Understanding HVAC Superheat: Common Issues with Calculating Target Levels

[jelanier]

I was trying to calculate my target superheat when I ran into a bit of a problem. My indoor return wetbulb temperature is 60F and the outdoor ambient condenser temp is 90F. These figures apparently don't compute in a calculator application or a chart. With 60F wetbulb, the charts and calculators will only allow ambient condenser temperature of 80F. What is the explanation?

 

[russ_watters]

I'm not sure I'm quite following; those are two different airstreams/states. So the numbers don't get mixed together. So, what, exactly, are you trying to do?

 

[jelanier]

Hard to explain in a few words, but I will try. When charging the A/C with refrigerant the idea is to maximize efficiency while also protecting against the possibility of liquid refrigerant in the return line. The conditions change with temperature. The target superheat is based on an existing condition which takes into account what will happen as conditions change. When the target superheat is known, the suction pressure (converted to temp 'boiling') is compared with the measured temperature of the return line. This is the superheat value to be adjusted to equal the target value previously calculated.

Your response is exactly why I ask the question. The indoor wetbulb measurement depends on indoor return temperature and humidity of the indoor air. The ambient is the air temperature entering the condenser outdoors. The systems are obviously related. I am trying to understand the "limit" that I came across.

(Oh, I am not trying to do anything. I have already done it. I did it under conditions that were not mathematically problematic)

Thanks

 

[russ_watters]

Hard to explain in a few words, but I will try. When charging the A/C with refrigerant the idea is to maximize efficiency while also protecting against the possibility of liquid refrigerant in the return line. The conditions change with temperature. The target superheat is based on an existing condition which takes into account what will happen as conditions change. When the target superheat is known, the suction pressure (converted to temp 'boiling') is compared with the measured temperature of the return line. This is the superheat value to be adjusted to equal the target value previously calculated.

Ok...

Your response is exactly why I ask the question. The indoor wetbulb measurement depends on indoor return temperature and humidity of the indoor air. The ambient is the air temperature entering the condenser outdoors. The systems are obviously related. I am trying to understand the "limit" that I came across.

The indoor and outdoor conditions don't have to bear any relation to each other.

(Oh, I am not trying to do anything. I have already done it. I did it under conditions that were not mathematically problematic)

Maybe I'm missing something: what sort of chart are you plugging the numbers into? When you plug in the two numbers that work (what numbers?), what output does it give you? Can you provide a link to a sample chart? I assumed in my first post that it was a psychrometric chart...

This link may describe what you are trying to do:
http://www.achrnews.com/articles/92392-proper-use-of-superheat-measurements

Here's a sample chart:
http://web.fscj.edu/Mark.Bowman/handouts/Proper%20System%20Charging.pdf

The outdoor ambient temp tops out at 105F for this particular chart, but some of the values are blanked-off, including where 60 and 105 intersect. I think it is just telling you the conditions you are after are not achievable.

 

[jelanier]

Basically, there are 2 heat exchangers connected by refrigerant that changes states. The heat exchangers are in different environments. I am sure it would be possible for me to derive a suitable equation to find limits. The fact that I did measure temperatures that are theoretically out of range or "not achievable" indicates a slight flaw in the target superheat calculation. The problem is that it takes considerable time for the total system to reach equilibrium. As long as the ambient temperature is changing, equilibrium will not be achieved.

Again, I am not "trying" to do anything. I have already charged the system. I was looking for an explanation, but after thinking about it a bit, I think my equilibrium latency statement (above) explains why. (BTW, your links are on subject)

Thanks