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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----- 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. 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 933 Road 101 Jeffersonville, Vermont 05464 USA hvac@xxxxxxxxxxxx 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----- A typical
design is to zone the floor compartment according to aspect and allow the
individuals to adjust the swirls. 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----- Do any of you
out there have any experience designing an underfloor air ====================================================== =====================================================You
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