Excessive High Side Pressure On small True 404a Frezer
synopsis R-404a system running high liquid line pressure 310 to 320 psi, no load onfreezer, 0*F case temp. 80*F ambient air in room, 28psi suction 10*F superheat full colum of liquid in sight glass slowly charged till glass cleared, it's 11:30 i may have forfgot other info I'm tired now!! haha
Let me start off with this, the model is a TUC-48f little 2 door undecounter thingy. This Freeser has had a "troubled history" lots of service calls over the past 6 years of operation. I looked back through the records of all the service done on the machine, this customer actually had good records!
Most of the service on the machine had to do with blocked condenser and/or blocked air flow to condenser. I think we can all agree that that situation is hell for these little systems with the newer refrigerants and POE.
Over a year ago or maybe better on two the system had a blocked cap tube so it was opened and cleared by blowing nitrogen through it. It worked dandy for 7 months, then same problems re-occured. I assume it was the same tech but this time the cap tube had been replaced(and it was obvious) system worked again for almost a year again.
System running in a vacuum on low side so i preformed a prwessure test on nitrogen and system held 250psi for over 2 days with out budging, no leak right? (Itwas during that time i looked in to the history on freezer)
After having a grand old chat with one of the techs at true basically comming to the conclusion that with the history we probably had a contaminated system from the POE oil breaking down under the high heat situations and thus blocking the little bastard um i mean cap tube!
I was in the mood to experiment with this little freezer so i decided that I would get rid of that little cap tube and install a TXV and reciever, so I did. the idea sounded fun but once i got nto it i was reminded how i hate working on little reachi-ins who really like sharp sheet metal cuts anyway haha!
So now I have the freezer running and there is excessive head pressure 310-320psi on the liquid line, suction is 28ish psi and superheat is around 8*F
case temp is is steady at -5*F to 0*F
system is still running on the temperature control but i was thinking of throwing on a pressure control in there to cycle it.
any ideas on what to look for?
If the oil has started to break down the only thing to do is change it out and flush the system. Then get the customer set up on a regular maintenance so machine stays relatively clean.
Why did you put in a TXV? Did the factory recommend it?
What is the condition of the condenser - greasy, bent fins, etc. ?
Bless your heart! I guess things must be slow in the great White North... eh?
I had to service the same model unit yesterday, same pressures.
All I had to do was degrease & rinse the condenser.
I admire your enthusiasm trying to re engineer a True. Try to keep it simple, the suction should be approximately 16 psig. The fan shroud (cardboard) should be in place and the FD/DT switch is working.
As far as high head now with a receiver, I would guess the liquid is backing up in the condenser?
Based on my experience I've learned not to try to reinvent the wheel
I try to to keep them O.E.M....... It's hard to make chicken salad out of chicken sh*t.
I know you've thought about it but, all that work and a hundred dollars worth of parts for a $ 10.00 cap tube
I hope your Customer and Boss appreciate your initiative.....so who is going to pay for all the labor hours.... or was this under warranty?
If your really trying to invent or backward engineer, why not try to invent a permanent cleanable air filter that True can use to keep those floor vacuums clean? Or.... maybe relocate the condenser coil....or a ducted fresh air inlet?
Good Luck on your endeavor. Keep us posted on your findings.
Condenser is new looking, unit is in a really clean location not kitchen situation!
In fact things are not really that slow in my area at all anyone need a job?? haha but i'm doing this on my own like i said experimental so the only time is my own which i don't mind, i like to mess with things some times, why not!!
as I said earlier the cap tube has already been replaced before and for the record i pay about $6 for them not $10 haha
as for cleaning this system and the labour involved in doing that I was talking to a true tech and that involves dumping the oil from the can and cleaning system and flushing/replacing the accumilator and then you still have a cap tube system.
as for the high head, possibly caused by liquid back in the condenser, that would be due to an over charge and I charged this by the sight glass, maybe that was a mistake, perhaps i may try to suck a bit back out and try to drop the head by adjusting charge and add by pressures and line temperatures, what ya think?
again this is a little project, the customr is paying a little but not nearly for what time is put in to it, I started this job as a project/experiment initally so it's understood with the customer. I already told her that if it happens again with the other 2 units she has it may be worth replacing the units haha I do do maintenance with this customer on a 6 month frequencey and usually there is not much dirt on the condensers, great location Medical food service so one would expect a clenlieness to be aparent!
just for the record True tech says in ideal situations they like to see 7-9psi back and 225-230 head on these 404a frezers, makes sense on the P/T.
As I see it, by converting this system to a TEV with receiver you now getting much better performance from a generously sized evaporator as indicated by the much higher suction pressure and lower superheat than the original factory design with a cap tube feed would allow. The higher suction increases the compressor capacity immensely by about twice is factory spec and with that, the total heat of rejection that the relatively small condenser must handle is also nearly doubled.
If you look at the discharge pressures that you are seeing as compared to what True said they should normally be and their corresponding saturated condensing temps, you can see that the factory condenser TD at say a 75ºF ambient is approximately 25ºF whereas your new TD looks more like about 50ºF....or double what it used to be. That's just what I would expect to see if the compressor capacity is doubled.
