2-stage brain storm

[Redwolf]

Feel free to post any insight you might have on the listed subject.

I've been wanting to build a 2 stage for quit some time now. Only issue is pump availability. Seeing there are not any 3 cylinder compressors less than 5hp 220V 3phase. The only logical solution is to use two K body type (Semi Hermetic) pumps.
Challenges I see are..

1) Oil Return - could use the ports below the suction valve.. you know the ones Copeland uses to vacuum out the compressors. Just tee them together then run to the oil separator.
2) Oil pressure - about the only way I can see around it is to use two oil pressure controls. Even then I'd have to tap into the oil supply channels in the case. However I'd like to do this with only one control.. but I'm not seeing how as of yet.
3) Cylinders - I'd have to modify a valve cover on one of the pumps to keep only one cylinder actually pumping. Basically a permanent unloader on the other. But I'm wondering if this could be a problem down the road with oil, overheating, basic ware and tear. I've never seen a pump run completely unloaded forever before. Could have both cylinders on unloaders and just switch them on a timer. Led/Lag situation.
Running the compressor at only 1/2 speed is another option.. but I don't see it as a logical one. Oil return becomes another issue.
4) Desuperheating Valve - There is very little info on these. Over or under doing it can destroy the system.
5) Given components - Accumulator, Oil separator, liquid sub-cooler, Water cooled condenser, 2 solenoid valves, etc.

[captaincascade]

i have also thought about building one of these, oil return shouldnt be an issue with a k type. id say use poe and have yer oil return piped to the first stage. maybe gary can help us on this one? Ill ask my dad tomorrow if i get a chance, beccause i know before there where 2 stage compressors they used 2 compressors.

i think if you pipe it right the oil level will be ok. 1 bit of information i recall is that you should set your txv on 1st stage to 10f superheat, nothin special just somethin i remember

[Redwolf]

You mean the setting the Desuperheating TXV to 10F superheat?

Haven't made up my mind on what refrigerant to use yet. Like to try out 410A with a set up like this. Seeing around 100btu/lb @
-75F evap would be a sight to see. But justifying 210 bucks for a jug just doesn't do it for me. Prob go with 502 and Zerol 150. That way when I can get ahold of some. Changing the oil becomes much easier.

Im not really worried about oil return to the two systems. More worried about runing one cylinder on a two cylinder pump, and oil pressure safty.

Another thing I thought about was start-up. Seeing the power company charges you for the max you used in the month. So I'll do a few second timeing between stages to get around that. Same way we do with "Part-winding Start" systems.

[captaincascade]

yes, the desuperheating txv 10f superheat.

on some of the lyophilizers i work on, they use two stage compressors that get to -80c with 404a.

[DaBit]

I can't help with the semihermetic details unfortunately. But regarding the desuperheating TEV: are you sure you need it? With R404a/R507 you don't; desuperheating the first stage with a (part of an) aircooled condenser will do just fine. Didn't check R502 and R410a, though I don't expect problems with 410A either. But I can check if you want.

[Redwolf]

Yes you do.. or you could end up with super super high discharge tempatures and to large of a volume of gas for the second stage to pump. Kinda like the guys who are running 404A in these little units their building. Or even worse R-22
120 condensing and 20Hg. Oil break down, mechanical failure, burnt valves, etc.. just waiting to happen.

Someone today told me to just take out one of the suction reeds on the second stage pump. Hence just recirculating its own suction gas. Basicly a hole in the piston. Older refer guys.. gota love em.

As for 410A.. It seems Copeland is saying NO. You can't put 410A in a cast iron hermetic. The casting can't take the pressure (650psig max). What about all the R-22 rack chillers out there. Guess in 15yrs chiller guys are going to make a killing on replacing all those compressors to screws or scrolls.

[DaBit]

You may indeed be right. I just checked R404a discharge temperature for the second stage with gas entering at 25C, compressing from 1.3 bara (-40C equivalent temperature) to 18 bara (40C SCT). I set the compressor isentropic efficiency to 50%.

