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Thread: DDR

  1. #1
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    DDR

    Hi all,

    I was being taught how to do a start up on a rack by a well respected J-man in our company and i am still unclear about some things and what better place to get all answers to my ?s than here.This rack was a zero zone rack with scroll compressor, all regulator vales was of the A8 type. Dont have model /ser#'s, just have a question on how the ddr should be set and where should my diff pressures should be measured.
    I observed him putting the system into defrost and then setting the valve, is this correct? Also when i asked where is differential is measured i was unclear of his explanation.I thaught valves of this type should be set with no cases in defrost and the differential should be measured betwen the liquid header and dischrage header? Any replies are thanked in advance and please explain how this should work.

  2. #2
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    I've always set them with no systems in defrost and measured the differential at the access ports at the valve.

  3. #3
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    What he said ^^.But you need to have the valve in the operating mode ie flip the dip switch ect when nothing is in defrost.Clear as mud right?
    I love the smell of phosgene first thing in the morning:

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  4. #4
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    A few points of advice, because a lot of guys get this WRONG!

    1> Use the same gauge to measure both pressures (inlet and outlet) There can be enough inaccuracy between gauges to cause a bad reading. What I have done to make this easier is to install ball valves on each hose of my gauges. This allows me to isolate each hose and set a valve like that easily.

    2> As crackertech said, make sure the valve is activated and that no systems are currently defrosting. I normally close the valve feeding the defrost header at this time.

    If this is an A82BL type of valve, you may have to get a little creative in finding ports on both sides of the valve.

  5. #5
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    jpsmith1cm great post get your count up and apply for pro. I use a manifold
    with 2 high side gauge's on it as long as they read the same it's good to go.
    I love the smell of phosgene first thing in the morning:

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  6. #6
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    Quote Originally Posted by jwc64 View Post
    I've always set them with no systems in defrost and measured the differential at the access ports at the valve.
    Where should the diff be measuered, between defrost header and liquid header am i correct??? The defrost header should be higher than the liquid to allow reverse flow ??? So would it be correct to measure diff at those two points?

  7. #7
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    Quote Originally Posted by jaybee View Post
    Where should the diff be measuered, between defrost header and liquid header am i correct??? The defrost header should be higher than the liquid to allow reverse flow ??? So would it be correct to measure diff at those two points?
    take it right at the valve.
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  8. #8
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    Quote Originally Posted by crackertech View Post
    take it right at the valve.
    At the valve. Im sorry to what am i compairing this reading to get a differential?

  9. #9
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    Have you read this yet?
    Attached Images Attached Images
    I love the smell of phosgene first thing in the morning:

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    Educational forums are open.

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  10. #10
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    Quote Originally Posted by jaybee View Post
    At the valve. Im sorry to what am i compairing this reading to get a differential?
    Most of the racks I work on have an access port at the inlet and outlet of the valve. So you would be measuring the diff across the valve itself.

  11. #11
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    Quote Originally Posted by crackertech View Post
    Have you read this yet?
    thanks guys for your replies I know understand. Had to think about it .Thanks again!

  12. #12
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    Quote Originally Posted by crackertech View Post
    What he said ^^.But you need to have the valve in the operating mode ie flip the dip switch ect when nothing is in defrost.Clear as mud right?
    Agreed, just to speed it up a bit... calabrate high side guages first, place guages across valve, put system 1 in defrost and close the hot gas ball valve on system 1, check differential , adjust valve, open hot gas ball valve system 1. Watch oppertion during defrost. Terminate defrost system 1.

    Note:
    On new installs I usually check one of the furthest systems from the rack after setting defrost differential, defrost temperatures, length of defrost, if it terminates ?

  13. #13
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    Thanks for the compliment, crackertech.

    I am personally too lazy to hike back to the truck to get a special set of gauges just for measuring high pressure differentials. Then, you have to have a set for low pressure differentials, regualar pressure service, R410a high pressure service. Too many gauges getting knocked around and damaged and too many hoses getting tangled all the time.

    By adding the ball valves directly to my manifolds, it allows me to have only 2 sets of gauges and still make all pressure measurements that I ever need to make. A normal pressure set and a high pressure (R410a) set.

    Just the way I see things.

    As a final check on defrost, you can monitor temperature of the liquid line at the case (or at the rack depending on how the system is piped) If you have the correct reverse flow, the temperature of the liquid line will drop off sometimes below 32 degrees, then slowly climb back up. Once the line reaches 70-80 degrees, depending on the case, you have reached the point at which the defrost needs to terminate. This works for Hussmann Kool-gas and Hot gas type defrosts.

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