R-134A compressors in R-404A applications

[herefishy]

Hi,

The Captain asked me a question, and I answered it. I thought it would be interesting to others on the forum, so I'm posting my response to his question: regrading R-134A compressors in R-404A applications.

_____________________________________________


If you reference, say Copeland, compressor application data (particularly the older material) you will note that most compressors are rated for a R-502 low temp application or a R-12 medium temp application. Do you have the Copeland compressor manuals? If not you could likely reference the catalogue of perhaps United Refrigeration. Now, the references per compressor that I illustrate, you may not be able to find readily in new literature, due to the differences of the lubricants for the different temperature ranges and refrigerant blends. But when all lube was mineral oil, A single compressor would be rated for different refrigerants at different suction temperatures. These days, just the oil contained in the crankcase will change the model number itself. Actually, the only difference between a R-134A compressor, and a R-12 compressor, is the lubricant!

It has been common practice in the ice cream merchandiser manufacturing industry to apply low-temp R-12 compressors (-10 SST) in very low-temp R-502 applications (-40SST) for Ice cream merchandisers. I personally experienced this while servicing some MasterBilt ice cream merchandisers. I was confused about the equipment when I referenced the Copeland information on the compressor, which was only rated for LOW TEMP R-12, but the manufacturer dataplate indicated R-502 ! The compressor was not at all recommended for a R-502 application (by Copeland).

I called the factory (Copeland) and inquired as to the situation. Copeland indicated that NO, they do not spec (the compressor) for the R-502 application, but they were aware of such applications by these manufacturers and warranted the compressors, nonetheless. The Ice cream boxes to which I refer are static evaporators (you know, the horizontal ice cream boxes with the slidey doors).

It is common for compressors that are appropriate for a low(er) pressure refrigerant in a medium temp application, also be appropriate for a low temp application with a high(er) pressure refrigerant. As such, in the case of the ice cream merchandiser, the compressor acceptable for the low temp R-12 application (-10F) was perfect for the very low temp R-502 application (-40F)

I apply the same principle with the stuff we are screwing with, here. Any mfgr data that I have available to me, does not rate any compressors below minus40F. As you may notice, we are obtaining minus60F saturation temperatures in the CPU cooling applications. Applying the principle of the ice cream merchandiser to the CPU chilling, we should be able to utilize a low temp R-134A compressor in a very low temp R-404A application. The key is decreasing the suction gas density by reducing the evaporating temperature in order to prevent the compressor from being overloaded. This is why modifying the cap tube in a prommy conversion from R-134A to R-404A is so important.

The capacity of a refrigerant control, be it a cap tube or TEV, is dependant upon the pressure drop across the device. The higher the pressure drop, the greater the capacity of the (cap tube). When converting to R-404A, the cap tube (in the prommy application) is exposed to a higher PD resulting in higher capacity which will exceed the compressor rating resulting in damage (or poor performance). The cap tube must be made more restrictive in order to stay within the original system capacity, which also results in the desired (lower) evaporating temperature that is the object of using the R-404A.

[]JR[]

:toast:

Nice explanation

]JR[

[DaBit]

You are omitting one thing: what about the elevated discharge pressures? Usually our condensing temps are low enough to stay just within manufacturers limits, but with R404a you exceed the specs for, example, a Danfoss NL11F already at 32C condensing. Which is not much for many of us.

I also wrote something about using R507 in the Danfoss here (http://www.icecoldcomputing.com/text/show_page.php?id=51).

[herefishy]

Hi DaBit,

This is simply what occurs in the real world.

I am reading right now, out of a 1995-96 United Refrigeration catalogue a KAJ*0100 compressor rated for 0F to 55F R-12, and at the very same time; also, -5F to -40F R-502

At 0F R-12 the compressor is rated @ 5,000 btuh.

At -40 R-502, the compressor is rated @ 2,700 btuh.

[baker18]

GReat info............

thanks
baker18

[herefishy]

Mind you, that as you decrease evaporating temperatures, or change an application, certain restrictions might apply (when different applications are specified by the manufacturer).

