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I am in the process of choosing a compressor for a low temp. condensing unit I am having installed. I am being given the choice of a Copeland Scroll or Semi-Hermetic compressor. With similar btu ratings (29,000 at -25C), the scroll has an EER of 5.8 vs 4.6 for the other, but has a mass flow of 349 (lb/hr) compared to 585 lb/hr for the semi-hermetic. I am a bit confused which is the better choice in terms of energy efficiency and actual cooling capacity.
You answered your own question, mate. If they have the approx. same Btuh rating, that makes them have the roughly the same capacity. The EER is the efficiency, so the scroll is more efficient. The flow rating is displacement, which has to do with the application.Quote:
Originally Posted by rickmor
Here is an interesting question that does not have a readily apparent answer.
If you want to see how confusing the compressor ratings are, this is a good example.Quote:
Originally Posted by rickmor
If the capacity is the same (Btu/hour or kW) for each compressor, then looking at the mass flow is a good indication of how these may not be comparable.
The mass flow for the semi-hermetic compressor is about 67% greater than the scroll compressor for the same operating conditions.:eek:
Something has to be the cause of this discrepancy. This could be caused by one of two things (assuming the refrigerant is the same for both compressor ratings).
Different rating conditions for subcooling and suction superheat or, a huge difference in volumetric efficiency.
I would carefully look at the specific rating conditions used for each compressor to be sure they are identical. If not, then you are comparing apples and oranges.
I have seen ratings based on huge differences in subcooling and superheat to cause an increase in the performance capability of compressors. In that regard you may see a much smaller compressor appear to have the same capacity as a much larger compressor!
This does not pass what I refer to as a smell test!
Look at the compressor displacement also (cubic feet per minute or cubic meters per second). The compressor displacement should be very close to each other. A minor difference would indicate the increase or decrease due to volumetric efficiency between the two compressors.
A scroll would also be more cost effective, since budgets are always a concern.
Flow is flow and that equates to capacity. I would like to see the model numbers of the compressors because I think we are seeing ratings at different conditions. I question the actual cooling capacity, and thusly, the EER. R22 would provide the capacity you mention at a displacement of 585 lb/hr at the temperature in -25 F, but the 349 lb/hr flow rate seems more representative of -25C.Quote:
With similar btu ratings (29,000 at -25C), the scroll has an EER of 5.8 vs 4.6 for the other, but has a Mass flow of 349 (lb/hr) compared to 585 lb/hr for the semi-hermetic. I am a bit confused which is the better choice in terms of energy efficiency and actual cooling capacity.
US Iceman raises a good question. Perhaps it is not an error with temperature conversions, but more a stinky issue with credits and such issued for marketing values that only serve to confuse us poor folks. Let's get the model numbers and run them on Cpcalc or whomever's program is available.Quote:
Something has to be the cause of this discrepancy. This could be caused by one of two things (assuming the refrigerant is the same for both Compressor ratings).
The semi-hermetic uses 502 while the scroll uses 404A. The displacement of the scroll is listed at 727 Cu.Ft. per Hr (CFH) while the other is listed at 1615 CFH, but both have nearly identical cooling capacities at 29,400 (scroll) and 29,200 (semi) btu at Evap/Cond -25/105 and RG/Liq 65/105. The power consumption of the scroll is listed at 4.95kw vs. 6.4kw for the semi. Bottomline is what compressor is more efficient given its actual cooling capacity? Are these compressors even comparable in terms of actual cooling capacity? The model no. for the Copeland Scroll is ZF18KVE-TF5 while the semi-hermetic is 9RB1-0765-TFC. I got the spec. sheets for each model from the Copeland website.
Hi, Rickmor. Btu ratings and mass/volume flow are invariable. But EER could change in different conditions and in different applications the semi-hermetic may become more preffered. Also semi-hermetic has bigger resource than scroll and it easily repairs.
