TEV superheat setting and low temperatures

I am just wondering: does the TEV superheat setting change with evaporation temperature? It seems logical to me if it does, presuming that the same refrigerant is used in the thermostatic element.

Say, the TEV manufacturer adjust the TEV for 4K/7F superheat at -20C / -4F evaporation on R404a. At that moment, the additional closing force from the spring equals 0.48 bar / ~7psi vapour pressure.

Now, when evaporating at -50C, R404a, same TEV, same spring setting, that same spring induced closing force would cause the superheat setting to rise to 9.5K / 17F.

The resemblance of these calculated numbers and actual numbers of TD between SST and secondary liquid temperature, measured in my R507 liquid chiller, is scary. I always assumed that the increasing TD came from less efficient heat transfer at low temperatures, but this might be another cause.

Can somebody enlight me? Or am I just kicking in open doors, and didn't I do my homework correctly?

Superheat decreases with a decrease in evaporator load. It does not increase.

But how does TEV superheat setting change with SST?

I doubt it will stay the same since the line in a PT chart for any refrigerant is not a straight line.

Thus, it seems to me that superheat spring force will have more or less effect as evaporator pressure changes, since that force is constant, and evaporator pressure change due to temperature change is not.

DaBit... The valve superheat is set at design temperature. Any other condition than design, the superheat will be different. At a higher SST, the superheat will be higher. At a lower SST, the superheat will be lower.

Every expansion valve should be adjusted at design temp when installed.

Quote:

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Originally posted by herefishy
At a higher SST, the superheat will be higher. At a lower SST, the superheat will be lower.

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Why? To me it seems the other way around. PT charts are steeper at higher SST, so less temp increase is needed for a certain pressure increase.

Quote:

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Why? To me it seems the other way around. PT charts are steeper at higher SST, so less temp increase is needed for a certain pressure increase.

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DaBit, your understanding of the 'L' thermostatic charge (same refrigerant in the TEV bulb as the system, and liquid charged) and static superheat is quite correct. Keep in mind here that you are looking at a "static" condition, i.e., no change in valve stroke. Gary's comment of increasing load leading to increased superheat is also correct. Increased load requires increased flow thru the TEV which requires the valve to stroke further open which requires higher superheat

You will find examples of static superheat curves on page 7 of Sporlan Bulletin 10-9. You will see the 'L' charge behaving as you would expect.

Ahhh, that was the missin link. The valve stroke.

Thanks Prof!

What is a valve stroke?

Here we all go again with TX valve superheat debate, there are many posts on the subject.
magoo

Hi Da bit.
your reporting power is 8, and you have not figured out TXV super heat yet. What the hell is going on here.
Round about now a good time for Gary and Des A and Mad Fridge and Us Iceman to chime into this post.
grump.
Are you a office bound computer tech.,?

Magoo, post is 7 years old (!!!) and one can learn more t then others in their whole life. Suggest you search once for posts of DaBit, you will be surprised.

Thanks Peter_1
really really should look at original post dating. Thanks again.