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Thread: Cv

  1. #1
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    Cv

    So this week I decided to try to get a better handle on CV, to use as a trouble shooting aid. But, my sorry skull doesn't want to process the information I'm reading.
    Basically it almost seems really easy to calculate a CV for a coil, at least some guys I've talked to said... oh all you need to do is know the pressure drop across the coil and convert it to psi and run the standard CV formula.
    I find that difficult to believe , especially if the pressure drop is really low. How does the valve control properly then, it must need a certain pressure drop across it for the characterizing ring to perform.?
    Doesn't it need say coil pressure drop added to valve drop?
    But if one figures in valve drop, one would need to know the valve FIRST!
    Confused...so keep it simple.

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  2. #2
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    I used to sell industrial controls. I suggest you read through some Masonelian materials to better undersand valves, Cv, cavitation, etc.
    HTH
    Attached Images Attached Images

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  4. #3
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    Quote Originally Posted by hearthman View Post
    I used to sell industrial controls. I suggest you read through some Masonelian materials to better undersand valves, Cv, cavitation, etc.
    HTH
    Thankyou @hearthman .
    I really was not getting the pressure drop/flow/CV relationship, and forgot that the flow doubles as the pressure quadruples.
    And just couldn't grasp the valve sizing based on a 1 psi drop when I kept thinking of a 3-5 psi drop.
    It's clearer now.
    So if theres a coil with say 5ft wpd, and I need 15 gpm wide open.
    5 χ2.31 = 2.16
    x a 1.4 saftey factor = 3.02 psi.
    Cv= 15χ(sqrt of 3.02)
    CV = 8.6
    At 1 psi deltaP, moves 8.6 gpm wide open
    At 3 psi moves 15 gpm wide open.
    Seems pretty important to know actual pressure drop.
    So I'm not clear on the pressure drop....is it accurate to use published coil DP and multiply by 1.4?
    Belimo suggests 3-5 psi DP be used if design conditions are not known....and I think I read somewhere not to calculate Cv below a 4 psi DP.?


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  5. #4
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    Check out the SystemSyzer app for iphone/android from B&G. Makes easy work of such calculations. Cv is simply a constant used to specify the flow at a 1psi pressure drop. Get a digital pressure gauge with a manifold if your looking to take accurate PD readings across coils, valves, chillers, boilers, etc.

  6. #5
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    CV is KindaSorta like refrigerant flow and restrictions and pressure drop controlling ref/oil flow velocity. The "total" resistance/restrictions to flow helps in picking the proper compressor/circulator moving the proper amount of GPM's of water or moving the proper amount of Freon. So, choosing a Long Radius 1 1/8" vs a Plumbers Ell(tighter than a Short Radius Ell) has different "press drops". Then ALL parts ,like a LLSV have their own "pressure drop" or Cv to water. I think.

    At one of TACO's Factory Classes they defined Cv this way. Cv is the amount of GPM's needed to produce a 1psi pressure drop thru a device. Where 1psi = 2.31' of head. So, a 3/4" valve has a Cv of 4.9. So,(flow divided by Cv)2 x 2.31' =additional head. KindaSorta.

  7. #6
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    Quote Originally Posted by nutradesman View Post
    Check out the SystemSyzer app for iphone/android from B&G. Makes easy work of such calculations. Cv is simply a constant used to specify the flow at a 1psi pressure drop. Get a digital pressure gauge with a manifold if your looking to take accurate PD readings across coils, valves, chillers, boilers, etc.
    Quote Originally Posted by TechmanTerry View Post
    CV is KindaSorta like refrigerant flow and restrictions and pressure drop controlling ref/oil flow velocity. The "total" resistance/restrictions to flow helps in picking the proper compressor/circulator moving the proper amount of GPM's of water or moving the proper amount of Freon. So, choosing a Long Radius 1 1/8" vs a Plumbers Ell(tighter than a Short Radius Ell) has different "press drops". Then ALL parts ,like a LLSV have their own "pressure drop" or Cv to water. I think.

    At one of TACO's Factory Classes they defined Cv this way. Cv is the amount of GPM's needed to produce a 1psi pressure drop thru a device. Where 1psi = 2.31' of head. So, a 3/4" valve has a Cv of 4.9. So,(flow divided by Cv)2 x 2.31' =additional head. KindaSorta.
    What I'm wanting to be able to do is approach a system with say 30 VAVs with variable flow.
    Some have trouble meeting SA setpoints.
    So I'd like to calculate the required CV for those VAVs.
    Currently many have a CV of 1.9 with same size valves and some with 2gpm required and some with 4 or 5 or 6 or whatever, all the way to one with 12 gpm required.
    So my problem is to figure the system pressure drop and calculate Cv for the control valve after that.
    I'd like to be able to definitively say that the valve CV that's currently installed, is wrong and this is what should be installed and why.
    (Proved with the simple Cv formula)
    Using a loop circulator, currently controlled by DP or possibly Supply pressure only (not sure) gives a fairly constant pressure drop. So I'm hoping to find original design data but dont have much faith that I will.


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  8. #7
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    Knowing the Press Drop across EVERYTHING is going to be "gotta know info".

