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View Full Version : Is the TEV mis sized?



kengineering
25-02-2006, 02:37 AM
I have a problem with a open ( self select) style beverage cooler. I uses 404a and is cycled with a low presure control. The recomended cut-in pressure is 82 and cut-out 52. When it cycles on the presure goes from 82 to 58 then rises to around 65 then a bigsurge down to the off cycle pressure. I have forced it to run and it eventually gets to temperature ( 38 degrees F ) at a pressure of 42 psi. I suspect an incorrectly sized TEV what do you think? Ken

Andy W
25-02-2006, 08:18 AM
Is the evaporator a finned or plate evap, fan blown or gravity fall. Sometimes easier to fit a stat such as an elliwell to control or pump down.

Peter_1
25-02-2006, 09:26 AM
Giving pressures doesn't sya you anything about proper sized TEV. You have to measure the superheat.
Only this will tell you if the TEV is wrong.

I think compressor is too big for the attached evaporator or vice-versa.

Andy
25-02-2006, 11:01 AM
Hi Ken,
if the valve has not got pure liquid at it's inlet you will have the same effect. Take on proper log and post it and we can help. Saturation pressures, superheats and subcool.
Kind Regards. Andy

kengineering
01-03-2006, 01:25 AM
I am still having trouble understanding the measurment of superheat. This is how I proceded. I recorded the sucton presure at the service valve as 60 PSI. Converted it to temperature for 404A is 23*f. I attached a temperature probe to the outlet of the evap using foil tape with cork gum insulation and read the temp as 34*f. which tells me I have 11*f of superheat. However since the pressures and temperatures are always changing I could also read the same 34*f and a suction pressure of 52 which converts to 17*f and a superheat of 17 degrees. How do I determine which to accept or am I going about this the wrong way. Ken

US Iceman
01-03-2006, 03:42 AM
Converted it to temperature for 404A is 23*f. I attached a temperature probe to the outlet of the evap using foil tape with cork gum insulation and read the temp as 34*f. which tells me I have 11*f of superheat.

I did not check the saturation temperatures but you are doing it correctly Ken.

How fast is the suction pressure changing? You have to wait until the system is down to the normal operating temperature before you start to adjust the TXV.

The suction pressure should be relatively stable with a slow decrease in pressure until the desired evaporating pressure is reached.

Is the evaporator pressure changing quickly due to flash gas in the liquid line, and then the liquid line fills up with liquid and the pressure increases rapidly? If this is case, do not try to adjust the TXV, until the other problems are corrected.

I think we will need some more input from you Ken.

kengineering
03-03-2006, 12:10 AM
Is the evaporator pressure changing quickly due to flash gas in the liquid line.

I have a full sight glass and other than that I'm not sure how to check for flash gas or what steps to take to insure I don't.


I think we will need some more input from you Ken

What information should I provide?

Ken

US Iceman
03-03-2006, 05:07 AM
What information should I provide?

Compressor capacity
evaporating temperature
condensing temperature
TXV capacity & model number

That's a good beginning.

The flash gas I mentioned would be periodic bubbles or foam in the site glass. If the site glass stays clear, then no flash gas is present.

kengineering
04-03-2006, 04:47 AM
I will get get it begining of the week.

Dan
05-03-2006, 01:11 AM
"When it cycles on the presure goes from 82 to 58 then rises to around 65 then a bigsurge down to the off cycle pressure."

Hmmm.. quote buttons didn't work. No matter. In saturated temperatures, you have "It goes from 36 deg F to 22 deg F then rises to around 26 deg F then a big surge to the off cycle. This would make a wonderful Professor Sporlan or Fridgetech challenge. Not moisture, because you can force it to run and achieve temperature. I have to assume if you are achieving temperature and not mentioning that you short cycle, that the compressor is not badly sized.

What on earth is a big surge to the off cycle? Forcing it to run makes everything work okay? What is the time-line? A graph of temps and pressures would likely tell the story. We risk a bad observation on your part, Ken. But nonetheless,

Nice puzzle!