In most instances, extended range applications may have some parameters regarding maximum suction return temperatures, or maximum condensing temperatures (much like the concerns that DaBit has expressed), but in the indoor applications of the prommies, and the characteristics of these systems (lineset length, etc) the OC cooling applications typically meet these criteria.

[captaincascade]

ok this is a little off topic, but tell me if im wrong.

say yer usin a 1/3 hp compressor using r-22. its runing in 20 inches of vacuum and the evap is at -60c.

in running in such a deep vacuum on such a small evap, with such a small compressor. wouldnt the capacity be extremely low?

running like .026 cap tube, and alot at that, isnt that just basicly starving your evap?



and herefishy,
i was so flattered that you wrote that whole thing for me, and now i just feel dirty:D :D :D :D

[DaBit]

Now another thing to think about. Many people here are using salvaged high temp compressors (A/C compressors) in a low-temp application. Most of these high-temp units are suction gas cooled. If the density of the suction gas decreases (with other words: we run the thing in a vacuum), cooling might be compromised, right? On the other hand, produced heat is less since the compresor has to work less hard.

How do you think about this? I decided not to mount a suction gas<->liquid line heat exchanger on my high stage to give the (airconditioning) compressor as much cooling as possible. So far it works. The compressor gets hot, but not extremely hot.

[bowman1964]

Well a good read bud.and the truth.

you know there isnt but a few differances in compressors.the stoke and displacement of the unit.and then speed of which it opperates.some have heavyer stator winding than others to help disaplate heat and some have larger oil capicity to help with the heat also.
but i think you have said this before if i am not mistaken.
a smaller compressor usally works better.and this is because it has smaller stroke and displacement which is alot easier to pull a vacuum with a low displacement compressor than with a high displacement one.the larger a compressor you got to has a larger piston lets say.and we will use a piston as a example.the larger piston it has, the more surface area it has to seal off to create a vacuum.so in turn a larger compressor has to work harder to create the same vacuum as a small one.but then you get to the trade off......Garys words...LOL

but really he is right everything is trade off's,ballance between how much vacuum you need and how much gas you have to move( or displace) to cool a specific heat load.

again nice read

[herefishy]

Originally posted by captaincascade
say yer usin a 1/3 hp compressor using r-22. its runing in 20 inches of vacuum and the evap is at -60c.

in running in such a deep vacuum on such a small evap, with such a small compressor. wouldnt the capacity be extremely low?

There is what is often referred to as the "rule of thirds". If you have a 1hp compressor in a high-temp (air-conditioning) application, you will realize approximately 12,000btuh of refrigerating capacity (1 ton). Funny enough, this is not by design, but is a coincidence that a 1hp motor driving a compressor can produce somewhere around 1-ton of cooling capacity @ +45F evaporating temperature.

Now, the same 1hp @ +20F evaporating temperature will produce somewhere around 2/3 ton refrigerating effect (9,000btuh), and in a low-temp application (-10F evaporating temperature) you may see around 1/3 ton refrigerating capacity (4,000btuh). As you saw in my example previous, @ <-40F, you will see about 1/2 of the last 1/3rd (1/6 ton) of refrigerating capacity (just a guess from my recent observation).

This is just ballpark stuff however (that the reefer guy uses in the field), and assumes that the compressor is properly applied, or even capable of performing the task for which it is commissioned, and does not hold true across the board, so-to-speak.

by the skipper
... running like .026 cap tube, and alot at that, isnt that just basicly starving your evap?

Well, your evap would only be "starved", if the load exceeded the capacity of the cap tube/system. If you designed a 150watt refrigeration system, and it was exposed to a 300watt load, would it be starved? Or would it just be misapplied?

The refrigerant can/does exist as a saturate in a vacuum (just at a lower temperature like the -60F as you indicated). You've noticed that many become concerned when they see ice on the suction line going back to the compressor, "Am I flooding back?", well, not necessarily.... At a -30F saturation temperature, you could form ice at +20F (50F superheat), and actually be overheating and/or damaging your compressor.

