Capacity Control- Does it really worth?

[lana]

Dear Experts,

I would like to open this discussion and know different ideas about compressor capacity control.

As all know, capacity control is a way to match evaporator load with the compressor load. The main benefits are:

1- Energy saving
2- Relatively constant suction pressure
3- Precise fluid outlet temperature
4- Less compressor cycling

Now the set backs are :

1- Oil return problems at part loads
2- Power consumption reduction at part load is not exactly equal the capacity reduction (where screws are the worst)
3- TEV is selected for full load capacity. At part load TEV is oversized

My main concern here is the oil return problem at part load.

Now lets consider a different approach which is to use multiple circuits (separate systems) for a given capacity. For example, for a total cooling capacity of 100kW instead of using one compressor with capacity control, use 4 x 25kW compressors with 4 individual cycles.

In this case we will have 4 step control with the same percentage of power reduction.
There will be no oil return problem at part load and also, TEV for each circuit will operate as designed.

Nowadays we see huge chillers with 4 or 6 individual circuits for the very reasons.


Another scenario is where we need precise fluid outlet temperature which requires compressor capacity control. In this case we have to sacrifice some technical issues to get the job done. No argue in this case.

I have seen a few people and companies who used capacity control methods just to show off "technology", but they encountered very serious problems, especially with oil return.

My question is this : Does it really worth to complicate the system with some real technical problems to gain a little bit in power savings? Of course where other alternatives are available.

Really appreciate your comments and ideas.

Thanks a lot
Cheers

LANA

[Tesla]

Hi Lana
You have forgotten to mention the Turbo cor compressor which is oil less there fore has no oil return issues. Also with this compressor there are no losses in the heat exchangers from the oil insulating the transfer surfaces. Then the scenario of using multiple compressors which is pretty good but it is done a little different so if we had 100kW we would be better off to use 4 compressors of say 10kW, 15kW, 25kW, and 50kW. Then we could have more stages to match the load for efficiency.
1; 10kW
2; 15kW
3; 25kW
4; 10 + 25 = 35kW
5; 15 + 25 = 40kW
6; 50kW
7; 10 + 50 = 60kW
8; 15 + 50 = 65kW
9; 25 + 50 = 75kW
10; 10
11; 15
12; and so forth
As you can see we would have 12 stages of capacity but in reality would probably use 3 compressors and play with the numbers for what six compressors are available to achieve the best match.
With variable this that and the other we loose around 10% in the drive through heat losses etc. Then there are now VSDs with an option of DOL used as a backup if the drive fails of at 100%.
I have seen many oil return problems in systems out there usually designed by someone - well who shouldn't have. We should be using EEVs over TEVs as they overcome many problems and increase efficiency.
I think these newer technologies should be used more often but are not due to a trade of between capital and running costs. We really need to get more into life cycle costing.
All in all this whole process can become complicated when all that is required is a simple old system to do the job, no doubt we will improve. Coming from a technician background I find some qualified engineers don't know enough to design a good reliable refrigeration system.

[lana]

Dear Tesla,

Couldn't agree more.

Thanks a lot for your input.
Cheers

[NoNickName]

lana and Tesla, each electrical machine consumes energy, the inverter drive is one electrical machine, ergo the inverter drive consumes energy.