And
“M” is for “More than one way to get the job done.” In this
two-school tour, we start with an underfloor ventilation design and
geothermal energy center in action. Then we take notes at a school
with displacement ventilation, radiant floor heating, and radiant
ceiling cooling. Round out the lesson with discussion of ASHRAE
62.1-2007 guidelines and the Wells-Riley equation for infection and
ventilation, and your next school project is practically getting
smarter already.
Areview
of recent media coverage on airborne diseases such as H1N1 and
various influenza strains indicates increased public awareness and
sensitivity to the spread of such maladies. The classroom is an area
where our children spend the greatest amount of time in close
proximity to the highest number of potential infectors. Diseases are
spread by a variety of means, such as surface deposition and skin
contact, spray droplet transfer, airborne, etc. However, only the
airborne portion falls directly under the control of HVAC design
engineers, installing contractors, and facilities O&M staff.
Recognizing this, how can we evaluate the ability of our systems to
maintain appropriate IAQ?
ASHRAE Standard
62.1-2007 “Ventilation for Acceptable Indoor Air Quality”1
provides design and operation guidelines that generally falls into
three categories:
- Sources of outside air (regional air quality, location of air
intakes, etc.)
- Quantity of outside air
(function of people count and building floor area)
- Delivery of outside air to the breathing zone (zone air
distribution effectiveness)
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| Figure
1. A
radiant heating and cooling system with a DOAS was used at Georgetown
Elementary School, resulting in high IAQ and low energy consumption.
(Photo courtesy of GMB Architecture & Engineering.) |
|
As
we think about where we capture outside air used for ventilation, we
need to consider potential contaminants, such as vehicle exhaust
fumes that are discharged near intake louvers, grass clippings and
other biological contaminants that might enter louvers at or near
ground level, other intakes that might entrain rain or snow and not
provide adequate drainage to prevent biological growth, and the
proximity of air intakes to other sources of contaminants, such as
exhaust fans, cooling towers, etc.
Indeed,
even the U.S. Environmental Protection Agency (EPA)2
has weighed in on the concern of providing appropriate IAQ within
classroom areas. The EPA has recommended consideration of central
HVAC AHUs in lieu of some configurations of unitary,
classroom-located HVAC units. These classroom-located units may not
always provide consistent IAQ because their fans may be tuned for
acoustics, they may be unable to provide higher levels of air
filtration, and they have multiple outside air intakes and condensate
drain pans that act as sources of contaminants, etc. A review of
recently constructed educational facilities appears to indicate that
the design team and the building owner increasingly consider these
issues; however, it is still very possible to apply unitary equipment
in a manner that can address these concerns.
If
we consider the application of ASHRAE Standard 62.1-2007 in the
design of an outside air delivery system, we see the ventilation
quantity requirement being driven by multiple components: the number
of occupants, the floor area of the zone, and the HVAC system’s
effectiveness at delivering this outside air to the breathing zone of
the occupants. Consider this example: a 1,000-sq-ft classroom of 34
students aged 9-plus years and an educator (total of 35 people).
Table 6.1 within the Standard indicates an outdoor air rate of 10
cfm/person and an area outdoor air rate of 0.12 cfm/sq ft of the
zone. Thus we need to deliver the following volume to the breathing
zone of that classroom:
35
people x 10 cfm/person + 1,000 sq ft x 0.12 cfm/sq ft = 470
cfm
Having established
this air volume requirement, we need to consider how effectively we
can deliver this air to the breathing zone. Table 6.2 in the Standard
defines zone air distribution effectiveness. (Ez)
indicates this effectiveness can vary from 0.5 to 1.2, depending on
the location of the air supply and return and the relative difference
between the supply air temperature and the room air temperature. If
we apply the two extremes in ventilation effectiveness from Table
6.2, we see a significant variance in the amount of outside air that
we need to condition and deliver to the zone:
If
the Ez is
0.5, our outside air requirement is 470 cfm / 0.5 = 940
cfm.
If the Ez
is 1.2, our outside air requirement is 470 cfm / 1.2 = 392
cfm.
