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Thread: MP 39 (401A)
03-05-2012, 11:21 AM #1
MP 39 (401A)
Does anyone use this for a low temp box in the -10 Degree range and if so do you have a particular expansion valve suitable for low temps that you can remember? I working on a box with this setup and the expansion valve says its for that type of refrigerant but I'm having a hard time understanding the pressures when I gauge the system and getting the box below 10 degrees. I tried doing a search for this but no one really covers mp 39 in any real detail.The only decision in life is to decide what to do with the time given to you
03-05-2012, 12:41 PM #2
This topic has had extensive discussions over the years here on HVAC-Talk. If you enter the right keywords in the site Search, you'll find lot's of them.
Here's one of the more recent ones:
Generally speaking, MP-39 (R401A) and its cousin R409A are best suited for medium & high temperature R12 applications. Although it will work on low temp R12, there is only one which is designed for that temperature range...ie, MP-66 (R401B) and it performs much better.
03-05-2012, 12:43 PM #3Professional Member
- Join Date
- Nov 2006
It was My understanding that MP66 is better for low temp applications. As far as the pressures go, just get a pt chart.
03-05-2012, 01:12 PM #4
Here's a link to info on MP-66 from DuPont:
03-05-2012, 01:36 PM #5
The thing that suck is I have already been adding the 401 a back into the system and trying to set up the tev to work with the pressures is confusing me. If you wanted a box to maintain that range temperatures (-10 - 0) that would mean that my average evap. temp would be the same right? And then I would figure in the glide of 6, because I think they say its actually <11.5 right? It would be added to the superheat? Too much math and I'm getting confused. Not too often do I have to do this but this is a first for 401a. Anyone have a better way to explain this? I feel I'm not doing it right.The only decision in life is to decide what to do with the time given to you
03-05-2012, 02:24 PM #6
First, the TXV's sole job in the system is to control refrigerant flow to the evaporator according to the superheat of the refrigerant vapor leaving the evaporator. It's not a pressure regulator.
Secondly, if the refrigerant temperature inside the evaporator were the same as the box temperature, how could there be any heat transfer? The evaporator temp must always be at a lower temperature. With a walk-in box, this temperature difference (TD) is usually about 10ºF whereas reach-ins usually run around a 15ºF TD. (I'll use a 10ºF TD in the following for simplicity.)
DuPont T/P Chart: http://www2.dupont.com/Refrigerants/...ofit_guide.pdf
With a 0ºF box running R401A and a 10F TD, you should expect to run an evap temp of -10ºF. That would correspond to a saturated pressure of 2.8 psig. At that point the TXV should be set to control the superheat to approximately 6-8ºF leaving the coil...ie, the suction line temp at the sensing bulb should read around -2ºF to -4ºF.
When you use a T/P chart, the saturated temps in the lower ranges typical are the dewpoint temperatures, which already accounts for the refrigerant's glide.
03-05-2012, 02:33 PM #7Professional Member
- Join Date
- Jan 2010
Others are R408 & R422C used for R502 replacement. Low temp. Was your box refrigerant originally R12 or R502 ?
03-05-2012, 03:11 PM #8
Good deal, I think I see the light. I'm currently adjusting the sh, adding refrigerant as the box cools. That seem normal? Lots of frost to. Seems to be going in the right direction.The only decision in life is to decide what to do with the time given to you
03-05-2012, 03:55 PM #9
What exactly is this system you're working on? Is it a walk-in box with a remote condensing unit with a receiver or a self-contained reach-in with no receiver?
If it's a walk-in system with a receiver, you should have a sightglass in the liquid line after the filter-drier. For proper charge, just clear the glass (unless it has a Headmaster condenser flooding control...which is another matter).
If it's self-contained with no receiver, the system would be critically charged...meaning you need to weigh-in the refrigerant. There should be a factory recommended charge stated on the data plate for the box. With R409A, since it's less dense than R12, will require less weight than the factory charge amount. This is determined by the density ratio of R409A to R12:
Density Ratio = 76 lbs/cu ft / 81.8 lb/cu ft = R12 Weight X .93
03-05-2012, 03:57 PM #10
03-05-2012, 07:55 PM #11
Its an old bohn evaporator that had r12 in it, converted a while ago, then sat for about a year when some other contractor tried condemning it. Owner said the contractor reported it had multiple leaks and a bad compressor, quoted him a really high price for replacement and repair. So coming into it I had my doubts. But today I finally was able to get the box down to 6-8 degrees. I played with the valve and charge and ended up with a suction of 8 and a head of 110 and my superheat as measured at the condenser was 8-12. This was after removing refrigerant until I seen flash in the sightglass for at least a 20 min period. Everything was done for at least 20 minutes at a time, including expansion valve adjustments and refrigerant adding/subtracting. But little by little I added refrigerant until the sg cleared and then that's when I started adjusting the tev. I'm just trying to understand why they went with 401a to begin with and was really unsure if it could maintain what they wanted. I was already committed to the 401a because I had ordered it from the supply house and broke the seal before I started having doubts that it would work. Funny how it all works out though. I'll check tomorrow to see if the box has reached proper temps. My question now is how do I get down to a 2-3 suction without causing flash in the sightglass?The only decision in life is to decide what to do with the time given to you
03-05-2012, 08:30 PM #12
Stop playing with the charge trying to affect a change in suction pressure.
Just charge to a full SG. Add a pound or two more to be sure it'll stay clear and forget about it for the time being.
Set the TXV superheat leaving the evaporator, not at the entrance of the condensing unit. The location of the TXV's sensing bulb is precisely where I'm referring to.
Once all this is done, please remember this advice. You cannot force the suction to where you want it to be, as it will always tend to go to a point at which the system is balanced with the load imposed upon it.
That basically says, "What you'll see is what you'll get."
03-05-2012, 08:42 PM #13
like he said, charge until glass is clear. then 6-8 deg s.h. at the bulb. copeland wants 20 at the inlet of the compressor, sometimes thats hard to do depending on the length of the lineset. focus on the s.h. at the bulb. once i get it to 6 at the bulb its usually very close. ill then adjust very slightly until i get it to its most stable point.
thats all the unit will do at that point. squeezing down the txv to lower your suction in attempt to get a colder evap will only starve the evap and make the txv hunt