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Thread: TXV Superheat
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14-08-2003, 11:28 AM #1
TXV Superheat
We are all taught the proper TXV superheat settings, but it is important to remember that these are the superheats to be expected when the refrigerated space is at design temperature.
Unfortunately, when we get there, the space is above design temperature. That's why they called us. We should expect the superheat to be a little higher, but how much higher?
Maybe Prof Sporlan can help us with this. Is there a chart somewhere or a rule of thumb that we can use to judge whether the superheat setting is somewhere in the ballpark if the space temperature is above design temp?
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14-08-2003, 05:01 PM #2Is there a chart somewhere or a rule of thumb that we can use to judge whether the superheat setting is somewhere in the ballpark if the space temperature is above design temp?
You might be amused to find a short article on the subject of adjusting TEVs in the April 2003 issue of Contracting Business... written by a well known personality within the hvac/r industry.Prof Sporlan
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14-08-2003, 05:48 PM #3
Hmmmm... I was afraid of that.
I wonder if a procedure could be worked out? Maybe something like reducing the evap airflow to reduce the heat load until the TXV starts modulating?
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14-08-2003, 06:45 PM #4
Reducing evap airflow to the point where suction pressures become "normal" for the system would be the way to do it. Should be not problem if one does this well.
Prof Sporlan
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14-08-2003, 07:32 PM #5
Can the Prof give a link to the article in Contracting Business for those of us not on that side of the pond?
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14-08-2003, 08:06 PM #6
So, for example, if we have a freezer designed for 0F @ 10F TD, we can block off airflow until the SST is -10F to check and perhaps even adjust TXV superheat.
Sounds like a winner.
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14-08-2003, 11:54 PM #7
A very insightful thread..... thank you.
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15-08-2003, 02:24 AM #8
It would, of course, help if the airflow was reduced evenly across the face of the evaporator, which could be done reasonably well with fan speed control. Using cardboard may give less than ideal results, however....
The Prof could not find a link to his article, however.Prof Sporlan
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15-08-2003, 03:55 AM #9It would, of course, help if the airflow was reduced evenly across the face of the evaporator, which could be done reasonably well with fan speed control. Using cardboard may give less than ideal results, however....
I think what the professor worries about is that a distributor circuit could deliver the wrong message to the TEV if the air flow favors or disfavors a circuit when airflow is not evenly distributed.
My guess is that this consideration becomes less important when you are lowering airflow by constricting ducted air such as in the air returns of display cases and air conditioners.
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15-08-2003, 02:14 PM #10I think what the professor worries about is that a distributor circuit could deliver the wrong message to the TEV if the air flow favors or disfavors a circuit when airflow is not evenly distributed.Prof Sporlan
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16-08-2003, 02:27 AM #11
Since circuits are horizontal, the entering air could be blocked so long as we block vertically (top to bottom), and avoid blocking horizontally.
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16-08-2003, 04:12 AM #12Since circuits are horizontal, the entering air could be blocked so long as we block vertically (top to bottom), and avoid blocking horizontally.
Blocking the return air changes not only the air flow velocity across the evaporator, but also, where the air crosses the evaporator. To me, it invites unnecessary confusion. And because we are changing things we have to consider all sorts of possible nuances.
I have seen some different circuitings of unit cooler coils. I am not sure all circuits are always balanced... I see return bends skipping a row, I have seen tripod return bends, where flow from two circuits become a single circuit, etc.
But I have to ask why? You can use the same piece of cardboard on the opposite side of the coil and achieve a guaranteed balanced reduced air flow across the evaporator.
A vertically restricted intake air flow may induce more air past a leaky tubesheet that influences TEV operation.
I think best practice is to restrict discharge air flow. It changes little about how the air will flow over the evaporator . Restricting intake airflow changes a lot of things regarding evaporator behavior.
I have no good argument, but if I have a piece of cardboard, I think the smart thing to do with it is put it on the air-discharge side of a unit cooler.
I think maybe that blocking any flow should be done from the discharge and not from the return... air conditioning, supermaket cases, condensers.
Messing around with the air that is entering a heat exchanger should preserve an equal air flow through the heat exchanger. I think the best way to do it is not by blocking the air or fluid entering the heat exchanger, but by blocking the air or fluid that is exiting the heat exchanger.
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16-08-2003, 06:05 AM #13
There are various ways to do this, and some will be better for some circumstances than others. It's all good.
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16-11-2007, 07:23 PM #14
Re:Books
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Thank YouLast edited by frank; 16-11-2007 at 08:14 PM. Reason: Please do not post e-mail addresses in the open forum
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17-11-2007, 02:07 PM #15
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24-04-2011, 02:51 PM #16
Re: TXV Superheat
This is an old thread, but I was researching TXVs and superheat and found the article you referenced. It really helped out a co-worker working a job that had confounded several others over the past week. Thought others might enjoy your article here:
http://contractingbusiness.com/residential/cb_imp_5549/
The error chart from Brainerd compressor is on page 14 here:
http://www.brainerdcompressor.com/_p...d%20Manual.pdf
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