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Thread: r12 to mp39

  1. #1
    Join Date
    Nov 2003
    Location
    South Carolina
    Posts
    1,167
    Ive got a small refregeration unit that holds 17 ounces of r12 that im dropping mp39 in to. Its one of those 2 door type units..2 evaps...its basicly a prep table with 2 doors below it. This unit doesnt have a high side access. I dont know why they dont put access on these little suckers. The origional call was the evaps freezing up..i suspected low on freon even though im having great difficulty finding the leak..its very minute..i got a slight reading with my detector but even with soap bubbles have not located it. Anyway I weighed in 12 ounces of mp39 before it started going off on the thermal on the compressor. Should i just go back in and weigh in 10 ounces or less...i know mp39 runs at higher head. Hes only using this thing another 3 months..i guess i can put a saddle valve on the high side although i dont like them. I cant stand drop ins with the different pressures..i dont consider that a drop in.

  2. #2
    Join Date
    Sep 2001
    Location
    East Stroudsburg, PA
    Posts
    13,215
    You're not topping off the 12 with 39, are you? That's not going to work on something that small.

    A high side access usually isn't necessary on that type of unit.

    MP39 is usually 20% smaller charge than 12, which indicates that you're actually overcharged at 12 oz.

    What made you suspect a low charge? Usually, when a unit of that size is losing gas, even slowly, it'll be about flat by the time a tech gets there.

    Icing evaps can be caused by a lot of different things.

  3. #3
    Choose a different gas, weld on C & D access valve on hi side. Replace drier.

    Pull vacuum down to 500 and let it sit thirty minutes. If it doesnt hold you either got slight moisture or a leak.
    Lotta moisture would never pull this low. Sizeable leak would never pull this low.


    Using a doggone micron gage and a suitable sized vacuum pump with clean oil .... it's a combination that cannot be beat for making sure you do NOT have a small leak in a very small system.

    Either that ... or throw some nitrogen in the box. Let it sit overnight with 150# pressure.
    Not a pound higher though. Evap manugfacturers dont like you going over their limit on specs. And 150 is the MAX on evap coils.


    If your in a hurry and the costomer doenst wanna put much time or money into the box, (which is a wise move, considering it's age), top off the current amount of refrigerant and add Spectronics Florescent dye.
    This would only work with a system you have not already pulled the charge and introduced MP into.
    By now, your already into some bucks on this box.


    Try tip # 1 or # 2.

    Ah what the heck ... tell him to scrap it and sell em a new system

  4. #4
    Join Date
    Nov 2003
    Location
    South Carolina
    Posts
    1,167
    Vacume has been pulled and all procedures to find the leak so far have failed..we are talking about the minute among minutes leaks here. The main problem is this unit will not be in use very long so understandably the owner isnt gonna put alot of cash into this thing. Therefore installing driers ect..really isnt feasable..this really was sort of a gas and go kind of call although i did want to make sure the leak wasnt as such that it wouldnt last a week. I did a thourough check of this system to find the leak and came up empty. I guess ive overcharged slightly at 12 ounces so i will tweek it some tomorrow. If mp39 isnt the route to go in this system then what freon of choice should i use...409a and mp66 both would yield higher discharge pressure and temp as well as higher low slide. As far as i can tell its the only one that gets somewhat close to r12 without going sky high on the high side. I have no doubt this is a situation with a leak because I did loose vacume over a 4 hour period but only slightly...fans are turning...no obvious signs of restriction...this is cap tube orifice..so i think just over time its gotten low. No this wasnt capping off...i recovered what was in there then weighed in 12 oz of 39. The one thing i hate about refer is its too dam expensive to carry every type of drop in for r12. R12 i do need to get a micron guage as i havent yet...seems like every week there is another piece of equipment i need to get and dam i been in business awhile.

  5. #5
    Join Date
    Mar 2004
    Location
    Gone
    Posts
    5,340
    Hack:

    You mean you have been putting in all those goodman without a micron guage, shame on you......lol

    madeinusa


  6. #6
    Join Date
    Nov 2003
    Location
    South Carolina
    Posts
    1,167
    Aww cut me some slack...i assure you i get all the air out...lol.

  7. #7
    Join Date
    Jun 2001
    Location
    San Diego, CA
    Posts
    1,149
    heck, 3months -
    I'd be careful to verify superheat when it's at box temp.
    If it's ok - let 'er ride.

  8. #8
    Join Date
    Sep 2002
    Posts
    186
    Hack
    I been trying 416 on those little boxes with good luck. When I used 39 and couple months later the cond 1/2 pluged overheating and freezing evap problems after switching to 416 no more service call on that unit I stop in there from time to time no more problems. I even put it in a r12 freezer just to see if it would work that was 6 months ago.Its still -10Ί Its at a monthly service contract store.
    That micron guage is a must I couldn't work without it.
    Because chicks dig it.

  9. #9
    Join Date
    Jun 2001
    Location
    Michigan
    Posts
    12,077
    This is why if it is a cap tube, superheat to compressor is best measurement to get correct amount of gas in system. 15 degree superheat at box temp, you can be sure your pretty well charged. In something that small, I never try to weigh in anything. Unless I got someone with me and trying to impress them with my dial a charge glass cylinder. Ha Ha.

    I always get asked how much to start with when adding gas out of a vacuum to a cap tube fed system. Casue you will need to adjust again at box temp. What I do, and it works like a charm is. Add refrigerant to the system till you get that old plus thirty rule on high side. Then watch it pull down and watch your compressor superheat. There were a few times where I was dead on. Most others I had to add just a tad. I don't beleive in that plus thirty for anything else any longer but it's just to ball park the initial start of charge for small systems like such.

    I like to add, it is a fine art to getting good at fixing these small litlle buggers.

  10. #10
    You'll want to watch your frost line coming from the evap when the box is nearing temp.

    Never tried 416. Maybe I'll look into that tomorrow.

    Be careful with that term you used a minute ago; "Gas N Go".
    We are not performing abortions here, we are craftsmen. We do excellant work. We are not "Hacks".

    If a customer requests gas n go....tell em it's be four-fifty. And then do the job correctly and surprise them with the bill, which will be lower than $450.

    Do NOT give in to the demands placed upon you by someone who dont know the biz.


    Not your Momma, not your manager, not your customer.

    ................... only you can prevent shoddy workmanship.


  11. #11
    It has been awhile since I worked on a cut table. But I think I remember the one I worked on had a defrost timer, and a defrost heater. That might be the source of your problem.

    You sound like you are coming into the job with a HVAC approach, refrigeration is a whole different ball game.

    The blend I've used most is 409A, and that has been very successful for me.

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