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24-06-2016, 11:34 AM #1
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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24-06-2016, 01:41 PM #2
Re: R134a Low Subcooling, High Superheat
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24-06-2016, 01:53 PM #3
Re: R134a Low Subcooling, High Superheat
Last edited by Gary; 24-06-2016 at 02:11 PM.
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24-06-2016, 02:10 PM #4
Re: R134a Low Subcooling, High Superheat
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24-06-2016, 02:18 PM #5
Re: R134a Low Subcooling, High Superheat
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25-06-2016, 08:34 AM #6
Re: R134a Low Subcooling, High Superheat
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.
Last edited by yash7; 25-06-2016 at 08:50 AM.
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25-06-2016, 08:36 AM #7
Re: R134a Low Subcooling, High Superheat
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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25-06-2016, 08:38 AM #8
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25-06-2016, 12:52 PM #9
Re: R134a Low Subcooling, High Superheat
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25-06-2016, 12:59 PM #10
Re: R134a Low Subcooling, High Superheat
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25-06-2016, 01:13 PM #11
Re: R134a Low Subcooling, High Superheat
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25-06-2016, 01:39 PM #12
Re: R134a Low Subcooling, High Superheat
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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26-06-2016, 04:27 PM #13
Re: R134a Low Subcooling, High Superheat
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?
Last edited by yash7; 26-06-2016 at 04:38 PM.
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26-06-2016, 04:30 PM #14
Re: R134a Low Subcooling, High Superheat
Last edited by yash7; 26-06-2016 at 04:39 PM.
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26-06-2016, 04:33 PM #15
Re: R134a Low Subcooling, High Superheat
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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27-06-2016, 12:38 AM #16
Re: R134a Low Subcooling, High Superheat
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?Last edited by Gary; 27-06-2016 at 12:51 AM.
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27-06-2016, 05:23 AM #17
Re: R134a Low Subcooling, High Superheat
Last edited by yash7; 27-06-2016 at 05:49 AM.
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27-06-2016, 11:28 AM #18
Re: R134a Low Subcooling, High Superheat
Can you confirm the suction pressure ; because 3 bar seems to be very high for 8 C water temperature.
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27-06-2016, 02:41 PM #19
Re: R134a Low Subcooling, High Superheat
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.
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27-06-2016, 03:08 PM #20
Re: R134a Low Subcooling, High Superheat
3 bar would make the saturation temperature about 9C. How is that possible given 8C water temperature?