Discharge temp would be about 180C in this case. I am not sure if this is too high, imho it is just within limits.

Limiting this temperature can also be done by increasing the 2nd compressor inlet pressure. When the inlet pressure is increased to 2.5 bara (-25C equivalent temp), discharge temp drops to 155C, which is certainly within limits.

Thus, you don't need to spoil capacity on a desuperheating TEV/captube, but you need to balance compression distribution over the two compressors carefully.

[herefishy]

Originally posted by Redwolf
... Kinda like the guys who are running 404A in these little units their building.

Hi redwolf....

The application of nominally rated low temp, medium pressure refrigerant, compressors in high pressure refrigerant, *very* low temp applications is common. As I have often referred to the Ice Cream Merchandiser industry that utilizes (Copeland) compressors rated for low-temp (R-12) applications in *very* low temp (-30F to -40F SST) (R-502)applications. You cannot find a (high pressure) refrigerant application for the (applied) compressor, at all!

Compressors are typically only rated down to a -40 F SST, at a minimum. The R-404A applications that we are spec'ing run a -60FSST. If you would refer to mfgr data, you would see that many compressors are rated at say, medium temp R-12 AND low-temp R-502.

There ya' go.

This is not at all to negate your intial point, none-what-so-ever, however I take this opportunity in order to NOT dilusion the active participants who engage the activity that you describe. I personally assure that the discharge temperature of R-134A -30F to R-404A -60F applications does not exceed approx. 145degF discharge temperature under extreme conditions that it may encounter.

Cheers!

[Redwolf]

Oh wow.. I can't believe you had to point out the OEM data.

There is alittle more to it than just discharge tempatures. Compression ratio, Volumetric efficiency, Oil return, among others.
Have you ever used an enthapy chart? Dont know so I have to ask. So next I have to ask as to how you came to this conclustion, with in a real world refrigeration system?

Now I can't remember if its add 30F or 50F to the temp reading on the discharge line to get a close discharge temp.

[captaincascade]

30

jokin, right?

[DaBit]

Originally posted by Redwolf
There is alittle more to it than just discharge tempatures. Compression ratio, Volumetric efficiency, Oil return, among others.

Compression ratio is high. Way too high. This drops the compressor efficiency to a point where it would be unacceptable for commercial refrigeration. However, we shoot a bug with a cannon; we use heavy compressors to cool a tiny load. Compression ratio itself won't destroy the compressor, only lower it's efficiency.

Oil return: with the low gas flow, we need to consider this. But so should anyone designing a normal refrigeration system. Gas speeds in 1/4" lines is always high enough to ensure oil return. Gas speed in a 3/8" line is good enough for everything but a straight vertical rise.

The only issue I am worried about is hermetic compressor cooling. Many use A/C compressors, which rely on the suction gas for cooling. Low-temp hermetics suffer less from this since circulated gas volume is insufficient to cool the motor even in a normal application. So I suppose they dump most of the heat through the shell.

BTW, may I point out that I did use a log(p)-h diagram to get to the discharge temps mentioned?

[herefishy]

Originally posted by Redwolf
I have to ask as to how you came to this conclustion, with in a real world refrigeration system?

Yes. When servicing a MasterBilt horizontal ice cream merchandiser employing copeland semi-hermetic compressors. I had a very problematic machine. I referenced the compressor data (because every trunk-slammer in town had worked on the machine before I was called out) and the mfgr data indicated the compressor for only a low-temp R-12 application! But the box data indicated a R-502 refrigerant charge! Good Greif!

I called the box mfgr., and they told me that was the compressor to be in there. That wasn't enough for me. I called Copeland, and they said that they knew that these mfgrs were applying these compressors as such, they just don't spec the compressors for the application. Copeland did not have a problem with the application by these mfgrs.

