cooling coils is low, extra chilled water
is circulated per ton of cooling. Building
owners may suffer from rising supply-water temperature. The central plant
may suffer from low return-water temperature.
Buildings that are directly connected
may experience improved coil performance since return water is not permitted
the chilled-water supply temperature
(CHWST) to the building is 45 F, but it
rises in the building as high as 57 F.
Supply air temperature rises to 62 F,
resulting in excess air flow, humidity
concerns and excess fan energy consumption. Delta T within the building is
far below the 12 F design Delta T for
Figure 2. Effect of Rising Chilled-Water Supply Temperature (CHWST) on Coil Capacity, Flow Rate and
need to deliver the chilled water
required to serve the load.
Hydraulic gradient analysis is one
technique that may be used to evaluate
distribution system design and operation
but is beyond the scope of this article.
Using this technique enables an analyst
or engineer to see in many systems that
there is plenty of differential pressure
in buildings close to the central plant to
serve the load without running building
pumps (provided that Delta T in excess
of design is achieved). Return-water
temperature control valves, when operating properly, consume all the pressure
between the building return and distribution return headers but do little to
provide accurate flow control at individual loads.
Flow Control Industries Inc.
to blend with supply at the building;
however, if return-water temperature to
the chilled-water plant is less than design,
system capacity (pipes, pumps, chillers,
and thermal storage) may be unnecessarily limited. Low Delta T across cooling coils in directly connected systems
causes the excess flow to be processed
at the central plant by overflowing
equipment, permitting supply-water
temperature to rise, or by running additional chillers.
Return-water temperature control is
a very common decoupling strategy
employed in buildings served by large
district cooling systems. By separating
the building flow from the primary or
secondary distribution flow, chilled
water is re-circulated in buildings until
enough heat has been gained to produce
adequate return-water temperatures
(and Delta T) for the distribution pipes
and the central plant.
Figure 1 demonstrates the adverse
effect of return-water temperature control in an actual system. In this example,
While this technique has been used
as an intermediate step to generate the
desired temperature rise at the plant or
through the main distribution pipes, it
is fraught with potential problems for
customers. When return water is permitted to blend with supply, CHWST increases. This commonly leads to building
performance issues including wasted
pump and fan energy, lost coil capacity
and poor humidity control. On a system
level, pumps used in buildings close to
the plant may starve buildings downstream of the differential pressure they
Factors Influencing Delta T
Through the cooling load range, the
Delta T performance of a properly
selected coil can decline for a number
of reasons. Rising CHWST and poor control valve sizing and performance are
among the factors that will adversely
affect chilled-water Delta T performance
(dirty fins and coils, low supply air temperature, improper piping and over-pumping are a few others).
Chilled-Water Supply Temperature
Figure 2 illustrates how Delta T,
flow and capacity of a 45/55-split ARI-certified cooling coil vary with a change
in the CHWST. At 100 percent design
flow for the coil, capacity is dramatically
reduced when the CHWST rises to 48 F.
The coil is capable of producing a high
Delta T when low CHWST is maintained.
Figure 3. Pressure-Independent Modulating Two-Way Control Valve and Hydraulic Gradient.
Flow Control Industries Inc.