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VFD with 3-way valves follow up

in my first posting on the original previous thread- to plug the bypasses. Glad we think alike! But I have reservations, each situational to each job.

This is what I posted:

<i>It MAY (just MAY) be possible to disconnect the bypass port and use the body as a 2-way. You can ask your ATC contractor or whomever supplied that valve if that can work. That is the "poor man's approach" to show a benefit but the stroke aspects and Cv ratings may not be ideal. If not doing this method you have to replace the valves with 2-ways.</i>

It is a good way to see if you are on the right track and for lowest first cost. Valve stroke and hysteresis may get a little wide though because of longer stroke times. (Characterized ball type control valves are less of an issue than piston types.)

Good call, Tim!



  • Dennis KunkleDennis Kunkle Member Posts: 34

    Thanks for the responses. Allow me to explain further. I'm learning as I go on this new HVAC system. Yesterday was one of those not hot/not cold days. As I was checking out the computer (Metasys), I see that the chilled water system is enabled and holding the water at 42 degrees. The CWS is enabled at 55 degrees OA temp. But because there's no call for cooling at the 5 air handlers, all 5 3-way valves are closed. Yet, the CWS pump is showing 100% output. So I'm thinking that this would be a time for the VFD on the CWS pump to slow down. Not so, apparently, according to the ATC guy. He didn't understand why there would be a VFDs on the pumps with a 3way valve set-up. The 2 VFDs cost the contractor $4K plus labor and it seems odd that the engineer called for something unneeded. The ATC guy said that the 55 degree set point for the CWS could be tweaked up a bit if I wanted.
  • Brad White_185Brad White_185 Member Posts: 265
    That is another way to illustrate the same issue, Dennis

    I would bet that if you forced the 3-way valves open (imposing the higher pressure drops of the coils onto your pump system) your VFD's might kick in or at least your DP would increase.

    Do the VFD's in fact respond to a DP signal? At least was that loop provided? If not then I really have to wonder what the drives do besides manual adjustment, trimming the pumps to a given manual point.

    With the valves closed (in bypass) the coil DP is invisible, the path to the return can be as much as 15 feet less (the DP of the coil and hook-up). Your apparent need for flow will be greater by whatever the pump curve says when you back out that PD from the system curve.

    Nothing wrong with providing VFD's so long as you are going to use them (and you still can). At least the cost of the starter is amortized into that. And as stated, there is the manual adjustment feature (if you ever see a surplus) which can be used until automatic control is instrumented.

    Keeping 3-way valves with VFD's is an easy (rookie) mistake to make because the principles are not understood. Easy to miss if that was the case.

    BTW: When you say "CWS system" you mean "Chilled", not "Condenser", right? I use CHWS and CWS to distinguish between the two, not that it is the only way, just to be clear on the terms.

    Now, if your CHW system is keeping 42 F and there is no load, you have excellent turndown and probably running on hot gas bypass maybe? I am suggesting that the control setup be a combination priority, that the CHW plant run when the ODT is (50---55F adjustable) AND any one or two air handlers are calling. By that I mean that the discharge air sensor or return air enthalpy sensor is above setpoint. Just another thought to save energy and equipment wear and tear.
  • tim smithtim smith Member Posts: 2,315
    Brad, & others, another way to skin this one

    What about just plugging off the bypasses on the 3 ways. Now you effectively have 2 way valves w/ no control changes on the valve side of the world. Then install VFDs and DP controls and away you go. Hmmmmmmmmmmmmmm.
  • Brad White_185Brad White_185 Member Posts: 265
    Oops NM

  • tim smithtim smith Member Posts: 2,315
    oops, sorry Brad, missed that in first post

    Don't want to steal your thunder, leave that for the spousal unit. :)
  • Dennis KunkleDennis Kunkle Member Posts: 34

    I just spoke with the engineer who is claiming that there should be 2-way valves and "it's not to spec". So I'll have to keep you posted on the progress. Currently we're in the 30 day punch list period.
  • Brad White_185Brad White_185 Member Posts: 265
    I think that

    is a relief! (Well, maybe not...)

    Let us know how it all works out. I am hoping the flange to flange dimensions are the same...
  • Brad White_185Brad White_185 Member Posts: 265
    \"steal\" my thunder? :)

    Susan IS my thunder. :)

    No worries, Tim, just one cannot read all posts so worth updating to the new thread to keep current.
  • paul_79paul_79 Member Posts: 91

    maybe you have an answer for me but i thoght that the chiller had to have a fixed flow through it or am i mistaken also what type of chiller , recip ,centrifical screw, ect.
  • Dennis KunkleDennis Kunkle Member Posts: 34

    It's a brand new York 76 ton chiller with the compressors below it, fans above it, and a tank in the center. As I understand it now (Big meeting next week!), the specs called for a VFD on the chiller pumps, but 3-way valves were installed at the air handlers. Those valves, apparently, are incompatable with the VFD.
  • paul_79paul_79 Member Posts: 91

    i would talk to your york chiller rep and make sure that you can run a vfd on a chilled water line. normally you could only do it if the chiller is piped in a primary secondary piping arrangment. the primary through the chiller at a fixed gpm and then the secondary off of a drive. it sounds like the engineer is trying to save money by installing drives but he may be messing up your chiller operation.
  • Brad White_187Brad White_187 Member Posts: 13
    Variable Primary Flow

    is done all the time but usually on much larger centrifugal chillers...76 tons is possible if it is a Turbo-Cor but a recip or scroll probably does not have the turndown curve.

    P/S may be the way to go, pretty adventurous for that small tonnage.

    My $0.02

This discussion has been closed.


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