If I look at Copeland compressor data information (right now), I see

model KAM*-0075 (3/4 hp)
R-12 @ 0deF SST = 3,670 btuh / 630watts power input
R-502 @ -40 SST = 2,030 btuh / 540 watts power input

As far as high discharge temperature concerns, I have just finished a MachII R-404A process (be it right or wrong) and I have loaded the machine up by attaching the evaporator to a 250degF condensate heater pan and increased the evaporator temperature reading to almost 0C (-59C to -62C idle temperature) and my discharge temperature did not rise above 145degF. That is a benchmark that I monitor when charging these systems as such. The discharge temperature is a primary criteria for reasonable operation.

When I scratch build a machine, I have employed Tecumseh TPA compresors rated from 950 btuh to 1050 btuh @ -10degF SST R-134A for the -50/-60F SST R-404A applications. I will tell you, that those compressors run *very* cool, seem very happy in their assembly, and perform very well.

[herefishy]

Originally posted by DaBit
Compression ratio is high. Way too high. This drops the compressor efficiency to a point where it would be unacceptable for commercial refrigeration.

DaBit, if the compression ratio is "way too high", then it is so in every low-temp R-404A application that exists in the world. You say the compression ratio is way too high... way too high in relation to what?

The compression ratios of these little machines is lower than the compression ratios of the Ice Cream storage at the convenience store on a 110degF day in Texas, bangin' 275psig head pressure.

[DaBit]

Originally posted by herefishy
DaBit, if the compression ratio is "way too high", then it is so in every low-temp R-404A application that exists in the world. You say the compression ratio is way too high... way too high in relation to what?

To the normally accepted range for reciprocating compressors. Danfoss recommends to use these compressors with a compression ratio of 9:1 or lower.

In fact, system efficiency drops very fast when the compression ratio increases.

The compression ratios of these little machines is lower than the compression ratios of the Ice Cream storage at the convenience store on a 110degF day in Texas, bangin' 275psig head pressure.

At those conditions COP is almost zero

[Redwolf]

Oil return for these set ups is strange at first glance. The seporator is on the 2nd stage pump. Then returns the oil back into the suction line of the same pump. The way to balance oil in both is to use a high side float which will over flow into the booster pump. The high stage pump ends up running a higher case pressure.

.......For this system design set point......
-high side psia / low side psia = compression ratio.
(110F)287.13/(-80F)6.18 = 46.46:1
-For a 2 stage you can take the same number and square root it. Giving you the ratio for both stages = 6.81:1
-Interstage pressure = square root of Evap pressure x Cond pressure = 42psia(-8F SST)

Herefishy, You haven't givin what all was need to see your conclusions. I'm not saying your wrong or even trying to behead you. For I know in some instances this is possible. However, can continue this in another tread.

[Redwolf]

Most Lowtemp 404A boxes are running a -20F evap with say 110F condensing. (Adverage norm.. Not taking in the 115F roof temp days) Ratio would be about 9:1. Very easy for low temp pumps to do.

[captaincascade]

i'm more of a hands on type. Yes I do think things through first, but if i get an idea in my head ,especially about freezers, i usually have it built in a week or two.


I say build it. Fix the problems. Get her goin

[Redwolf]

All in due time. Still working on a matching set of compressors.

[Redwolf]

An oil level control is basicly a low-side float...
I was thinking of using one of these but just turning it upside down. Then it should work as a high-side float. See any reasons why this wouldn't work?

[Redwolf]

Found a couple of nice 1hp 220V 1phase Extra low temp comps. KALB-0100-CAV-205
Now for the boring calculations. However at first guess I think she'll do around 2600 btu/hr @ -70F

[DaBit]

Would you like to share the calculations with us? I am interested...

[Redwolf]

Ah sure.. Once I get it down in my head. Stoecker has a strange way of puting his books togeather. Very Engineerish.

[DaBit]

Ah, Engineerish. My second mother tongue :P

[Redwolf]

Well crap.. They guy I was going to get the XL K bodies from re-niged on me. So I'm contemplating using 3 Tecumseh RK screws

[DaBit]

I have no idea what the difference is. Can you enlighten me?