cv factors

Bob M.

Does anybody know of any good articles on CV and picking the right mixing valves?

Thanks

hr

R. Bean is the guy

to ask about valves and flow. He has done a number of great RPA seminars on this. Maybe he can foward some info to you.

"Modern Hydronic Heating" by John Siegenthaler has a chapter on fluid flow in pipes that is very helpful also.

Simply put Cv is defined as the flow rate of 60 degree F water that will create a 1.0 psi pressure drop through the valve. A valve with a Cv of 4 would require a 4 gpm flow through it to create a 1 psi drop across it.

Valves should be selected using this number not the pipe size.

hot rod

hot rod

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Tony Conner

Cv Factors

And that includes check valves.

Riles

Bean is the Cv info man

I second Hot Rod's comments. I have been in two different sessions with Robert Bean and he is all over Cv since he discusses controlling flow.

Here's another obvious bit of info; 1 P.S.I. equals 2.31 Feet of Head (Since a lot of Pump Curves List Feet of Head vs GPM).

RB_2

Cv

Short & sweet...

A competent designer selects a circulator, which operates close to middle of its performance curve for optimum efficiency.

A competent designer selects pipe diameters to achieve a 3.5-ft/sec velocity, the mid point between 2ft/sec and 5 ft/sec…the target range.

A competent designer selects the mean temperature (the mid point between supply and return) as the design temperature for selecting heat terminal equipment.

…and when it comes to selecting control valves a competent designer selects the valve to operate in the mid position…so it has room to modulate open or close.

A valve is too big when over the season it continually operates near its closed position.

In fact an oversized modulating control valve is no better than trying to control a system with a hand operated gate or ball valve.

How do we select a valve so it operates in its mid position?

We look at the hydraulics or the distribution of pressure losses in the system it is controlling.

We say a control valve has control when under design condition it represents between 30% to 50% of the circuit head loss.

Imagine this…under maximum load if the valve is oversized, a few degrees of valve rotation is sufficient to meet the target temperature. If only a few degree of valve rotation is required under maximum load what is the rest of the valve opening for?

In reality the valve capacity is never used…it never gets to open up fully because its not needed and if it isn’t needed…why buy it?

The Cv tells us what the pressure loss is through the valve at a given flow rate so if we know the head losses in the circuit being controlled then we can pick a valve with a Cv which will provide us a pressure loss equal to or slightly less than the circuit it is controlling. I.e.: If we allow 20 ft of head loss for a zone, the control valve should represent between 6ft and 10ft.

The next question is...what if you can’t find a valve in this range?

The answer…a balancing valve is required...another topic.

The above is short synopsis of control valve theory, which hopefully answers your question.

Incidentally the same theory applies to using injection pumps vs. injection valves or zoning with pumps vs. zoning with valves...it can not be ignored.

As a final thought…you’d never find a designer debate the need to do heat loss calculations and then translate them into flow rates and head losses per zone…but if one is not prepared to ensure what one has done on paper is created in the field by installing the right equipment and verifying the flow and head losses are as per design…why do the calculation?

Just something for all to think about.

RB

as far as articles on Cv you can down load a complete contolling hydronics manual from our web site.

Ignore the Euro terminolgy...the theory doesn't change because we cross the ocean!

http://www.cd.danfoss.com/HeatingApplications/

enjoy.