Rack cannot reach low or medium setpoints

Hello,
Working on a problem that has me stumped. I'm sure anyone trying to help will probably be interested in more details that I don't list below, but hopefully this is enough to encourage suggestions...

Supermarket recently performed remodel and added case lineups to existing racks. Rack A added 3 low temp systems/circuits along with a 20hp low temp satellite compressor. Also changed out a medium temp 10hp comp for a 15hp. Rack B added 1 med temp system to med. suction header to handle 8 single deck meat cases (dual temp) with no other changes. No changes to evap cooled condenser. Condenser circuits completely separate for Rack A and B. Hot water reclaim on Rack A.

Here's the problem: Rack B will not reach setpoints of 8psi for low nor 37psi for med. (R22) except when in defrost. All compressors will run and keep rack around 14 and 45psi. Cool gas defrost initiating in any low temp circuit will enable rack to reach low and med setpoints relatively quickly, then begin staging compressors like normal.

During defrost the pressure readings of discharge, drain leg, pressure regulator valves (stepping down @ 10psi), receiver, liquid feed and liquid reducing valve all seem to read and operate properly.

The most confusing thing to me is during normal operation discharge psi = 190 but my drain leg psi = 195 or slightly more! How is this possible? I've checked multiple times on multiple days and the discharge pressure is always at least 5psi less than drain leg psi - and I'm using the same guage. Liquid cores are newly changed out - no pressure drop. No suction filters. The pressure regulator valve readings on in and out are equal during normal ops. Liquid reducing valves read equal in and out. Discharge temp = 212, drain temp = 92. All eprs operating properly and there is no seepage of solenoid valves of cool gas into headers.

Thought it may be a load problem so I ball valved 50% of the low temp load (3 large circuits) before low temp began to satisfy. Med temp never satisfied.

Rack has check valve between low and med header for helping low temp if it exceeds med temp pressures.

A rack has higher discharge temps and drain leg temps but it appears to be running fine - even though I'm concerned with a 170 degree discharge temp on the sat comp and that they added the extra hp without addressing the condenser. (Open for suggestions on reducing this disch temp since it's another critical problem in my opinion)

WHEN IN DEFROST SYSTEM WORKS LIKE A CHAMP.

Hopefully someone has some suggestions on what additional items to further inspect and that I'm overlooking something simple.

(As I re-read this I'm wondering about the new meat cases, dual temp, into the med header...)


Thanks all,
HD

hi mr hd welcome to re forums. its always good to hear from the cousins.
some questions and observations.
how did the pack work before the remodel? have the problems existed since the remodel only? who designed it??? were pipe sizes increased with new duty?

is the check valve a new addition? if it is there to assist if med temp cannot cope then it would be open constantly. more sense would mean the med temp assists the low temp at high pressures. there would have to be a 5 psi differential between the 2 pressures for the valve to operate effectively.

if any low temp circuit on defrost acts to assist the pack, what changes during the defrosting phase?
with cool gas defrost, it is normal for a liquid line differential valve to modulate via the operating solenoid to maintain a receiver pressure higher than the liquid line pressure, is this happening? up to 20psig is normal (from memory.) i presume this is the 10psi step down you mention. can you define the difference between pressure regulating valve and liquid reducing valve please. please define your circuit from low temp discharge onwards please.
defrost operation should follow this:
liquid pressure reducing valve modulates closed to maintain difference between liquid and receiver. higher pressure gas from receiver flows through solenoid or threeway backwards through evap to check valve @ 5 psig opening, bypassing tev into liquid line. this is to maintain cool gas pressure (now condensed) flow at a higher pressure than the liquid. is this your system? separate drain leg indicates some differences.
how is the cool gas flow controlled, threeway valve or separate solenoids on each refrigerant phase supply?

i don't understand either how the discharge pressure can be less than the drain leg pressure. is it definately not hot gas defrost?
where are you taking the discharge reading and drain leg reading from?
it does sound like duty is part of your problem, but give more detail please.
170 degree discharge indicates high suction superheat on pack A. but it is not excessive imho. unless you are now using centigrade in which case bye bye compressor.
good luck and keep coming back.

I'm not a pack expert, or even a novice, but would suggest a careful look around all of the new pipework to ensure that all is connected correctly.

Sometimes you look but do not see.

I wish you luck with it and welcome to the forum.