cap tube or TXV?

[star882]

For my science fair/engineering project, I'm building a phase change CPU cooler. I plan to use something like a 1/6HP compressor and R134a. I'll use it on an old Celeron (633MHz to 950MHz or 1266MHz) for practice, but I will later use the unit with another machine (not sure, but expecting about 80-150W load).
How much more does a TXV cost over a cap tube? What are the exact benefits?

If I use a TXV, what size TXV is most suited for my application?
If I use a cap tube, what size and length should I use? Keep in mind that I do not know the exact load and I will likely move the cooler to a different system later on.

[Brettbeck]

I think the captube would be cheaper, and as you will only be having a small load, you could have longer captube which will give better idle temps.

Not sure about the TXV though, I don't know much about them :( . I'd like to learn if anyone would be kind enough to explan a little about them :).

[tim-]

tvx works as an thermostat for waterradiators at home, the flow increans when it's get hotter and reduces when it's get colder. as on a radiator you can adjust the tempture you want.

to make this possible without any electrition they use a sensor connected to the valve with a cupper pipe which is filled with a gas which reacts as the gas in the system. this gas depends on what gas it's made for. I'm not sure if it's r134a in a specific r134a valve. I just know there is different gasses depending on what gas it's in the system and they acts really freaky on other gasses then it's made for.

[jinu117]

TXV cost goes up due to need for receiver, oddly sized tubings you probably didn't have to buy, etc as well as price of TXV. Besides, even the smallest orifice I have found tend to be too big for direct die system. I suggest sticking to cap tubing for direct die.

[THE JEW (RaVeN)]

TXV cost goes up due to need for receiver, oddly sized tubings you probably didn't have to buy, etc as well as price of TXV. Besides, even the smallest orifice I have found tend to be too big for direct die system. I suggest sticking to cap tubing for direct die.

I realize that this is vapor, but would your opinion change if this was the chilled water section (ie. is a TXV better suited to a water chiller)?

[star882]

as you will only be having a small load, you could have longer captube which will give better idle temps.
So what happens when I switch it to a higher load? Would I have to take it apart to shorten the cap tube? If I use a cap tube sized for a high load on a low load, I would get excess liquid refrigerant on the low side, right?

[Blaster]

I realize that this is vapor, but would your opinion change if this was the chilled water section (ie. is a TXV better suited to a water chiller)?

yes :)

[jinu117]

So what happens when I switch it to a higher load? Would I have to take it apart to shorten the cap tube? If I use a cap tube sized for a high load on a low load, I would get excess liquid refrigerant on the low side, right?

In all honesty, I don't think anyone want to tune a unit below 150w now a day. And it's just matter of finding optimal cap tubing length to make sure you don't get excess by much to make it worse :)
As for chiller, I won't say TXV will work better flat out as in temp but for over all system efficiency, yes... :)

[star882]

So if I use the system with a 150w captube on a 30w load, do I simply undercharge it to prevent floodback? And when I put it on a 150w load, I simply add some refrigerant?

And if the load somehow decreased dramatically, would it cause damage to the unit? Would a suction line accumulator be enough to protect it?

[bazx]

So if I use the system with a 150w captube on a 30w load, do I simply undercharge it to prevent floodback? And when I put it on a 150w load, I simply add some refrigerant?

And if the load somehow decreased dramatically, would it cause damage to the unit? Would a suction line accumulator be enough to protect it?


I think you should try a setup with both as this is the best way to learn
After all this section is all about experimentation

Make a thread and compare the two methods side by side

It would make a good read

[SexyMF]

So if I use the system with a 150w captube on a 30w load, do I simply undercharge it to prevent floodback? And when I put it on a 150w load, I simply add some refrigerant?

I found if there isn't enough gas then you just don't get the pressure up enough for the system to function. Adjusting the condensor airflow helps a little to raise the highside pressure to get things going with less gas.

Also, adding refrigerant after you have removed the guages and broken the vacuum isn't easy. You can get hoses which hold refrigerant in them but you would still need to vacuum the hoses, fill it and then connect to the system etc

I would also go for a 1/5 or 1/4HP compressor. I found the price was not much between them.