The cap tube length establishes the PD (pressure drop) across it, and further establishes the evaporating temperature of the system. Hopefully the compressor, evaporator, and condenser are also properly applied for the desired affect... and the load is actually what we anticipate it to be.


by DaBit
Now another thing to think about. Many people here are using salvaged high temp compressors (A/C compressors) in a low-temp application. Most of these high-temp units are suction gas cooled. If the density of the suction gas decreases (with other words: we run the thing in a vacuum), cooling might be compromised, right? On the other hand, produced heat is less since the compresor has to work less hard....
...I decided not to mount a suction gas<->liquid line heat exchanger on my high stage

I was reading through my Copeland application engineering literature, investigating the Captain's 2-stage compressor and interstage cooling scenario, when I ran across some other information specifically stating that air-conditioning compressors are NOT made to withstand "refrigeration" applications, AT ALL! And I agree with the speculation regarding subcoolers/heat exchangers. I hate them. If a system is properly designed (i.e. has enough condenser coil) there should be no need to short-circuit the refrigeration system in such a manner, unless there is a particular design application for such use. I have seen the snake oil salesman come into town (during the Jimmy Carter administration) and sell a whole load of heat exchangers with the promise of increasing system efficiency and saving money/electricity. The customer started losing compressors, left and right!

Typically, high or medium temp compressors in low/lower temp applications require that suction gas returning not exceed perhaps +40F or maybe lower (in applications that are specified), parameters such as I have eluded to earlier.

by Bowman 1964
.... a smaller compressor usally works better.and this is because it has smaller stroke and displacement which is alot easier to pull a vacuum with a low displacement compressor than with a high displacement one.

I don't think that I said that, but I relate your statement to the volumetric efficiency of the compressor. The A/C compressor is mfg'd to compress high(er) density refrigerant, and has larg(er) valve ports in order to (have enough room) for the higher density gas. As the gas density (application) decreases, and the suction pressure drops, the relative amount of refrigerant left in the cylinder at the top of the stroke which will expand on the suction (down) stroke decreases the volumetric efficiency of the compressor due to the higher compression ratio resulting. This decreases mass flow, as you say, and does affect compressor cooling and in the case of the A/C compressor makes it nearly impractical (from an efficiency point of view) for a low temperature application (in most cases).

The "size" of the compressor would be dictated by the capacity required for the application.

[DaBit]

Originally posted by herefishy
I was reading through my Copeland application engineering literature, investigating the Captain's 2-stage compressor and interstage cooling scenario

Ah, the captain uses a 2-stage compressor?

, when I ran across some other information specifically stating that air-conditioning compressors are NOT made to withstand "refrigeration" applications, AT ALL!

I am aware of that too. Still, many people here use them, and it seems to work.

And I agree with the speculation regarding subcoolers/heat exchangers. I hate them. If a system is properly designed (i.e. has enough condenser coil) there should be no need to short-circuit the refrigeration system in such a manner, unless there is a particular design application for such use.

Not short-circuiting; it is trading heat. Extract here, dump there.

It is the same as with every single component: you should be aware of why you use them, and what the impact of using them is. If you just throw them in, you can't blame the poor device for causing problems.

Personally, I have seen significant increases in system and especially evaporator capacity when using them. Especially with our ultra-low temperature applications, a lot of refrigerant flashes off in the evaporator without contributing to cooling the load. The percentage of refrigerant needed for this increases with decreasing evaporator temperatures, and it is about 35% at -40C/-40F R507 evaporation, 32C liquid into the TEV. Thus, only 65% of the injected refrigerant actually cools the load. Do a mass flow calculation to see for yourself.

Now, if we do heat suction gas<-> liquid line heat exchange, we decrease liquid temperature while we increase suction superheat. The decreased liquid temperature benefits our waty-too-small evaporators, while the increased suction superheat usually does not bring us into problems.

The last point is probably what caused compressor failure. Increased suction superheat means less dense gas and thus less cooling (for suction gas cooled hermetics), and more important: it drives up the discharge temperature. Possibly up to the point where the oil starts to decompose.

