Superheat/subcooling differences from walk-ins to A/C??? - Page 3
Page 3 of 3 FirstFirst 123
Results 27 to 37 of 37
  1. #27
    Join Date
    Dec 2004
    Location
    Sunny So Cal!
    Posts
    649
    nothing to add here, just want to get in a the case of beers (potentially) flying around- sounds like a bet- cmon Andy step up!
    Look, just do your job, stay outta my way and we'll get along fine.

    Teach your kids to respect themselves and others with your actions- these little baboons will imitate you like it or not.

  2. #28
    Join Date
    Jul 2000
    Location
    Guayaquil EC
    Posts
    10,249

    I agree with Dow & the Others that the 80-85% Rule is BS.......

    ....especially if one appreciates from whence the rule comes to us.

    The refrigerant manufacturers like DuPont, Genetron (Honeywell), etc. came up with guidelines for retrofitting existing systems to the blended refrigerants that in my view were overly simplistic in that these rule-of-thumb figures like 80-85% of the original charge weight can totally confuse the average Joe out here in the field. It pains me that this confusion persists even now.....well over ten years after first having to deal with this issue.

    Genetron has a good P-T chart that includes the refrigerant densities and the formula for calculating the new refrigerant weight based on the density ratio of the new vs the old refrigerant:

    Genetron P-T Chart

    If you take the time and effort to sit down and calculate the proper ratio for an R12 to R401A (MP39) retrofit you will find that the percentage is not 80-85% but rather 91.4%. If you're going to R409A it's 93.1%. If you insist on using a percent rule for charging, please at least use the right numbers to get there, but as Dow said.....if you're dealing with a cap tube or fixed orifice metering system you still have to get your suction superheat right or the charge is not correct regardless of the density ratio rule. The ratio will get you in the ballpark but if you don't fine tuned the charge on your cap tube/fixed orifice system by superheat afterward, then you simply haven't finish the job properly.

    When the subject turns to a typical commercial refrigeration system retrofit to a blend the manufacturers rules-of-thumb really fall short in my opinion. They make overly generalized (and erroneous) statements that you should not clear the SG when charging with a blend and that bubbles in the glass a perfectly normal. Any of us who have worked in commercial refrigeration dealing with these refrigerants knows this a bogus statement so we ignore it and do what we know to be right......ie, if we have a receiver/TEV system, clear the glass and add for condenser flooding as required.

    If the sightglass still has bubbles flowing through it the refrigerant is not completely subcooled and therefore what Dow said about flash gas and the TEV will occur. If there are bubbles in the SG with a blend it means that your refrigerant is in the glide region. One or more components may be subcooled but not all of them are so there will be flash gas at the TEV.....not good. The manufacturers say if you clear the glass you will overcharge the system....what crap. You cannot overcharge a receiver/TEV system until the receiver is full and causes the refrigerant to stack up in the condenser.

    There's another camp out there that says you have to charge to 15-20F subcooling and ignore the SG. I say this methodology scrawled on the side of a Larkin 5 HP condensing unit retrofitted to R401A from R12. It was going off on high head. I had to remove about 40 lbs of refrigerant out of there before the charge was correct (including low ambient for this area). A typical refrigeration condenser with no subcooling loop can at best give you between 5-10F subcooling. If you get more than that your condenser is stacked from either the Headmaster of from overcharging.

  3. #29
    Join Date
    Nov 2000
    Location
    Memphis TN USA
    Posts
    6,945
    You are not going to get much subcool out of a receiver system.
    AC condensing units sit on the ground with the evap 20 ft up in an attic most of the time. Walk-ins almost always have evap below CU. ACs need more SC. Walk-ins don't need much.

    There are more details in refrigeration. Discharge temp is more important. Coil TD is realy measured in AC almost always measured in Refrigeration.

    Refrigeration has defrost, etc.

    Take AC out of your mind and think about refrigeration systems. Read everything Sporlan, Copeland and Heatcraft. They will get you thinking on the right path. puts out.
    If the superheat ain't right it ain't charged right.

  4. #30
    Join Date
    Apr 2005
    Location
    where the beer flows like wine
    Posts
    2,871
    Originally posted by kim
    You are not going to get much subcool out of a receiver system.
    AC condensing units sit on the ground with the evap 20 ft up in an attic most of the time. Walk-ins almost always have evap below CU. ACs need more SC. Walk-ins don't need much.

    There are more details in refrigeration. Discharge temp is more important. Coil TD is realy measured in AC almost always measured in Refrigeration.

    Refrigeration has defrost, etc.

    Take AC out of your mind and think about refrigeration systems. Read everything Sporlan, Copeland and Heatcraft. They will get you thinking on the right path. puts out.
    I disagree, sub-cooling plays the same role in refrigeration or air conditioning systems, as liquid refrigerant moves through the liquid line friction and static head losses do occur, this will cause refrigerant to flash, sub-cooling roles is to guarantee that only liquid refrigerant will reach the inlet of the metering devise regardless the pressure drop. The same factors affect both system in a similar way.

