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Originally Posted by Six

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.

Originally Posted by twophase

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?

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.

Originally Posted by icemeister

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.

Originally Posted by louiee

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.

Originally Posted by Six

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.

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

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

Originally Posted by Phase Loss

why not pipe a Normally open solenoid valve around the TEV and wire it to cycle with the compressor?

That won't mess with TEV operating capacity & ought to give ya some rapid pressure equalization on the off cycle.

I thought about that, too. That's probably plan C.

why not pipe a Normally open solenoid valve around the TEV and wire it to cycle with the compressor?

That won't mess with TEV operating capacity & ought to give ya some rapid pressure equalization on the off cycle.

Originally Posted by icemeister

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.

If you go with a cap tube for a bleed, try Tecumseh's capillary tube selection program and select one for a percentage of the total capacity:

http://boxload.tecumseh.com/SelectCapTube.aspx

Thanks icemeister. I have ran some numbers in the Danfoss cap tube program and it looks like it actually won't be a very good solution due to the small cooling capacities. To get 10% capacity through the bypass requires at least several meters of the smallest listed size (0.026" IIRC), and I don't really want to go much higher than that. I was considering one of those hard start kits as an alternative solution, but if rotaries generally will start against backpressure, then that solves the problem easily. I'll probably get one of those Supco 3 in 1 kits for the 1500W compressor anyway, since I don't have the original overload clicker switch, but I may need to find one for it anyway since I don't know if the compressor has internal thermal overload protection. It says "thermally protected" on the label, but I don't know if that specifically refers to the external thermal/current overload it would normally have that is missing, or if there is additional internal protection. It's a Rechi precision compressor and the manufacturer's website is more or less useless.

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.

If you go with a cap tube for a bleed, try Tecumseh's capillary tube selection program and select one for a percentage of the total capacity:

http://boxload.tecumseh.com/SelectCapTube.aspx

Just add a start kit to your compressor so it can start against pressure.

Bypass with cap tube not a good idea. Find a manufacturers chart and get one that bleeds internally. If you want to use a cap tube..... use a cap tube and not a TEV. Either that or TRY to figure out the combined capacity of the two. (you are an engineering student.) Supco has good literature on cap tube design and contrary to popular opinion cap tube is is a load sensitive device to some degree. Properly designed, it might be just what you need for the price of a bit of tubing and no TEV!

Originally Posted by twophase

I am trying to maximize flexibility for the system to accommodate as wide a range of loads as is reasonable (anywhere from liquid cooling gaming computer hardware, general space cooling, rapid temp pulldown of party drinks, to operating a small scale working ice rink model for a school project). Seems to me that a TXV would be the more flexible and safer solution for the compressors. I plan to use balanced port valves to further stabilize operation across as wide a range as possible.

Just a little background on the project...the compressors are rated for 1300W and 1525W cooling at 7.2 deg C and they will run separate refrigerant circuits that share the same tube for the shell and tube evaporator and condenser. I have a programmable microcontroller to use as the control system to control the temp monitoring, cycling, staging, startup sequence, anti-recycle timers, etc., etc. The condenser is liquid cooled to provide flexibility in different cooling options, but it will normally implement a liquid to air radiator with a speed controlled DC fan using the liquid line temp as feedback to control head pressure over a wide range of ambient conditions.



Yes, exactly. From reading manufacturer's literature, the port seems to be non checked and (at least for parker/sporlan valves) is sized as a percentage of nominal valve capacity, all built into the valve body itself. It seems that I can replicate the same effect by mounting a length of cap tubing the same way as you explained for the check valve, since cap tube is cheap and easy to deal with.

I'm not referring to internal versus external equalized valves, chosen depending on the pressure drop across the evaporator.

Yes, a small check valve from the liquid line to the TXV distributor should do it.

Be careful. putting it in backwards could be costly.

I am trying to maximize flexibility for the system to accommodate as wide a range of loads as is reasonable (anywhere from liquid cooling gaming computer hardware, general space cooling, rapid temp pulldown of party drinks, to operating a small scale working ice rink model for a school project). Seems to me that a TXV would be the more flexible and safer solution for the compressors. I plan to use balanced port valves to further stabilize operation across as wide a range as possible.

Just a little background on the project...the compressors are rated for 1300W and 1525W cooling at 7.2 deg C and they will run separate refrigerant circuits that share the same tube for the shell and tube evaporator and condenser. I have a programmable microcontroller to use as the control system to control the temp monitoring, cycling, staging, startup sequence, anti-recycle timers, etc., etc. The condenser is liquid cooled to provide flexibility in different cooling options, but it will normally implement a liquid to air radiator with a speed controlled DC fan using the liquid line temp as feedback to control head pressure over a wide range of ambient conditions.

Originally Posted by commtech77

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.

Yes, exactly. From reading manufacturer's literature, the port seems to be non checked and (at least for parker/sporlan valves) is sized as a percentage of nominal valve capacity, all built into the valve body itself. It seems that I can replicate the same effect by mounting a length of cap tubing the same way as you explained for the check valve, since cap tube is cheap and easy to deal with.

I'm not referring to internal versus external equalized valves, chosen depending on the pressure drop across the evaporator.

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.

Why not just go with AXV's?

Sent from my C771 using Tapatalk 2

cap tube as a bleed port for TXV?

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!