ASHRAE Journal - April 2020 - 72

```COLUMN ENGINEERS NOTEBOOK

CHW Flow (gpm)

Designers and operators may occaFIGURE 6 Coil heat transfer equations.
sionally come across campus chilled
TD2-TD1
LMTD=
water building connections that use
Q=U×A×LMTD
ln(TD2 /TD1)
a bypass line that is often combined
with a control valve to selectively
TD1=Leaving-Air and EnteringQ=Amount of Heat Transferred Btu/h
Water Temperature Difference
U=Heat Transfer Coefficient Btu/h·ft2·°F
divert building return water to
at the Coil, °F
2
A=Effective
Surface
Area
for
Heat
Transfer
ft
blend with campus chilled water to
TD
=Entering-Air
and Leaving2
LMTD=Log-Mean Temperature Difference
Water Temperature Difference
deliver warmer chilled water to the
Across the Coil Surface, °F
at the Coil, °F
HVAC systems within the building
(Figure 5, Page 70). This bypass line
FIGURE 7 LMTD calculations for changing entering water temperature.
is also known as a decoupled direct
building connection or "bridge"
EAT
EAT=
83.0°F
connection.
LWT
The author speculates that designEAT
31.9
LAT
28.0°F
26.7°F
°F D
ers that incorporate this feature into
T Ai
rsid
EWT
LWT1=
e
their campus chilled water system
13.7°
56.7°F
F DT
LWT2=
building connection are attemptWater
LWT
LAT=51.1°F
side
55.0°F
16.7°
ing to increase chilled water ΔT by
9.1°F EWT =42.0°F 11.1°F
F DT
2
1 Water
315 gpm
supplying warmer chilled water to
side
EWT1=40.0°F
EWT TD = LAT-EWT 245 gpm
TD2= EAT-LWT
encourage warmer chilled water
1
return temperatures. However, this
At 16.7°F DT Waterside, 31.9°F, DT Airside:
logic does not track with how heat
26.7°F -11.1°F
LMTD=
= 17.8°F
exchangers function (Figure 6).
ln(26.7°F/11.1°F)
While maintaining constant airAt 13.0°F, DT Waterside, 31.9°F DT Airside:
side conditions, increasing the
chilled water supply temperature
28.0°F - 9.1°F
LMTD=
= 16.8°F
ln(28.0°F/9.1°F)
will reduce log-mean temperature
difference (LMTD) and result in
FIGURE 8 Chilled water supply temperature vs. chilled water flow rate through coil (constant EAT and LAT
higher chilled water flow as the
conditions).
HVAC system attempts to maintain
350
leaving air temperature setpoint
through the chilled water coil
300
(Figures 7 and 8).
250
Chilled water flow will start
to increase, and initially the
200
U-factor will also increase some150
what over a very limited range
as a result of higher water-tube
100
velocity. This may result in the
50
coil being able to maintain leav0
ing air temperature setpoint
38
39
40
41
42
over a limited increase in chilled
CHW Supply Temperature (°F)
water supply temperature, but at
the expense of chilled water ΔT.
However, as the chilled water supply temperature
the coil leaving air temperature will rise, resultbecomes warmer, the coil will inevitably be unable
ing in a loss in control of supply air temperature
to maintain the leaving air temperature setpoint;
setpoint and reduction in coil cooling capacity. See
72

ASHRAE JOURNAL

ashrae.org

APRI L 2020

```
https://www.ashrae.org/

ASHRAE Journal - April 2020

Contents
ASHRAE Journal - April 2020 - Intro
ASHRAE Journal - April 2020 - Cover1
ASHRAE Journal - April 2020 - Cover2
ASHRAE Journal - April 2020 - 1
ASHRAE Journal - April 2020 - Contents
ASHRAE Journal - April 2020 - 3
ASHRAE Journal - April 2020 - 4
ASHRAE Journal - April 2020 - 5
ASHRAE Journal - April 2020 - 6
ASHRAE Journal - April 2020 - 7
ASHRAE Journal - April 2020 - 8
ASHRAE Journal - April 2020 - 9
ASHRAE Journal - April 2020 - 10
ASHRAE Journal - April 2020 - 11
ASHRAE Journal - April 2020 - 12
ASHRAE Journal - April 2020 - 13
ASHRAE Journal - April 2020 - 14
ASHRAE Journal - April 2020 - 15
ASHRAE Journal - April 2020 - 16
ASHRAE Journal - April 2020 - 17
ASHRAE Journal - April 2020 - 18
ASHRAE Journal - April 2020 - 19
ASHRAE Journal - April 2020 - 20
ASHRAE Journal - April 2020 - 21
ASHRAE Journal - April 2020 - 22
ASHRAE Journal - April 2020 - 23
ASHRAE Journal - April 2020 - 24
ASHRAE Journal - April 2020 - 25
ASHRAE Journal - April 2020 - 26
ASHRAE Journal - April 2020 - 27
ASHRAE Journal - April 2020 - 28
ASHRAE Journal - April 2020 - 29
ASHRAE Journal - April 2020 - 30
ASHRAE Journal - April 2020 - 31
ASHRAE Journal - April 2020 - 32
ASHRAE Journal - April 2020 - 33
ASHRAE Journal - April 2020 - 34
ASHRAE Journal - April 2020 - 35
ASHRAE Journal - April 2020 - 36
ASHRAE Journal - April 2020 - 37
ASHRAE Journal - April 2020 - 38
ASHRAE Journal - April 2020 - 39
ASHRAE Journal - April 2020 - 40
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ASHRAE Journal - April 2020 - 42
ASHRAE Journal - April 2020 - 43
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ASHRAE Journal - April 2020 - 47
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ASHRAE Journal - April 2020 - 53
ASHRAE Journal - April 2020 - 54
ASHRAE Journal - April 2020 - 55
ASHRAE Journal - April 2020 - 56
ASHRAE Journal - April 2020 - 57
ASHRAE Journal - April 2020 - 58
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ASHRAE Journal - April 2020 - Cover3
ASHRAE Journal - April 2020 - Cover4
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