For energy saving scroll is much better and that is friendly invironmental,but in my experiences its easy to maintain semi hermetic, not so complicated..
Hi Rickmor ,
I wonder if you are looking at a USA sourced copeland Semi / Scroll Or a Copeland Europe Sourced Semi / Scroll.Copeland is now owned by Emerson as we all know & has 2 Websites - 1 for the USA / 1 for Europe.BOTH have got Copeland Software Selection programmes that you can input Operating conditions into & the software will give you the EXACT answer of m3/hr , EER's , Amps , etc etc.Be Aware that Copeland USA uses slightly different RATING conditions from Copeland Europe , and if you are sourcing a model from differing copeland sources , your figures will reflect this.Join the Emerson website with membership & you should be given full access to the copeland compressor selection sofware.Very Very Handy ,and you can iron ANY problems out with selections quickly.
Cheers.
scroll its cheaper well if should b
Quote:
Originally Posted by rickmor
Semi is allways better in low temp you have a stornger stroke speciely colder then -10F
I'm laughing my head off.Heres Wineman again.Wineman , I'm amazed that you find time to get off that Yacht of yours , and leave a message or two specially when theres so many things to do in Spain...Or is now "moored in Lancashire" ?
The obvious pros/cons between semi/scroll choices are scrolls are generally cheaper and are a 'disposable' item but semi's more expensive but maintenance friendly.
Does anybody think there will be any shift in popularity as scrolls are getting bigger (30hp/50kW) and can be 'remanufactured' (J&E Hall Re-Scroll)?
I don't think there will be a shift to scrolls in supermarket or low temp refrigeration. We have tried them and they have a high failure rate. The Carlyle and Copeland semi-hermetics we use are more forgiving and longer lasting in more diverse conditions than scrolls. Our dehumidification units here in Georgia had two 15 ton scrolls in tandem. Because of the failure rate we now changed the new units to four seperate circuits with 7.5 ton scrolls and they work great. Sometimes I think that the bigger the internal scroll platters get the harder to balance the internal parts gets. Just an opinion.
Semies all ways for the win, they can be fixed, near never break, and will survive a low yeild nuclear blast!
You just can't beat a well installed semi!
To answer your question in terms of energy efficiency, the scroll is the better choice ;)
howzet I would go with the semi for long term use due to that it can always be repaired as with the scroll if it packs up it is useless an it must be dumped,but also saying that a scroll has less moving parts and can last longer if commissioned propley
I sense a certain degree of frustration out there ,which I share, with scroll .... from static to transport; retail refrig, residential and commercial a/c and so on. It fascinates me from the marketing point too ....who drove this business model? Did the 2 major scroll OEM's satisfy a real demand when they came up with it OR did they pioneer scroll tech first , then go try sell the whole HVACR world on the concept of volumetric efficiencies, "wear in, not out" and so on ?
From what i seee in the fieldDealing Mainly with Copeland Scroll VS copeland Semi in the Cooler/Freezer applications, Failure rate after 5 years nearly 100% for Scrolls with nearly 0 for semihermetic recips.
Maybe the heat loads here but likley improper applications, too high superheat? Mostly motor burn out on scrolls.
This can't be! sorry, the adaptive scroll is much more rugged than the semihermetic and a high failure rate should be investigated, possibly caused by high discharge temperature that is the one thing that a 100% volumetric efficiency compressor will not tollerate.Quote:
Originally Posted by CHilly1
But going back to the original post. Can you send me directly the two models you are comparing, I will send you correct comparisons. One operation point is not enough unless you know your system will be usually working very near those conditions. Also give me info on you application because if you must sometime go lower than -25 I doubt the Scroll will be more efficient not because of the technology but some manufacturer's design choices.
i once had a conversation with an area manager for safeway and they worked out that the failure rate was far higher in scrolls compared to semis,but at a copeland factory he was told they were never designed for low temp as such was the need to fit demand cooling into the head.also the new controls for scrolls are ten times better than they used to be and iam seeing alot less failures now
Always a new product has a higer failure rate than another that's been around. I've seen many failures myself but many can be tracked to people not fully understanding the technology. I can't beleive semi hermetics can beat orbitals in relative figures (divided by the total number out there).Quote:
Originally Posted by fowlie
Semi hermetics were not used in low temperature either when they first came out eons ago! People tend to stretch the technology after trying it until a smart guy uses it for something else.
