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Goober
29-11-2015, 09:06 PM
Hi All,

I want to select 4 - 6 compressors of the same model for a system that has two evaporating points with out having to install 2 suction headers, only 4 degree C difference in the SST's.

The loads are follows, 75.6kW@-8 Degree C and 81kW @ -4 Degree C.

What jiggery pokery do I have to do with the math to give me the total load that would be on the Rack, when I would have the Suction Header running at -8 and EPRS on the branches that run at -4?

Goober

RANGER1
30-11-2015, 10:06 AM
Goober,
I can't give you the calculation but think 15% reduction in capacity of 81kw -4 deg Csst system if run down to -8 deg C sst (65.7kw R404A -8 deg C sst 45 deg C sct).

Otherwise rack or compressor supplier would have selection table with capacities or software selection programme like Bitzer can be downloaded.


https://www.bitzer.de/websoftware/Calculate.aspx?cid=1448879906034&mod=HHK

RANGER1
01-12-2015, 09:14 PM
Goober is this what you are looking for, or something else like all the calculations?

I just need some encouragement as this site appears almost dead & no one seems to care.

I joined up with that US HVAC Talk, but they are off with the fairies.

They do have a lot of other things though, like downloads, articles of interest with the industry.

Goober
02-12-2015, 01:16 AM
Sort of, just more getting my head around what calcs you would do. But a colleague said to just add the the two capacities together and base the compressor selection on the lower SST. Which sort of makes sense.

Bitzer software doesn't have the function for Split capacity/different SST's. (Well actually it does, but not in the way I'm trying to think how to go about it)!

I know what you mean about this site, seems to be dying a slow death and being taken over by AC queries, mostly on VRV/VRF systems. Should rename it Air Conditioning-Engineer.Com

Magoo
02-12-2015, 04:17 AM
Hi Goober.
the capacities you quote is that evaporator capacity required , or at the compressor allowing for suction line losses.
magoo

Goober
02-12-2015, 04:38 AM
Good question, at compressor capacity, already takes into account Suction Line losses.

Glenn Moore
02-12-2015, 12:20 PM
Hi goober
Your colleague is right about the selection. The only point you need to consider is how you control the rack, as with eprs modulating your suction will be a bit erratic, so I would suggest the lead machine is invertor type to prevent the fixed speed machines constantly cycling. P

Glenn Moore
02-12-2015, 12:22 PM
Hi goober
Your colleague is right about the selection. The only point you need to consider is how you control the rack, as with eprs modulating your suction will be a bit erratic, so I would suggest the lead machine is invertor type to prevent the fixed speed machines constantly cycling. P

Magoo
03-12-2015, 03:58 AM
Hi Goober,
do a loading profile over a 24 hour period, summer then winter, so as you can select numbers of compressors required and stepping stages for capacity control. Alternatives would Bitzer mini screws several and VSD control. Carel do a rack controller I think, simple and off the shelf type system.

Glenn Moore
03-12-2015, 08:32 AM
Hi Goober
Danfoss have a large range of pack controllers . They have a new range called AK-PC 351,551& 651. These all can drive variable speed, digital scroll, cope land stream, etc it makes the pack control very flexible and easy to set up

Magoo
04-12-2015, 02:14 AM
Hi Glenn.
you are correct with comments with Danfoss systems
Down side is here in NZ they are supported from Melbourne Australia, not exactly down the road so to speak, total shame really as good equipment. Try getting ammonia related spares and it is always ex Australia, via Singapore via Europe and will probably comes from China..Every point clipping the ticket so super expensive. ( check were things are made usually PRC here, including Bitzer ).
Not too flash when hanging on the phone with a plant stopped. Basically I have scrubbed anything Danfoss related. Sorry. Hence referral to Carel that are based here down the road. Literally.

Glenn Moore
04-12-2015, 09:32 AM
Hi Magoo
That answers why I used to get calls from oz about fridge problems from old uk lads. Maybe there's an opportunity to become a Danfoss kiwi agent.
The most important one I did for oz was for the Sydney Olympics they couldn't get the beer coolers to work around the site due to the high ambient temperature of the beer with the long pipe runs around the site The compressors would not pump due to the high suction gauge causing the suction valve reeds to flutter . Got my old mate Digger Wood out of the brown stuff with that one ,probably saved the Olympics as there's nothing worse than warm beer, I'm still waiting for my crate of beer for sorting it.Maybe it stuck in Singapore customs ha ha best regards Glenn

Magoo
04-12-2015, 11:07 PM
Hi Glenn,
Ozzies love their beer teeth cracking cold. You would have definitely saved the day.