constant vs variable volume

[ACD1]

I'm used to designing variable volume systems.

I have a food production facility with a warehouse/loading dock side of house, a large area with a lot of steam being used, and another large area with a lot of sensible heat on one side of the room - an area with insulated, cold equipment - and another area with more heat.

The loading dock area has several rooms for storage adjacent. I thought about a variable volume system for these spaces - but it would require multiple, possibly large VAV boxes for the loading dock area. My superior said, "It's process. Make it a constant volume system."

Can someone explain to me the ins and outs of constant volume systems, when you can use them?
When am I violating the intent of the energy code? When am I not?

For systems serving multiple spaces, you can't use variable volume because of pressurization, correct? The fact that you can't control exactly where that return comes from without say, phoenix valves on the return as well?

If it's a single zone served by a single unit, is there any harm in making that particular space a variable volume unit?

 

[GT-EGR]

You’re bosses direction is typical for designers with the older mindset who don’t want to deal with all this fancy new variable controls that are now very standard. The statement that it is a process is one of the most common, and used much more often than it should be.

Energy code comes at this in two different ways. It wants to limit wasteful useage of total airflow by requiring VAV systems to slow down fans, and it also wants to limit simultaneous heating and cooling by only allowing reheat to be applied to a certain % of VAV box airflow.

Unless you have a real and critical pressurization component, constant exhaust makeup requirement, and/or dehumidification component to this process, I would guess that there isn’t a good reason to not use VAV systems. Even with VAV you can control pressure by also making your return system have a tracking VAV box to follow the supply airflow.

Even single zones are required to be VAV after they reach a certain capacity - requiring you to reduce your total airflow when you are at partial load.

Another decent justification sometimes to stick with constant volume is if you have real physical limitations on an existing system.

 

[ACD1]

GT-EGR:

Thanks so much for the response. I'm trying to get as much input as I can.

I was thinking last night about it and thought that thermal dispersion controls would really help ensure the proper amount of building pressurization air regardless of turndown. Just wonder if they're available on packaged units. Of the three areas I describe above, the large area I describe above with "with a lot of sensible heat on one side of the room - an area with insulated, cold equipment - and another area with more heat" since it's all really one big room - could be served by a single packaged variable volume unit, with different VAV boxes to serve the areas with very different sensible loads within the space. The return for this space is a non-issue, since it's all one big volume.

The loading dock area is a different story, as more of the "dirty" side of the facility, I started thinking that constant volume wasn't such a bad idea, to ensure a buffer when the loading dock doors are open. Maybe add a backdraft damper for when the doors are shut. I suppose the different storage, maintenance, and dock areas could have return valves associated - but there would probably be a first cost issue with the client. Or maybe, the pressurization of these areas just isn't that critical to require them. I'm really only familiar with Phoenix valves when you really want to keep pressurization to a space constant, and don't know how accurate regular return valves are for this purpose. Probably accurate enough, I'd imagine.

The other large area with a lot of steam being used will most likely have a wet exhaust system, but the loads will vary greatly depending on the amount of product in process. I think you could have this be set as constant, and ensure that the single unit serving this space would have that exhaust cfm (plus appropriate pressurization) programmed in as an offset between the supply and return fans.