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VAV Diffusers, Packaged Rooftop Units, and BAS
A “newish” concept comes out for public bid
am currently in the midst of estimating the digital controls for the
mechanical system that is to serve a stand-alone three-story facility.
The facility is to be used as a salon, although I don’t know if that
information is pertinent to the story. Anyway, I thought I’d
write about it because it is the first time I’m seeing such a system.
That says something, as I’ve been in the biz a long time and it’s not
often that I see a new mechanical systems design concept, one that you
read about in the monthly engineering and design journals, actually
come out for bid!
The facility is to be served by eight packaged rooftop units. Each unit is to be a VAV unit, in that the unit will come from the factory with the controls and mechanical appurtenances required for it to operate to provide 55-degree air at a duct pressure of 1.5 inches W.C. (in rough terms). The rooftop unit manufacturer is to provide any “loose controls” that need to be field-mounted, such as a discharge air temperature sensor and a duct static pressure transmitter.
The eight rooftop systems are consistent in design and mechanical layout, so I’ll simply discuss a single system. So we have the VAV rooftop unit, which traditionally would serve a bunch of VAV and/or fan-powered boxes. Terminal units, if you will. Instead, the unit serves a bunch of “VAV diffusers”. These diffusers are equipped with actuators that can accept a control signal, and modulate in response to it, to proportionally control the amount of conditioned air delivered into the space being served. For any particular rooftop unit, there may be a dozen or more VAV diffusers being served.
But that’s not all. The space served by each rooftop unit is broken down into zones. For the unit in question, the space is divided into four zones. The supply duct from the rooftop unit branches off, into four smaller duct runs. Each duct run has what is referred to as a ‘pressure control valve”, or PCV. Similar in concept to a cooling-only VAV box, with a motorized damper that can modulate in response to some control signal. Downstream of each PCV are the VAV diffusers. For the sake of simplicity, we’ll say that each PCV on this particular system serves four diffusers.
So in summary, the VAV rooftop unit feeds four pressure control valves, which in turn each serve four VAV diffusers. That is the mechanical description of the system. Now for the controls.
As stated earlier, the rooftop unit is equipped with VAV controls that allow it to operate as such, in the simplest of terms, to blow 55-degree air at a suitable pressure. The unit uses its own microprocessor-based control system to perform all facets of unit operation, from fan control to cooling, and everything in between. As we will be proposing to install a full-scale Building Automation System (BAS), we will interface to the rooftop unit via a BACnet communication module that will come as part of the rooftop unit’s control system. This module will allow the BAS to provide monitoring and supervisory control of the rooftop unit. Control is limited to time-of-day scheduling, night setback, and other supervisory functions. Monitoring would be everything that the rooftop unit can give us via the BACnet connection, and will include trending and alarming of critical values.
For each PCV, we are proposing to field-install a damper actuator and a digital controller, as we’d typically do in the case of a VAV box. However, instead of installing a space temperature sensor and wiring it to the controller, we will be installing a static pressure transmitter downstream of the PCV and before the VAV diffusers. The PCV will operate to maintain a suitable downstream branch duct pressure, suitable for the VAV diffusers to do their thing and not be “overpressured”.
Now we get down to the diffuser level. Each PCV serves four diffusers. The plans call for one of the diffusers to be the “master”, and the remaining diffusers to be the “drones”. The plans show for the space temperature sensor to be wired to the master, and for the master to be wired to the drones in a serial fashion. Seeing as we’re proposing to install a networked digital control system, we’re planning on doing things a little differently, however the end result will be the same, if not a lot better. In essence, we will be installing a zone temperature sensor, one for each “master”, however we will be terminating the zone sensor back at the PCV controller. The PCV controller will have enough inputs to accommodate all sensors required. In this case, we will wire four space temperature sensors back to the PCV controller, for the four zones of temperature control.
The PCV controller will also have enough analog outputs to accommodate each “master” VAV diffuser. In practice, we will be wiring out from the controller to each of the four masters, using up an analog output for each instance. From each master, we will wire out in a serial fashion to all associated drones. The analog output will source a voltage signal, which we can run the master VAV diffuser and all associated drone diffusers from. In this manner we can control the VAV diffusers to maintain zone temperatures, as read by the zone temperature sensors.
Racapping the sequence of control…the rooftop unit operates, for all occupied periods, to maintain cool supply air at a pressure suitable for the pressure control valves to do their thing. The rooftop unit feeds four pressure control valves, and therefore four zones. Each pressure control valve operates solely to maintain a pressure suitable for the VAV diffusers to do their thing. The four VAV diffusers associated with each pressure control valve (one master and three drones) operate in unison; their dampers are modulated to proportionally control the amount of cool air into the zone, as a function of the temperature sensed at the space temperature sensor, and of the zone temperature setpoint. In simple terms, the further (higher) the space temperature is from desired setpoint, the more cool air is allowed into the zone by way of the VAV diffusers.
Seems like a good design, at least on paper. But in the end, is it really? My first question is, is it an economical alternative to traditional VAV? We’re so used to doing things a certain way, why would we want to try something different in the first place. Of course that’s a hypothetical question, and there are all kinds of good answers to it. Still, as I sit here determining the costs involved, I can’t help but think that this building could be heated and cooled in a traditional manner, at a lesser cost and in a less complicated manner.
I hope to procure this project. My intention is to write about it again sometime down the line, after the project has been completed, and discuss the details of the project, including the controls engineering and design phase, the installation phase, and the commissioning phase. I also hope to spend just a little time comparing it with a traditional VAV project, to see how it compares, in a financial sense as well as in a practical sense. With any luck, I’ll be writing this follow-up article in a year from now!
Tip of the Month: New mechanical design concepts come to light all the time. We in the industry are to determine whether or not these concepts lead to practical alternatives to the tried-and-true methods of the past. My tip for this month is to embrace anything new that comes your way, and not be hesitant to learn and understand it. There are very good reasons that new concepts are developed. Not all of these will prove to be practical in the end, but some of them will, and to keep an open mind is to have one leg up when the new idea of the day becomes the next big thing in HVAC!
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