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SYS-APM001-EN Chiller System Design and Control 67
Plant configuration
Consider a series arrangement for small VPF applications.
When the plant consists of only two chillers and expansion is unlikely, you
can simplify control by piping the evaporators in series. Doing so avoids flow
transitions because the water always flows through both chillers. The series
arrangement requires careful selection because the pump must be sized for
the pressure drop through both chillers. However, the extra pressure quickly
decreases (by roughly the square of the flow rate) as the flow rate slows. For
example, at 80 percent of design flow, the evaporator pressure drop is only
64 percent of design. Given this operating characteristic, a VPF design may
permit a slightly higher system pressure drop than a comparable primary–
secondary system without a noticeable penalty in operating cost. “Series
Chillers” on page 44 discussed these arrangements in greater detail.
19
Note: To further reduce the system
P, lower the required rate of chilled-water
flow through the system by increasing the temperature difference between the
supply and return. Plants that supply 40°F [4.4°C] chilled water based on a
T of
16°F [8.9°C] or more are increasingly common.
Assess the economic feasibility of VPF for single-chiller plants.
Although most VPF applications consist of two or more chillers, variable
primary flow also offers potential operating-cost savings in a new or existing
single-chiller plant. Instead of a bypass line and flow-sensing devices,
minimum flow through the chiller can be maintained by three-way valves.
(Use enough three-way valves to assure that the minimum evaporator-flow
rate of the chiller is always satisfied.) This simple approach will reduce
pumping costs while providing the chiller with enough chilled water.
To quantify the savings potential of variable versus constant primary flow in
a single-chiller plant, we examined a two-story office building in St. Louis,
Missouri. The HVAC system includes a 50-ton scroll chiller and a 5-hp chilled
water pump. Figure 39 illustrates the results of our analysis. Although the
absolute savings are not large, variable primary flow did reduce the cost of
operating the chilled water system by more than 6 percent … enough to
warrant further investigation. The difference in installed costs is a variable-
speed drive, a differential pressure sensor, and a pump controller.
For more information, refer to the Trane
Engineers Newsletter, “Don’t Overlook
Optimization Opportunities in ’Small’
Chilled Water Systems” (ADM-APN009-
EN).