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Thread: minimum head pressure?
01-22-2011, 07:47 AM #1
minimum head pressure?
It's the time of year for low ambient problems.
The big chain that we do work for runs very low head pressures. Currently, on R-22 racks, we set the A9 at 105, the A8 at 115 and the condenser fans to control at 70 degrees SCT. The R404a racks are set at the same SCT settings. 135 and 145 off the top of my head.
I know these pressures are very low.
Now for the question.
Walking up to a rack with no legends, no mfr data or anything, how can you determine the lowest safe setting for these valves?
01-22-2011, 09:58 AM #2
trial and error
maybe not an exact science, but it works.
for example: Start with a 60* minimum SCT, if its too low bump it up to 65* then 70* and so on...eventually you will find the minimum allowable set point for that store.
however not all stores will be the same due to various reasons.
I am sure there is some mathmatical equations...but without getting too fancy, the above method works.
01-22-2011, 10:15 AM #3
Trial and error is a tried and true method, but I was hoping for something more exact. A once and done type fix.
When in doubt, I normally just set them a bit high and go.
01-22-2011, 01:04 PM #4
Copeland's Low Condensing Temperature Bulletin AE-1334 is probably a good place to start if you want to compile a checklist of stuff you need to consider when dealing with low condensing temperature operation.
01-22-2011, 01:16 PM #5
I don't work with racks as you guys do... you are the masters of that. I believe the minimal is 70* . I could be wrong. I seem to remember that in class a while back."Time to nut up or shut up!" Tallahassee - Zombieland
01-22-2011, 06:31 PM #6
01-23-2011, 12:29 PM #7
01-24-2011, 11:49 PM #8
witnessed a 404a LT enviroguard rack running 70psi head(since yesterday) today no temp alarms., i wish i pulled the graphs from the E2!!
there are a few things to take in to account on this topic, main things to keep in mind is the operation of the TX valves, the rack dosn't care(usually) how low the head runs, it's the TXV's that care,
TXV capacity that is lost in lower head pressures(pressure drop across valve) can be gained back with increased subcooling, throw in balanced port TXV's and it helps out even more
MikeSig removed by mod. G-Rated site
01-25-2011, 02:45 AM #9
70# head is good for now but how is the oil return doing in the long run?
we do have a chain that runs low head pressures. but all cases/boxes are fitted with pulsed electronic expansion valves. no oil return issues because when the valve pulses open its full flow.
01-25-2011, 01:50 PM #10
this is not the long run, this equipment is designed for 100F condensing so needless to say it has a wide operation range, on the other hand these systems are maintining 10F subcooling on a 90F day and head stays under 275psi, Oil Return is ok for 90% of the year i would assumeSig removed by mod. G-Rated site
01-26-2011, 11:49 PM #11Professional Member
- Join Date
- Oct 2008
I've read somewhere Copeland recommends a minimum compression ratio. I think it's 2 but I'm not 100% on that.
01-29-2011, 01:07 AM #12
Who likes 60psi head pressure haha
Here is is....... I was in the area so i went up on the roof to get the graphs!
LT rack R404A
What is graphed from Bottom to top,
HUssmann RL Frozen food doors
outdoor ambient temp, (drops to almost -10F)
LT10 hussmann FWEG D/T island in MT mode
have a look on the 23rd to 25th and how low the discharge pressure got and held(55 to 65psi) and compare that to the temps in the red line for temp in frozen food doors
there was a little bit of suffering in the D/T island, temp rose a bit but still didnt break 40F!!! it's using the Sporlan ORIT+ CV colenoid dual temp system(which sucks) i would think if this one ran its own EEPR it would have ran a lot smoother like sys#2 doors, now i wanna see the graph for the lt2 eepr! haha
MikeSig removed by mod. G-Rated site
01-30-2011, 06:45 PM #13
Hey mikey does that rack have a subcooler? If it does do you turn it off during winter? I seem to find on the second stage off the subcoolers in cold ambients it just floods the hell out of the rack. just woundering if you come across subcoolers flooding back type problems?