Valves' Cvs 1

[dynamo78]

Hello fellow members,
I just want to recall my HVAC knowledge if it still applies.
Say, I have a cooling coil with 33Ft. PD @ 125 GPM, my old school knowledge says that I should a control valve with a CV of 8-9. Will that change if I have VFD in the chiller system?
I appreciate any comment to help me boost my confidence in HVAC
Thanks.

 

[HVAC-Novice]

There were many rules of thumb, none based on science. You need to size for a pressure according to those rules. Some say to at least have the pressuredrop of the coil, or the branch, or the entire system.

Much simpler to just use PICVs. Always work correctly and no problem sizing as long as you select for the correct flow rate.

 

[dynamo78]

Well, The reason for being is that at 1 psi pressure drop the flow would be 8GPM, and to simulate the coil flow at 125 GPM and 15psi PD, the PD across the valve should be about 15psi PD too. right?
Thanks Novice.

 

[moideen]

I'm not sure if I fully understand what you're asking. The flow rate divided by the square root of the total pressure drop (gate valve + strainer + balancing valve + coil) yields the required CV. So, 125/3.87=32

The problem with the world is that intelligent people are full of doubts, while stupid ones are full of confidence.
-Charles Bukowski-

 

[dynamo78]

Thanks Moideen. Got it. By definition, Cv is the flow per 1psi PD across the valve. in this case, it's GPM. yeah, I agree with you. I should have included the PDs on the fittings too. I agree that the CV should be 15 as you have shown. Now if the Supply CHw side to the coil is closed, the bypass is still showing a full flow (100% bypass). That's where my question of " Does having a VFD on the pumps negates the Cv?I ask this because the pump will drop flow accordingly thru BMs or /VFD.
Sorry for my very basic knowledge.
Again thankyou for the input.

 

[nuuvox000]

What type of control valve are you talking about? Just a simple two-way open/close? Three-way with bypass? I'm not sure why you would want your pressure drop so high on this valve.

 

[dynamo78]

Sorry for the confusion for not having a well defined question. Based on your input, I was able to research what I want and needed to know.

Thank you for all the inputs.

 

[BronYrAur]

wait a minute, why would we take into account to PD across the coil, strainer, etc when selecting the valve Cv? We would just take the PD across the valve, correct?

 

[HVAC-Novice]

For a regular control valve, you need some control authority and that requires some decent pressure drop. so the Cv is determined to have the valve fall within a pressure drop range. How much pressure drop do we need? No idea, but old wife's tales (and that basically is what it was based on) recommended to try to match the coil loop pressure drop. but this is NOT based on actual science. just a rule of thumb that gives you a reasonable result. You may as will just decide to use 3 or 5 psi as target to determine Cv.

But really, in 2023, no one ever should use regular control valve. It all should be PICV and you don't have to worry at all. Cheaper, easier and more or less fool-proof. This whole thread basically is about how to adjust the carburetor in a car, while we should talk about injection systems.