Smarter to group the circuits by similar suction temperatures and set up the racks that way.
A good example of what the op was referring to can been seen in Albertsons stores. They have an icecream cake case thathas to be kept at -20f. The low temp rack was di idwd into two main sections. A -12 group (for 0f cases), a -22f group (for -10f cases ex ice cream). The compressors used a split suction header and common liquid feed. Attached to the -22f group was a single compressor that ran the ice cream cake case. The case ran off of the common liquid, but suction was piped directly to the single compressor. Between the cake freezer and -22 suction header was a ori valve. This valve would open when the came creezer came out defrost. It would allow the -22 suction group pull on the case to bring the temp back down, and close off when the load was handled. I think this compressor ran at 10 or 12 psi. On 404a.
This whole system was flawed as the high temp rack powered a subcooler for low temp rack. If the high temp rack went down, so did the low temp. The low temp rack was under powered. Have seen this in 9 different stores. Ice cream is soft beacuse of weak valves on produce rack....
Produce high satellite runs the subcooler at about a 37-40 degree evap with a target of 55 degree leaving liquid.
I've got a rack or two that are DEAD without the subcooler and a couple of racks that I'd like to ADD a subcooler to to increase the capacity just a WEE bit.
Ive never understood using the subcooler for capacity. I always thought it should be used for efficiency only..but in not the guy in the choo-choo hat....
There is always a "what if" scenario, which can cause an entire rack to go down. What if there's a leak on the LT rack; what if you loose a compressor on the LT rack; what if you loose a condenser fan motor or two in the summer time on the LT rack; etc.
The reason the subcooler is on the MT rack is this: You've got LT compressors operating at between 4-5 HP per ton and MT compressors operating between 2-3 HP per ton. So you use a compressor operating at a much more efficient condition (MT rack) to reduce the mass flow on the LT rack.
You can use a LT compressor to operate the subcooler, but why? You will reduce the mass flow to all of the TEVs on the system, but you're still spending the same amount of horsepower to accomplish it.
And you're right...the subcooler is typically added to increase the efficiency of the system. But on a system that was poorly designed, adding a subcooler (if there's sufficient capacity on the MT rack to add this load) will give the LT rack more capacity. It doesn't make the compressors any more efficient to operate when the system's liquid is subcooled, but it does reduce the mass flow requirement, meaning that less compressor capacity is required.