Prep table raised rail starving/serpentine separation?
Working on a Randell Model 8411M, 111" long, 4 lower doors and full length raised rail. Original tag says 134A, but has R404 comp and tev's. Bottom works fine. With the bottom stat turned off, so only the Top Rail runs: Top rail only frosts the first half of the serpentine circuit. Fresh charge, dry, solid liquid, 10 degrees superheat, Suction Pressure 22#/-15degrees, head 240#. Pump down to 8".
When I turn on the lower air coil unit, the suctions raises to about 65#, and the bottom refer cycles normal, but the top rail never freezes past past the half way position.
I would like to see more pump down, but the capacity with the bottom refer turned off, should be enough to run the rail. Any thoughts?
I've serviced a couple of these and to my recollection, the top rail wall never fully frosts. I take this to mean the rail's evaporator tubing covers only the upper portion of the wall.
Originally Posted by Boise500
As for the R404A, it sounds like someone converted it from R134A sometime in the past. No biggie...I like R404A better.
Randell says the air temp in the rails should be 25 - 30 degrees, it is running at about 50 degrees. They have all the holes filled. And stock with pre-cooled pans.
Which design is this - Top rail with fans to circulate, or just a static coil?
The static coils in my experience will ice completely over top to bottom - maybe 8-10" high.
What controls the temp for the top rail, a T-stat or pressure control? Is it set correctly?
You have 10* SH at the compressor or outlet of evap coil? And is that with just the top running?
The only tiny fan in the static rail, is in a 12" box, that sits loose in the rail, and plugs into an electrical socket in the rail wall. When plugged in, it also closes a circuit which allows a thermostat (in this same box) to energize the solenoid for the rail. This thermostat stays closed. The solenoid, for the rail stays open, and never shuts off.
The rail has 3, 360 degree passes around the rail, per Randell. It only frosts the first pass (top), the top pass, and half of the 2nd pass (middle pass).
The 10 degrees superheat is at the outlet of the rail evap, with the bottom refer solenoid closed. Only the rail is running while testing.
Could be separation like you said, or possibly the insulation is saturated with water which is killing your heat transfer ability. If you let it run for a while (like an hour) it still won't frost up the rest of the way?
I haven't seen the evaporator tube layout drawings for these tables - where did you find it?
It sounds like a restriction, but your SH is too low for that.
Or contact problems with the evaporator coil, but I would think your SH is too high for that.
I'm wondering if it's an accurate superheat measurement.
When checking your SH, you have a probe at the outlet of the evaporator, but where are you getting your pressure reading from - the compressor?
Does this table have the liquid line and suction line soldered/taped together?
Pressure reading at comp, five feet of line from evap outlet, using 3 month old Fluke 52 II with clamp on probe, on sanded clean suction line.
Randell explained the circuitry to me.
Liquid line not soldered/taped to suction.
Randell assured me that these cabinets have never had separation problems. Have anyone experienced otherwise?
The new 404a 1/4 ton low temp tev, has a tendency to hunt, which means low load, which makes me suspect separation,...versus water saturated insulation which should be heavier load. Rail gets turned off every night, and surfaces warm up to room temp.
Never had a seperation problem on them. How long ago was the comp changed? And why? Could you possibly have an oil issue in that Circut? If previous comp junked because of a low charge or something else caused oil to get pumped out and not returned you could have alot of excess oil looking for a nice cold place to hang out and cause you to have bad thoughts about a unit. Might explain some of your symptoms. Just a thought. Good luck.
Also never had a separation issue.
Possibility of mismatched components (compressor)?
Have you tried backing out the rail TXV to lower the superheat? (10* seems high for what I would suspect)
Gentlemen, thank you so much for you input. I talked to the Randell Factory again this morning. The first conversation since new gas, tev,...etc. They explained the manufacturing process of this rail, and they are proud of it. They have seen rare failures. Sharing the frost patterns, the low load on the valve, the recommended superheat, they think the only thing it could be, is separation. This unit was born in 1999, as a 134A for Papa Murphy's. The factory later went to 404A. This unit was later converted with a factory supplied kit, comp/tevs/etc. for conformity reasons, the customer just wanted it to be like the newer ones (404a).
Due to the rail circuit starting at the top, and running downhill, Randell says there should be no chance of oil build up in the rail. Although with the symptoms, oil migration was a legitimate issue.
Unfortunately, the present owner of this unit, is a neighborhood Pub, which is just a year old. And as typical, they have financial challenges, like so many, plus they're just appreciative and nice folks. So just to be a good steward of our trade, and appreciating you guys for brain storming, I am "Paying it Forward" by not charging these folks any labor, and only charged them the wholesale cost of parts. Thank You.
A noble thing to do. Thanks for sharing.
BTW- what superheat do they recommend at the outlet of the rail?
They're saying separation, and my only concern is, wouldn't you have SH that is too low?(Perhaps 2-4*) Very little of the coil is actually doing any work, so you should have a fair amount of liquid at the outlet that hasn't done much.
I would assume a properly working unit would have SH in the range of 8-12*, but that's just my hunch.
Good to know. Never seen a seperation but of coarse no one can ever say they have seen it all in this trade. Will file this for future reference. I'm still sceptical however with the hunting and SH. I also would think it would be lower with seperation like Doleman. Never the less good job following through and helping someone out.