[SexyMF]

As for capillary line 0.031" is just easier to work with than 0.026".

[DetroitAC]

tvx works as an thermostat for waterradiators at home, the flow increans when it's get hotter and reduces when it's get colder. as on a radiator you can adjust the tempture you want.

to make this possible without any electrition they use a sensor connected to the valve with a cupper pipe which is filled with a gas which reacts as the gas in the system. this gas depends on what gas it's made for. I'm not sure if it's r134a in a specific r134a valve. I just know there is different gasses depending on what gas it's in the system and they acts really freaky on other gasses then it's made for.

That's really pretty close, you've got the jist...but
The txv controls the evaporator exit superheat, not the temperature. If the superheat is too low it decreases the flow, if the superheat is too high it increases the flow. This can be basically thought of as feeding the evaporator the "amount it needs", or maybe "keeping it full" (although that implies full of liquid and that's not quite right). OK, now that I have that disclaimer in there...it keeps the evaporator supplied with the amount of liquid it requires to match the load. A better evaporator can use more liquid than a worse one, a higher load on an evaporator will require more liquid than a lower load.

The obvious benefit to this is that it is a true feedback system, and it will work with variations in load, inlet subcooling and evaporating pressure.

The gasses in the head, the way the valve works...it's about as complicated as an entire refrigeration system. Let's just leave it at that.

[Brettbeck]

TXV cost goes up due to need for receiver, oddly sized tubings you probably didn't have to buy, etc as well as price of TXV. Besides, even the smallest orifice I have found tend to be too big for direct die system. I suggest sticking to cap tubing for direct die.

Could I ask what a reciever is? I've seen them on diagrams before but I dont know why they are used.

Also, is TXV the same as CPEV? Or is there a difference?

[DetroitAC]

Could I ask what a reciever is? I've seen them on diagrams before but I dont know why they are used.

Also, is TXV the same as CPEV? Or is there a difference?

A receiver is just a tank with the outlet at the bottom, inlet at the top. Sometimes both connections are at the top, and the outlet uses a pickup tube that goes to the bottom. There are various reasons why they are used, and the size of the receiver depends on the use. In large systems a receiver can be used to hold the entire refrigerant charge while servicing some part of it. The receiver can hold reserve refrigerant to compensate for leaks (automotive), and for wide variations in operating conditions. It also serves as a phase separator somewhat. If a condenser is supplying a saturated mix of vapor and liquid to the receiver, only the liquid will flow to the TXV.

A CPEV is a Constant Pressure Expansion Valve, you can think of a TXV as a Constant Superheat Expansion Valve. A CPEV is much simpler, really close to an air pressure regulator. It feeds refrigerant to hold the evaporator inlet (since that's what the valve senses) at a constant pressure. Pressure too low=the valve opens further, pressure too high=the valve closes further.

Neither valve will hold precisely it's target values, there must be some bias based on flowrate, and both will wander around a bit due to the slight variations in conditions that are always occurring.

[Brettbeck]

A receiver is just a tank with the outlet at the bottom, inlet at the top. Sometimes both connections are at the top, and the outlet uses a pickup tube that goes to the bottom. There are various reasons why they are used, and the size of the receiver depends on the use. In large systems a receiver can be used to hold the entire refrigerant charge while servicing some part of it. The receiver can hold reserve refrigerant to compensate for leaks (automotive), and for wide variations in operating conditions. It also serves as a phase separator somewhat. If a condenser is supplying a saturated mix of vapor and liquid to the receiver, only the liquid will flow to the TXV.

So in systems that are built for our purpose a reciever isn't really necessary?


A CPEV is a Constant Pressure Expansion Valve, you can think of a TXV as a Constant Superheat Expansion Valve. A CPEV is much simpler, really close to an air pressure regulator. It feeds refrigerant to hold the evaporator inlet (since that's what the valve senses) at a constant pressure. Pressure too low=the valve opens further, pressure too high=the valve closes further.

Neither valve will hold precisely it's target values, there must be some bias based on flowrate, and both will wander around a bit due to the slight variations in conditions that are always occurring.

