[bldg-sim] VAV control of underfloor air

[Kirchhoff, Eric]

Regarding the VFD control, what I suggested is not new.  You let the VFD be
the application specific controller (ASC, by some manufacturers
terminology).  ABB, Graham, and others do this regularly.  The EMCS is
hooked up to the VFD "controller," The VFD works to maintain its programmed
set point, which is decided by the EMCS.  The VFD will continuously
communicate with the DP sensor, while routinely communicate with the EMCS
main system.  This arrangement is beneficial to the overall EMCS system in
that it minimizes traffic on the cable, and frees up the main system from
having to send speed control signals to the VFD on a continuous basis as
well as continuously monitor the DP sensor.  It is very similar to an ASC on
a VAV box.  The main system does not monitor, continuously the temperature
in each zone and then command the damper and valve to adjust accordingly;
the ASC monitors the zone temperature continuously, adjusts accordingly, and
then reports to the main EMCS system to inform it as to the current
situation as well as receive new instructions.
 
I did not mean to imply that the VFD was a controller unto itself, rather an
integral part of an EMCS system, however I am making the VFD work more for
its money.  If you look more into this, and ask VFD manufactures, you will
find this is becoming more the standard practice.  Interestingly though,
mechanical designers do not show this arrangement, or are not aware that
what they show is not what they are receiving.  What they are actually
getting is a VFD that functions the way they asked, but is cheaper to
install and works better.
 
Eric Kirchhoff, PE
Project Engineer
 
 
-----Original Message-----
From: Tom Anderson [mailto:hvac at cx-assoc.com]
Sent: Monday, November 04, 2002 8:16 AM
To: EKirchhoff at semprasolutions.com
Cc: bldg-sim at gard.com
Subject: Re: [bldg-sim] VAV control of underfloor air
 
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 <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
<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
<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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