R22，direct expansion system

[COLD WANG]

i have meet a DESIGN project :
r22,direct expansion system: the evaporate temperatur is -45C, THE ROOM TEMPERATE IS -40C,FREEZING COIL CAPACITY IS 120KW,THE FREEZING ROOM IS 150M FAR AWAY FROM THE MACHINE ROOM.I am very worry !so far ,150m,r22 direct expansion system can do ?:confused:
help!

[Pooh]

Hi
you will not be able to use R22 as the suction pressure will be in a vacuum unless you fit an air purger also the compression ratio is 18 to 1 which means you should concider two stage if you have to use R22. The other concideration is the pipe work length even with minimal pressure drop you will struggle to get the evaporating temperature you are looking for.

Ian

[COLD WANG]

thanks for your reply ,
the condensor is water cooled,condensing temperatur is +40 c,screw compressor must be two stage?
the suction pipe can be sized minimal pressure drop 2k,but the oil return problem is very trouble.
direct expansion system can do ?150 metre so far away from machine room.
we will install a air purge .
and subcooling liquid line ?

[winfred.dela]

r22,direct expansion system: the evaporate temperatur is -45C, THE ROOM TEMPERATE IS -40C,


R22 system for -45C Tevap will always have potential oil problems.

I use R22 sytem only for up to Tevap = -25C.
For Tevap -30C to -55C, i suggest you use R-507.



THE FREEZING ROOM IS 150M FAR AWAY FROM THE MACHINE ROOM.


If this is a direct expansion system, you will have oil return problems. You should try to convert it to pump recirculation system.

[Samarjit Sen]

For Te -45 Deg. C with R 22 you should go in for a two stage system. However with a distance of 150 mt. firstly you have to design the pipe lines very carefully and secondly the oil return shall be a problem.

Let me think about the system and give you some remedies.

[Peter_1]

R22 at -45°C is running in vacuum, especially with the additional pressure drop in the long lines, so not advisable.

Fact you're using a screw doesn't change it's suction pressure.

Anyhow, you need 2 stage with R22 at teh temerpatures you're working on, otherwise you will have excessive discharge temperatures (+/- 130°C).

If your lines have a speed of +/-10 m/s (+/- 175 mm or +/- 3 5/8) and are inclined towards the compressor (1 cm/m), then I don't see a problem for oil return. If you need to rise for some reason, install an oil trap.
Monitor on start up and for some time - speaking of days- the oil in the sump because some oil will allways remain in the system.

Install also an oil separator.

Isn't there a possibilty to install the compressor near the cold room and install the condensor at 150 m? This is already a much better solution.

[US Iceman]

Isn't there a possibilty to install the compressor near the cold room and install the condensor at 150 m? This is already a much better solution.


That is an excellent idea Peter. The pressure loss on the longer high pressure gas lines would be more tolerable. The only problem I might see with this is the long liquid lines returning liquid back to the TXV.

If the liquid line is installed outdoors for the long distance, it should be insulated to protect from heat gain, which could cause the liquid to flash off.

Some type of subcooling here and the insulation would solve that problem.

[Peter_1]

Or install also the reciever together with the compressor and add some sort of mecahnical SC with a PHE nearby the compressor.

[COLD WANG]

thanks peter 1 for your reply ,thanks very much !
bizter one stage screw comp with eco and oil cooling with r22, discharge temp can reach (+/-130c)? i cant believe it .you can explain ?
the suction horizontal line are level or pitched in direction of the compressor ,oil can be transported with normal design velocities ? i think so ,the long horizontal distance is no problem. how much normal design velocities is? +/-10 m/s?
Double risers are necessary
i also think that IN this project should utilize two stage screw compressor for -45C suction temperature .but so long distance the suction pressure losses is 1K ?
and thanks you for giving me a good advice, but the condenser is installed 21metres higher than compressor , the hot gas riser will result in another problem with oil or so ?

[taz24]

R22 at -45°C is running in vacuum, especially with the additional pressure drop in the long lines, so not advisable.

.

Hi peter.

Where did you get the figures from that say 22 is on a vac at -45? :confused: coolpack say that at -45 pressure is 0.8bara.

I agree with what you say but amconfussed about the pressure.

Cheers taz:)

[Peter_1]

[COLOR=black]bizter one stage screw comp with eco and oil cooling with r22, discharge temp can reach (+/-130c)? i cant believe it .
How I got the 130°C? Isentropic compression of a refrigerant :confused:

If you give in your actual working parameters in the Bitzer software, it gives 214°C (!) (417°F) without cooling for a semi-hermetic screw.

With additional oil cooling is this of course something different.

Taz24, -45°C is indeed 0.8 bar (11.603 psi) but absolute, so running 0.2 bar (2.9 psi) in vacuum.
I know this figures right of my head because we have a 2-stage Mycom piston running on R22 and we need to hold it carefully just above -41°C ( -41.8°F) with an unloader on the pistons.

[COLD WANG]

thanks peter 1,
i blieve that the temperatur is 130 deg c ,and i think a two stage screw comp is necessary.
and another question :if the comp install on roof 15 meters high ?the riser can bing back the oil ?

[Peter_1]

Cold Wang,
This is indeed a better solution.
A height of 15 m is not that much but you have to take some precautions like you said.

If you have partial load, install at least two risers and the smallest one is selected for the smallest load.

If you have no partial load on the screw, I should split the suction flow to the compressor in 2 equal up-going risers, departing from a T and both with an oiltrap.

Should it be me, then the suction pressure should be limited just above atmospheric pressure - easy with a screw if you have an unloader slide - and I should install evaporators working with a very small DT (5K, perhaps 4K), eventually working with an EEV.

Working below atmospheric pressure can cause a lot of problems.

