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Currently there is no "optimized static pressure
control" for fans/ducts in DOE-2. However, J. Hirsch & Associates
has funding to implement such a mechanism in DOE-2.2 within the next few
months. The funding is provided by Southern California Edison as part of
their "Energy Design Resources" program (http://www.energydesignresources.com/).
These enhancement will be incorporated into both DOE-2.2 and eQUEST (not
PowerDOE), and will be made available to the user community at no
charge.
This control mechanism will be similar to the head
reset controls already implemented for variable-flow pumping systems in
DOE-2.2, where the pump head setpoint can be reset based on the hourly head
demand of the worst-case valve. The actual pump head is then a function of the
worst-case valve head, plus the additional variable-flow head losses of the
loop piping and the central plant equipment. This control
sequence maximizes the potential savings of variable-speed pumps, as the
required speed of a pump is dependent on both the flow as well as the head
requirement; with the head requirement usually being the dominant factor.
(The maximum head of a pump varies roughly as the square of the impeller speed;
if the head requirement does not decrease as the flow drops off, then the speed
cannot decrease significantly).
For fans/ducts, the simple curve-fit that varies
fan power as a function of flow will become obsolete. Instead, DOE-2.2 will
keep track of the static requirement of the worst-case VAV box, as well as
ducting, coils, filters, etc. Each component must be accounted for
separately, as the head of all of these devices varies differently for a given
flow. (Some components, such as ducts and sound traps, experience
turbulent flow, and their head varies as the 1.85 power of flow. Other
components, such as coils and filters, are more laminar, and their head
typically varies in the range of the 1.1-1.6 power of flow. Still
others, such as a VAV box without an intelligent DDC system to monitor
damper position, will have a constant head requirement; i.e. a constant head
setpoint out toward the end of the ducting.) These changes will require
DOE-2.2 to know about the flow through each component in the system, the
component's head characteristic, as well as the fan map for a given type of fan
(forward-curved, airfoil, etc.) Based on flow and static requirement,
DOE-2.2 will then be able to determine the fan speed and power
consumption.
In the meantime, you can approximate "optimized
static pressure control" by generating a new curve that causes power to fall off
more rapidly with flow than the default curves. However, this is not a
straightforward task, as you would have to calculate by hand all of the
algorithms we have designed for DOE-2, and repeat the calculations for a series
of flows. Perhaps someone has already done this exercise, and can share
their curves with you.
Steve Gates
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