TEV - Pressure limiting element

[herefishy]

On occasion there have been remarks regarding the "proper application" of pressure limiting TEV elements, particularly by those who are most versed in refrigerant control. However I don't recall the criteria ever having been qualified.

I have a low temp application that employs a compressor with a maximum evaporating temperature rating of 0degF. Aside from all of the other existing problems and exclusions in the system (by others) I consider this a candidate for a "ZP" charge (Sporlan-speak), in consideration of the potential load coming out of defrost and the close margin between the max rating of the compressor and the design temperature, even in consideration of fan delay (when it is to be re-employed if ever it was).

Now, if the compressor was rated for up to +20degF or so, I would not be so concerned, and perhaps consider the application of a "pressure limiting" refrigerant control to impede the available capacity of the CU for quick pull-downs.

Is my line of thought correct, or am I a candidate for further enlightenment?

[Prof Sporlan]

The complete thermostatic charge designation would include the refrigerant code. So if we are using R-404A or R-507, the appropriate charge would be the SZP or PZP respectively, where 'S' and 'P' are the Sporlan (and Alco for that matter) refrigerant codes for R-404A and R-507 respectively. As it turns out, the SZP and PZP charges are identical, since the saturation curves of R-404A and R-507 are close in the low temp range.

These thermostatic charges have an "air test" MOP (maximum operating pressure) of 45 psig, which works out to about 35 psig on the system. On an air test fixture, the TEV is only slightly open, whereas on a system the TEV will be stroked open much further, causing "air test" MOP reading on the fixture to be greater than operating MOP. In addition, valve adjustment and how hot the sensing bulb actually gets during defrost will affect operating MOP. If one adjusts the TEV in the opening direction, it will increase MOP. Conversely, if one adjusts the TEV in the closing direction, it will decrease MOP. If the bulb gets particularly warm during a defrost, which often is the case with electric defrost, expect MOP to increase.

Values for system MOPs for common thermostatic charges are published in the Sporlan literature.

Assuming we have R-404A, expect a nominal 35 psig system MOP, but to be safe, expect it could go up to 45 psig under the right conditions. Corresponding temperatures would be 2°F to 11°F. Might be a tad heavy for your compressor.....

A better choice may well be the Sporlan SZP35 charge, which has a 35 air test MOP, and a 25 psig operating MOP. If your design evaporator pressure is 20 psig (-15°F) or lower, this charge in the Prof's humble opinion, is the safer one to use.

Of course if your are looking for precise suction pressure control following pulldown or a defrost, use a crankcase pressure regulating valve and a non-MOP charge for the TEV, e.g., Z charge.

[Dave Goodings]

Herefishy
You could use a TEV with a crankcase pressure regulator which you can adjust to suit the current draw of the compressor after a defrost i.e. rated current, or you can use a MOP TEV which will be non adjustable and may be slower for pull down after defrost to suit your application, so a MOP valve will be cheaper but not as flexible as fitting a CPR hope this answers your query??

[Prof Sporlan]

so a MOP valve will be cheaper but not as flexible

Precisely...

[herefishy]

HHHHHMMMMMMMMM................

...............very interestink, I think............



I'll have to think about this.

I want to know the story behind the developement of the MOP element, before I really illustrate myself as the ASS that I am!.....

Prof......?

[Andy]

Hi, Herefishy
I agree with Marc, with the additional comment that a CPR will still work well on a single compressor system, but MOP valves not not my prefered fitment on multi evaporator systems, more hassel than they are worth, especially on low temp R22 (usually two stage) where the extra suction superheat is not welcome.
Regards. Andy.

[Prof Sporlan]

I want to know the story behind the developement of the MOP element, before I really illustrate myself as the ASS that I am!.....

LOL! MOP charges aren't too difficult to understand. They are vapor charged at a warm bath temperature, typically 90°F to 100°F, and the charge constituents are design to condense at the desired MOP point. When the charge is in its vapor state, the TEV is operating at its MOP. When the charge is condensed, the TEV is operating in its design evaporator temperature range.

Non-MOP type charges are typically liquid charged, which does not allow an MOP.

Perhaps the finest explanation of thermostatic charges may be found here considering the Prof authored the majority of this literature...

Setting your CPR near the MOP point will lead to instability.

Very true. As a result, TEV manufacturers only recommend using TEVs with non-MOP charges when a CPR is used.