This yields quite
a variation in air volume requirements, which will translate to
significant differences in the capacity and first cost of the HVAC
system and its related energy consumption. Ideally, we should focus
on system configurations that provide Ez
values as high as possible, typically 1.0 or better for systems that
are commonly applied in educational facilities.
Applying
the Wells-Riley Equation3
provides an interesting backdrop as we consider the spread of
airborne diseases. This equation, developed in the 1940s, provides an
estimated rate of infection as a function of the rate at which
infectious material is discharged into the space, exposure time, air
movement (dilution), outside air quantity (dilution/removal), and
effectiveness of return air filters at capturing infectious materials
(removal). This equation has some challenges as it is based on a
fully mixed indoor air environment, and its application on system
configurations that may employ stratification (such as displacement
ventilation [DV]) is probably not accurate.
Additional
research is needed on the ability of DV systems to capture and remove
warm infectious materials expelled via breathing. European studies
have shown lower concentrations of particles in the breathing zone
when DV systems have been applied, but there is some speculation in
the industry with regard to the system’s ability to adequately
remove these airborne contaminants, since these systems rely on
natural convection to transport these warm materials upwards where
they are removed from the space.
If we design
HVAC systems following ASHRAE Standard 62.1-2007 and adjust the
outside air quantities to reflect Ez,
we will see that the Wells-Riley Equation predicts virtually the same
rate of disease spread for all systems. If we keep the overall volume
of outside air constant between systems, we will see projected
reductions in infection rates associated with HVAC systems
that:
- Have
higher Ez
(including some overhead systems, most underfloor air distribution
(UFAD) systems and DV systems)
- Have
higher ability to remove contaminants via exhaust (dedicated outside
air systems [DOAS] with 100% outside air and recirculating systems
that have better filters or the ability to kill the infectious
materials)
- Have higher air turnover
rates that dilute the infectious material by mixing it with more
air
In our own design
practice, we have seen lower than state-average absentee rates in
multiple schools we have designed with HVAC systems ranging from unit
ventilators (both ducted and non-ducted), overhead VAV, UFAD, and
DV/radiant heating, and cooling configurations.
However,
the attendance rate information lacks granularity, in that we are
unable to categorize the cause for the school absences into illness,
family vacations, etc. Based on this broad experience, we have
adopted an approach of applying systems with high ventilation
effectiveness and DOAS with 100% outside air that is not recirculated
between classrooms. Two examples follow.
Quincy Elementary School
|
|
| Figure
2. Air
temperature uniformity at the radiant floor. (Figure courtesy of GMB
Architecture & Engineering.) |
|
This
was one of our first projects to employ a central geothermal energy
center, which in this case includes five Waterfurnace 30-ton
water-to-water heat pumps providing 45°F chilled water for cooling
and 120° hot water for heating. A vertical closed-loop earth heat
exchanger provides the heat source and sinks for the building. A
single, dual-wheel, passive desiccant energy recovery unit from SEMCO
Pinnacle functions as a DOAS unit and provides 62° to 65° supply
air to each classroom. Each classroom then has a UFAD plenum with a
hot water reheat coil to temper the supply air. The supply air is
then distributed into the classroom via multiple VAV floor terminals
from JCI/York FlexSys. Non-classroom spaces have dedicated
water-to-air geothermal heat pumps.
The UFAD
system with a raised access floor over a concrete slab offers unique
opportunities for using thermal mass as a flywheel to reduce peak
load demands on the geothermal heat pump system. During the heating
season, the DOAS operates during unoccupied periods in a
recirculation mode (no outside air) and reduced air volume
(approximately 30% of typical design) while the reheat coils at the
raised floor plenums maintain a plenum (not space) temperature of 68°
to 70°. Conversely, in the cooling season, the DOAS unit can operate
at night to precool the concrete slabs in the classrooms to 70° to
72°.
