ASHRAE Journal - September 2020 - 34

TECHNICAL FEATURE

Wet
-B

FIGURE 3 The Sea Level Psychrometric Chart shows the boundary of Climate

200

Tem
p

erat

ure-

°F

180

140

120
70

60%
RH

100

40%

60

Room Target Range 70°F - 75°F DB
40% - 60% RH

50
45°F DP/DB/WB
40
Seattle Winter Design
23°F DB/22.1°F WB
Supplemental Heat
Climate Zone 1
10

30

Climate Zone 2

Room Load

Hum Adiaba
idifi tic
45°
cati
FW
on
B

30

40

55°

40

FW

B

Outdoor Air Hydration
20

Climate Zone 3
5°F DP
50

60
70°F 75°F 80
Dry-Bulb Temperature (°F)

A Wet-Bulb Heat Recovery Economizer
For more than three decades, air-to-air heat exchangers have been used to recover building generated heat
and comply with the minimum winter ventilation
requirements of ASHRAE Standard 62.1.19 These conventional systems use the VAV 55°F (13°C) air delivery
setpoint of the building. Figure 3 demonstrates a novel
method to deliver 70°F (21°C) air at 40% relative humidity using heat recovery and adiabatic cooling/humidification (AC/H).
Since VAV systems operate at their lowest flow rates
on the coldest days, heat recovery from the building
is required to furnish the code-minimum outdoor air
to the building. The code minimum airflow rate may
be more than that needed at 45°F (7°C) DB to meet the
cooling load in core conference rooms and north-facing
perimeter zones (in the northern hemisphere). VAV terminal reheat will be required for those mixing boxes.
34

ASHRAE JOURNAL

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60
45°F DP

Heat Recovery
23°F

80

45°F DP

Humidity Ratio-Grains of Moisture Per Pound of Dry Air

160

80

RH

Zones 1, 2 and 3 for the 22 cities listed in Table 1. All ambient conditions in
Climate Zone 1 require supplemental heat to produce a 45°F dew-point (DP)
building delivery with 100% outdoor air. Climate Zone 2, between 23°F dry bulb
(DB) and 45°F wet bulb (WB) are ambient conditions where a high sensible
efficiency heat wheel may be used to deliver a 45°F DP off the adiabatic cooler/
humidifier (AC/H) without supplemental heating. In Figure 4, Dampers A and
B modulate to produce a 45°F WB mixed-air condition delivered to the AC/H
with all outdoor air. Climate Zone 3 contains all the ambient conditions between
45°F WB and 55°F WB when the heat wheel is off and bypassed on the supply
side with Damper B open and Damper A closed. In Climate Zone 3 all outdoor air
is cooled and hydrated to a leaving air condition that is allowed to float between
45°F DP to 55°F DP. Sensible cooling is produced with a cooling energy efficiency
ratio (EER) ranging between 80 to 100 with refrigeration off.

ulb

SEPTEM BER 2020

90

100

110

120

ASHRAE Research Project RP-1515 demonstrated that,
with good aspirating ceiling diffusers, the turndown of
a VAV terminal to 10% flow will not lose flow control and
will not result in "dumping" of cold air into the space.
On the contrary, overcooling complaints in the buildings
tested went down, eliminating cold room issues while
saving fan energy.20
In many northern hemisphere cold climates, high-rise
buildings with south-facing glass experience design
cooling loads in January and February due to the low
angle of the sun and the radiant energy on the southfacing side of the building.21 A 45°F (7°C) DB delivery
to these VAV terminals will provide comfortable indoor
environments with reduced fan energy during these
peak cooling loads.
When we can hold indoor RH in winter at a minimum
of 40%, room thermostat setpoints may be reduced
to 70°F (21°C) or lower, saving heating energy. This
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ASHRAE Journal - September 2020

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