How an expansion tank works?

[Raadster]

Ive seen the use of expansion tank in auto cascades and lower stages in regular cascades, i understand why there is a need for them since this systems are working with higher pressure gases, but what i wanna know is how the affect the system once its properly charged.

So my understanding of an expansion tank is to keep BP low enough while the system isnt in use so that the compressor can start, this BP is obviously lowered by the extra internal volume the expansion tank adds to the system.

My question is ive seen expansion tanks used in lower stage systems that are connected to the suction line using a short piece of captube. Now apparently when the system if off the captube will allow gas to enter the tank thus adding that extra volume and lowering system BP.

These are the things i don't get, when you are charging your system you will have gas enter that expansion tank, so your charge isn't the amount that is just enough for a system without that extra volume, but has that extra volume already being used. Doesn't that defeat the purpose?

Also if you added some valve to stop any gas entering the expansion tank while charging, and then opened that valve once the system was properly charged wouldnt there be a certain amount of gas always present thereafter remaining in the expansion tank?

For example if your BP was 150psig while the system was off and you turned the system on, the suction side pressure for the system is say 30psig, wouldn't enough gas be sucked out of the expansion tank until it had an internal pressure equaling that of the suction line.

This would mean that the expansion tank would therefore be taking a certain amount of charge out of the system that we could never retrieve unless the suction side ran in a vacuum?

Any clarification or links to material explaining this would be appreciated.

[Raadster]

I had that post properly formatted with paragraphs and yet it comes out as a block of text, sorry for that but i dont know why it happened.

[GXMPLX]

Ive seen the use of expansion tank in autocascades and lower stages in regular cascades, i understand why there is a need for them since this systems are working with higher pressure gases, but what i wanna know is how the affect the system once its properly charged.

So my understanding of an expansion tank is to keep BP low enough while the system isnt in use so that the compressor can start, this BP is obviously lowered by the extra internal volume the expansion tank adds to the system. .

Not really. The main purpose of the expansion tank is to provide an expansion for the gas so that when the system stops and is at ambient temperature it won’t blow up because of the pressure!

Having high system pressure is not a problem to start a compressor but having high differential pressures (low high) is. So the compressor starts unloaded if system pressures are equalized at startup.

Not Back Pressure, the whole system is at the same pressure (usually).

My question is ive seen expansion tanks used in lower stage systems that are connected to the suction line using a short piece of captube.

Now apparently when the system if off the captube will allow gas to enter the tank thus adding that extra volume and lowering system BP.These are the things i dont get, when you are charging your system you will have gas enter that expansion tank, so your charge isnt the amount that is just enough for a system without that extra volume, but has that extra volume already being used. Doesn't that defeat the purpose?

ALso if you added some valve to stop any gas entering the expansion tank while charging, and then opened that valve once the system was properly charged wouldnt there be a certain amount of gas always present thereafter remaining in the expansion tank?

For example if your BP was 150psig while the system was off and you turned the system on, the suction side pressure for the system is say 30psig, wouldnt enough gas be sucked out of the expansion tank until it had an internal pressure equaling that of the suction line. This would mean that the expansion tank would therefore be taking a certain amount of charge out of the system that we could never retrieve unless the suction side ran in a vacuum?

Never add a valve to the expansion tank this would break pressured vessel safety code and it is really dangerous should the valve fail!

No, if you multiply the volume of the tank times the density of the refrigerant at low temperature you’ll see it is only a few grams that fill it.

It doesn’t affect the amount of refrigerant during operation. There is no liquid in the tank ever.

The manufacturer gives instructions to charge the system that will (generally) include the amount in the tank, even at ambient temperatures this amount is small (but not negligible like at low system temp).

Get refrigerant data and do this math, if you don’t have tables tell me what refrigerant you use.

Any clarification or links to material explaining this would be appreciated.

I’m sure you’ll get lots of links before I can post mine when I get to the office.

[Raadster]

Im copying a build of a autocascade which uses a 1.5hp compressor to cool 150w to -100c.Gases Used will be R123, R22, R23, R14 with a little R50.So your saying that during operation the gas present in the expansion tank wont have a factor on operation?My only worry is that since ill have much smaller amounts of the higher pressure gases in the system that there will be a unknown mix of gases in the expansion tank, which will affect system performance.

[GXMPLX]

Please do me a favor and don’t do this!

The problem of copying this type of system is that you won’t be able to copy what you can’t see and these are really dangerous.

You have to calculate system idle pressure, forces on the different parts, additives for the refrigerant and many others with the compressor.

Believe me, charging the compressor is the least of your problems!

At least I will do you a big favor and not help any further!

[Raadster]

I got all the info on the build down to the exact charge requirements.Worst case scenario is the pressure's build up to a dangerous level at which ill have a safety cut off valve.System will be pressure tested to 350 psi before charge, i cant see what could go wrong.

[GXMPLX]

..., i cant see what could go wrong.

Exactly my point!

Please get saturation temperatures of R23 and read ASME´s Pressure Vessel Code FIRST!

If you get to understand it you will START to learn what could go wrong!