cap tube as a bleed port for TXV?

[twophase]

twophase, I have some thoughts on this. Tried to PM you but could not. PM me with contact info.

I couldn't PM you either. Maybe I'm not of sufficient posting status yet. Why not just post it here. Maybe it will be useful to someone else, too.

Trane uses a check valve that Ts off the liquid line and bypass's the TXV. Works just fine.

I'm assuming the check valve is installed to allow flow from high side to low side, or else it kind of defeats the purpose. Is there a special check valve used for this, since if the check valve is not restrictive enough, it will drop too much pressure from the liquid line. That is the biggest concern if it turns out the compressors can't start against backpressure and a start kit doesn't work and I have to use some sort of bypass like this. The biggest problem I have found out is, since my nominal capacity (for the smaller stage) is only 1100W, 10-20% of that is very small. Dancap was giving me roughly 9 meters of 0.026 cap tube for 100W with R22 at 7 deg C evap temp, 50 deg condensing temp. I don't really want to buy and deal with 30 ft x 2 of cap tubing. With a multi ton unit, it is easy to make a bypass that will flow 10% capacity.

I'm now just crossing my fingers that the compressors will start one way or another without a bypass. The solenoid valve idea is probably the best if they don't. One portable dehumidifier I had used one to bypass the cap tube for defrost, so they do exist for this size application. Of course the best way is to just get a TXV with a bleed port, but my project budget won't really allow for paying full new price for 2 txvs probably special ordered from somewhere, as I have not yet found a retailer that stocks any of that specific type and size, let alone any on ebay. It's a personal/school project, so ebay junk is acceptable and preferred. I know txv systems this small are not that common.

[craig1]

A rotary wouldn't require the high starting torque of a start capacitor/potential relay when used with a TXV...as I understand it...as they're similar to a scroll in that respect.[/url]

Rotaries won't start unequalized without a start cap. Try it with a window unit sometime. It needs to be off a few minutes.



A scroll compressor normally won't start unequalized either, but they have an internal equalization feature. on shutdown the scrolls seperate and allow the pressure to equalize. A checkvalve at the discharge port prevents the whole charge from equalizing through the compressor.

[twophase]

Been busy lately and hadn't had time to respond, but thanks for everyone's input. Sounds like a 3 in 1 is what i'm going to do.

[bunny]

Hey guys. I'm an engineering student working on a small liquid chiller project. I'm trying to find a couple R-22 expansion valves in the 1/3 to 1/2 ton range and I need them to have a bleed port since my compressors are PSC type rotaries. I'm scrounging around ebay since I don't want to spend a fortune on retail priced valves and it has proved very difficult to find them with a bleed port (either that or nobody properly identifies the valve numbers). Is there any reason why I couldn't just braze in a length of small diameter cap tubing parallel with the valve to act as an external bleed port, choosing a small enough dia/long enough length so as to not significantly affect valve operation, but large enough to be able to recycle the compressor after a couple minutes? Seems to me like a simple way around the problem.

Thanks in advance!

The typical bleed port is within 5% - 10% of the TEV's nominal capacity. Essentially, the bleed port is an engineered leak in the valve's seat, accomplished by notching the seat and enlarging the flow path. When the system shuts off the TEV will normally close, stopping refrigerant flow. The bleed port (notice in the seat) is an open flow path even with the valve piston seated in the port.

A 10% bleed on a 5 ton valve gives you 5.5 tons of capacity. There would be little difference in doing this or in piping a bleed port in parallel with the TEV, providing it isn't larger than 5% or so. If you have a distributor where would you pipe it in? It really ought to be piped into each circuit inlet, so the TEV can respond to the increased liquid flow in the evaporator and still maintain SH.

Why not just add a start kit to the compressor?

[bunny]

Are you talking about a valve that would equalize pressure on off cycle ?

That is usually done with a check valve that mounts at the TXV distributor and at the liquid inlet of the txv valve. Not on the valve itself.

Some TXVs are externally equalized too.

External equalizers do not allow system pressures to equalize during the off cycle.