Alright, last week I was working on this Rack. The engineering chart mounted on the wall said it was a R-402A (HP80) suction group. However the Rack itself was labeled in several locations that it had R-408A dated as far back as 2009. The Rack design is a +15*SST per the engineering chart.
The EMS programming was setup to run as a +15*SST R-408A, which is a 44# set point.
The problem was called in as, multiple cases are freezing product. This Rack at one time was setup to float suction pressure set point...but overtime has been been disabled by circuits being setback up into EMS Full control, EPR's being stemmed back into control and T-stats being setback into control range.
After spending some time setting those temperature controls back below range and letting the case temperatures be controlled by Rack suction pressure, re-enabling floating suction set point and controlling it with my meat cases discharge air sensors. I had every case on that suction group running too cold and freezing product in short order
My suction set point had floated up 10# which made it a 54# set point or 24*SST R-408A
My Float circuit set point was 29*, and even running at max float set point 54#, my cases were running actual discharge air at 25*-26*....Now I either have the most efficient cases on the planet that can maintain a 1*-2* TD or something is not right.
I go down to the cases and start checking superheat using my R-408A PT chart and start reading superheat values that are in the negative 2*-4* range...now I know open air cases do not run 1* TD's and negative superheat values are impossible. I've come to the conclusion that whoever labeled the Rack as R-408A in 2009 was an idiot.
Go back to the machine room, re-adjust all suction group parameters to run on R-402A set points.
+15*SST 53# with a max float 63#.
Now everything starts jiving like is supposed to do. Cases are running 8*-10* TD's, superheat values are positive and the rack starts purring along, you can actually hear it run smoother.
Attached is graphs of the suction pressure and some case temperatures. Notice how much tighter the suction pressure and case temperature control is while operating under a properly operating floating suction.
But on to the original title of the post...MIXING REFRIGERANT...when does it become a problem? This rack above has a 800# Total Charge. This year it has already had 400# R-408A dumped into it due to leaks. However, the PT relationship is obviously dominate with R-402A. There are also R-507 racks out there that I know of when a guy dumped in 200# R-22 as a "tracer" that do not have any issues...maybe it matters on the smaller stuff, but when hundereds of pounds are involved. I know it's not good to do, but I call BS on it being that big of an issue. And I base this on experience...whats your experiences/opinions?