R134a Low Subcooling, High Superheat

This is regarding the refrigeration system for a water cooler. The evaporator is immersed in the water to be cooled. The following information is available at 25 mins after starting the compressor:
When water temperature was 7.8 deg. C
Evaporator Inlet: 0.7 deg. C
Suction: 23 deg. C
Discharge: 72.6 deg. C
Condenser: 53.6 deg. C
Liquid Line: 54 deg. C
Compressor Power consumption: 100 W
Cooling capacity (averaged over a period of 20 mins): 62 W
Ambient: 35 deg. C

Note that steady state wasn't achieved yet. The cooler cuts off cooling beyond this point. Earlier, we thought that undercharging is the issue. But the condenser and evaporator pressures are high, and that doesn't support this theory. The throttling device is capillary tube and condenser is wire and tube type. Any suggestions on the possible reasons for this problem?

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Originally Posted by yash7

The cooler cuts off cooling beyond this point.

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Why does the cooler cut off?

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Originally Posted by yash7

Ambient: 35 deg. C

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This is the only problem I am seeing so far. The system seems to be working very hard because of the high ambient temperature.

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Originally Posted by yash7

But the condenser and evaporator pressures are high...

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This is not data, it is opinion. Why not tell us what the pressures are and let us decide if they are high or not?

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Originally Posted by yash7

R134a Low Subcooling, High Superheat

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Low subcooling is meaningless on a cap tube system.

When the water temperature is high, the superheat should be high. This is normal.

The cooler cuts off as the water cooler is designed to make sure that water temperature doesn't drop below 7 deg. C. The users don't prefer water colder than that.

Well, the pressures were not monitored during the test, due to some problems. However, from the evaporator and condenser temperatures, we can say that the evaporator pressure was nearly 3 bar and the condenser pressure was nearly 14 bar

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Originally Posted by Gary

Low subcooling is meaningless on a cap tube system.

When the water temperature is high, the superheat should be high. This is normal.

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I did not fully understand this. Why is low subcooling meaningless? Doesn't it mean that vapour can enter the throttling device and lower capacity significantly?

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Originally Posted by yash7

The cooler cuts off as the water cooler is designed to make sure that water temperature doesn't drop below 7 deg. C. The users don't prefer water colder than that.

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Throughout this thread you have said there is a problem. The system pulls down to the desired temperature and shuts off. So, where do you see a problem? What problem?

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Originally Posted by yash7

I did not fully understand this. Why is low subcooling meaningless? Doesn't it mean that vapour can enter the throttling device and lower capacity significantly?

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Yes. It limits the flow , presumably to what it should be. So what? From a diagnostic point of view low subcooling is meaningless.

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Originally Posted by yash7

Well, the pressures were not monitored during the test, due to some problems. However, from the evaporator and condenser temperatures, we can say that the evaporator pressure was nearly 3 bar and the condenser pressure was nearly 14 bar

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You say condenser temperature is 53.6C and liquid line temperature is 54C. This gives you negative subcooling which is not possible. So... you really don't know what the saturated pressure/temperatures are and you really don't know what the subcooling is.

Heat rejected by the condenser is heat absorbed by the evaporator plus heat of compression. If we look at the heat being rejected by the condenser, this gives us a rough indication of the heat being absorbed by the evaporator.

We can tell a lot by subtracting the ambient temperature from the saturated condensing temperature. This is the condenser TD.

Given standard sizing, a light heat load gives us a TD of about 11K and a heavy load gives us a TD of about 20K. Your numbers indicate 19K, but I suspect it is a little higher. In any case, it seems about right given a 35C ambient. The system is pumping a very heavy load.

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Originally Posted by Gary

Throughout this thread you have said there is a problem. The system pulls down to the desired temperature and shuts off. So, where do you see a problem? What problem?

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The problem is actually the COP. It is very less. The compressor is consuming about 100 W and the rate of cooling is very slow. The average cooling capacity is about 62 W and it took nearly 25 mins to pull down the temperature from 35 to 7 degrees C (0.8L water). As per the compressors' datasheet, for 54.4 deg. C condenser temperature, and -10 degrees C evaporator temperature, the power consumption is 100 W and the capacity is 135W. I had expected that at nearly the same condenser temperature, and higher evaporator temperature, the capacity should be even higher and that low/no subcooling, and the subsequent reduction in flow rate by the throttling device are lowering the capacity. Where am I going wrong?

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Originally Posted by Gary

You say condenser temperature is 53.6C and liquid line temperature is 54C. This gives you negative subcooling which is not possible. So... you really don't know what the saturated pressure/temperatures are and you really don't know what the subcooling is.

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I know that negative subcooling is not possible. I had attributed the higher value compared to condenser temp. to sensor accuracy. Thermocouples usually are accurate to +-1 deg. C. My point was that the amount of subcooling, in any case, is very low, or zero.

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Originally Posted by Gary

Given standard sizing, a light heat load gives us a TD of about 11K and a heavy load gives us a TD of about 20K. Your numbers indicate 19K, but I suspect it is a little higher. In any case, it seems about right given a 35C ambient. The system is pumping a very heavy load.

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Can you elaborate more on standard sizing? I am a novice in system design. One of the possible solutions thought of was that probably the condenser is not being able to dissipate enough heat to the surroundings, and therefore, providing larger area to the condenser might solve the problem.

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Originally Posted by yash7

One of the possible solutions thought of was that probably the condenser is not being able to dissipate enough heat to the surroundings, and therefore, providing larger area to the condenser might solve the problem.

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We can't tell because we don't know what the saturated condensing temperature is... and for the same reason, we don't know what the subcooling is... or the condenser TD is. We are just guessing, because you didn't measure the high side pressure. I have based my opinions on your stated condenser temperature. What if the actual saturation temperature is in fact higher?... perhaps much higher?

Nonetheless the system is functioning sufficiently to get the job done, despite high ambient temperature. The system is working fine, given the conditions. Why are you trying to fix what isn't broken?

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Originally Posted by Gary

Why are you trying to fix what isn't broken?

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The problem is the capacity. As I said, the rate of cooling is very low and unsatisfactory for the end user. We would ideally want the device to deliver a faster cooling even at a higher ambient. Hence, I need help in identifying what is lowering the capacity.

Can you confirm the suction pressure ; because 3 bar seems to be very high for 8 C water temperature.

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Originally Posted by yash7

The problem is the capacity. As I said, the rate of cooling is very low and unsatisfactory for the end user. We would ideally want the device to deliver a faster cooling even at a higher ambient. Hence, I need help in identifying what is lowering the capacity.

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For accurate evaluation, we need accurate measurements. And since we can't see the system, we need to know exactly how each of those measurements were obtained.

For example, you say the condenser temperature is 53.6C. We know this is not pressure converted to temperature, so what is it? Condenser surface temperature? Return bends?... middle?... bottom?... top?... inlet?... outlet? Center face? What?

And the liquid line temperature. Is this measured in an area where the line is surrounded by ambient air? Near the condenser outlet? Near the cap tube inlet? Somewhere in between?

We can't see it from here. We can only see your description and we must rely on your measurements.

3 bar would make the saturation temperature about 9C. How is that possible given 8C water temperature?