I think the answer here is to increase you superheat to more closely match that of what you would get with the cap tube setup, which I dare say will be much higher than what you have now. I would guess that a True freezer like this would actually run compressor superheats in the 50ºF+ range. (Yes that's higher than what the compressor makers would like to see, but it's more in line with what True sets them up for.) This will lower the suction pressure, decrease the compressor capacity and as such the condenser TD and subsequent discharge pressure will drop back to a more comfortable level.
I think this little rig will run pretty sweet once you getthe bugs out.
PS......As for the low pressure control, I'd suggest sticking with the OEM temperature control. If anything, add a manual reset HP control. If they had installed one at the factory in the first place you likely wouldn't have been doing anything to this unit other than a condenser cleaning.
Last edited by icemeister; 01-19-2007 at 09:51 AM.
ckecked on this freezer agaon today, the last time i checked it out i reduced the charge by dumping a little liquid into a jug to reduce head pressure i also upped the superheat settging on the TEV
checked it out and I had 250 head and 12psi back. the sight glass was low and cabinet temp was around 32F, by the look of the coil it looked to have a low charge, TEV was frosted and so was first part of coil
I added a little 404a and brought the liquid level in the glass up sone but still not full, I have a few different readings and case temop was dropping
rome readings after adding more juice
Suction 22psi 43F
Reciever outlet 300psi 104F
Condenser outlet 106F
Condenser inlet air 79F
Condenmser outlet air 102F
I'm starting to think that this just may not work out for me and i may just throw back in a $10 capilary tube but I really don't want to haha
maybe i need an orface after the TEV or something, im usinf a sporlan 1/4 404a TEV, or like was mentioned a bigger condenser coil?
any ideas or is wheel reinvention time over? hahaha
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I have been watching this thread for a while and I do not understand it at all.
Why did you ever feel that 310 to 320 head was too high?
I mean WTF that is NOT high for 404 at all. 350 would make me raise an eyebrow but anything under 300 would be more of a redflag.
Saturation temps of 100 to 130 are not totally out of line for kitchen equipment though something at the 130 level is workin real hard! The TEV you installed is set out of the box with a pressure drop design that is at 100 SCT
It is an old rule of thumb and not really a good one but the 30 over ambient is not outragous on little boxes like that.
I jsut thought that those pressures were too high for that little tecumseh in there, it does have a high starting torque motor but that's a lot of pressure to try to start under. if for some reason it had to (it is controled with a temperature control so if it cut out and door was opened for a pit it may wanna try to start up again and not be able to start with such a head pressure there, and with out the cap tube it won't equalize that quick, That is kind of why I was thinking of adding a pressure controll as stated earlier.
I know i shouldn't judge by factory specs but now that the system has been bastardized originally new out of the box pressures are according to a true tech in the 7-9 and 225-230 ball park(not sure of ambient but I'd assume 70F maybe. basically this is why i have some concern on this system and how it's operating after the mutilatiion
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Good luck to ya
I don't have any good advice for what you have gotten yourself into but I was on a "no freezing" call and it was a TRUE 3-door reach-in freezer. It was also a 404a unit. I pluged in the unit to get it started and the Evap fans didn't come on right away so I looked at the wiring dia. and saw that it was controled by the defrost termination switch but all the readings checked out. After an hour of trying to find what was keeping the fans from coming on it dawned on me that the switch was normally closed and the pressure sw. was actually the component controlling the fans. The unit was almost empty and after replacing the low side service valve and wieghing in the critical charge the unit droped the temp and purrrd like a dream. I hope it comes out good in the end for ya but sometimes you need to take a step back and not get into the system so deep like I sometimes think I need to. lusky
If your not getting the results you desire then change. People change from either desperation or inspiration.
You may consider trying to adjust the txv. it could be overcharged with the txv not fully adjusted open. You could try using 134a in the system, i think it runs at a lower pressure than 404a and uses POE oil.
Never thsught of the R-135 A avwenue, never used or seen it used in a low termp applicatjion buty i have seen 12 used in that application so one may think anything is possible, I'm gonna have to pull out the P/T and have a look, maybe I'll give it a shot, i got piles of 134a kicking around
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I wouldn't advise getting deeper into this mess by trying R134A. It will run, but you'll lose 30-40% capacity right off the bat and you'll need to change the TEV as well.
Did you try adjusting the evap SH higher like I suggested earlier? I really think that will make this puppy work.
Doh on the bulb charge haha
Originally Posted by icemeister
yeah i screw it almost all the way
the first time i adjusted the SH the next time i went back to it the breaker had tripped some time over the weekend.
i has my supco tem,p recorder in it too haha too bad that recorder didn't have pressure and amperage inputs haha
just on a side note with the capacity loss with 134 but the gain of having a TEV i'd probably be in the same level as far as capacity with 404 and a captube ya think, i doubt i'll change it but it wouldn't be that difficult, all i got to fo is buy a 134a power element for the sporlan TEV(edit: and maybe the orface too), I'd name that sporlan system but i forget what they call it when ya can mix and match valves with a few parts
Last edited by MikeySq; 01-30-2007 at 08:51 PM.
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