VAV air system mixed with constant volume diffusers

Brad White

Hi Luke-

I suspect your 1.5 inches is too high for one thing. If you assume (as I must for I am not there) that the ductwork downstream of your furthest VAV box imposes maybe 0.20 inches, your VAV box itself might impose 0.50 inches if tightly sized, then add another 0.25 inches as a control pressure margin, you will only need 1.0 inches or less <i>at that point</i>, the inlet of that furthest VAV box.

That is another 0.50 inches you have to knock down. Most of my VAV systems (12,000 to 50,000 CFM or more) can operate at an initial setting of 1.25 inches or less.

A blast gate (the kind where the plate seals into a pocket, not the cut-off type used in dust collection, which would leak badly) can take off 100% of the pressure (if closed naturally) and anything in-between. I have used them and bored a hole with a Greenlee punch in the center as an orifice plate. You can find tables which can tell you how large a hole will pass how much air at a given pressure to guide you. If cut too large (happens), a pivot shutter on the upstream side, secured by a Tek Screw, will adjust this precisely. Such a deal.


Key is, when you do this, it can and will create noise. This is why you need to install these as far upstream from the diffuser as possible to allow the noise to dissipate.

Here is another tip, not that you asked: Next job you design, install the duct as a "ring duct". This shares pressure between say east and west exposures and allows the system to run at even lower SP settings. The furthest VAV box is effectively served by two mains coming from different directions. You can get away with 0.75 inches or so. Key is though, control the pressure at or near the furthest VAV box, not the usual "75% from the fan" measuring point.

What do you think?

Luke_4

Can a VAV system have constant volume diffusers?

Did anybody experience a VAV air system mixed with constant volume diffusers? Does it work if the constant volume diffusers are less than 20% of the air flow? In my case there is an entire floor served by one air handler with variable speed fans. The air system has VAV boxes but in the same time has a number of diffusers tapped off ahead of VAV boxex. These diffusers which represent about 20% of the entire air flow don't have a VAV box and are noisy. Any suggestions how to corect the problem at this stage?

Brad White

CV on VAV

I think you are finding a common mis-application. In any VAV system, every diffuser must be governed by an upstream regulating device (VAV box).

As you are finding out, those unregulated diffusers are becoming noisy but also pass air entirely dependently on the pressure in the ductwork. Add to that, VAV systems tend to run at higher static pressures in order to maintain control ("some pressure to work against", I call it.)

The only mitigating factor is the duct static pressure control: If you are using a VFD with electronic proportional signal controls, the pressure can be held fairly constant. Constant pressure begets constant flow, all other things being equal.

[I will confess to having intercepted a VAV duct main using this constant pressure principle by installing a blast gate and diffuser where needed. But I have to add, this was an ad-hoc solution and was cooling a small server room where noise was not an issue. I would not do this in an occupied space and there is no control either.]

The correct solution is to place VAV boxes of similar type upstream of these diffusers.

Absent that, there are self-contained even self-powered VAV diffusers. Thermal Products, Rickard are two names and there are others.

http://www.vav-diffuser.com/square/vfsa94-100/vfsa94-100.htm

http://www.rickardair.com/sec4.asp

Some manufacturers use 24V controls and are probably more accurate and adjustable via a wall thermostat but the self-contained ones use a bulb and capillary not unlike a TRV on a radiator.
I would also install a good balancing damper or blast gate at the main take-off to "knock down" the pressure so that the diffuser does not have to throttle against higher duct pressure than necessary.

Regardless of what I did, I would check your duct static pressure control, the heart of it all. Make sure it is smoothly-responsive but also set it as low as possible, enough to get the air to the furthest diffuser with a very slight margin. (On large systems this can even be re-set lower during low-load times.)

Point being, the lower the pressure the lower the energy costs but more importantly, the less noise potential.

Hope this helps!

Brad

Luke_4

It does help

Brad thank you so much for sharing your experience. I know is an odd situation and not many people experienced a VAV system mixed with constant volume diffusers. The majority of the constant volume diffusers are serving spaces which are not critical like toilets, storage rooms but few of them do serve open offices. I tried the thermal diffusers. They require very low static ahead of the diffuser 0.5"-0.4" and cannot withstand a 1.5" static pressure. What is your experience with blast gate damper? How much static you can cut without making the diffuser noisy?
Again thank you for your help!

Brad White

Oh, another thought...Iris Dampers

A company called Continental Fan Manufacturing (CFM- cute huh?) make what are known as Iris Dampers which come in manual and motorized models.

These have pitot ports on either side of a variable orifice (like an eye iris) which are great for setting flows. Better than the blast gate with orifice and not too expensive. The blast gate orifice approach is good when you have to punt though.

Just remembered the iris dampers.

Brad