[COLD WANG]

Thanks peter 1, your advice is very helpful to me, thanks a lot!
Could you give me some advice? master.
1: in this case, whether it is best to use reciprocating comp or screw comp? both types are viable but which is best? i don't know.
2:If using reciprocating comp , I think that two stage is necessary. but for screw comp one or two-stage can do ? Two-stage for screw is more efficiency but expensive and one stage is simply but inexpensive. So lower temp I prefer two-stage installation. I am right?
3:One-stage installation for screw cant unload to 50% capacity because of ECO , 75% is the limits? If it is to run a part load, screw with eco isnt efficient. THAT IS RIGHT?
4: a two-stage installation is the best choice in low temp. Power consumption for such a two-stage unit is about the same for both compressor types, depending on plant size. SABRO TELL US THIS .i have a question : IF using reciprocating comp should we prefered a 2-stage compound machine or separate 2-stage machine.


Best regards, cold wang

[Peter_1]

If I put your numbers (-41°C/40°C) in the software of Bitzer, then a screw gives a COP of +/- 0.8 and a piston 1.19, even 1.3 for a 2-stage.
So in that case a piston.
If we compare price...pistons.
If we compare available place and/or weight: screws.

A one stage screw will do the job. You're right about the efficiency of a 2-stage compressor if you're working that low evaporating temperatures.
It all depends how much you will save on the electricity bill compared to the costs you have to make. If the supplementary cost can't be paid within 5 to 7 years, then it isn't worth to do these extra costs. That's how I calculate it.

A screw has its best efficiency when working at full load. If you need to work regularly at partial load, better install 2 screws or even better, some piston compressors.

The needed capacity (120kW) is in my opinion too small to use screws.
I remember - correct me if I'm wrong - that a compound installation performs almost the same as a 'normal' or a standard manufactured 2 stage, only you have to calculate it all yourself. I should say that a compound is more prone to failures.

If you ask me: I should install at least 2 or even more independent 2-stage-systems on R404a. If one fails, your freezer will still keep running - although on a reduced capacity - and for a storage freezer, this is the most important thing.
But also, with the R404a, you can evaporate at almost -47°C at atmospheric pressure(limited at -41°C for R22), so you then can choose evaporators with a DT of 6K.
If R404a shouldn't be available in your country, then R502 will be a better choice then R22.

[taz24]

Taz24, -45°C is indeed 0.8 bar (11.603 psi) but absolute, so running 0.2 bar (2.9 psi) in vacuum.
I know this figures right of my head because we have a 2-stage Mycom piston running on R22 and we need to hold it carefully just above -41°C ( -41.8°F) with an unloader on the pistons.

Peter you are right.:)
I will now shuffle off holding my head in shame:)

cheers taz.

[US Iceman]

I remember - correct me if I'm wrong - that a compound installation performs almost the same as a 'normal' or a standard manufactured 2 stage, only you have to calculate it all yourself. I should say that a compound is more prone to failures.


I would agree with that.

I might also add the use of screw compressors at very high pressure ratios will decrease the expected life. This type of operation is very hard on bearings in screw compressors.

A two stage (or compound) recip compressor system would be much better for longer life and lower energy use.

Also as important as the above, is the need to have some equipment redundnacy as Peter explained. If the system is used for process cooling, this need becomes more important.

Shutting down a process cooling system because of equipment failure is a very costly mistake.

[COLD WANG]

Hi Peter ,hi us iceman

Do you mean the comp install on roof 15m higher than blast evp is a good solution?
What is an EEV?
If this is a direct expansion system, you will have oil return problems. You should try to convert it to pump recirculation system?

[Peter_1]

It's indeed a good solution, well at leastn taht's what I think.
EEV = electronic expansion valve, but you don't need it really
If you install the U-bends (I suggest 3 of it), then you won't have problems with oil return. In your case, I should install 3 independent compressors with their own suction lines, so in that case you don't need double risers because it's then an on/off setup.

[COLD WANG]

thanks peter !very much !your adevice is really more helpfull to me.
but the custom sugest installing ***** parallel compressor rack systems ,so the comps are parallel running ,the suncion line is common one , not 3 independent compressors, instead ,3 parallel two-stage recip compressors.your adevice?
in this case, your mean dont need EEV ? NEED TEV?(MECHANIC)?
THANKS A LOT FOR YOUR REPLY!

[Peter_1]

In case of a rack with 3 compressors. Install 3 suction lines of equal diameter or 2 and the smallest one has the capacity of 1 compressor.

I should use ordinary TEV's.

What gas will you using? R22 is not a good choice. R404a or R507c is better and R502 is then even better then R22.

2 stage is more expensive but the COP is +/-20% better. I should install 2-stages.
Advise your client not to install a rack for the reasons mentioned above. Price will be +/- the same but you have Maxim redundancy with independent one.

[COLD WANG]

Hi Peter ,
1:a coalescing oil separator are far better than separators using only mist pads or baffles; most of oil can be removed from the stream by the separator and return to comp, the bizter oil separator is coalescing oil separator?
2:at temperatures somewhat lower than -40c , oil viscosity may be significant ,bitzer using what grade oil?


3: today iI ask york for calculating the cop of the same screw comp with r22 and NH3, they tell me in -45c/+40c ,the cop OF r22 is bigger than THAT OF NH3,and they tell me one-stage screw comp

Discharge temp is 85C ,NOT 130C .AND I ASK ANOTHER FACTORY IN OUR COUNTRY, THEY TELL ME IS 83C.???

[US Iceman]

You should try to convert it to pump recirculation system?


A pumped recirculation system would reduce the oil return problems since the additional liquid helps to transport the oil.

However, this would raise another issue of oil recovery off of the pumped vessel.

This is possible, but it adds another layer of complexity to your system.