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  1. #1
    rackrookie is offline Regular Member - bad email address Contact the Admin
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    Rack hot has solenoid ajdustment question

    I´ve been told that the large throttling valve located in the discharge line of the compressors activates whenever there´s a circuit in desfrost mode: it partially blocks off the path of the gas, redirecting it towards the defrosting cicuit. Also, I have noticed that whenever the throttling occurs, head pressure and temperature rise. Knowing this, is it posible to increase the defrosting power of the hot gas by adjusting this throttling valve? Some of the evaporatoirs of this rack are not getting effective defrosting: seems to me like the hot gas reaching them is not hot enough.

  2. #2
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    the differental valve can be adjusted to a point. what you need to ask is why some of the coils are not getting defrosted. are these coils on the same circuit? If they are are they the furthest out or the closest to the mechanical room. most people just crank the valve higher with out looking at the full system. how are the other circuits on the rack doing? if they are defrosting ok then its a system problem and adjusting the valve will not do much of anything except cause problems with the other circuts.

  3. #3
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    Quote Originally Posted by chilly123 View Post
    the differental valve can be adjusted to a point. what you need to ask is why some of the coils are not getting defrosted. are these coils on the same circuit? If they are are they the furthest out or the closest to the mechanical room. most people just crank the valve higher with out looking at the full system. how are the other circuits on the rack doing? if they are defrosting ok then its a system problem and adjusting the valve will not do much of anything except cause problems with the other circuts.
    chilly is right jacking the defrost valve could cause more trouble.Did you
    check defrost term wireing ect? Their is such a thing as setting the def
    valve to high and case dose not defrost but defrost will terminate.
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  4. #4
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    Why not measure what pressure temp your getting at those loads by forcing them into defrost and seeing if it is falling off from the rack to load. It is not that uncommon, especially on long runs where you can drop off on pressure and you lose the heat, so bumping it up is the thing to look at possibly.

    And only I would authorize that after looking at everything else. Alot of times the caes specs will tell you the temp they want for hot gas.

    But before even going there, I'd be doing everything else first, proving and disproving the over all correct operation. Based on the cases specs. Number of Defrosts a day, Duration, termination, drip time, fans, often there is a electric heater in the drain area most guys will miss. Are we getting the flow, to the load and back to the rack liquid header.

    What's the condition? Have you visually seen the build up. Is it ice, frost, any type of pattern, such as front to back, side to side, is there alot of whore frosting on product. I mean what is exactly going on?

    I have a store which has an abortioned HOT gas system, converted from electric, when store was relocated. I have been servicing the store now for about a year, and it has been a fight the whole time having to redo stuff, add stuff, and find that balance with the rack and the hot gas. Hot gas is a great system, but you need to take the time to prove out it's operation. It takes time.

  5. #5
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    Quote Originally Posted by Dowadudda View Post
    Why not measure what pressure temp your getting at those loads by forcing them into defrost and seeing if it is falling off from the rack to load. It is not that uncommon, especially on long runs where you can drop off on pressure and you lose the heat, so bumping it up is the thing to look at possibly.

    And only I would authorize that after looking at everything else. Alot of times the caes specs will tell you the temp they want for hot gas.

    But before even going there, I'd be doing everything else first, proving and disproving the over all correct operation. Based on the cases specs. Number of Defrosts a day, Duration, termination, drip time, fans, often there is a electric heater in the drain area most guys will miss. Are we getting the flow, to the load and back to the rack liquid header.
    I agree with whats being said here. But i would like to add, you need to confirm on which is ending the defrost, termination stat, or time. If its time, are the defrost periods to short. If its the defrost limit stat, at what temp is the returning liquid. If the liquid is not at least up to 50f, its proving that its terminating to early.
    You want me to do what, with that thing!!!!!!

  6. #6
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    what type of case's are not getting a complete defrost and are the fans running or do they shut off and are they supposed to shut down

    case manufacturer's have certain specs to follow

    hot gas systems should have some sort of temperature termination like a closure and/or temp probe and most controls have a minimum setting like 10% of time for defrost so if the controls is bad and reading a closure/bad temp it could terminate prematurely with still having ice on the coil and the seed is planted for the tree to grow

    the hot gas valve (DDR discharge differential regulator) only provides a differential for gas flow and the proper setting are determined by a few factors like location of motor room, length of piping ,coil size,needed lift to keep flow of hot gas and having the improper setting can cause the issue's you are seeing having a short run and a small coil with a higher pressure setting can cause a higher temp reading on on closure and quick termination with ice still on coil so a lower setting is needed for slower more methodical defrost with a longer drip time but this might affect a longer run so you have to look at the whole picture


