2 Compressors In A Single Stage?

[Shingoshi]

to eliminate the oil level issue on hermetic compressors in paralell; you simply drill a hole in the bottom of the shell, and drill a hole at the upper part of the compressor, or use the service pipes to equalise the gass pressure. By doing so the oil flows freely from one compressor to the other. If one compressor is not running, ther wil be no problem since the compressor high pressure valves are shut due to the pressure difference.

I just got confirmation of my compressors being shipped now. Can't wait to see these things. Having two York 2-ton compressors in parallel should be a blast!

Shingoshi

[sdumper]

I just got confirmation of my compressors being shipped now. Can't wait to see these things. Having two York 2-ton compressors in parallel should be a blast!

Shingoshi

off topic please post in your own thread.

[Shingoshi]

http://cgi.ebay.com/COPELAND-SCROLL-...d=p3286.c0.m14

Tandem Scroll Compressors. This is how you build a tandem compressor system. Notice the oil equalization piping at the bottom with the sightglass. Imagine building a gigantic autocascade with this setup

So was this also off-topic? It was a point raised here, not in my thread. And I believe it was already commented on. I simply stated that this wasn't a concept only to be left up to the imagination. What's the point in people providing ideas, if no one is expected to use them? I gave no further details here about it. Instead, I did that in my own thread. Sorry I ruffled you.

Shingoshi

Thanks [XC] gomeler: I thought I read the title here as "2 Compressors In A Single Stage?"

[[XC] gomeler]

Easy guys, no need to get up in arms. Shingoshi was just informing us that he purchased two compressor for a tandem compression experiment which is relevant to the last month's discussion in this thread.

[Shingoshi]

I'd think you'd have to desuperheat/cool the discharge. And then you'd risk sending liquid refrigerant at your other compressor.
But two stage compression does exist, so it has to work in some way, unless the systems are rated for dealing with that sort of work load.
If you want to play with two, try two in parallel.

Would it be possible to use one compressor before the condenser and one after it? I'm thinking that if you use a refrigerant that wouldn't condense but lose most of it's heat at room temperature (from the first compressor), you should still have vapor entering the second compressor. The second compressor then would need effective subcooling to get the refrigerant to condense to a liquid. Or does this then qualify as a two-stage system?

Shingoshi

[mEsUsah1]

Would it be possible to use one compressor before the condenser and one after it? I'm thinking that if you use a refrigerant that wouldn't condense but lose most of it's heat at room temperature (from the first compressor), you should still have vapor entering the second compressor. The second compressor then would need effective subcooling to get the refrigerant to condense to a liquid. Or does this then qualify as a two-stage system?

Shingoshi

The system you are talking about is a two-stage system. You should know what you are doing if you atempt this. If the "condenser" (really a gas cooler) is to efficiant you wil have liquid out of it. But the good thing about this if you do it right is that you will have a lower spesific volume, and lower gastemperature. That means lower displacement on the sencond stage, and lower power consumption overall. It's a good idea to have a liquid seperator after the gass cooler (middle pressure cooler) to avoid a broken compressor. In industrial cooling this is actually a normal way to do things (mainly because you will have lower gas temp into the condenser, pluss a lower power consumption)

[Shingoshi]

As always, click on my images to see the links they point to!

The system you are talking about is a two-stage system. You should know what you are doing if you atempt this. If the "condenser" (really a gas cooler) is to efficiant you wil have liquid out of it. But the good thing about this if you do it right is that you will have a lower spesific volume, and lower gastemperature. That means lower displacement on the sencond stage, and lower power consumption overall. It's a good idea to have a liquid seperator after the gass cooler (middle pressure cooler) to avoid a broken compressor. In industrial cooling this is actually a normal way to do things (mainly because you will have lower gas temp into the condenser, pluss a lower power consumption)

I'm guessing you'd want to use something like this:

So then the question now becomes,
1.) Which refrigerants are pretty much guaranteed not to condense at room temperature? R-744?
2.) Specifically, which refrigerants need very low temperatures to condense? I know that R-14 is a candidate.
3.) Should both compressors be a different size? The first one larger than the second?
4.) Could such a system function with only a single refrigerant? Or would you still need another seeding refrigerant (like R-600a) to work for the subcooler downstream from the second compressor?
5.) Could you have a combination of refrigerants all incapable of condensing at room temperature? (That seems to make sense)
6.) So which refrigerants require an even lower condensing temperature than R-744/CO2?

Shingoshi
EDIT: Sorry for repeating myself. I keep re-editing my text, and I wind up with redundant questions.

EDIT: I always learn so much from these guys!


EDIT: I think this applies to this subject as well:

[Shingoshi]

I got my new compressors today. I've posted pictures of them in my thread.
The Towers of York

[IMG=http://img27.imageshack.us/img27/5177/compressortwins.th.jpg]

Shingoshi

[mEsUsah1]

So then the question now becomes,
1.) Which refrigerants are pretty much guaranteed not to condense at room temperature? R-744?
2.) Specifically, which refrigerants need very low temperatures to condense? I know that R-14 is a candidate.
3.) Should both compressors be a different size? The first one larger than the second?
4.) Could such a system function with only a single refrigerant? Or would you still need another seeding refrigerant (like R-600a) to work for the subcooler downstream from the second compressor?
5.) Could you have a combination of refrigerants all incapable of condensing at room temperature? (That seems to make sense)
6.) So which refrigerants require an even lower condensing temperature than R-744/CO2?

