ASHRAE Journal - March 2020 - 6

LETTERS

Engineer's
Notebook:
Control Valve
Selection

The Author Responds

Thank you for the overview on
traditional control valve sizing and
fundamentals (in the Engineer's
Notebook column in the January
ASHRAE Journal). Conventional
control valves allow flow to change
due to system pressure variations
even without load change. With
few exceptions, mixed air HVAC
coils show that fluid side ΔT goes
up at part load while maintaining
leaving air design temp. I would
challenge the author and our HVAC
industry to consider that if we
don't improve our control valve
type and selection methods we'll
continue to see systems with low
ΔT, that waste 30% - 40% of system
flow rate and result in poor central
plant operational cost to process
that higher than needed flow rate.
As ASHRAE heads towards net-zero
energy, why doesn't ASHRAE set a
strategic goal to solve low ΔT syndrome at coil level, and endorse
using any control valve strategy that
optimizes the coil performance for
accurate leaving air temperature
and simultaneous fluid side design
ΔT or higher at all loads (minimizing the flow and equipment needed
to serve the load)? ASHRAE should
also encourage monitoring the
return coil temps with sensors that
can be monitored (at least on critical coils), tracked and compared so
that best practices of control can be
assured and repeated.
Dan Holtmeyer, Member ASHRAE,
Omaha, Neb.
6

ASHRAE JOURNAL

ashrae.org

My experience, and mathematics show that control valve fluidside ΔT goes up with reducing flow
and load, especially in old-school
180°F/160°F heating systems that
can produce nearly 100°F ΔT at light
loads. This assumes that the valve is
not hunting. I don't think I've seen
a research report about this, but
I would be very interested in the
results of one-not anecdotal information, but a true research project.
Perhaps something sponsored by
TC 1.4, or 6.1, or even Guideline 36?
I haven't seen many hunting complaints in my years in the industry,
but others may have very different experiences, and as engineers
we generally are only called when
there is a complaint, and a reheat
coil cycling from heat to no heat
seldom upsets room temperature
enough to cause those complaints
(that's how most house furnaces
work). My company generally monitors the SAT off of VAV boxes, but
not the return water temperature,
except at the mechanical room level
where we believe it is cost justified
for our clients. My belief is that
PIC valves are justified if valves
are hunting, and not if they are in
control.

Jeff Boldt, P.E., Fellow ASHRAE,
Madison, Wis.

n Jeff Boldt's "Engineer's
Notebook" needs a bit of scrutiny
and shouldn't be taken at face value.
I'll touch on a few of the points
that concerned me. Unfortunately,
Journal publication rules limit the
length of a response, so the majority
of my original text is published at
www.ashrae.org/ashraejournal.
The concept of coil thermal characteristics is inadequately discussed.

MARCH 2020

The column is lacking, hiding the
much larger implications of the subject. For example, look at Figure 1, a
cooling coil characteristic that does
not lack the many required data
points that the article's does not
present.
These other unmentioned items in
the article are really required when
a designer or a controls contractor begins the process of specifying
or "making" this small sub-system
operate properly.
It gets more complicated, and the
column glosses over the potential
coil issues. Figure 2 shows how a VAV
air handler coil with mixed outdoor
and return air might react.
The point is, and has probably
gone unwritten for too long, that
the generalized coil characteristic
that many have referred to over the
past 60 years isn't wrong, but also
doesn't cover all of the applications
that designers think it does. It has
a major effect on control and valve
selection.
Issue 2: The column's statements
on control logic need to be clarified as well. Proportional feedback
control logic (which is the majority of what HVAC has relied on
for control) relies on the premise
of there being a linear response
to the control error. The column
"muddies up the water" and is
confusing. The online text provides a more complete explanation, albeit much shorter than a
textbook.
Issue 3: The industry has three
basic valve characteristics defined
and the column implies specific
manufacturer curves as being
industry definitions, even publishing some wrong data for one of those
represented.

http://www.ashrae.org/ashraejournal

ASHRAE Journal - March 2020

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