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12-09-2015, 10:36 AM #1
Strange cooler operation (high superheat)
Hi Everybody, hope you all are fine !
Please, I am posting here a case that is puzzling me
here is the case
I have an evaporator in a cold room with following set points
- air inlet temperature 0.32F
- evaporating temperature -27.4 F
Now starts the strange things.
I am measuring a superheating of 15K.
Yes, 15K !!
I made some verifications
- theoretical, with evaporator manufacturer, to see capacity in working conditions
- measuring air inlet/outlet temperatures. with manufacturer air flow I calculate a capacity similar to the one
all let convey that I can open the valve, so that I can get more refrigerant and therefore more capacity
But when I do this, I have fluid in compressor. All the compressor start to frost. Not only the beginning, the whole compressor.
The evaporator starts to frost in all rows
My interpretation is that there is a problem with the valve.
Please, could anybody give me an opinion about ?
as a general question.
What would happen if an expansion valve is not working properly ?
Many thanks !
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12-09-2015, 04:28 PM #2
Re: Strange cooler operation (high superheat)
Is the room down to temperature? If not then wait until it is and then take another set of readings.
15K superheat, OK, has it changed from when you last checked the system?Brian - Newton Abbot, Devon, UK
Retired March 2015
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13-09-2015, 10:27 AM #3
Re: Strange cooler operation (high superheat)
.
What is the refrigerant?
Where are you measuring the suction pipe temperature?
To get true evaporator superheat you need to measure both
the pressure and temperature as near to the outlet of the evap
as possible.
If you are measuring the pressure at the compressor there will always
be a slight difference. Also the refrigerant is important, is it a blend
or a single refrigerant? Is it a 400 series refrigerant or are you using R22?
Blended refrigerants (400 series) have a glide in the evaporating temperature,
it is very important that you know which side of the glide you are measuring.
If you are working with a blended refrigerant (400 series, zeotropic) you always
measure evaporator superheat from the Dew point temperature or sometimes it
is refereed to as the Vapour temperature.
Rob
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13-09-2015, 02:31 PM #4
Re: Strange cooler operation (high superheat)
Hello, sorry for not being precise
- refrigerant is R507A, therefore no glide topics
- the most important element is that all the compressor starts to frozen when I open the valve, so that I get liquid going to the compressor, with the risk of damaging it.
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13-09-2015, 08:38 PM #5
Re: Strange cooler operation (high superheat)
Then don't open the valve.
Brian - Newton Abbot, Devon, UK
Retired March 2015
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13-09-2015, 10:20 PM #6
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14-09-2015, 07:42 AM #7
Re: Strange cooler operation (high superheat)
Hi Paul
Couple of checks to make:
Is the evaporator coil clear and air flow is correct?
Measure pressure and temp at evaporator as Rob says?
Check there is sufficient refrigerant in system.
Record suc & discharge pressures at compressorI love the smell of Ammonia in the morning!
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15-09-2015, 04:17 PM #8
Re: Strange cooler operation (high superheat)
It's really strange the fact that with 15K superheat you get the vapour refrigerant temperature (evaporator outlet) at the same temperature the air inlet...meaning on theory you have a lot of lack of refrigerant flowing inside the evaporator. Where are you measuring the suction superheat? If you measure after a liquid subcooling heat exchanger with the vapour passing through it or if you measure the SH near the compressor of course an higher SH will happen.
To make progress is never good enough, I want to do better and better and better
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15-09-2015, 08:06 PM #9
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15-09-2015, 09:16 PM #10
Re: Strange cooler operation (high superheat)
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16-09-2015, 10:25 AM #11