1,2)This is kind of andvanced stuff but anyway. You don't look for a refridgerant that does not condense in room temperature. You can do this with every rifridgerant. You just "get rid of" the gas superheating (cooling the gas, so it gets closer to condensing temperature at the spesific pressure).

3) Usually the first compressor (that sucks superheated gas from the evaporator) is close to 3 times larger than the sencond compressor. This is due to the lower spesific volume on the gass after the intercooler.

4) The difference between cascade and traditional two-stage system is that you just have one refridgerant in a two stage system. (The same refridgerant runs through the entire system). In a cascade you can have the same refridgerant, but they run in a independent loop that heat exchange (The 1st. stage is condensing against the 2. stage evaporator).

5) You chose refridgerant based on the pressure and temperature you want the system to have. Things like oil blend capability, and oil types is also a factor. The critical pressure/temp (the maximum pressure that a refridgerant can have in order to condens/evaporate) is a major factor.

6) Nitrogen has an extremely low critical point. (34bar/-147c) But this is not what you want to consentrate on.

Hope this helps!

[Shingoshi]

1,2)This is kind of andvanced stuff but anyway. You don't look for a refridgerant that does not condense in room temperature. You can do this with every rifridgerant. You just "get rid of" the gas superheating (cooling the gas, so it gets closer to condensing temperature at the spesific pressure).

3) Usually the first compressor (that sucks superheated gas from the evaporator) is close to 3 times larger than the sencond compressor. This is due to the lower spesific volume on the gass after the intercooler.

4) The difference between cascade and traditional two-stage system is that you just have one refridgerant in a two stage system. (The same refridgerant runs through the entire system). In a cascade you can have the same refridgerant, but they run in a independent loop that heat exchange (The 1st. stage is condensing against the 2. stage evaporator).

5) You chose refridgerant based on the pressure and temperature you want the system to have. Things like oil blend capability, and oil types is also a factor. The critical pressure/temp (the maximum pressure that a refridgerant can have in order to condens/evaporate) is a major factor.

6) Nitrogen has an extremely low critical point. (34bar/-147c) But this is not what you want to consentrate on.

Hope this helps!

1+2.) Let's concentrate on R-744 as my baseline for this discussion. I need to look at how much pressure I need to get the condensation point low enough to work with. I need to take the time to convert bar to psi to see what I'm working with here. My main concern is that I don't want too much pressure inside my gas coolers. Specifically because they weren't designed for this (They're actually very large 32"x12"x3" Turbocharger Intercoolers). So I don't want the pressure inside of them to be higher than say 200psi. I want to use my smaller compressors to bump the pressure up from there.

3.) My two compressor sizes are 19.6cc and 7.75cc respectively for the Matsushi+a (York) and Rechi. That's about a 2.5x difference between them.

4.) I have a total of six compressors. Two York and four Rechi. See my link I posted above to see them together. I want to use the two York (Panasonic/Matsu) compressors in tandem/parallel for my first stage. I simply want to them to provide for the system's volume/load capacity. I then want to divide the four Rechi compressors into two separate tandem/parallel groups. The first tandem group will be used to bump the pressure of the refrigerant up coming from the gas coolers. The second tandem group will be used to subcool the refrigerant leaving the first group, passing the heat back to the outbound refrigerant.

So there will be two completely separate refrigerant loops in the system. The first loop will have the combination of both the York and Rechi compressors. This loop will likely have R-744 as the main refrigerant. The Yorks will be (first-stage) before the gas cooler, and the Rechis will be (second-stage) after the gas cooler. The second independent loop will have only the last two Rechi compressors to provide the subcooling. The subcooling loop will likely have R-600a as the refrigerant. The two loops (main flow and subcooling) will run in counterflow. The inside subcooling Rechi loop is moving refrigerant in the opposite direction against the main incoming refrigerant loop. Instead of the subcooling loop having it's own condenser, it will use a heat-exchanger to cool the subcooling refrigerant with the refrigerant coming from the evaporator of the main loop. This will also be done with the two Rechi compressors in the second stage.

I'll leave 4+5 alone for now.

Thanks for your time!
Shingoshi

[mEsUsah1]

I imagine you have made some autocaskade/cascade systems. When you build two-stage systems you have to think a bit different. And R744 (CO2) is really different from the usual refridgerants. I'm sorry but i'm not used to working with psi (i live in europe). R744 has a critical point of 73,6bar (31.1 °C). In other words to make a traditional system with condensing, and subcooling you must have a pressure below 57bar(20°C).

This is how a typical transkritical 2-stage R744 the system is buildt. As you can see the pressure in the middle pressure reciever is quite lower than the gas cooler pressure.




This system solution does not requier external cooling. and will give a good cooling capasity with low mass-flow. And therefor a low compressore displacement. Sub cooling before the evaporator vent is not neseserry in so small builds. You recomand having a suctiongass-heat exchanger on the low pressure compressor. Leting the middle pressure liquid heat exchange with the suction gass from the evaporator. This will superheat the gass before the compressor and subcool the liquid before the evaporator. And then secure your low pressure compressor from "liquid-hammering" (destry the compressor).

EDIT: Usually you have a 2 times larger dispacement on the low pressure compressor i R744 systems.

The valve between the gas cooler and the middle pressure reciever is important if you plan on running transcritical. If you don't have it you'll have a low evaporator performance.

I do not recomend building a sub-critcal system with R744 if you dont heat exchange the condensation with another refridgeration loop (cascade)

A solution for subcritical R744 is something like this:


This is a good way of doing things but requier a lot of automation + oil return systems.

Hope this helps.