TEV with or without equalizing line

[Peter_1]

Let's discuss the following: can you use a TEV with internal equalizing line on an evaporator with a distributor in a conventional storage cold room with a relative stable load?
If so why?
If you can't, why not?
Can a TEV with internal eq. work as well as a type with external eq. on a coil with distributor?
Is the choice for an TEV with or without eq. line determined by the construction of the evaporator or more by the application?

[Tradewinds]

Hi Peter,
My understanding is that you should always use an externally equalised TEV on large evaps (large pressure drop) or when an evaporator has a distributor fitted for a multi-pass coil.

The reason for the external equalisation line is to take an accurate reading of the evaporating pressure actually in the heat exchanger. You may find that there is a 1bar pressure drop across the distributor and then another smaller PD across the evap. As the pressure required to act on the TEV is upstream of the distributor it would therefore not be equal to the pressure in the evaporator.

Does that make senses? I think I need a diagram to explain it better.

[monkey spanners]

My understanding is that for the TEV to work correctly it needs to know the temperature and pressure at the end of the evaporator, so if the pressure is near enough the same at the start of the evaporator as it is at the end then a internally equalised valve will work ok.
If there is a pressure drop over the evap then an externally equalised valve is needed.

We could fit an external valve on either evaporator and it would work ok, so we need only fit external valves but being lazy we prefer to be able to only have to make two connections if needs be :D

Jon :)

[Peter_1]

We have a certain evaporator which will see almost always the same heat-load for a certain refrigerant at certain room temperatures (cold room application)

This said, DP over the coil will be always the same. So, you then can use a TEV with internal equalization and you regulate the SH-spring till SH is again the desired value. Once regulated, this SH will stay the same because DP will stay the same.

The only difference you will have when using an internal type on a distributor is that the valve will not 'see' the DP over the coil and regulate therefore a too high SH when you leave the valve in the factory setting as delivered. You only have to regulate the additional DP.

The internal valves are slightly cheaper and easier to install them.

someone disagree with this?

[monkey spanners]

I'd just fit an external one i think as i couldn't be bothered with waiting for it to reach design temp and then start setting the superheat up. Though i can see the point you are making.

I would also worry that at some time in the future under certain conditions it may run with too low a superheat.

I tend to fit braze in valves now and i don't think it would take much longer to braze another connection once you've got the torch lit.

I guess like a lot of our work it would depend on the actual equipment and its use and the best valve to fit would be best decided on site with all the relavent information in front of you.

Jon :)

[Tradewinds]

I can see what your saying Peter. I guess it depends if the range on the spring pressure is large enough to allow for the pressure drop across the distributor then it may work in stable conditions. Personally, I would fit an ExTEV so that I knew it was right. The additional time it would take to fit the valve is minimal compared to adjusting superheat settings.

[Peter_1]

I'd just fit an external one i think as i couldn't be bothered with waiting for it to reach design temp and then start setting the superheat up. Though i can see the point you are making.
I see it exactly the same way Jon, otherwise you have to wait till you come at +/- the design conditions to regulate the SH. With an external one, you can leave mostly leave it (99% pf the cases) the way the manufacturer delivered it to you.

[mad fridgie]

"relative stable load", you could just a fixed orifice and critical charge.
Unless you know your actual pressure drop before installation, you could just end up starving the evap.
You need to know your pressure drop at max SCT and minimum SC, so that you know max flash gas, then max pressure drop.