PEV and TEV help

[craig588]

I have never used them before and they are confusing me. Why are they better than a cap tube? They are only giving me problems. Sometimes the first stage of my cascade will go down to -50C loaded, and other times it will stuggle to reach -15C loaded. The second stage is even crazier. The pressures randomly move around, seemingly having no relation to the temps. Sometimes I can see a massive vacuum on the 2nd stage low side, other times I have pressures of 150 PSI, when it is working at its best I see pressures in the 20PSI range. Usually when there's a vacuum in my second stage my evap temps are shooting up to the positives, untill I hear a pop and a whoosh and suddenly they drop from 0C to -70C in less than 5 seconds and continues untill it levels out at around -90C. That is not always the case though, sometimes the temps are fine while it is running in a vacuum. Does anyone know what I need to do? Can people just post all of the information they know about PEV and TEVs?

[Stewie007]

Theres more to a TXV than many people realize.

First, you need to tell me what kind of TXV you are using. Is it Internally balanced? 1 Inlet and 1 Outlet?

Or is it externally balanced 1 Inlet, 1 Outlet, 1 Exualizer Connection (3 connections)

You seem to be witnessing the symptoms of TXV Hunting. The variable load is throwing your pressures all over the place and your TXV is having a tough time compensating. But first, answer my two questions :)

[craig588]

It has one in and one out. The blub is on the second stages entrance to the HX if I remember right. (It's all boxed up right now, it's either the 2nd stage entrance or the first stage exit, but I'm pretty sure it's the second stage entrance)

That is for the first stage TXV (I'm supposed to use an X for expansion?)

I don't think it's a charge issue, I charged with one pound of R404a, but the compressor was getting a little cold while loaded so I let a little bit out and the compressor is hot now. When the second stage is off it gets a little cold, but I don't think that matters much because the second stage is going to be on 95% of the time that the first stage is on.

[berkut]

you have to fine tune the TXV to work correctly, it takes time and a LOT of experience but eventually you will love them as much as i do ;)

PEV's.... the POP indicates you have humidity contamination in your system... a PEV does not pop. Its job is to set the pressure to a given value, whatever the flow.

The compressors should be ~60C hot

[craig588]

Chilly1 did all of the work on the second stage which is where the "popping" is coming from. I don't know where to get ethylene so I guess I can't do anything about it.