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desA
15-04-2009, 06:09 AM
I'm currently working through a new air-to-water heat pump design, for South-East Asian operating conditions.

I'd like to ask opinions & thoughts on whether a suction accumulator is essential in the system.

There seem to be two opposing viewpoints in the Copeland Application Engineering Bulletins:

1. AE-1300-R1 (rev: June 1996) "Application guidelines for 1.5 to 4 ton Quantum ZR*K3 and ZR*KC model Copeland compliant scroll compressors"

Accumulators
Due to the Compliant Scroll's inherent ability to handle liquid refrigerant in flooded start and defrost cycle operation, operation, no accumulator is required for durability below system charge levels listed in the Scroll Compressor Application Diagram (Table 1). However, large volumes of liquid refrigerant which repeatedly flood back to the compressor during normal off cycles or excessive liquid refrigerant floodback during steady operation can dilute the oil in any compressor to the point that bearings are inadequately lubricated and wear may occur. To test for these conditions, see the section entitled EXCESSIVE LIQUID REFRIGERANT FLOODBACK TESTS at the end of this Bulletin.

The Table 1 cut-off limit for using an accumulator is 10 lb (4.54kg).

2. AE-1247 (Dec 15, 1975) "Suction accumulators for heat pump applications


One of the most critical areas of air to air and air to water heat pump application is the proper control of liquid refrigerant under low ambient heating conditions.
System design must maintain a delicate balance between sufficient flooding to adequately cool the compressor, while avoiding excessive flooding which would adversely affect lubrication. When coil defrost is required, the compressor is exposed to sudden surges of liquid that can create extreme stresses in the compressor.
Both laboratory testing and field experience indicate that a properly designed suction accumulator can provided excellent protection against both potential hazards.

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My thoughts
My expected gas charge (from experimental tests) will be around 1.2-1.3kg of R-134A. This is far less than the 4.54kg recommended compressor cut-off limit.

Under South-East Asian design conditions, evaporator coil frosting is not expected as a routine occurrence (if ever).

My question
In the experience of RE members, what would your sound advice be on whether to use a suction accumulator, or not?

Links to additional field information, references, literature would be gratefully appreciated.

Brian_UK
15-04-2009, 11:23 PM
You have actually answered your own question by stating that the refrigerant charge is below the compressor limits so in theory you should be OK but ask yourself these questions...


Will you be including heaters to ensure that refrigerant will not migrate back to the compressor?

Are you concerned about the additional cost of fitting an accumulator?

Are you 100% certain that your system will never allow liquid refrigerant back to the compressor?

desA
16-04-2009, 08:48 AM
Hi Brian, thanks for your helpful comments. I'll answer piecewise below.



Will you be including heaters to ensure that refrigerant will not migrate back to the compressor?

No heaters to be included, since operational temperature zone lies in the 'safe zone' to exclude oil mixing in the refrigerant. We're pretty hot out here (typically >25'C), evap runs ~+10 to +12.5'C.



Are you concerned about the additional cost of fitting an accumulator?

Not the cost, but more the suction line pressure-drop (can accommodate), & more importantly, the additional refrigerant trapped in the accumulator itself - increasing the system refrigerant charge excessively. Also the available suction accumulators from Alco seem to be overly large, in my view - compared to refrigerant system charge. It's just one more problem, if it's not 100% necessary.



Are you 100% certain that your system will never allow liquid refrigerant back to the compressor?

This is the tricky part, in that the Copeland information seems to imply that as long as the total system charge remains below the 10 lb (4.53kg) limit, that the lower shell acts as a form of accumulator itself.

With a small refrigerant charge around 1.2-1.3kg, my thoughts are that perhaps the shell is essentially large-enough to deal with the no-accumulator situation.

I have a test rig in my lab with no accumulator & some units of similar design were shipped to Australia by others, in the past. In the main, they seem to have run ok, except for a few gremlins - one of which was compressor failures (2 out of a batch of some 30 machines). I had put this down to lack of control on the compressor discharge temperatures, where water had been allowed to rise up to something like 80'C, before action was taken (not advisable). Another issue (1, or 2 machines) were TXV failures - perhaps incorrect installation, or condensing coils set up incorrectly & a very poor liquid return design (water-hammer).

The designer's dilemma, I guess. I'm beginning to err on the side of being conservative & installing the accumulator more for peace-of-mind, than for pure functional requirements.

Has anyone seen an air-to-water heat pump without a suction accumulator?

nike123
17-04-2009, 01:24 PM
Make it without accumulator and do test as described in bulletin. If design passes tests, then you don't need accumulator.

nike123
17-04-2009, 01:31 PM
Has anyone seen an air-to-water heat pump without a suction accumulator?

All latest (not R22) Galetti air to water heat pumps are without accumulator. And many others. In fact, if you have R407C refrigerant in system, you cannot have accumulator because of refrigerant fractionation.

desA
17-04-2009, 04:48 PM
Thanks so much, Nike123 - to the rescue again. Much obliged.

Actually, an issue also cropped up with finding the correct vertical suction accumulator which could allow adequate spillback to the device from the compressor suction - in the event of shutdown. They were all too high for the scroll compressor I'm using.

Wasn't too happy about using a horizontal acc.

Ok - now I'm happier.

JohnAC
04-09-2010, 08:25 AM
greetings from Qatar!

just new in RE. working on DX split units installation 15TR w/ 35m vertical lift (outdoor above indoor) scroll compressor 2-circuit constant capacity. is an suction accumulator required? im already fixing traps 1/3 intervals of the vertical line? please help

Peter_1
04-09-2010, 05:57 PM
desA, I have a case for the Court pending on 3 Carrier chillers (330 kW each, air to water) without SA.
On 3 machines +/-10 compressors (Maneurop/Danfoss scolls) broke in a timespan of +/- 1.5 to 2 years.

I should go for you conclusion I'm beginning to err on the side of being conservative & installing the accumulator more for peace-of-mind, than for pure functional requirements.

In almost every small HP split (no Copeland scrolls), you always see a SA.
But you can use perhaps also a 'post-SH-coil' in the suction line (some rows in you aircoil, just a wild idea from me right now) perhaps cheaper.

EEV on the new ones?

nike123
04-09-2010, 06:10 PM
greetings from Qatar!

just new in RE. working on DX split units installation 15TR w/ 35m vertical lift (outdoor above indoor) scroll compressor 2-circuit constant capacity. is an suction accumulator required? im already fixing traps 1/3 intervals of the vertical line? please help

John, open new thread with your question, since this doesn't have anything to do with this thread subject.


http://www.mcquay.com/mcquaybiz/literature/lit_systems/AppGuide/AG_31-011_120407.pdf

desA
05-09-2010, 07:53 AM
desA, I have a case for the Court pending on 3 Carrier chillers (330 kW each, air to water) without SA.
On 3 machines +/-10 compressors (Maneurop/Danfoss scolls) broke in a timespan of +/- 1.5 to 2 years.

I should go for you conclusion I'm beginning to err on the side of being conservative & installing the accumulator more for peace-of-mind, than for pure functional requirements.

In almost every small HP split (no Copeland scrolls), you always see a SA.
But you can use perhaps also a 'post-SH-coil' in the suction line (some rows in you aircoil, just a wild idea from me right now) perhaps cheaper.

EEV on the new ones?

Thanks so much for the update, Peter.

I must say, I have also come to the same conclusion in regards to SA's. In many applications they really do provide the safety margin required for variable load patterns.