ASHRAE Journal - September 2013 - 21

Technical FeaTure
Fea

Septem ber 2013

ashrae.org

Sensible Wheel

Enthalpy Wheel

humidity control using dual-energy
Figure 1 Recovery with wheels.
recovery, and produces a leavingair temperature in the range of
Bleed Air
Exhaust Stack
approximately 63°F to 70°F (17°C
Pre-Conditioning Coil
to 21°C). This system is combined
Cooling/De-Humid Coil
Roof
with air recirculating supplemental
ERU
cooling devices: typically fan coil
Wheel
Wheel
units, chilled beams, or other simi(With
Neutral
To General
lar devices. In hood intensive rooms
System
Exhaust
care should be exercised to avoid
Only)
C
Louver
P
interference between these devices
From
C
C
Outside Air/
and proper hood performance.2
Louver
AHU
Dual energy recovery consists of two
Supply
energy wheels, one enthalpy and
Hazardous Exhaust
General Exhaust
one sensible, for the first recovery
Dedicated Shafts for Each Floor Area
method. Under the second recovery
Supply
Hazardous Exhaust
method, two glycol coils are substiGeneral Exhaust
tuted for the wheels. The two glycol
Shaft Bottom (typ)
Fire/Smoke Dampers (typ)
coils in this case can be described as a
wraparound/runaround system. The
glycol flows in series from the second
exhaust. In this case, the exhaust must comply with
coil, where the supply air is heated up to neutral condiSection 510. In accordance with this section, hazardtions, then wraps around the chilled water coil (providous exhaust systems shall comply with the following
ing dehumidification) to the first glycol coil (serving to
requirements:
precondition the outside air).
1. Under Subsection 510.4, hazardous exhaust systems
Based on the previous variations, there are a total of
shall not share common shafts with other duct systems,
four different combinations, with two energy recovery
except where such systems are hazardous exhaust sysmethods each applied to two different air-handling
tems originating in the same fire area.
systems:
2. Exception 2 requires that hazardous exhaust
1. Energy wheel method of recovery with a traditional
ductwork manifolded together within the occupied space
system (Figure 1; with first wheel only);
2. Energy wheel method of recovery with a neutral air must originate within the same fire area.
3. Subsection 510.5.7 requires that hazardous exhaust
system (Figure 1; with dual wheels);
3. Glycol coil method of recovery with a traditional sys- duct systems extend directly to the exterior of the building.
tem (Figure 4; with first glycol coil only); and
4. Finally, 510.6.1 prohibits the use of fire and smoke
4. Glycol coil method of recovery with a neutral air sysdampers.
tem (Figure 4; with dual coils).
These requirements are illustrated in Figures 1 and 4.
A method of recovery using heat pipes is not examined
as this system is expected to have characteristics similar A design using wheels for energy recovery, and taking
the hazardous exhaust through the wheels for recovery
to glycol runaround but with slightly higher recovery
effectiveness, due to the higher heat content of refriger- purposes, could violate code if the AHU/ERU are located
in the penthouse instead of on the roof (or exterior of
ant verses a glycol solution.
the building). Similarly, a design using glycol for energy
Code Considerations
recovery, and taking the hazardous exhaust through
For the mechanical designs proposed in this article, the the coils for recovery purposes, could violate code if the
fume hoods and similar exhaust follow the International ERU are located in the penthouse. If a proposed design
Building Code and International Mechanical Code
approach does not treat the fume hood exhaust as haz(IMC-2009 as example) classification for hazardous
ardous, it is recommended that the engineer obtain
ASHRAE JouRnAl

ASHRAE Journal - September 2013

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