  5. #31
    Originally posted by smilies
    Originally posted by t&mechanical
    They only refrigerant you can clear a sight glass on are R22, r12, 502.
    12, 502 are out dated and very seldom you find r22 in a walkin, you do see them but not often

    All blends should never have a clear sight glass, you charge 80 to 85 % of your existing charge when converting to a blend.
    FALSE!!!!! If you have a receiver, full column of juice.
    On critically charged units with s/g's you will see bubbles.
    I'm only quoting what all manufactures of blend refrigerants tell you in thier charging instructions for their refrigerant

  6. #32
    Join Date
    Nov 2000
    Location
    Memphis TN USA
    Posts
    6,945
    Originally posted by hvacpope
    Originally posted by kim
    You are not going to get much subcool out of a receiver system.
    AC condensing units sit on the ground with the evap 20 ft up in an attic most of the time. Walk-ins almost always have evap below CU. ACs need more SC. Walk-ins don't need much.

    There are more details in refrigeration. Discharge temp is more important. Coil TD is realy measured in AC almost always measured in Refrigeration.

    Refrigeration has defrost, etc.

    Take AC out of your mind and think about refrigeration systems. Read everything Sporlan, Copeland and Heatcraft. They will get you thinking on the right path. puts out.
    I disagree, sub-cooling plays the same role in refrigeration or air conditioning systems, as liquid refrigerant moves through the liquid line friction and static head losses do occur, this will cause refrigerant to flash, sub-cooling roles is to guarantee that only liquid refrigerant will reach the inlet of the metering devise regardless the pressure drop. The same factors affect both system in a similar way.
    I don't work on any rack systems, so most systems I see do not have huge elevation changes or long line sets. The ones I have seen did not work very well. The ones that did work had LL/SL subcoolers.
    If the superheat ain't right it ain't charged right.

  7. #33
    Join Date
    Jun 2006
    Posts
    1,105
    Wow lots and lots of conflicting statements in this thread!

    Refrigeration and AC are the same thing with some equipment and design differences, that is all.

    AC uses a high TD to get rid of moisture, refrigeration uses a lower TD to reduce the rate that the system dries out the product stored. It depends on what product is stored to determine the TD.

    AC has a higher evaporator temperature, typically anyway.

    AC has more wires

    Refrigeration takes more horse power to get the same BTU transfer because of the lower temps and TD.

    Refrigeration uses devices to protect the compressor like a CPR.

    Pretty much all things that are employed in one part of the trade are used for both ac and refrigeration because they are on and the same, the difference being only the temperatures they operate and the equipment used to obtain those temps.

  8. #34
    Join Date
    Apr 2002
    Location
    Ann Arbor, MI
    Posts
    2,985
    Originally posted by t&mechanical
    They only refrigerant you can clear a sight glass on are R22, r12, 502.
    12, 502 are out dated and very seldom you find r22 in a walkin, you do see them but not often

    All blends should never have a clear sight glass, you charge 80 to 85 % of your existing charge when converting to a blend.
    Dupont pubishes a nice retrofit guideline for their service replacement refrigerants: http://refrigerants.dupont.com/Suva/...pdf/h75336.pdf

    In addition the providing estimates for initial refrigerant charge for the various replacement scenarios, they also do state in this guideline: "attempting to charge until the sight glass is clear may result in overcharging the refrigerant."

    Actually, they meant "overcharging the system."

    I understand the purpose for this statement, but I find it a disservice to many mechanics who do not understand the differences in thermodynamic properties between the service refrigerant and the refrigerant it is designed to replace.

    You want to see a clear sight glass with any refrigeration system employing a TEV. Period.

    But with some systems you can't get quite there without running excessive head pressures.

    Why does this happen? R-401A (MP39), for example, has a much higher heat of rejection than R-12. If you have a marginal condenser with your R-12 system, you'll likely see problems converting it to R-401A. Solution? Undercharge it so that you can get your condensing pressures in line.

  9. #35
    Join Date
    Jan 2004
    Location
    Earth
    Posts
    4,879
    Good advice Andy.

    I have seen few sight glasses that did not eventually clear. I have left many glasses flashing only to return and see a clear glass when the load came down.
    A Diamond is just a piece of coal, that made good under pressure!

  10. #36
    Join Date
    Apr 2002
    Location
    Ann Arbor, MI
    Posts
    2,985
    Good point about the high load condition. I'll modify my statement:


    You want to see a clear sight glass with any refrigeration system employing a TEV operating at design conditions. Period.





  11. #37
    Join Date
    Jan 2004
    Location
    Earth
    Posts
    4,879
    I'm a full colum of liquid man myself.


    But the real world is not always that easy. When they give us gases the quality of 12 and 502, or customers who maintain and replace their equipment. It will be.
    A Diamond is just a piece of coal, that made good under pressure!

Page 3 of 3 FirstFirst 123

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event