119 - 30 = 89 F.

And here are the numbers you gave for after you added the first pound of refrigerant:

Suction Pressure: 52 psig
Head pressure: 270 psig
Evaporator Superheat: 15 degrees fahrenheit
Subcooling: Not given, but assumed to be similar to above
Compressor superheat: 16 degrees fahrenheit

Looking up the new head pressure gives

123 F for 270 psig

So now the evaporator temperature would have been 28 + 15 = 43 F, the compressor out temperature would have been 123 + 16 = 139 F, and the condenser out temperature would have been 123 - 30 = 93 F.

Do you see what happened? Because of the head pressure increase, the high side saturation temperature went up by 4 degrees. But the suction pressure stayed the same, so the evaporator saturation temperature stayed the same; but now the refrigerant coming into the orifice tube was 4 degrees warmer, which translated into higher evaporator out superheat.

Another way to look at it is that, with the liquid coming into the orifice tube 4 degrees warmer, after expansion the mixed saturated liquid/vapor inside the evaporator would have started further along in the phase change, i.e., higher quality. That means less heat transfer during the phase change and more for superheat.

Excellent post. Thank you.