ASHRAE Journal - April 2015 - 31

TECHNICAL FEATURE

diverges after the fan into a separate cold
deck and hot deck. The single fan system
generally requires active heating of the hot
air deck, mixing of hot and cold airstreams
at the terminal and allows air that is not
dehumidified to enter the space through
the hot deck.
A discussion of the minimum outside air
portion of the two fan dual duct system,
the function of the air-handling unit warm
deck heating coil and terminal reheat coil
will follow later after the establishment of
the energy use advantages of the system.
With the two fan dual duct system, the
cold deck produces only the amount of
cooling supply air needed to meet the load
requirements of the spaces. The remainder of the supply air quantity required to
meet the minimum airflow requirement
is recirculated, HEPA-filtered air from the
warm deck of the unit. Note the heating
coil in this air tunnel is not energized for
the vast majority of system operation.

FIGURE 1 VAV air unit.

Economizer

H H

M
Fans

Minimum OA

HEPA

55°F

Cold
Supply

M
Return Air

FIGURE 2 Dual-duct air unit.

H C

M
Air
Blender

Economizer
M
Minimum OA

Heat
Wheel C
C
M

The VAV two-position system produces about 54°F (12°C)
supply air (50°F [10°C] coil leaving temperature plus 4°F
[2°C] fan heat) 24 hours a day, all year, to be distributed to
the VAV reheat terminals. During the unoccupied periods,
when the space humidity requirements can be relaxed,
the discharge temperature can be reset to 60°F (15°C). The
quantity of supply air is generally 20 ach during occupied
periods and 10 ach during unoccupied periods.
With the VAV two-position system providing more air
to the space than required to meet the heat load, the
temperature of the space would be lower than desired.
Reheat is introduced to maintain the space temperature
setpoint. The reheat quantity would be zero if the cooling capacity of the supply air at 54°F (12°C) exactly meets
the cooling requirements of the space. This is, however,
almost never the case.
The design cooling load of the typical (not robotic or
hybrid) operating room requires only about 12 ach of cooling capacity to maintain a general operating room temperature of 65°F (18°C). Also, this is the design heat load
with all equipment in operation at nameplate capacity.
There is some diversity, over time, of this equipment load.

55°F

Cold
Supply

Service Access
Return Air
H

M
Fans

Smoke Purge

VAV System Operation

HEPA

M
M

M
Fans

HEPA

72°F

Recirculated Air
(Neutral Temperature)

An even larger factor in reducing the average heat load
from design is that the operating room is not in use for
a large portion of the occupied period and the only heat
load may be the small load of the general lighting.
When the space equipment heat load is not equal to
the cooling capacity of the supply air, all the difference
in capacity must be offset with reheat to maintain space
temperature. To meet the actual cooling load of the
space, extra cooling energy is spent to cool all the air circulated and then reheat is added back into the airstream
to compensate for the overcooling of the supply air. The
sum of the overcooling and subsequent reheating is
more energy than the actual cooling load of the space.

Dual Duct System Operation
As stated earlier, the design of the two fan dual duct
system leads to the cooling capacity matching the heat
load of the spaces and requires no reheat to maintain
space temperature in most cases. Both the VAV and dual
duct systems require approximately the same amount
of fan power since both require 20 ach of circulated air
during occupied periods.
There are some savings in supply fan power of the dual
duct system since the two air tunnels operate at part
APRI L 2015

ashrae.org

ASHRAE JOURNAL

ASHRAE Journal - April 2015

Table of Contents for the Digital Edition of ASHRAE Journal - April 2015