There has been a lot of buzz on Control Trends lately on Cv so I thought I would do my best to explain what Cv is and how does it relate to HVAC systems.
NOTE : I am not attempting to be exact on definitions only practical in field use of the Cv information.
Based on water (media for most HVAC applications)
Photograph by Bill Jones
How many GPMs?
Cv is the flow of water through a valve at 60 degrees F in US gallons/minute at a pressure drop of 1 PSI. Easy enough right? So if you have a Cv of 25 and your media is water at 60 degrees F and a pressure drop is 1 pound you will get 25 GPM out of the valve. Be careful it’s not that easy in the “real world” of the HVAC system.
Real world engineers when they develop a HVAC water system throw a wrench into this easy equation by calculating different pressure drops for valves, equipment, and then add in the problem of valve actuation types.
Over the years a safe bet was to size a HVAC valve was to use the calculation for Cv as 1/2 the gallons per minute using a 3 PSI drop. Other words if you needed a valve that could handle 10 GPM of water at a 3 PSI drop (used by most engineers) you would pick a valve having a Cv of 5. This system still works today as a RULE of THUMB because most contractors do not know what the system pressure drop was originally designed for when ordering or replacing a valve.
Engineers today are now designing systems using a 4-5 pound pressure drop. Why you ask? More and more of today’s HVAC systems are modulating systems which means valves are being asked to move on a constant basis somewhere between full open and full closed to maintain closer temperatures in the conditioned space and to save costs. Using a 4-5 pound pressure drop allows for better modulation when accuracy is an upmost concern.
Now follow me here… When using (2) two position (on/off) actuators with ball valves size for “pipe” size and as long as you select a ball valve that has a Cv rating of 1/2 your GPM rate you will be good.
Modulating valves need to be considered carefully. Because flow through coils, to properly maintain temperatures in the conditioned space, is critical to the building design valves must NOT only be sized via pipe size. Actual GPM flow is the best way to size modulating valves. Piping is considered as part of the equation along with pressure drop and flow. If you size a valve with too large or too small of a Cv (even if the pipe size is right) the valves will not supply the proper amount of flow causing system temperature fluctuations.