Racks and diversity

[mad fridgie]

Hi to all the guys who are at the workface.

I understand in the UK and EU, that most (not all) supermarkets have large numbers of dedicated self contained rack systems, each having 8 or 10 scroll compressors each (manufactured by every man and his dog) a couple of questions
1 when these racks load and unload, do they change the duty rotation of the compressors, or do they just run in a fixed sequence?
2 Averagely over a year, how many compressor are running all the time (one would expect more in summer than winter) primarily LT systems
3 What is practically the minimum number of compressors running.
4 Do they have head pressure control, if so what is the normal method, (fan cycling, fan VSD, mechanical control of the refrigerant "valve"
cheers
Mad

[fowlie]

hi mad fridgie

all the packs now will run all the comps in a set sequence like 1-8 but will average the comps to run the same amount of hours,mostly only maybe three to five comps will run at anyone time due to low loads because of akv valves on cases and packs allways being oversized to begin with.
its quite ushall for only one or two comps to maintain suction target pressure or one if its the large scrolls.As for fan control most are fitting these ebm low energy fans that are controlled by hp transducer control generally keeping hp between 150/180psi.Not much difference in loads on lt systems as mostly glass door freezers and stores have decent hvac systems,plus it doesnt get very hot in summer.ht loads do increase in summer but not to much,mostly due to condensors undersized to save install cost but now are being increased as customers are realising this is a problem.
hope this is of some help

[mad fridgie]

Cheers Fowlie, good info, excellent!!

[lawrence1]

Hi mad fridgie,
here one supermarket chain uses say 4 bitzer comps on any rack each comp with 50% unloaders,,,,,each comp does equal runtime every 24hrs while condensing is by EC EBM variable speed fans,,,all fans run together from around 100rpm to 1400 rpm,,,,extremely efficient,accurate and the head pressure graph shows almost a straight line.Controller is Emerson E2.

[mad fridgie]

Hi mad fridgie,
here one supermarket chain uses say 4 bitzer comps on any rack each comp with 50% unloaders,,,,,each comp does equal runtime every 24hrs while condensing is by EC EBM variable speed fans,,,all fans run together from around 100rpm to 1400 rpm,,,,extremely efficient,accurate and the head pressure graph shows almost a straight line.Controller is Emerson E2.
Sounds like IGA, this is very similar to what I was used to in the UK (a couple of big racks LT and MT) "O" so many moons ago, apart from the electronics are a bit more advanced nowadays :D

[NoNickName]

1) yeah, the lead / lag. At least the newest ones, with microprocessor control.
2) All of them in summer time at high condensing pressures. 1 or 2 in winter time at night time.
3) 1, although oil separation may be an issue at the lowest refrigerant flow rates.
4) They tend to keep the condensing pressure floating to make advantage of a increase in subcooling at night / low ambient. Here the mileage varies a lot.

[D.D.KORANNE]

Electronic rack controllers are offered by many companies which handle compressor operating time & various functions to make it simpler to operate racks/packs .an issue that should be addressed is oil return to compressors . Remote mounted air-cooled condenser call for proper receiver design & discharge piping .

[mad fridgie]

Thanks to all, I am not designing a rack system, but need to design a control method, the info given has helped.

[lawrence1]

mad fridgie,
Go for the Emerson E2,,,you can't go wrong.

[lowcool]

what if compressor capacities vary ?

[D.D.KORANNE]

what if compressor capacities vary ?
usually compressors are selected basis equal capacity except for one compressor which will have to a part load ; example ;
170 kw rack will have 30kw x 6 compressors each having identicle displacement .

[Silhouette]

The supermarkets are now starting to use Co2 sub and transcritical systems which use Bitzer or Dorin semihermetic compressors.
The first compressor is on a VSD and the others run fully loaded but are rotated to equalise the run hours.

Scrolls are still used on the majority of R404a systems being installed and the controller will use a rotation based on run hours.

Condenser/gas cooler control is usually EC fans with a 0-10Volt control feed based on a floating head pressure control.

Scroll packs typically run in winter with 1 maybe 2 comps but in summer might be at full capacity depending on ambient temperatures, most packs in the UK are now designed for a 35 degree C ambient, 42 degree C condensing.

[750 Valve]

what if compressor capacities vary ?


No problems, program into controller the HP or KW of the compressors and it will utilise them accordingly to meet PID loop within controller, you can fine tune the cycling and PID values to maximise use of an uneven rack.

[Peter_1]

1) ....
3) 1, although oil separation may be an issue at the lowest refrigerant flow rates....
to counteract this, we switch on 4 times a day all at once the SV's to have a max gas flow in the lines, pretty much the same as the VRV's are doing.