ASHRAE Journal - January 2020 - 58

COLUMN ENGINEER'S NOTEBOOK

FIGURE 4 Pump system control loop.

Butterfly
Equal Percentage

Ball

100
90
80
70
60
50
40
30
20
10
0

VFD

DP
2-8
psid

Coil

40
60
Stem Travel (%)

100

for preheat coils or AHU cooling coils with substantial
dehumidification.
Cooling coils are a bit closer to linear devices because
the DT between the incoming water and leaving air temperatures is not as large. For this same reason, HW coils
are more linear when HW supply temperature is reset so
water and air temperatures are closer.
Control system logic (particularly proportional control
logic) is normally based on the assumption that there
will be a linear response between valve motion and temperature change. Obviously, coils do not have a very linear response to flow rate. Most of you probably learned
as young engineers that the solution is equal percentage
(EP) control valves. That's pretty good advice, but I'm
going to delve into the details. I used CCV in the figures
as an abbreviation for characterized control valve, which
are also called V-port ball valves.
Normally, we control pump speed from a differential
pressure sensor near the end of the system (Figure 4) and
reset the setpoint based on polling valve positions.
The ANSI definition for control valve rangeability is
"The ratio of the largest flow coefficient (Cv or Kv) to the
smallest controllable flow coefficient (Cv or Kv) within
which the deviation from the specified flow characteristic does not exceed the stated limits."
The ANSI definition for Equal Percentage valve characteristic is, "An inherent flow characteristic which, for
equal increments of travel, will ideally give equal percentage changes of the flow coefficient (Cv or Kv)."
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0 - 20 psid

Coil

FIGURE 5 Heating coil + control calve combined at fixed valve pressure drop.

Heat Delivered (%)

0 - 20 psid

COURTESY IMEG CORP.

Linear
CCV

COURTESY IMEG CORP.

Flow (%)

Quick Opening
Balancing

100
90
80
70
60
50
40
30
20
10
0

Equal Percentage
Quick Opening

40
60
Stem Travel (%)

Linear
Ideal

100

In my opinion, the interaction of these definitions
alters the equal percentage curve based on valve rangeability. The equal percentage curve gets very steep when
rangeability is extremely high.
Control logic, especially the proportional component
of most PI or PID loops works best if moving a control
valve produces at least a somewhat linear response.
But a linear response seldom occurs in practice. Figure 5
shows the combination of valve and coil for a typical heating coil. In all cases, quick opening valves are
undesirable as control valves, and are normally recommended for on/off service only. Linear valves are most
often globe valves or high temperature V-port ball valves
used for steam applications, where each pound of steam
delivered produces ~1,000 Btu of heat regardless of flow
rate. In hydronic applications, butterfly valves have the
closest characteristic to linear.
Jeff Boldt, P.E., is managing principal-director of innovation and quality at IMEG
Corp. in Madison, Wis.

COURTESY IMEG CORP.

FIGURE 3 Control valve characteristics at fixed valve pressure drop.

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ASHRAE Journal - January 2020

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