It also depends a lot on the refrigerant. R290 and R404a, for example, are not very sensitive to increased superheats, while, for example, R22 and especially R23 are. When simulating the refrigeration cycle of my cascades' low stage with R23, it very soon became clear that adding a SG<->LL HX to the R23 circuit would have caused the death of the compressor due to skyrocketing discharge temperatures.

Again, draw a few cycles into the PH-chart (or have CoolPack do it for you), and watch the results of adding a SG<->LL HX, and make yourself familiar with the side effects.

Especially in low-temp applications with HC or HFC refrigerants they can increase COP with 5-10% while increasing superheat and subcool, possibly preventing nasty things like liquid slugging and vapour bubbles in the liquid line. Not that these cannot be avoided by good system design, BTW.

It's not snake oil, it is just another tool which must be applied properly. It is not a magic solves-all-your-problems kind of device.

Typically, high or medium temp compressors in low/lower temp applications require that suction gas returning not exceed perhaps +40F or maybe lower (in applications that are specified), parameters such as I have eluded to earlier.

I am feeding my A/C compressor -30C/-22F suction gas, and until now it seems to be quite happy with it (as happy as it can be with 'square bearings' :rolleyes: ). Is there a bottom limit on suction gas temperature?

[Gary Lloyd]

In a closed loop system, mass flow is equal at all points (conservation of mass flow). In other words, the cap tube dictates the mass flow of refrigerant throughout the system.

If we go to a higher pressure refrigerant using the same cap tube, the mass flow is increased due to the higher high side pressure pushing more refrigerant through. This increased flow can overload the compressor under heavy load (during pulldown).

We can compensate by reducing the refrigerant charge, but then we are pushing a liquid/vapor mixture through the cap tube in order to reduce flow. Cap tube sizing is the key.

Everything is balances and trade-offs.

[Gary Lloyd]

Discharge temperature depends upon compressor inlet superheat. Adding a suction/liquid heat exchanger has no ill effects on the compressor, but high compressor inlet superheat does. This can be disastrous on a TXV system in particular, because the superheat is controlled at the coil outlet rather than the compressor inlet. This must be compensated for.

Everything is balances and trade-offs. (Thinking I should add this line to my sig) :D

[Gary Lloyd]

A compressor that is designed for low pressure refrigerants and/or smaller capacity has smaller valve ports (among other things), because it is designed to pump less mass flow. All else equal, this will allow it to run at a lower low side pressure/temperature. But since when has all else been equal... LOL

Everything is balances and trade-offs. :D

[captaincascade]

i think chocolate chip are the best kind of cookies, and thats all im tryin to get at here

[herefishy]

Originally posted by captaincascade
i think chocolate chip are the best kind of cookies, and thats all im tryin to get at here

Maybe I should've made it clear, that the Captain merely asked me, "Why (herefishy) likes low temp R-134A compressors in a very low temp R-404A application?", since he had heard that I have professed this. (i.e., why I like butterscotch... LOL :p )

I didn't mean to make you feel "dirty", Captain.

The Skipper was by no means asking me for advice, and I guess in light of a lot of exchanges on this forum, I am only relaying my opinion and my experiences (as right or wrong as they may be). As a matter of fact, I was learning something about 2-stage compressor applications during my browsing. The Captain only served to heighten my interest in the subject, by means of speaking of things that I have no practical experience of.

BTWFWIW, I don not know that (and I never said) the Captain used a 2-stage compressor on his cascade.

see ya'

[captaincascade]

lol, i guess its hard to tell when someones joking, when you cant hear or see them say it.

i like feelin dirty.

vote yes on prop 83

prop 83 is a motion to create laws that would restrict those in the baking industry from; manufacturing, distributing, dunking, chewing, ect.... of any cookie other than a peticular specified cookie. this cookie shall be chocolate chip. there is no longer a need for any other cookie.
the people have spoken.

What? im bored.

oh and i used 2 1hp copeland hermetic compressors

[WxChaser]

Mmm cookies!

Nice easy reading material. Now where are the Bayer? :toast:

[sadcat]

This thread should be moved to Xtreme baking.