The HVAC system is extremely quiet,
providing classroom acoustical readings well under NC-30. This
facility provides many of the benefits of a geothermal heat pump
system with centralized HVAC equipment to simplify maintenance,
reducing the number of condensate drain pans and eliminating moving
parts in the classrooms other than the hot water reheat control
valves and the underfloor air VAV terminals. The facility uses less
than half the state average for energy consumption for elementary
schools, yet it is fully air conditioned including even the gym,
cafeteria, and media center in addition to all of the classrooms.
Probably fewer than 20% of the elementary schools in this state can
make that claim.
Georgetown Elementary School
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| Figure
3. Isothermal
temperature contours show uniformity within the room. (Figure
courtesy of GMB Architecture & Engineering.) |
|
This
project builds upon the successful system concepts piloted in the
above project and employs radiant heating and cooling with a DOAS —
a system concept that we believe represents some of the best
contemporary thinking with regard to high thermal comfort, high IAQ,
low HVAC-related noise, and excellent energy performance.
The
central plant energy center is again based on water-to-water
geothermal heat pump technology, provided by Trane.
However,
in this application, we are applying chilled water in a 58° to 62°
range, instead of the 45° in the project above, due to the
application of suspended radiant cooling panels within the
classrooms. This “warmer” chilled water allows us to use water
directly from the geothermal vertical closed-loop earth heat
exchanger without operating the heat pump compressors for a portion
of the cooling season. When this water is too warm to use directly,
the geothermal heat pumps can produce the 58° to 62° chilled water
at very high energy-efficiency ratios.
Zone
heating is supplied via PEX tubing embedded within the concrete slabs
of the classrooms and fed with 100° to 110° hot water. Our
experience with radiant floor heating in educational facilities over
the past two decades has shown us that this system can provide very
high comfort levels with lower energy consumption when properly
designed and operated.
Displacement
ventilation is provided from a single DOAS system, from SEMCO
Pinnacle, similar to the project above. In lieu of the 45° chilled
water coil we have applied a water cooled DX refrigeration unit, from
MultiStack, coupled with the geothermal heat pump loop to provide the
cooler coil temperatures needed to maintain the supply air moisture
at approximately 48 grains during the cooling months. This unique
application employing the desiccant wheel in the DOAS allows lower
supply air dewpoint conditions than would typically be possible at
the design cooling coil temperatures. No issues with condensation on
the radiant cooling panels have arisen, even during periods of
Michigan high relative humidity weather.
The
supply air is distributed to the classrooms via a concrete tunnel
below the building slab. Careful attention was paid to the design and
construction of this duct to reduce the potential for moisture to
enter the air distribution path and an anti-microbial coating was
applied to the concrete. The air enters the classrooms via
displacement ventilation diffusers, from Price, designed to be
encased within the classroom casework or to be wall
mounted.
Extensive computational fluid dynamic
(CFD) modeling was used during the design process to determine if the
ventilation air traveling across the warmed floor slab in the heating
mode would effectively reach a student in the far corner of the room.
DV uses low velocity air (typically 40 fpm or less at the diffuser)
that is slightly below the room temperature setpoint. This cooler air
then flows across the floor (similar to water filling a bathtub), and
the convective plumes associated with body heat pulls the ventilation
air up through the breathing zone.
While in
the breathing zone, the air conveys the warm contaminants from the
occupants upward towards the ceiling where they are captured and
exhausted from the building. No air is recirculated within the
classroom or between the classrooms. Instead, 100% outside air, which
is filtered and conditioned, provides a fresh indoor air environment.
A recent visitor to the building was inquiring about the DV system,
and the receptionist asked if he was there to visit their “new air
building.” He asked her what she meant by that statement, and she
indicated that her own children typically had several colds during
the winter months in their previous school, but the frequency of
their colds diminished significantly since they began attending here,
which she attributed to the “new air” brought into each
classroom.
Cited Works:
1.
ANSI/ASHRAE Standard 62.1-2007 “Ventilation for Acceptable
Indoor Air Quality.”
2. Environmental
Protection Agency/EPA Indoor Air Quality website;
http://www.epa.gov/iaq/schooldesign/hvac.html#Selection
of HVAC Equipment
.
3. Wheeler,
Arthur E., “Better Filtration for Healthier Buildings” ASHRAE
Journal, June 1994.