So both a TXV and CPEV can replace capillary tube? I allways thaught that a TXV/CPEV were used in some cascades so that you can change the flow of refrigerant into the HX, enabling you to tune them easily for the load of the HX. Is this true?

Sorry if this is OT :( .

[epion2985]

So in systems that are built for our purpose a reciever isn't really necessary?



So both a TXV and CPEV can replace capillary tube? I allways thaught that a TXV/CPEV were used in some cascades so that you can change the flow of refrigerant into the HX, enabling you to tune them easily for the load of the HX. Is this true?

Sorry if this is OT :( .

You want a reciever if you are using a valve since they ussualy need a solid column of liquid refrigerant.

To answer your second question to use capillary line instead of a valve (any valve txv, cpev etc) is like using a wooden wheel on your car. Its the wost metering device out there and we have evolved past it a long time ago. Only reason people still use it here and there is because they are dead broke or its a business choise to bring the cost down a little on these units as people who are buying them are broke/cheap bastards. Ie: (10m cap line ~$10-20, valve ~$50-80).

TXV's are great on chillers. Cpev's are nice on DD units because you can adjust the restriction on the fly without recovering refrigerant, cutting up the system, rebrazing, vacuuming, and recharging every time you need to make an adjustment.

Valves are really the way to go :)

[Brettbeck]

Ahh right. I see lots of people still using captube on cascades. And I must admit, if I ever build a cascade in the near future I would probably use captube due to how cheap it is. (Im only 16 at the moment).

But basicly you could use a CPEV in place of captube, and you can alter the flow of refrigerant (like changing the length of captube) to suit the HX best?
And by the sounds of it, TXV is used in water chillers?

[wdrzal]

For your first build use the K.I.S.S method. If you want it to work. you can experment later. You will have to aquire many new skills to build even the simplest system. :stick: Use a cap tube....:slap:
Expansion valves don't work well in very small systems 1/5hp and as detrotiot as eluded too, the use of them while beneficial when used and set up correctly, doing that is very complex,You must understand all the parameters of a system. :)

[epion2985]

1/5th is a bit on the small side. One would want to stay closer to 1/2 :)

[Big SturL]

Think a CPEV would be most suitable.

[johann]

Think a CPEV would be most suitable.

No I think a captube would be most suitable.

Lately, systems based on CPEV I see here is not impressive atall.

[epion2985]

Nothing wrong with a txv/cpev when properly sized and installed

[epion2985]

Exactly but that was my point 6cc is not the optimum size, 10-13cc is more like it. And if you must limit yourself to a no lic refrigerant I would go with r290.

[DetroitAC]

i think Tom and Jinu are right, the capacity of such a system is off the small end of the Sporlan TXV sizing charts. Perhaps there is a way that a TXV could be made to work, but it's a difficult development project, and i think not appropriate for a relative newcomer to refrigeration, and with her first system build. I think stick with a capillary, we know it will work.

[epion2985]

Thats an interesting idea. But there is expantion after the orifice in to about what, 1/2" pipe, then down to cap line then expantion again? Thats like having a metering device open up in to an evap then the evap dump in to another metering device.

[DetroitAC]

Hi Eric,

What would happen if you put a small (say 3 feet) length of 0.026" cap tube after a '00' orifice?

Tom

Ah, sorry I missed this. That's basically what i had in mind when I made the statement that it "could be made to work". My feeling is that it will be a lot bigger capillary than that (i.e. much shorter or much bigger diameter). I think you want just enough restriction after the valve so that the valve is in control, but enough restriction to put the valve in the range where it is within it's capacity. That's not an easy thing to do, since you have to apply a guess capillary, and then charge the system, and check whether the valve is in control, or you just have a fixed restriction. Yes I think it could be done, and I think it's a good bit of work.

[Xeon th MG Pony]

we use 1/8 tub to feed evaps that are in a tricky spots. Need to adjust the super heat setting accordingly on the TVX though.

[Marvin]

So a CPEV with a good thermometer is a good option for a 1/2 HP direct die ?

[DetroitAC]

What is the ID and length of the restriction in a '00' orifice?

Sorry Tom, I don't know, but the "orifice" is really just the seat of the seat and pin valve of the TXV.