[bldg-sim] VAV control of underfloor air

[Tom Anderson]

RE: "Additionally, to keep costs down, I would let the VFD control the
situation, not the EMCS.  Just have the VFD update the EMCS, and allow
the EMCS to reset the differential pressure sensor."

I do not agree with this concept.  Using stand alone VFD control loses
the integrated features of a building DDC system.
-How does one adjust and tune the VFD static pressure control?
-How does one deal with PID windup problems?
-When the system is commissioned, establishing the correct static
pressure control setpoint will be VERY difficult without DDC access to
VAV box damper positions.

The cost to control a VFD from the building DDC system will actually be
less when the VFD is connected to the DDC system using modBUD or LON
network... you only run one data cable from DDC controller to the VFD,
and you pick up all 60 or so VFD data points, some of which are quite
useful for diagnostics and monitoring energy use.


     Thomas E. Anderson
     President
     Cx Associates, Ltd.
     Building Commissioning Specialists
     http://www.cx-assoc.com
     933 Road 101
     Jeffersonville, Vermont 05464 USA
     hvac at cx-assoc.com
     Tel: 802-644-5616 Fax: 802-644-6797

"Kirchhoff, Eric" wrote:

>
>
> Although I have always been interested in designing such a system I
> have yet to do so.  However, that doesn't seem to stop me from adding
> in some anecdotal information from friends who have designed such
> systems.  First off (not from friends, this is my own observation), a
> VFD for an air handler tends to be nearly the same cost as a starter,
> except for the additional control point-that being the differential
> pressure sensor.  Additionally, to keep costs down, I would let the
> VFD control the situation, not the EMCS.  Just have the VFD update the
> EMCS, and allow the EMCS to reset the differential pressure sensor.
> Go ahead and monitor all the points you want, and at least do start /
> stop, status, and DP sensor reset.  Like Robert says, the VFD is a
> cost wise investment, even if you end up later on setting it up to be
> a constant speed system, the cost risk is low.
>
> Now I would like some additional information.  The IRR of 30% to 50%;
> is that just for the mechanical investment or everything?  Since this
> type of system uses a raised floor system, all the utilities end up
> running under the floor.  Code will dictate the use of plenum rated
> cable (even when run inside conduit), the floor system itself tends to
> be proprietary-or at best limited in number of suppliers--hence supply
> and demand comes into play driving the cost of the floor up and that
> is beyond the fact that it is already more expensive since it needs to
> be structurally capable of carrying the loads (including areas for
> very large filing systems), there are a limited number of suppliers of
> underfloor vav systems (and the quality varies), as a positive there
> is a greatly reduced amount of sheet metal used for ductwork, there
> usually is not a realistic coordination between cubical / office
> layout and diffuser placement (in large jobs, the eventual leasee is
> unknown hence their layout needs are unknown) causing problems by a
> maintenance staff that is unfamiliar with the system (despite
> training), and another maintenance problem are users who do not
> understand nor properly utilize the system hence they will block the
> diffusers (more often then the overhead type).  Plumbing may also be
> run in the floor space as well as other utilities causing a need for
> additional wall furrings then may be in a more traditional layout. Was
> there any account of these items in the IRR, or was it a theoretical
> value based only on the mechanical components?
>
> I do not mean to discourage anyone from using this system, I actually
> am very interested and excited to implement this type of system, and
> believe (instinctually) that it is an energy efficient alternative to
> the traditional diffusion method of airside systems.  The points
> listed above are things you should be aware of, and are questions to
> ask so as to minimize their impact.
>
> As Rob says Good Luck, and have some fun!
>
> Eric Kirchhoff, PE
>
> -----Original Message-----
> From: Robert Lord [mailto:rgl.lsbris at lincolne.com.au]
> Sent: Sunday, November 03, 2002 2:55 PM
> To: bldg-sim at gard.com
> Subject: [bldg-sim] VAV control of underfloor air
>
> A typical design is to zone the floor compartment according to aspect
> and allow the individuals to adjust the swirls.
> There are only a handful of large VAVs and the fan speed is varied
> according to duct static pressure (not plenum pressure).
>
> In most instances, I have found that the internal rate of return is
> between 30% & 50% and hence is a good investment. I am sure VSD
> pricing and energy pricing varies but a point to note is that the RH
> control is improved.
>
> Good luck!
>
> Rob Lord
>
> -----Original Message-----
> From: Molinini, Louis [mailto:lmolinini at syska.com]
> Sent: Saturday, 2 November 2002 8:37 AM
> To: bldg-sim at gard.com
> Subject: [bldg-sim] VAV control of underfloor air
>
> Do any of you out there have any experience designing an underfloor
> air
> system with VAV control.  The particular system I am looking at is a
> plenum
> based system with manually adjustable swirl diffusers (Titus/Krantz).
> I am
> curious if it ends up operating like a constant volume system or is it
> worth
> the expense of installing the freq drive and the Static pressure
> sensor or
> differential pressure sensor.  Any help is greatly appreciated.
> Thanks,
> Louis
>
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