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Thread: Prep Tables

  1. #1
    Join Date
    Apr 2010
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    Charleston, WV
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    25

    Prep Tables

    I have recently started working with commercial prep tables/counters can anyone give some advise? Residential or walk-ins and especially the electric side are simple for me but prep tables what kind of superheat am I looking for with cap systems. Any info or sites are appreciated as I want to learn all that I can I really like this trade.

  2. #2
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    Jul 2000
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    Guayaquil EC
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    Almost all self-contained commercial refrigeration is critically charged and is best charged by weight as indicated on the unit's data plate. If that's not available, then shoot for 20F-40F SH as measured about 6" before the compressor if it's a capillary tube system. If it's an expansion valve system with no receiver then charge to about 5F-10F subcooling at the condenser outlet.

    Remote units will typically have expansion valves and a receiver, so just charge to a full sightglass. If it has a Headmaster for low ambient head pressure control then you'll need more refrigerant based on the volume of the condenser coil.

    For some good info on servicing this stuff, go to post #12 here.

  3. #3
    Join Date
    Aug 2007
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    Winter Haven, FL
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    Dont forget to degrease/vacuum the condenser coil before charging.

  4. #4
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    Oct 2008
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    OFFICES IN : ARIZONA - NEVADA - TEXAS
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    Smile

    Be sure to check the condensate drain .. ! The situation I've seen the line is very small plugs up & then the fan throw the puke water everywhere !

    "Rock-n-Roll " Ain't noise pollution..

  5. #5
    Join Date
    Oct 2008
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    787
    Don't be overly concerned about superheat on a prep table. Those things do break some conventional rules by their design.

    What icemeister said is so true. Measure in the charge - no more and no less. Consult the manufacturer on expected pressures. Some of these units run such low suction pressures you tend to question an adequate charge. However, the manufacturer designs them to frost up, then defrost in the off cycle.

    Usually you'll find in prep tables that the t-stat is a pigtail style sensor jammed in between the evaporator fins to sense core temperature. With those styles, the t-stat cycles off well below freezing temp - near 20 degrees. Yet, the box temp gets happy in the 34 to 38 degree range.

    Diagnosis can be upside down too. You'd think a cooler that's freezing product has a bad t-stat. On the design mentioned above, if the system has a slow leak and the evaporator is no longer uniformly cooling, the box temp may still be readily cooled by PART of the evaporator but the t-stat isn't seeing it because the section of the evaporator in which it's sensor is installed may already be in a high superheat range due to the low charge. So, the t-stat may be seeing box temp instead of evoporator core temp.

    So, the unit keeps chugging right along to achieve that - YES - 20ish degrees, but can't get there. Meanwhile, the lettuce, tomatoes and pickle are suffering a case of frostbite.

  6. #6
    Join Date
    Apr 2010
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    Charleston, WV
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    25
    appreciate the help. I wieghed in acharge and had 1.6 on suction and 135 on dicharge, thought I had a restriction and almost replaced cap tube.

  7. #7
    Join Date
    Oct 2008
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    787
    Well, you've said little of the system you're working on but those pressures do seem low even if it's R134a. Check with the manufacturer on those pressures.
    I'm told newer systems using POE oil (for R134A) will break down and cause cap tube restrictions and imminent compressor failure if poor condenser upkeep lends to abnormally high discharge pressures and temp. Blockage forms on the cap tube walls, then symptoms of a restriction soon follow.

    Get a good look at the evaporator in cycle. Is it frosting uniformly as it should OR is it growing a glob of ice ON the cap tube or evaporator frost ONLY near where it feeds into the evaporator? If you oberve the latter, definitely a blockage.

  8. #8
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    Aug 2007
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    Winter Haven, FL
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    Did you compensate for you hoses when you weighed in the charge??

  9. #9
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    Apr 2010
    Location
    Charleston, WV
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    The unit uses 134a and required 8,75oz, I put in 8.9oz because of the hoses. I really do appreciate the input from all of you because if I don't know it I want to learn it and nothing beats answers from experienced Techs as far as I'm concerned.

  10. #10
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    Aug 2007
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    Winter Haven, FL
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    I think you are probably still short on refrigerant. How long your hoses are determines how much extra to add. A 5 foot hose holds 3 oz of liquid refrigerant.

  11. #11
    Join Date
    Jan 2010
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    6,324
    Quote Originally Posted by Bolling View Post
    The unit uses 134a and required 8,75oz, I put in 8.9oz because of the hoses. I really do appreciate the input from all of you because if I don't know it I want to learn it and nothing beats answers from experienced Techs as far as I'm concerned.
    Bolling,
    You can charge the 134a as a vapor and have better control of the charge. With such a small charge it take no time at all.

  12. #12
    Join Date
    Apr 2010
    Location
    Charleston, WV
    Posts
    25
    When I charged, I allow suction to pull remaining refrigerant from my lines and at 1.6 PSI I wouldn't have too much left in lines would I?

  13. #13
    Join Date
    Oct 2008
    Posts
    787
    Bolling,

    My manifold gages I have the red, high pressure hose with low-loss type connection to the equipment being serviced. I don't know what you're using.

    After the unit I'm servicing has been running, that red hose will have liquid in it from the system - usually several ounces; but exact amount depends on the hose length.

    Disconnecting your red hose from a unit always leaves a few ounces trapped in the red hose. You'll need to transfer that refrigerant back into the low pressure side of the through your manifold. Be sure that the non-condensables (air) are purged from your manifold.

    VTP99 is correct about R134A. Like R-12 & 22, it's a pure fluid type refrigerant (made of one molecule) so it CAN be fed into the system as a vapor through the suction service port with the system running. This is practical only on smaller systems but allows far more accuracy that is critical to those smaller systems.

    Back in the day (before the EPA and all these new, NON-ozone depleting refrigerants), charging a system as a vapor was common practice. However, almost ALL of the new refrigerants are mixtures and blends (two or more type molecules) which MUST be introduced into a system as a liquid through a static system's high pressure port so the refrigerant maintains the proper ratio of ingredients it's comprised of.

    In theory, blended refrigerants will separate when boiling off into a vapor. Since it's a blend or mixture of two or more components, each component of that refrigerant will evaporate at different rates. If you recharge a system with a blend or mixture as a VAPOR, what you put into the system is no longer the true and actual mixture of that refrigerant when it left the factory. Same goes with what's left in your bottle.

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