CEMP-E
TI 811-12
18 August 1998
(e)
Minimum cooling capacity.
(f)
Peak demand limit target.
(g)
Equipment priority schedules.
(h)
Length of sliding window interval.
(i)
Instantaneous demand.
(j)
Maximum space temperature during occupied periods (cooling).
(k)
Minimum space temperature during occupied periods (heating).
(l)
Space temperatures.
(m) Equipment constraints.
(n)
Cooling and heating operation.
(o)
Demand limit setpoints
(2)
Program outputs.
(a)
Calculated percent load point.
(b)
Demand signals.
(c)
Start signals.
(d)
Stop signals.
(e)
Setpoint adjustments.
c. Application notes. The demand limiting program is used in conjunction with scheduled start/stop
and optimum start/stop programs. Standard demand limiting steps appropriate to summer and winter
operation have been established and listed in guide specification CEGS-16935. The designer will assign
each sheddable load to electrical demand limiting steps based on installation requirements. The designer
will consider the impact of demand limiting on building habitability, occupant comfort and productivity. In
general, demand limiting actions having the least impact on operations will be scheduled to occur first.
Demand limiting designs which include shutting off ventilation to occupied buildings shall incorporate air
quality sensors or other features to prevent or alert occupants to potential discomfort.
13. CHILLER DEMAND LIMITING PROGRAM. One specific application of demand limiting is chiller
capacity control. Centrifugal chillers are normally factory equipped with an adjustable control system
which limits the maximum available cooling capacity; thus, the power the machine can use. An interface
between the UMCS and the chiller controls allows UMCS to reduce the maximum available cooling
capacity in a demand limiting situation, thereby reducing the electric demand without completely shutting
down the chiller. The method of accomplishing this function varies with the manufacturer of the chiller.
The chiller percent capacity is obtained by monitoring the chiller current input. When a chiller is selected
for demand limiting, a signal is transmitted, reducing the chiller capacity. The chiller demand limit
adjustment is performed by shunting out taps of a transformer in the control circuit or by resetting the
control air pressure to the chiller compressor vane operator or by potentiometer adjustment at the chiller
control panel. As further need arises, signals are transmitted until the demand limiting situation is
corrected. Extreme caution will be exercised when applying this program to chiller demand, since
incorrect control can cause the refrigeration machine to operate in a surge condition, potentially causing it
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