TXV Superheat

[Gary]

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?

[Prof Sporlan]

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?

Unfortunately not, or at least not anything like a charging chart for a cap tube or restrictor system. If the TEV is maxed out during pulldown, superheat will effectively be the TD of the evaporator, and it will provide no insight as to the correctness of the valve setting. If the TEV isn't quite maxed out, superheat should fall below the TD of the evaporator, which in theory could provide some insight as to the correctness of the valve setting. Problem here is the TEV is much more difficult to model under these conditions. Unlike a restrictor or cap tube, the TEV has gradient, and it modulates.

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.

[Gary]

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?

[Prof Sporlan]

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.

[frank]

Can the Prof give a link to the article in Contracting Business for those of us not on that side of the pond?

[Gary]

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.

[herefishy]

A very insightful thread..... thank you.

[Prof Sporlan]

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.

[Dan]

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....

On unit coolers, obstructing the discharge air evenly is probably best when a fan speed control is not available. The higher static pressure on the outlet of the evaporator will provide a more even flow across the evaporator, than just letting portions of the evaporator see differences in flow across different circuits.

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.

[Prof Sporlan]

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.

Bingo!

[Gary]

Since circuits are horizontal, the entering air could be blocked so long as we block vertically (top to bottom), and avoid blocking horizontally.

[Dan]

Since circuits are horizontal, the entering air could be blocked so long as we block vertically (top to bottom), and avoid blocking horizontally.

I can't come up with a good argument for your suggestion, other than why bother. Blocking the discharge changes nothing about the entry of the air over the coil (I am assuming we are talking about a unit cooler).

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.

[Gary]

There are various ways to do this, and some will be better for some circumstances than others. It's all good.

[Suta Eugen]

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Thank You

[Gary]

My books are available at:

https://www.merchantamerica.com/tmethod/

[Hokiewolf]

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