ASHRAE Journal - May 2020 - 8

LETTERS
Engineer's Notebook:

Optimizing
Campus
Chilled Water
Connections
Thank you for the April column
with several important tidbits
for designers on the benefits and
importance of maximizing chilled
water T to reduce pumping and
system power. The column fails to
mention that the coil control valves
ability or lack of ability to control to
leaving air temperature and provide
simultaneous coil design return
temperature would be the root cause
of applying various chilled water
connection and add additional
control scenarios. In theory, if the
control valves proportioned flow
exactly to air side coil load, the fluid
side T would always be a minimum
of the coil design T. Since the coil
air side surface is sized for 100% load
(ignoring safety factor with load or
coil selection), T should always
increase at part load and flow should
always be less than proportional
load equivalent allowing increase
return water temperature above
design. If T is below coil design at
the coil, then the control valves are
overflowing versus the proportional
load even if they meet leaving air
set point satisfactorily. Fluid return
temps lower than coil design should
first look at the control valve as the
root cause to fix. And as you stated,
the central plant gets what it gets
from the system.
Figure 12 in my view, should have
had the CHWR valve installed with
a small flow bypass to ensure adequate sensor flow, and the sensor
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itself should have been located on
the CHWR valve side of the return
pipe bypass tee to the supply side.
That likely would have eliminated
the need to remove the bypass pipe
from return to supply. The Figure 12
and 13 comparisons are using a relatively low constant dp (10 or 15 psid).
Controlling to the higher coil control
valve design dp would lower the
dp setting and lower pump power
further. The column doesn't explain
why the control dp was increased
for Figure 13. Was that to overcome
the pressure drop of the CHRW
valve in locked open position? If so,
eliminating the additional 5 psi drop
for 250 gpm flow rate would seem
to be worth removing the CHRW
valve completely and saving more
pump power. Does this also explain
the speed difference of the building
pumps of 67.5 versus 86% or were
the readings at different loads? I'd
take the Figure 13 solution as using
more pump energy than the Figure 12
even as higher return chilled water
was achieved.
Primary secondary (P-S) systems
properly applied can contribute to
lower pumping energy by allowing
the common pipe to take the served
zone pressure drop off the main system pumps and use smaller lower
hp pumps to serve the zone (constant or variable speed). Variable
speed building/zone pumps make
sense when the majority of the zone
pressure drop is variable with flow.
Less so, when a large portion is
fixed pressure drop. P-S also has the
benefit of reduced control valve differential pressure especially for coils
located close to the central plant
pumps. It can allow zones to have
different Ts and different mixed
temperatures (supply or return as

M AY 2020

needed). On existing or new systems, installing control valves that
control to setpoint and provide T
or higher at all load points as a byproduct of good control would be
first preference.

Dan Holtmeyer, Life Member ASHRAE,
Omaha, Neb.

The Author Responds
Thank you for sharing your
thoughts.
Regarding the root cause of chilled
water T shortcomings, not always
is it due to inadequate control valve
or control system performance in
the case of systems that are 100%
outdoor air or are utilizing an
outdoor air economizer. Airside
conditions can also significantly
contribute to diminished coil T
performance.
The above being said, erratic or
faulty control valve operation can
certainly have a detrimental impact
on coil T performance. The control would have to be very poor
indeed to impact T significantly, at
which point the inability to achieve
stable supply air temperature control should draw a red flag during
start-up, commissioning, and periodic trend reviews over long-term
operation. It's also likely that the
occupants would notice such poor
control and call in the issue at this
point.
The control differential pressure in
Figure 13 was raised based on direction from the plant operator, note
that AHU-1's control valve position is
at ~96% open under this condition.
Regarding the comment about
primary-secondary systems, if the
building pumps have VFDs, then
direct-coupled building pumps
would use less energy than a
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ASHRAE Journal - May 2020

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