    1. time
    2. temperature
    3. drip time
    3A.amount of defrost
    4. fans on or off depending on manufacturer's case setting
    5. infiltration of high humidity air (like a system design for normal use and its got the door open continuously)


    the numbers you get from a manufacture are pretty good guides but the variables will make you adjust each system accordingly and this is not a run and set it up and leave to never return as you adjust each systems you will need to get the rack balanced for most basic situations... a log of what was done will prevent another mechanic from adjusting the system back and repeat call a month later on the same system where all start to look foolish adjusting the system back and forth

    good luck
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  7. #7
    rackrookie is offline Regular Member - bad email address Contact the Admin
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    Thank you all for the very informative tips and advice. I´m going now to try first assimilate all of your opinions before going into actual testing. By the way, the evaporators (3) are located in a freezer room. The system is a Tyler Enviroguard med-low temp rack with hot gas two pipe defrost arrangement. I know for sure that the evaporator´s fans are controlled by a thermostat with the bulb attached to the suction line. The digital controller sets the defrost and drain timing. I don´t think there is another defrost termination device. I posted another case I am working at right know, but it is related to floodback after defrost. They belong to different racks.

  8. #8
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    Quote Originally Posted by rackrookie View Post
    I´ve been told that the large throttling valve located in the discharge line of the compressors activates whenever there´s a circuit in desfrost mode: it partially blocks off the path of the gas, redirecting it towards the defrosting cicuit.
    That large throttling valve is (like the dirtyboy said) a DDR (I've always known 'em as a DDPR).

    The purpose of this valve is not to divert hot gas out to the defrosting circuit at all. The entire reason this valve is utilized is to create a pressure differential accross the high side of the system so there is hot gas FLOW during defrost.

    If you understand what I'm talking about in the above paragraph - great! If not, please ask because if you don't know how reverse cycle defrost is designed to function - you're toast in trying to repair it.


    Quote Originally Posted by rackrookie
    Also, I have noticed that whenever the throttling occurs, head pressure and temperature rise. Knowing this, is it posible to increase the defrosting power of the hot gas by adjusting this throttling valve?
    As you now know, when any hot gas circuit initiates defrost, the DDPR engages and creates a pressure differential (roughly 20 psig - depending) just like it should.

    Quote Originally Posted by rackrookie
    Some of the evaporatoirs of this rack are not getting effective defrosting: seems to me like the hot gas reaching them is not hot enough.
    I would highly recommend you don't crank up the head pressure/discharge temp as a means of more efficient defrost. If the temperature becomes too high you can "shock" the evap coils and cause some major problems.

    You may possibly want to study the circuit in question and come up with an answer to questions such as these:

    • Is this circuit you mentioned in your subsequent post the only circuit having freeze up problems?
    • How many coils are not defrosting properly?
    • How is the defrost terminated? It almost certainly has to be terminated by some means other than time aka temperature.
    • Do you have enough refrigerant in the rack?
    Good luck with this - keep us posted.

  9. #9
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    Quote Originally Posted by rackrookie View Post
    Thank you all for the very informative tips and advice. I´m going now to try first assimilate all of your opinions before going into actual testing. By the way, the evaporators (3) are located in a freezer room. The system is a Tyler Enviroguard med-low temp rack with hot gas two pipe defrost arrangement. I know for sure that the evaporator´s fans are controlled by a thermostat with the bulb attached to the suction line. The digital controller sets the defrost and drain timing. I don´t think there is another defrost termination device. I posted another case I am working at right know, but it is related to floodback after defrost. They belong to different racks.
    It is something you may easily be able to determine by knowing what is available in the programming. W/I freezers are most commonly terminated based on coil temp, much the same as if using electric defrost. Depending on controller, it may be looking at a digital input, looking for contact closure of one or more in series klixons that close on rise of temp. Once that takes place, the controller terminates and drip time starts.

    It sort of reminds me to begin asking more specific questions before answering.

    One thing is certain, each application of a system can have many similair ways of control with little variations to that specfic peice of equipment and or hollistically different ways of accomplishing the same things.

    An example would be, that in a W/I Freezer, hot gas defrost, we don't want fans on during defrost, and were terminating based on coil temp, where as you may find a different approach completely on a single deck frozen food.

    Always always start by knowing specs. Then as the others have said, tweak to conditions by watching the system accomplish the tasks.

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