ASHRAE Journal - September 2020 - 39

TECHNICAL FEATURE

210
200
FIGURE 5 The Prehumidification and Morning Warm-Up Psychrometric
process using building recirculated air to recharge moisture capacitance before building occupancy after night or weekend building
shutdown. Reheat may be provided, as needed, at the air-handling unit
or at the VAV terminal box.

190

85

180

80

170
ulb T
emp

eratu

160
re-°

F

150
140

75

130
120

70

110
100

65

90
60

80
55°F DP

Reheat

55

55°

50

FW

60

B

45

50

Hum Adiaba
idifi tic
cati
on

40
35
30

20%

25

70

Hydration

40
30

RH

20

19°F DP
Preheat

20

25

30

35

40

45

50

55

60°F

Humidity Ratio-Grains of Moisture Per Pound of Dry Air

WetB

65

70

10
75

80°F

86°F 90

95

100

105

110

115

120

Dry-Bulb Temperature (°F)

increase to a 95% WBDE at 50% of design airflow, while
the static pressure will decrease to 0.05 in. w.g. (12 Pa).
The marriage of the heat wheels and AC/H in a VAV
design results in significant energy savings because
effectiveness increases and the parasitic losses decrease
with reduced air mass flow. The building owner benefits
from the increased energy exchange efficiency and outdoor air hydration during cold weather.

Morning Building Prehumidification
And Warm-Up Control Strategy
In cold climates, it is customary for buildings with daytime-only duty cycles to schedule a "morning warm-up"
cycle after night, holiday or weekend shutdowns, when
indoor temperatures are allowed to drop to 60°F (16°C)
or lower to save energy. Moisture migration from the
building during shutdown often allows indoor relative

humidity to drop to 20% or lower, especially after weekend or holiday shutdown periods. Morning warm-ups
should include building prehumidification prior to
building occupancy.
The AC/H component may be used with recirculated
building air to prehumidify the building. Figure 5 shows
the psychrometric process for humidification of the
building return air. In Figure 4, Dampers A, B and C are
closed, and the heat wheel is off with Damper D open
and the VAV supply fan on. The reheat after the AC/H
may be provided by a coil downstream of the AC/H in the
AHU or by the VAV terminal reheat coils in each zone of
the building.
A column from the September 2017 issue of ASHRAE
Journal entitled "It's All Relative"23 explains the mechanism by which vapor may be stored inside buildings
on hygroscopic materials. Water vapor molecules are
SEPTEM BER 2020

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ASHRAE JOURNAL

39



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ASHRAE Journal - September 2020

Table of Contents for the Digital Edition of ASHRAE Journal - September 2020

Contents
ASHRAE Journal - September 2020 - Intro
ASHRAE Journal - September 2020 - Cover1
ASHRAE Journal - September 2020 - Cover2
ASHRAE Journal - September 2020 - 1
ASHRAE Journal - September 2020 - Contents
ASHRAE Journal - September 2020 - 3
ASHRAE Journal - September 2020 - 4
ASHRAE Journal - September 2020 - 5
ASHRAE Journal - September 2020 - 6
ASHRAE Journal - September 2020 - 7
ASHRAE Journal - September 2020 - 8
ASHRAE Journal - September 2020 - 9
ASHRAE Journal - September 2020 - 10
ASHRAE Journal - September 2020 - 11
ASHRAE Journal - September 2020 - 12
ASHRAE Journal - September 2020 - 13
ASHRAE Journal - September 2020 - 14
ASHRAE Journal - September 2020 - 15
ASHRAE Journal - September 2020 - 16
ASHRAE Journal - September 2020 - 17
ASHRAE Journal - September 2020 - 18
ASHRAE Journal - September 2020 - 19
ASHRAE Journal - September 2020 - 20
ASHRAE Journal - September 2020 - 21
ASHRAE Journal - September 2020 - 22
ASHRAE Journal - September 2020 - 23
ASHRAE Journal - September 2020 - 24
ASHRAE Journal - September 2020 - 25
ASHRAE Journal - September 2020 - 26
ASHRAE Journal - September 2020 - 27
ASHRAE Journal - September 2020 - 28
ASHRAE Journal - September 2020 - 29
ASHRAE Journal - September 2020 - 30
ASHRAE Journal - September 2020 - 31
ASHRAE Journal - September 2020 - 32
ASHRAE Journal - September 2020 - 33
ASHRAE Journal - September 2020 - 34
ASHRAE Journal - September 2020 - 35
ASHRAE Journal - September 2020 - 36
ASHRAE Journal - September 2020 - 37
ASHRAE Journal - September 2020 - 38
ASHRAE Journal - September 2020 - 39
ASHRAE Journal - September 2020 - 40
ASHRAE Journal - September 2020 - 41
ASHRAE Journal - September 2020 - 42
ASHRAE Journal - September 2020 - 43
ASHRAE Journal - September 2020 - 44
ASHRAE Journal - September 2020 - 45
ASHRAE Journal - September 2020 - 46
ASHRAE Journal - September 2020 - 47
ASHRAE Journal - September 2020 - 48
ASHRAE Journal - September 2020 - 49
ASHRAE Journal - September 2020 - 50
ASHRAE Journal - September 2020 - 51
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ASHRAE Journal - September 2020 - 53
ASHRAE Journal - September 2020 - 54
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ASHRAE Journal - September 2020 - 58
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ASHRAE Journal - September 2020 - 60
ASHRAE Journal - September 2020 - 61
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ASHRAE Journal - September 2020 - 63
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ASHRAE Journal - September 2020 - 65
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ASHRAE Journal - September 2020 - 70
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ASHRAE Journal - September 2020 - 76
ASHRAE Journal - September 2020 - 77
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ASHRAE Journal - September 2020 - 79
ASHRAE Journal - September 2020 - 80
ASHRAE Journal - September 2020 - Cover3
ASHRAE Journal - September 2020 - Cover4
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