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Cv Flow Coefficient

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by jebatty, Nov 10, 2009.

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  1. jebatty

    jebatty Minister of Fire

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    Would appreciate a link or an explanation with a couple of real world examples as to how Cv is used in picking valves. Also, how to determine Cv. Thanks.

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  2. sgschwend

    sgschwend New Member

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    Taken from: http://www.smithvalve.com/technical/flow-coefficient.aspx


    Cv – Flow Coefficient = Index of Flow Capacity equivalent to the gallons per minute of water at standard temperature (60°F = 16°C) which will flow through a valve or fitting at a pressure differential across the valve of 1 psi. (the assumption is that a 1psi drop across the valve is not significant, therefore the Cv indicates the acceptable flow rate for the valve or fitting).

    Here is an online valve calculator:
    http://www.a-tcontrols.com/products/valve_sizing/index.html

    The calculator will return the Cv value giving the system values of pressure, flow rate and specific gravity (which is a ratio of density to water).
  3. emesine

    emesine Member

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    Look on page 2 of this document. There is an excellent equation there on how to calculate the Cv you need based on your heating requirement, your boiler hot water supply, and your return water.

    http://www.taco-hvac.com/uploads/FileLibrary/102-144.pdf


    Design load
    divided by
    500 X (boiler supply - radiant return)

    For instance:

    I need 100,000 BTU

    My storage water will hit my mixing valve at 110F
    My valve is set to 100F
    I am planning on loosing 10F over my radiant loops, so my return water is 90F

    100,000
    divided by
    500 X (110-90)

    Answer, Cv is 10 gpm.

    Hope this helps!
  4. jebatty

    jebatty Minister of Fire

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    I feel like a "blockhead" from the old Laurel & Hardy movie "Blockheads." So what happens, particularly in the case of a 3 way or 4 way mixing valve, if a valve with a higher or lower Cv is used than the tables recommends? It just looks like larger means less flow resistance and vice versa. Is there another effect of larger or smaller valve? I still am not getting my head around exactly what Cv means and how it relates to a hydronic system.

    Steve, for some reason the link your gave, which I also found while researching this myself, hangs my computer. Likely a java problem of some sort.
  5. sgschwend

    sgschwend New Member

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    Yes, my computer said it would load the page but the page used an old Java version.
  6. Gooserider

    Gooserider Mod Emeritus

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    I'm not sure just what the difference is between Cv and the "equivalent pipe length" concept, but I think they are similar... I don't think any harm would come from using a valve with to high of a Cv, and the links seemed to say that you wanted a valve with a certain MINIMUM Cv, but didn't say anything about maximum.

    Assuming that Cv for a valve is like "equivalent pipe length" then my assumption is that putting in a valve with to low of a Cv would be like splicing a length of smaller bore pipe into a line - you'd have excessive head pressure, and probably excess velocity through the pipe, with probable noise and possibly erosion of the valve parts.

    To large a Cv might cause some extra turbulence and a velocity drop going through the valve, but I don't think would do any major harm.

    Gooserider
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