ASHRAE Journal - July 2014 - 55

Technical FeaTure
Fea

or in only a few spaces, the sensible
cooling benefit of delivering the OA
cold to all other spaces served by
the DOAS may outweigh the heating
energy needed to prevent these few
spaces from overcooling.
At some load conditions, however,
it might be more economical to
reheat the cold OA centrally, using
recovered energy. In this case, one
should design the DOAS to deliver
the air cold, and only reheat when
needed in order to avoid individual
space/zone overcooling.

DOAS Supplies OA Directly to
Occupied Space

Dedicated
OA Unit

OA
CA

EA

CA

EA
A
Conditioned OA
Delivered Directly
To Each Space

CA
SA

SA

RA

RA

SA

CA

RA

Dedicated
OA Unit

OA
CA

EA
RA

Dedicated
OA Unit

OA

SA

SA

RA

Dedicated
OA Unit

OA
SA
B
Conditioned OA
Delivered to the
Supply-Side of
Each Local Unit

C
Conditioned
OA Delivered
to the Intake
of Each
Local Unit

CA

EA
SA

RA

RA

SA

D
Conditioned
OA Delivered
to Plenum,
Near Each
Local Unit

Total Energy
In this system configuration, the
Cooling Coil
Heating Coil
Fan
Recovery Devices
conditioned OA is delivered by the
Figure 1: DOAS with total-energy recovery delivering OA.17
DOAS directly to each occupied
space. Meanwhile, the local unit(s)
(such as fan coils, water-source heat
Dedicated
pumps, packaged terminal air conOA
OA Unit
ditioners (PTACs), small packaged
EA
or split DX units, radiant chilled
ceilings, passive chilled beams,
and variable refrigerant flow (VRF)
SA
equipment) located in or near each
RA
SA RA
SA RA
SA RA
space provides cooling and/or heatCA
ing to maintain space temperature
Floor by Floor
VAV Air-Handling Unit
(Figure 1a).
This approach easily ensures the
SA
RA
required OA flow reaches each zone,
SA RA
SA RA
SA RA
CA
and affords the opportunity to cycle
Floor by Floor VAV
off the local fan, or reduce its speed,
Air-Handling Unit
when cooling or heating from local
unit(s) is no longer needed in a given
zone. Since the OA is not distributed
Total Energy
VAV Box
Cooling Coil
Heating Coil
Fan
to the zone through the local unit,
Recovery Devices
the local fans do not need to operate
Figure 2: DOAS used with floor-by-floor VAV air-handling units14 (re-created).
in order to deliver OA to the conditioned zone. Therefore, the DOAS
can be activated during part-load periods without
equipment size (both airflow and cooling capacity) and
operating the local units.
hence also its capital and possibly operational energy
When the conditioned OA is delivered at a cold temcosts. However, to prevent uncomfortable drafts when
perature, rather than reheated to neutral, this conthe conditioned OA is delivered at a cold temperature,
figuration offers the opportunity to reduce the local
one may need to use high performance diffusers that
J u ly 2 0 1 4

ashrae.org

ASHRAE JouRnAl

55



ASHRAE Journal - July 2014

Table of Contents for the Digital Edition of ASHRAE Journal - July 2014

Contents
ASHRAE Journal - July 2014 - BB1
ASHRAE Journal - July 2014 - BB2
ASHRAE Journal - July 2014 - Cover1
ASHRAE Journal - July 2014 - Cover2
ASHRAE Journal - July 2014 - 1
ASHRAE Journal - July 2014 - 2
ASHRAE Journal - July 2014 - Contents
ASHRAE Journal - July 2014 - 4
ASHRAE Journal - July 2014 - 5
ASHRAE Journal - July 2014 - 6
ASHRAE Journal - July 2014 - 7
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ASHRAE Journal - July 2014 - I1
ASHRAE Journal - July 2014 - I2
ASHRAE Journal - July 2014 - I3
ASHRAE Journal - July 2014 - I4
ASHRAE Journal - July 2014 - 113
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ASHRAE Journal - July 2014 - 143
ASHRAE Journal - July 2014 - 144
ASHRAE Journal - July 2014 - Cover3
ASHRAE Journal - July 2014 - Cover4
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