Yes low temperature orbitals still have a long way to go. They also have big electrical motors, good for wide range applications but no good for efficiency in low temp. Hearsay open scrolls will go lower without scroll cooling through inyection. Time will tell.
Ever heard a scroll running on a vacuum after a LP control has failed on pump-down? Very nasty.
I got a service contract at a school a couple years ago. They had some small systems. One of them was a scroll pump down system for a cold room. The LP was set at 0.5 bar cut in and 2 bar diff :eek: It never cut out of course. The compressor ran in a vacuum when the solenoid valve closed until it got so hot it tripped on internal motor protection. It had run like that for over a year. I was impressed that it survived. It was a Copeland.
The guys installing these systems couldnt have been among the brightest guys. On one of the evaporators the temperature bulb of the TEV was clamped to the tube between TEV outlet and evaporator inlet :eek:
It was an internal pressure sensing TEV. Wonder how they got the superheat adjusted :rolleyes:
Scroll compressors :eek::eek::eek:
First please do not look at swept volume as a good indication of anything between scrolls and other types it does not compare due to different volumetric efficiencies.Quote:
Originally Posted by rickmor
You cannot compare compressor efficiency using different refrigerants unless you have information on how much of the difference is explained by the refrigerant only. Cannot use general refrigerant comparisons here.
But deffinitely can compare the pair compressor-refrigerant and it is not surprising it favours R502, the industry spent 30+ years optimizing compressors for its use. Please wait if you want to pull me by the nose and make me look at the figures in this example, its misleading. See below first.
Oh, yes! Definitely a BTU with R502 is the same as a BTU using R404a.Quote:
Originally Posted by rickmor
Sorry I missed this info completely before (My wife must be right, I'm reading with half my brain!).Quote:
Originally Posted by rickmor
I have these results from my CpCalc reference number P66-04R2.
9RB1-0765-TFC EER=4.6 BTUH/W @ -25/+105/+65 no subcooling.
ZF18KVE-TF5 EER=5.9 BTUH/W @ -25/+105/+65 Low temp Economized.
Great! The ZF18 is more efficient ... way wrong!
If you hit the definitios button in CpCalc you will see that Low temp Economized means the system has 68,4 F subcooling! Moreover this subcooling comes from using some of the refrigerant flow through the same compressor. Because the injection port is way in the scroll not affecting the suction flow, it does increase power consumed.
Recalculating with 68F subcooling the 9RB1 jumps to EER 6,4 BTUH/W.
This tells you that the pair 9RB/R502 is more efficient than ZF18/R404a but it tells you nothing about the compressors efficiency.
If I had to bet how will ZF18 compare to other low temperature compressors EFFICIENCY I'd bet against it for several reasons.
- It is the first generation low temperature scroll. Optimizing the scroll's shape for low temp is an ongoing process.
- The electric motor is to big, it works in a VERY wide temperature range, won't compare well against low temperature compressors.
- Scrolls like Screw compressors have fixed discharge/suction volume and suffer from under and over compression, this was improved a little placing a small check valve in the discharge the first models did not have.
But I have no doubt this will improve in the future, and maybe Scrolls success in other applications have delayed developments in this area.
Anyway I think ZF18 is a great compressor I've fount that pull-down time is shorter if you uset it (this also improves efficiency) because it does not overload and has good capacity at higher temperatures.
And you should definitely use it with Economizer, this does not make it more efficient (the system is the compressor not) but won't make you miss R502.