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  1. #1
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    Temp control question

    Something has been bugging me that I just can't understand, and my textbooks don't seem to address it. It may be a dumb question...

    Lets say we have a case that we want to keep at 38 degrees. Let's also assume we have an R-22 rack system where the suction setpoint is at 53 psig (which would give an evap temp of 28). But, let's say the manager of the department wants the case to be a freezer (it's farfetched, but it's for illustration). There's no way you can lower the suction setpoint to get 0 degree case temp, of course, because all the other circuits need medium temp suction pressure.

    Here's my question...even with the evaporator temp at 28 degrees due to the suction sp at 53 psig, if you just kept the solenoid open feeding the case with refrigerant constantly, couldn't you take it down to the required 0 degrees? Why not? Would the coils ice up? I know this isn't *normally* done, but I need to understand the "why" of it...

  2. #2
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    Think of your evaporator temp as the basement. There is no way you can chill your equipment or box or whatever to a temperature lower than your evaporator. Not in this reality.
    The views and opinions posted here are my own. They do not reflect the corporate policies of my employer and will most likely get me fired at some point.

  3. #3
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    Quote Originally Posted by Iceneck View Post

    Here's my question...even with the evaporator temp at 28 degrees. if you just kept the solenoid open feeding the case with refrigerant constantly, couldn't you take it down to the required 0 degrees? Why not? Would the coils ice up? I know this isn't *normally* done, but I need to understand the "why" of it...
    If you figure out a way to take a 28* evap coil and produce a 0* case...you have defeated the science behind physics and have redefined thermodynamics.

    Start thinking about refrigeration as a heat transfer process and this should become clear to you.

    Heat travels from hot to cold...at least on this planet. (heat transfer should be covered in your textbook within the first few chapters)

  4. #4
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    Heat transfer is the way I do think of it (when the refrigerant vaporizes, it takes in heat, and when it condenses, the heat is removed) but I thought...ahh, I don't know what I was thinking. I was thinking the heat would keep "going into" the refrigerant if the process was still occuring (i.e. more refrigerant was being fed into the evaporator).

    So, just for kicks...if one really wanted to turn one case on MT rack into a frozen case, you could lower the entire rack suction pressure to get 0 degrees evap temp for that case...and then put an epr on all the other cases to raise the evap temp? I know this would never be done, but it *could* be, right? Theoretically?

  5. #5
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    Quote Originally Posted by Iceneck
    Heat transfer is the way I do think of it (when the refrigerant vaporizes, it takes in heat, and when it condenses, the heat is removed) but I thought...ahh, I don't know what I was thinking. I was thinking the heat would keep "going into" the refrigerant if the process was still occuring (i.e. more refrigerant was being fed into the evaporator).

    So, just for kicks...if one really wanted to turn one case on MT rack into a frozen case, you could lower the entire rack suction pressure to get 0 degrees evap temp for that case...and then put an epr on all the other cases to raise the evap temp? I know this would never be done, but it *could* be, right? Theoretically?
    Until you blow the compressors up from running them outside design limits....

    Sure, though, theoretically, it will work.

  6. #6
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    If you wanted to choose the most expensive and time consuming way possible...

    sure why not, that would work.

  7. #7
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    Quote Originally Posted by Iceneck View Post
    Here's my question...even with the evaporator temp at 28 degrees due to the suction sp at 53 psig, if you just kept the solenoid open feeding the case with refrigerant constantly, couldn't you take it down to the required 0 degrees? Why not? Would the coils ice up? I know this isn't *normally* done, but I need to understand the "why" of it...
    couldn't you take it down to the required 0 degrees? No

    Why not? Heat transfers hot to cold. For heat to transfer, there must be a thermal difference between 2 objects. If the evaporator is at 28*, at BEST it could only absorb heat until a 28* equilibrium is reached. at which point no further heat transfer takes place...it is physically impossible for a 28* evaporator to lower the temperature of any medium below 28*. YOU MUST UNDERSTAND THAT

    Would the coils ice up? Yes

    I hope this helps you understand the "why".

    It's important to think things through logically. cause & effect. This stuff is 100% basic physics principles. NO magical devices. Things don't just happen Willy-Nilly.

    understanding the principles is one thing, then you have to be mechanically inclined enough to do the physical work, and that's a whole other game.

  8. #8
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    Quote Originally Posted by Phase Loss View Post
    couldn't you take it down to the required 0 degrees? No

    Why not? Heat transfers hot to cold. For heat to transfer, there must be a thermal difference between 2 objects. If the evaporator is at 28*, at BEST it could only absorb heat until a 28* equilibrium is reached. at which point no further heat transfer takes place...it is physically impossible for a 28* evaporator to lower the temperature of any medium below 28*. YOU MUST UNDERSTAND THAT

    Would the coils ice up? Yes

    I hope this helps you understand the "why".

    It's important to think things through logically. cause & effect. This stuff is 100% basic physics principles. NO magical devices. Things don't just happen Willy-Nilly.

    understanding the principles is one thing, then you have to be mechanically inclined enough to do the physical work, and that's a whole other game.
    Got it! Thanks....

  9. #9
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    Quote Originally Posted by Iceneck View Post
    So, just for kicks...if one really wanted to turn one case on MT rack into a frozen case, you could lower the entire rack suction pressure to get 0 degrees evap temp for that case...and then put an epr on all the other cases to raise the evap temp? I know this would never be done, but it *could* be, right? Theoretically?
    There is any number of island coffin-case line-ups at the larger membership stores that do just this. They typically have a switch at the rack that bypasses the epr. They've proven to be the service guy's pita. But the stores like them for seasonal promotions such as turkeys during Thanksgiving. Yes, it can be done.
    The views and opinions posted here are my own. They do not reflect the corporate policies of my employer and will most likely get me fired at some point.

  10. #10
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    It can be done...

    But we must agree that dropping the MT suction setpoint to achieve a 0* case temperature is NOT the ideal way to do this...?

    We are talking about running the compressors out of their operating envelopes AND installing EPR's on all other MT cases on that Rack that are not going to run as a freezer....this would be costly and insane.

    There is a better way to setup Dual Temp cases.

  11. #11
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    Quote Originally Posted by Phase Loss View Post

    There is a better way to setup Dual Temp cases.
    Split suction, right?

  12. #12
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    Quote Originally Posted by Iceneck View Post
    Split suction, right?
    nope

    I'd imagine you could divert the suction to either a MT or LT split suction header with a Valve...just never seen that.

    There's an even simpler way.

  13. #13
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    Quote Originally Posted by engineerdave View Post
    There is any number of island coffin-case line-ups at the larger membership stores that do just this. They typically have a switch at the rack that bypasses the epr. They've proven to be the service guy's pita. But the stores like them for seasonal promotions such as turkeys during Thanksgiving. Yes, it can be done.


    Yes but those are on LT racks.
    I love the smell of phosgene first thing in the morning:

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