View Full Version : Cv ratings
I have to select some three way valves for some air handlers, I have the gpm of the coils but I don't know how to select the cv of the valves. How do I calculate cv.
12-27-2005, 09:39 PM
Cv= Q / Sq Root of Delta P
12-27-2005, 09:48 PM
Originally posted by hvac3901
Cv= Q / Sq Root of Delta P
Man that helps alot, but could we have more detail? Please.
I ran into that term last year and drew a valve number out of a hat. Well the hat part was made up. ;)
12-27-2005, 09:52 PM
A valve with a Cv of 1.0 has the capacity to pass 1 USgpm (US Gallons per Minute) of water while sustaining a pressure drop across the valve of 1 Psi (Pounds per Square Inch pressure drop).
To find your CV rating you need to know the volume (GPM)
and the pressure drop across the valve, often times just sizing a valve to the pipe size of th eunit can get you in touble, if you give me those two numbers I'll give you the short math on it,
I am going to bed soon, if you post it I'll catch you in the morning,
12-27-2005, 10:07 PM
Sample if you have 1 PSi drop across the valve,
and 20 GPM across the valve, you will need a valve with a mid range CV of 20,
If you have the same flow with a pressure drop of 1.5 , you will need a valve with a mid range cv of about 16,
Thats off the top of my head, if you post the actuals numbers, I'll do the math for you,
12-27-2005, 10:10 PM
Forgot to answer your question, CV is an index, that indicates a valves capacity to pass fluids at a certain condition.
To calculate it the short way, you can use the formula they posted, that is not enough info to make a valve selection though,
[Edited by titleless on 12-27-2005 at 10:14 PM]
12-28-2005, 12:29 AM
ok someone educate me then please. titleless, thats psia.
knowing the design gpm.
the available pressure and the pressure drop of the coil (a detail that is needed), how is that not enough info?
[Edited by hvac3901 on 12-28-2005 at 12:32 AM]
12-28-2005, 06:16 AM
Closo off required, fluid type, normal position, fitting type,
More, but I got to go, busy day and running late already,
esssentially though the correct CV puts you pretty close,and lets you start choosing from a smaller number of valves,
12-28-2005, 07:26 AM
You need to set what an allowable pressure drop across the valve is.
Some valves may require a bit of a drop to function properly maybe a couple psi.
The pressure drop through the coil will vary with the flow but a 10 ft HEAD loss is not uncommon to pick as a max. Your valve will probably end up being a nominal pipe size smaller than the coil header connection based on a 10 ft headloss through the coil.
I usually use CV to figure out the valve pressure drop loss, however once you had that pressure drop selected and used to work out the head of your pump, you could see what the Cv was and start comparing other valves with a similar Cv.
If you do fire sprinklers, Cv is the same as the 'k' value of the sprinkler head. Pressure drop in psi = (GPM/Cv)^2
When you ask for pressure drop do you mean system (supply and return ) drop through the coil. I have the coil specs put it does not give me pressure drop.
12-28-2005, 07:48 AM
A chilled water coil, will have to have the data for GPM, entering/leaving air conditions, entering/leaving water temperatures, and the head loss.
Head loss will most likely be in ft of water, maybe there is a column entitled "WPD" in the rating table. This means a drop in pressure as the water flows through the coil, so it would be the difference between the supply and return to/from the coil.
Perhaps you are bored and your selection table is 'titleless'.
12-28-2005, 06:05 PM
If you do not have a lot of engineering data available, you can take your pump differential at the coil, and use 15% of this as your pressure drop through the valve,
12-28-2005, 07:02 PM
So lets ay, you have a 20 GPM coil,and your system/pump diff is measured at the coil as 40PSI,
15% of 40 is 6
take 20(GPM) divided by the square root of 6
about 8.16 cv,
So you need a valve with a CV rating of 8.16, I use +- 5% , because you won't find a valve to match the exact CV rating,
This was a fictional scenario, with made up input numbers,
12-28-2005, 08:01 PM
What about the CV rating on a two port steam valve that just closes off the flow of steam into a coil, I had a guy at a church that I work at claim the CV was to low on the valve to work properly. How would you size that? Thanks for your help Bob!
12-28-2005, 08:22 PM
Steam is a fluid also, there is more to know in order to choose the proper valve, but its teh same principle,close off would be critical in this application,
[Edited by titleless on 12-28-2005 at 08:25 PM]
12-28-2005, 08:36 PM
some of your valve manuf will have a sizing calculator on their website. last time i checked belimo had a calculator on their site. you will need pressure drop and gpm requirment and they will do the sizing for you. Good luck
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