Multi Stage Expansion

[amitsaxena]

Dear All,

If anyone have information about multistage expansion, then please share the info.

thanks
amit

[bibin777]

hi amit
An expansion engine having a cylinder with a piston and a head provided with a housing in which the housing is subdivided by a partition into a first chamber and a second chamber. The first chamber is in open communication with the cylinder by means of an opening and has at least one burner supported by the partition extending into it. The second chamber communicates with a reservoir for compressed air by way of a conduit and an inlet valve. The reservoir is connected with the cylinder by means of a non-return valve. Both chambers communicate with each other by means of a pressure equalizing conduit which also serves as a heat seal. Air and fuel are supplied to the burner to permit continuous burning.

[amitsaxena]

Thanks but i need if we have multistage expansion in HVAC systems.

thanks
amit

[sterl]

In refrigeration: High pressure liquid is metered to low pressure vessel via 2-or more reductions of pressure instead of a single. In a 2-stage system: High Pressure Liquid is first expanded to the intermediate pressure; then to the low suction pressure. Low stage compressors pump the vapor resulting from the temperature change from Intermediate to Low; not from High to Low. This improves circuit overall efficiency. Economizer on a screw or similar compressor does very much the same thing; but pressure at intermediate (side) port is not as consistent as it is in a 2-stage compressor arrangement.

[US Iceman]

If the context is refrigeration system this usually implies multiple expansion stages of liquid refrigerant from one pressure to another pressure.

If the context is gas compression this usually implies the use of an expansion turbine to gradually reduce the pressure from higher to lower to extract energy from the gas.

[amitsaxena]

Thanks for the replies, but i will make my question more clear

"Is there anyone who has tried two stage expansion in a simple vapor compression system (Single stage)"
As i am going to experiment this and expecting some suggestion if anyone can give.

Thanks
Amit Saxena

[US Iceman]

"Is there anyone who has tried two stage expansion in a simple vapor compression system (Single stage)"

Yes. However, we need to define what each expansion stage is, and, if it is being done for the proper reasons.

Consider a screw compressor in a single stage system (using a flash type economizer). The flash tank would be the first stage of expansion. Typically the liquid feeding to the evaporator(s) would be the second stage of expansion.

However, if you use a second flash tank and expand the liquid again at an intermediate pressure between the evaporator pressure and the side port pressure of the first flash tank... where does the flash gas go to?

The only pressure low enough for the flash gas from the second stage of expansion is the compressor suction pressure. And... you would need a back pressure regulating valve to control the pressure in the flash tank (to keep the second flash tank pressure above suction pressure). Otherwise you will not be able to push liquid from the second flash tank to the evaporator(s). The pressure would be too low.

If you do this, then a small part of the compressor suction volume flow is displaced by the flash gas of the second stage.

You could do the same thing and have a flash tank between the condenser/receiver and the economizer operating at side port pressure.

You will need to review the thermodynamics of these process and ensure you actually gain some benefit from doing either one.

[Josip]

Hi, Amit Saxena

Thanks for the replies, but i will make my question more clear

"Is there anyone who has tried two stage expansion in a simple vapor compression system (Single stage)"
As i am going to experiment this and expecting some suggestion if anyone can give.

Thanks
Amit Saxena

I am not sure, but are you thinking about using economizer system ... that is a single stage system using two expansion pressures ... applicable only on screw compressors..

maybe I am wrong but your question sounds like..

or you can have a few evaporating pressures/temperatures, but all of them you have to control with the back pressure valve and the lowest pressure should be the compressor's suction pressure all other must be higher ... here you can utilize screw and piston compressors too...

If we are wrong, maybe you can explain to us where are you going to use that kind of cooling system ... please describe your cooling process...

Best regards, Josip

[amitsaxena]

Dear Mr. Josip / Mr. Iceman,

Thanks for your suggestion.
Yes i am going to try this in a Single stage ref system having Recip type comp. As you people have guided me through the precautions need to be taken care,can you people suggest me like how much gain i can expect in cooling cap. or EER.

Mr. Iceman, I am going to circulate the flash gas from first stage to compressor suction using a pressure reducing valve which will try to elevate the suction side pressure slightly and provide rich mixture of Eva out ref vapor and Flash gas ref vapor also prevent any liquid entry to the comp.

I think till now i am on the right track but still i am trying theoritically to prove the improvement. If you can suggest something or otherwise after experimentation i will do the analysis to understand the same.

thanks
amit saxena

[US Iceman]

I'm still not sure what you are trying to do. If you are using a flash tank/economizer on a single stage recip. compressor you have some problems.

Can you make it work? Yes.

Will it have any benefit to compressor performance or energy savings? No.

If you take the flash gas to the compressor suction via a pressure reducing valve (from the flash tank) the mass flow the compressor will see at suction is essentially the same as if you used single stage liquid (from receiver straight to evaporator). Therefore, the compressor is pumping the same volume of vapor and the power input will be the same.

A second issue arises in that the liquid supply to the evaporator is now at a lower pressure and it is saturated (not subcooled) so you can have issues with liquid flashing in the liquid line or not have sufficient pressure to push liquid to the evaporator).

Multiple liquid expansion can be done, but to be effective it must produce some gain in greater compressor capacity or lower power input.

A long time ago a different version was tried with old slow-acting compressors. They used a side port on the cylinders for economizing I believe. The port was located in the cylinder where the pressure was low enough to provide a benefit much like an economizer cycle on a screw compressor.

If you only have one or two cylinders the piping could be fairly simple. For a 8-16 cylinder compressor the piping would be terrible!

[amitsaxena]

Thanks for your reply Mr. Iceman

Thanks
Amit Saxena

[Josip]

Hi, amitsaxena

Dear Mr. Josip / Mr. Iceman,

Thanks for your suggestion.
Yes i am going to try this in a Single stage ref system having Recip type comp. As you people have guided me through the precautions need to be taken care,can you people suggest me like how much gain i can expect in cooling cap. or EER.

Mr. Iceman, I am going to circulate the flash gas from first stage to compressor suction using a pressure reducing valve which will try to elevate the suction side pressure slightly and provide rich mixture of Eva out ref vapor and Flash gas ref vapor also prevent any liquid entry to the comp.

I think till now i am on the right track but still i am trying theoritically to prove the improvement. If you can suggest something or otherwise after experimentation i will do the analysis to understand the same.

thanks
amit saxena

I was reading and reading your post, but unfortunately it is not clear for me ... maybe due to language ....but this is one technical forum, could you, please, post a little scheme of your idea or ... maybe even better the present scheme of your plant ... then we can discuss it like a real refrigeration engineers

N.B. I have a feeling that you have some other problem (liquid return to suction) and you try to solve it on this way ... but, I can be wrong

Best regards, Josip