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Thread: Head scratcher.

  1. #1
    Join Date
    May 2001
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    Western Wa
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    Head scratcher.

    The chill system I am working with here consists of three 1985 Mc 048s with the evaporators in series. Yeah, I know, but that's the way it is. Single pass evaps.

    I was called upon to investigate why there is excessive DP across the first two evaps, but not the third. No problem, I thought to myself, this should be simple. I'll just use one gauge to measure pressures in and out at each evap. The gauge ports are all within two feet or so of the inlets and outlets.
    No pete ports, just places for gauges that can be valved off from the system.

    First I verified the report, and yes it is true. One PACO pump drives the loop, and the nameplate is 460 GPM @ 75'. The DP on the third one matches the original install DP specs and is at almost twice the nameplate GPM of the pump. The pump is currently running at almost 100' of head, so unless I'm losing my mind, it's actual flow should be considerably less that the 460 GPM @ 75' figure on the nameplate. The DP on the first two evaps is almost twice the original spec.

    The next thing I did was to open up the evap inlet of the first unit. I expected some sort of restriction, like half full of crap, from what I saw on the pump head and the first two barrel DPs. But it's clean as a whistle. The inlet and outlet are 8 inch, and it's necked down to 6 inch within inches of either end of all three evaps.

    So, after all this, I have to assume, at this point, anyway, that either there is a restriction I haven't found yet, or the pump has had the impeller replaced with a higher GPM impeller. Even that doesn't make sense to me.

    The only other thing I can think of is the six inch piping, and that doesn't really add up, either.

    So I left there today talking to myself but at least I wasn't saying "Huh?"

    Any wild guesses, word of wisdom, and even ridicule would be appreciated greatly at this point. I'll be back there tomorrow to put the piping back together and get the system running, even with the mysterious DP numbers.
    My credibility and sanity are both coming into question here.

    Thanks, folks...
    God Bless our Veterans

    God Bless the USA

  2. #2
    Join Date
    Jan 2004
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    1,174
    It would make sense that the dp would be higher on the first two but the 3rd one being twice the flow doesn't. The single pass pressure drops should not be more than a couple of pounds at the most so if you do have a different impellar you would see it in higher GPM's. What is the drop and does the leaving pressure of one chiller equal the entering pressure of the other.

  3. #3
    Join Date
    Feb 2009
    Location
    good old Europe
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    160
    Simplyfied, the cooler is a single piece of pipe. The volume/mass flow through all three evaporators in series must be the same! The pressure in the system consits of static and dynamic pressure. Pressure drop is a function of lenght, diameter, velocity, hydraulic friction etc. Assumeing that the physical datas are the same, the velocity in the pipe is the most affecting parameter. The velocity in a pipe at constant volume flow changes only with the diameter (this would indicate the restriction that you couldn´t find). I can only imagine, that some sort of Venturi effect (high velocity at the measurement port while the static pressure decreases) takes place. I would therefore go for a ultrasonic flow meter to check the flow rate and compare with the pump characteristic curve (DP).
    "Quality exists, when the price is long forgotten."

    Henry Royce

  4. #4
    Join Date
    May 2008
    Location
    Central Pennsylvania
    Posts
    438
    I too have seen the venturi effect happen and it messed with my brain. Pressure gauge on the outlet of an RTHC was pulling into a vacuum while there was obviously positive pressure in the pipe. Gauge was threaded directly into a pvc pipe instead of a tee, I assumed thats what was causing it to happen.
    If you did want to confirm the impeller size you can do so by dead heading the pump and reading your delta P at 0 gpm, then plotting that on the pump curve.

  5. #5
    Join Date
    Aug 2007
    Posts
    94
    Deadhead a Pump? Oh my god you will ruin it! I actually had an engineer tell me that not too long ago! I told him to step aside while I do my job!

  6. #6
    Join Date
    Jan 2004
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    1,174
    You won't ruin a pump by deadheading it briefly.I have seen systems where a pump has been deadheaded by a control valve etc to where the water temp was over a 100 degrees on friction alone and pump was fine.

  7. #7
    Join Date
    Mar 2003
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    7,321
    as you increase flow through a given constant, dp will go up. If it were me, I would be looking to get flow in order, then taking a second look at dp. who the hell knows what happened before you got there, but you need to get flow in check anyway for your chillers to do their respective jobs right. Give it a try.

  8. #8
    Join Date
    May 2001
    Location
    Western Wa
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    Well, kids, I think I have an answer. Not much hair left, anyway.

    Buried in the insulation blankets are three Vic T shaped strainers, an item that I haven't seen before. They look like Vic T's and I first thought they were just Ts with a valve for bleeding off air or something. I'll pull them apart when I get the gaskets and such.
    I did get one "Butterball" valve (I hate those things) to open in an attempt to blow down one of them, and then it wouldn't shut.

    So there I was, re-enacting the story of the guy trying to put the cork back in the pig's a**.

    You guys would have all laughed like heck.

    Thanks for all the input.

    I'll let you all know the end result when I get the parts and get back there, maybe next week sometime.
    Last edited by Randy S.; 05-12-2010 at 10:45 PM. Reason: spelling
    God Bless our Veterans

    God Bless the USA

  9. #9
    Join Date
    May 2001
    Location
    Western Wa
    Posts
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    One more thing...

    Am I the only one here who hasn't seen this type of strainer before?
    God Bless our Veterans

    God Bless the USA

  10. #10
    Join Date
    Mar 2010
    Location
    Morgan Hill Ca.
    Posts
    1,219
    Quote Originally Posted by Randy S. View Post
    One more thing...

    Am I the only one here who hasn't seen this type of strainer before?

    Super common on Victaulic piped jobs, much cheaper than a "y" strainer....

    GT
    If a day goes by and you have learned nothing, I hope you got a lot of sleep.

  11. #11
    Join Date
    Jan 2004
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    1,174
    I just pulled one and cleaned it today, so yes, I have seen them before.

  12. #12
    Join Date
    May 2001
    Location
    Western Wa
    Posts
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    Well I guess I don't get out much.
    God Bless our Veterans

    God Bless the USA

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