Page 312 - HVAC Pump Handbook

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Basics of Pump Application for HVAC Systems

Basics of Pump Application for HVAC Systems 309

3. Wire-to-water efficiency control. Field experience with best effi-
ciency control has proved that pumps still were not being sequenced
efficiently under all system conditions. This led to further evaluation
of pumping system operation and its refinement by wire-to-water effi-
ciency control, which utilizes the procedures listed in the previous
section for wire-to-water indication.
The wire-to-water efficiency determination can be used to select
approximately the points at which the pumps should be added or sub-
tracted. The transition points can be selected from the wire-to-water
efficiency tables and inserted into the pump-control program. The
actual pump operation can be checked by wire-to-water efficiency
indication, and the set points can be changed in the field until the
pumps are operating on the most efficient program. How the transi-
tion points are secured is shown in Table 10.7, data from an actual
test for wire-to-water efficiency.
The value of wire-to-water efficiency control is shown dramatically
in Table 10.5. If one pump were allowed to operate as high as 1625
gal/min, the input kilowatts would be 28.7 and the wire-to-water effi-
ciency 55.1 percent compared with two pumps operating at 1625
gal/min with an input kilowatts of 22.3 and a wire-to-water efficiency
of 70.6 percent.
4. kW input control. This control is described previously. Adaptive
control accomplishes the same adjustment in the set point, established
to stop one of the running pumps. Typical use of adaptive control occurs
in chilled water systems with a sizeable system head area, Fig. 10.18;
this is for the model building of Chap. 9 and Fig. 9.3. It is impossible to
establish a simple set point based upon one rate of system flow on a
system with such an appreciable system head area.
Adaptive control will be used in other control procedures herein,
such as variable condenser water temperature and add-subtract
points for boilers and chillers.
5. Adaptive control. The perfection of digital control has resulted
in the use of adaptive control to adjust set points to variable condi-
tions. As has been demonstrated in a number of figures in this book,
transition from one to two pumps, two to three pumps, and so forth
can be accomplished by wire-to-water efficiency or kW input control.
With adaptive control, the actual point of transition, such as 1100
gal/min in Fig. 10.19, can be continuously adjusted to secure the opti-
mum point of transition. This is accomplished by sensing the change
in kW input as transition is made, say from one to two pumps. If the
kW increases, the second pump should not be added. The next time the
requirement for a pump addition occurs, the adaptive control proce-
dure adjusts the set point up to 1300 gal/min as shown in Fig. 10.19.

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