Proper control for VAV air handler

[tdhardbar]

I am currently working on a project that has a VAV air handler.
The supply and return fans are pneumatically driven vane-axial fans. The controls are all pneumatics. The the old engineer on site did not maintain the pneumatic controls properly and they are all not functional. I am going to retrofit the controls to DDC controls on an as needed basis in order to maintain the equipment properly. I plan on using a transducer controlled by the DDC controller to operate the pneumatic vane-axial fans until frequency drives are in the budget. My question is air flow. I have 2 air measuring stations that I have cleaned and inspected. They have air srtaighteners and total and static pressure sensors evenly distributed and manifolded for averaging. I am installing a XL15C Plant controller and digital panel mounted pressure sensors. The pneumatic controls are hooked up with panel gauges that read CFM for supply and return fans, and one gauge reads Supply static pressure. Is it ok to use the air stations at the outlet of the supply and return fans to control the fans or are these stations only for the pneumatic square root extractors (which are not functioning and non-calibrateable) to give a readout of cfm for supply and return fans. Or should I put a static pressure probe 3/4 of the way down the supply duct and control only by static pressure.
If the latter is preferred I am wondering if I could use a math function in the programming and read the pressures in my air measuring station to calculate cfm as a guide.

 

[gepman]

thardbar
I am going to assume that you mean that the vane-axial fans have some type of pneumatic damper and not that the prime mover is pneumatic and not an electric motor. I am not familiar with an XL15C since I am from the industrial control world and not the building automation system world. I assume that the signal point to which you want to connect is ahead of the square root extractors and therefore they should be good locations from which to control and/or measure the flow of your fan. If you want to control your fan on flow then then you will need to use a math function in your DDC unit proportional to the square root, which you will need anyway to calculate the flow. If you want to control your fan on static or total pressure then you will not need the math function.

 

[absrbrtek]

Fix the dampers open 100% and put a VFD on the fan. Probably be cheaper and alot less headaches. Use a sensor 3/4 of the way down the duct to control the static air pressure. Sounds like the ststem is way over controlled for such a simple aplication.

 

[quark]

What control do you want? Is it constant flow or constant pressure? If you have VAV terminals, then many people prefer to have constant upstream pressure so that volumetric flowrate from the terminals will be maintained constant. If the terminals are controlled manually, then flow based fan control is good. You already have averaging pitot tubes and what you have to do is to calculate the velocity pressure once, corresponding to your duct size and volumetric flowrate, and then set the value. Your fan will run accordingly. Suppose, if you have a 2'x2' duct and your flowrate requirement is 3000 cfm then the velocity pressure should be 0.035" WC(by solving 3000 = 4005x(2x2)xdP^1/2