ASHRAE Journal - May 2020 - 9

LETTERS

primary-secondary pumping arrangement as the VFDs
would slow down if the available campus distribution
pressure is high. The direct-coupled building pumps
arrangement would perform even better under that
condition when they are provided with check-valve
bypasses so that the pumps can shut off when the coincident campus distribution pressure is sufﬁcient to meet
the building demand.
Following up on the comment about installing
control valves to impact coil ∆T, it is not possible to
control both supply air temperature and coil leaving
water temperature; there is essentially only one ﬂow
rate that will provide the desired supply air temperature at a given entering air and water condition. As
noted above coil airside conditions and unstable control should be investigated if a coil is not performing
as designed.

Nathan Ho, P.E., Associate Member ASHRAE,
Irvine, Calif.

Further, the component solely responsible for the efﬁciency of this energy transfer is the control valve at the
coil. It is the responsibility of the valve to set the ﬂow
rate of the chilled or heating water in order to meet the
airside requirements most efﬁciently. This is not to say
that the best control valve operated poorly will not do a
poor job controlling a coil. But a sub-optimal valve cannot achieve ideal performance, no matter the control
system.
I applaud both the author and ASHRAE for broaching
these topics and continuing the investigation and discussion around them. As energy efﬁciency becomes increasingly important, the tolerance of poorly performing systems is inexcusable. Movement toward fully understanding what makes high performance systems work using
data and science instead of old rules of thumb and long
disproven myths is the only way forward.

Chris Reed, P.E., Member ASHRAE
Woodinville, Wash.

 I read with great interest Nathan Ho's column

"Optimizing Campus Chilled Water Connections" in the
April ASHRAE Journal. I was curious as to whether the
column would dispel any of the common myths or poor
design practices that plague many, if not most, chilled
and heating water distribution systems. And, I am
pleased to say that, overall, the column does make some
good points regarding the fallacy of building interface
bypasses and the all-to-common myth of return water
temperature control strategies.
That said, I do feel that the column stops short on a few
key points. First is that implementing building pumping
strategies in order to meet building pressure requirements will decrease the overall pump energy by moving
the pumping from the plant to the buildings. While this
is true, by the pump afﬁnity laws, we know that reducing the ﬂow requirement has an order of magnitude
greater effect on pump energy than reducing the pressure requirement.
This alludes to the second, and most important,
oversight. It is stated in the column that "high ∆T is not
something that the central plant can 'make happen'
through control logic". This is also 100% true but where
the explanation falls short is that ∆T cannot be dictated
by the building interfaces either. The only location in
the system that T can be controlled is where the energy
transfer occurs between the air and the water, i.e., at the
coil - nowhere else.

The Author Responds
Thank you for the comments.
As discussed in the column, coil performance is directly
determined by LMTD conditions. The primary function of
the coil system is to operate to satisfy a thermal demand
and the associated hydronic control valve is a part of that
system by modulating ﬂow to achieve a target leaving air
temperature. There is essentially only one ﬂow rate that
will provide the desired supply air temperature at a given
entering air and water condition. In addition to the control valve's physical characteristics, how that valve is being
controlled e.g. loop tuning, is also essential to achieving
desired performance out of the coil.

Nathan Ho, P.E., Associate Member ASHRAE,
Irvine, Calif.

Industry News, From Page 7

AAON Builds HVAC Units for Hospital
TULSA, OKLA.-AAON built 2,200 tons (7.7 MW) of HVAC apparatus for a temporary hospital in Long Island, N.Y., in
early April. The New York City area was considered the
epicenter of the COVID-19 pandemic at that time.
New York Stock Exchange Lists Carrier
PALM BEACH GARDENS, FLA.-Carrier Global Corporation debuted
on the NYSE as an independent, publicly traded company on April 3 under the symbol "CARR."
M AY 2020

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