    Caleffi has some good info on Cv. Check out their "idronics Issue #8" from 2010 ,titled Hydronic Balancing.
    Caleffi -ThermoSetter Recirculating thermal balancing valve,,Series 1164A has some CV examples
    Caleffi ,Balancing valve,fixed orifice, Series #130, for a few more CV examples.

  9. #8
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    Quote Originally Posted by TechmanTerry View Post
    Knowing the Press Drop across EVERYTHING is going to be "gotta know info".

    Caleffi has some good info on Cv. Check out their "idronics Issue #8" from 2010 ,titled Hydronic Balancing.
    Caleffi -ThermoSetter Recirculating thermal balancing valve,,Series 1164A has some CV examples
    Caleffi ,Balancing valve,fixed orifice, Series #130, for a few more CV examples.
    Thanks Terry.

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  10. #9
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    I just ran across this real good Youtube video "What is Flow Coefficient Cv and how do we apply it to Hydronics". This is also presented by Caleffi and featuring Mr. Hot Rod Rohr.

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  12. #10
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    Quote Originally Posted by TechmanTerry View Post
    I just ran across this real good Youtube video "What is Flow Coefficient Cv and how do we apply it to Hydronics". This is also presented by Caleffi and featuring Mr. Hot Rod Rohr.
    Thanks again Terry.
    I had a nephew visit from NW usa.
    He's a wildfire hotshot, soon to be smoke jumper, and had a ton of entertaining stories. Good story teller too.
    Up till 2am so will have to dig into your suggested reading later.


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  13. #11
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    Cv flow coefficient equates the pressure drop and density of a fluid to its volumetric flow. It is technically valid only for incompressible (liquid) flow, though it can be used for vapor flow with reasonable accuracy if pressure drop is very low.

    Cv has been commonly used to determine solenoid valve capacity.

    I suppose one could use it to estimate static loss at an AHU, but I’ve never seen a coil manufacturer publish such numbers. No doubt they have much better ways to estimate air flow pressure drop at various CFMs across their coils.
    If we have data, let’s look at data. If all we have are opinions, let’s go with mine. – Jim Barksdale, former Netscape CEO

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  15. #12
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    Quote Originally Posted by icy78 View Post
    What I'm wanting to be able to do is approach a system with say 30 VAVs with variable flow.
    Some have trouble meeting SA setpoints.
    So I'd like to calculate the required CV for those VAVs.
    Currently many have a CV of 1.9 with same size valves and some with 2gpm required and some with 4 or 5 or 6 or whatever, all the way to one with 12 gpm required.
    So my problem is to figure the system pressure drop and calculate Cv for the control valve after that.
    I'd like to be able to definitively say that the valve CV that's currently installed, is wrong and this is what should be installed and why.
    (Proved with the simple Cv formula)
    Using a loop circulator, currently controlled by DP or possibly Supply pressure only (not sure) gives a fairly constant pressure drop. So I'm hoping to find original design data but dont have much faith that I will.


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    Wait are you trying to size the valve or figure out what the delta P of the system should be?

    Or what the delta P of the terminal unit / valve should be?

    Delta P set-point of the system is determined by system curve at typically 100 percent of design gpm with every load at 100% open.

    Delta P of a terminal unit is a function of GPM requirements which is a function btu requirement on the airside, which is a function of air vollume. If you dont have the design data, gotta reverse engineer.

    Inability to make target air temp could just as easily be a function of excessive air volume as compared to coil capacity. IE the reheat coil is designed for 1000 cfm of air, but your pushing 1250 for example


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  16. #13
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    Quote Originally Posted by heatingman View Post
    Wait are you trying to size the valve or figure out what the delta P of the system should be?

    Or what the delta P of the terminal unit / valve should be?

    Delta P set-point of the system is determined by system curve at typically 100 percent of design gpm with every load at 100% open.

    Delta P of a terminal unit is a function of GPM requirements which is a function btu requirement on the airside, which is a function of air vollume. If you dont have the design data, gotta reverse engineer.

    Inability to make target air temp could just as easily be a function of excessive air volume as compared to coil capacity. IE the reheat coil is designed for 1000 cfm of air, but your pushing 1250 for example


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    I dont think I knew what I wanted at the start of this thread .
    I went to the site yesterday, and verified a few years ago, a bunch of belimo control valves were changed to 1.9Cv. No one knows why.
    Delta P thru the strainer, coil, control valve , and automatic balancing valve, (ABV) was 15 psi.
    Loud as all get out.
    I have coil design wpd , design gpm , from the prints. Most of the control valves and ABVs have their data tag still.
    Belimo says to figure the valve/ coil components at 4 psi delta... so basically GPM/2 = Cv.

    I had balanced the VAVs prior to digging into the Cv because that needed doing anyhow.
    Inlet strainers have been checked on random units.
    Maintenance found a couple 3Cv control valves and we have a couple locations those can be very close to the correct size, so will proceed that way.
    I'm hoping that at the end of this, I'll be able to drop the pump hertz and save energy about 270 days/year.

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  17. #14
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    Keep in mind that next year ,w/ next years COLD COLD temps might still show up as a problem on a few zones.

  18. #15
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    Hows this? "For example a valve w/ Cv of 10 will flow 10gpm w/ a 1psi pressure drop"

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