More Efficient HVAC Ops: System Elements Important to Indoor Air
Quality
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Replacing traditional round tube heat exchangers with flattened
tube technology may enable the air conditioning industry to
provide comfort cooling using a new refrigerant, while
increasing efficiency, but not the system’s size.
A newly released Air-Conditioning and Refrigeration Technology
Institute (ARTI) report characterizes how flattened-tube heat
exchangers function under various environmental conditions and
pressures. “Flattened tube heat exchangers have received much
attention as a possible re-placement to traditional round tubes,
but until now little research has been done on the
thermal-hydraulic performance of flattened tubes under wet, dry
and frosted conditions,” said Elizabeth Jones, a project manager
with ARTI, which provided the funding for this project under its
HVAC&R Research for the 21st Century program. “This research
report addresses the fundamental science needed to allow the air
conditioning industry to engineer products using this
technology.”
The geometry of a flattened tube, compared with the traditional
round tube heat exchanger, allows for improved heat transfer and
thermal performance; increased coil and overall unit efficiencies;
substantial refrigerant charge reduction; and more compact and
reduced coil size.
In the ARTI report, University of Illinois researchers provide
analysis, modeling, and interpretation of air-side,
thermal-hydraulic performance for flattened tube heat
exchangers under wet and frosted surface conditions.
They make design recommendations to help improve the performance
of plain, wavy, strip and louvered fins for flattened tube heat
exchangers. They conduct a full assessment of the air-side
thermal-hy-draulic performance of flattened, round, and finned
heat exchangers. In addition, researchers developed a new method
to provide data on retention and drainage of water from the
air-side surface of flattened tube heat exchangers under a number
of operating conditions.
To read the final report, “An Assessment of the
State-of-the-Art and Potential Design Improvements, for
Flat-tube Heat Exchangers in Air-Conditioning and Refrigeration
Applications,” go to www.arti-research.org/research/completed/finalreports/20021-final.pdf.
EPA Guidance
The Environmental Protection Admin-istration’s Indoor Air
Quality Building Education and Assessment Model (IBEAM) is a
guidance tool designed for use by building professionals and
others interested in indoor air quality in commercial
buildings. It recognizes that there are significant spatial and
seasonal variations in the volume of air delivered by most HVAC
systems.
It says that HVAC operators must understand these variations to
know how to provide occupants with adequate outdoor air in all
spaces throughout the year. The ventilation features most
important to IAQ are the way in which supply air volume is
controlled, and the way in which outdoor air delivery is
controlled. In most HVAC systems, a portion of ventilation air
supplied to occupied spaces is outdoor air and a portion is
re-circulated air. The total volume of air is important for two
reasons:
Air movement contributes to thermal comfort. The lack of air
movement can create a sensation of hot/stuffy air.
In many Variable Air Volume (VAV) systems, outdoor air is a
constant fraction of the total supply air. Thus, the total
volume of outdoor air depends on both the outdoor air
fraction, and the supply air volume.
There are two major types of HVAC systems based upon the use of
airflow to control temperature — the Constant Volume (CV)
system, and the VAV system.
In a CV ventilation system, variations in the thermal
requirements of a space are satisfied by varying the temperature
of a constant volume of air delivered to the space.
A constant fraction of outdoor air will mean that a constant
volume of outdoor air will be delivered to occupied spaces. This
volume can be set to satisfy applicable ventilation standards.
CV systems are less energy efficient than VAV systems, but
controls for outdoor air delivery are simpler to manage.
In VAV system, variations in the thermal requirements of a
space are satisfied by varying the volume of air that is
delivered to the space at a constant temperature. VAV systems
reduce HVAC energy cost by 10-20 percent over CV systems but
complicate the delivery of outdoor air. VAV systems also
complicate pressure relationships in the building and make
testing, adjusting, and balancing more difficult.
HVAC Components
Many HVAC components are particularly important to maintaining
good IAQ. Tips for optimum functioning are listed below.
Coils and Drain Pans
• Malfunctioning coils, including dirty coils, can waste energy
and cause thermal discomfort. Leaky valves that allow hot or
chilled water through the coil when there is no demand waste
energy and create thermal discomfort.
• Cooling coils dehumidify the air and cause condensate water to
drip into a drain pan and exit via a deep seal trap.
• Standing water will accumulate if the drain pan is not
properly designed and maintained, creating a microbial habitat.
Proper sloping and frequent cleaning of the drain pans is
essential to good indoor air quality.
Humidification and Dehumidification Equipment
• Potable water rather than boiler water should be used as a
source of steam to avoid contaminating the indoor air with
boiler treatment chemicals.
• Wet surfaces should be properly drainedand periodically
treated as necessary to prevent microbial growth.
• Duct linings should not be allowed to become moist from water
spray.
Outdoor Air Dampers
Screens and grilles can become obstructed. Remove obstructions,
check connections, and otherwise insure that dampers are
operating to bring in sufficient outdoor air to meet
design-level requirements under all operating conditions.
Air Filters
• Use filters to remove particles from the air stream.
• Filters should be replaced on a regular basis, on the basis of
pressure drop across the filter, or on a scheduled basis.
• Fans should be shut off when changing the filter to prevent
contamination of the air.
. • Filters should fit tightly in the filter housing.
. • Low efficiency filters (ASHRAE Dust Spot rating of 10
percent-20 percent), if loaded to excess, will become deformed
and even “blow out”, leading to clogged coils, dirty ducts,
reduced indoor air quality and greater energy use.
. • Higher efficiency filters are often rec¬ommended as a
cost-effective means of improving IAQ performance while
min¬imizing energy consumption. Filtration efficiency should be
matched to equip¬ment capabilities and expected airflows.
Ducts
A small amount of dust on duct sur¬faces is normal. Parts of the
duct suscep¬tible to contamination include areas with restricted
airflow, duct lining, or areas of moisture or condensation.
Problems with biological pollutants can be prevented by:
. • Minimizing dust and dirt build-up (espe¬cially during
construction or renovation);
. • Promptly repairing leaks and water damage;
. • Keeping system components dry that should be dry;
. • Cleaning components such as coils and drip pans;
. • Good filter maintenance; and
• Good housekeeping in occupied spaces.
Duct leakage can cause or exacerbate air quality problems and
waste energy. Sealed duct systems with a leakage rate of less
than 3 percent will usually have a superior life cycle cost
analysis and re¬duce problems associated with leaky duct¬work.
Common problems include:
. • Leaks around loose fitting joints;
. • Leaks around light Troffer-type diffusersat the diffuser
light fixture interface when installed in the return plenum; and
. • Leaks in return ducts in unconditioned spaces or underground
can draw contam¬inants from these spaces into the supply air
system.
Exhaust Systems
In general, slightly more outdoor air should be brought into the
building than the exhaust air and relief air of the HVAC system.
This will insure that the building remains under slight positive
pressure.
. • Exhaust intake should be located as close to the source as
possible;
. • Fan should draw sufficient air to keep the room in which the
exhaust is located under
negative pressure relative to the surrounding spaces, including
wall cavities and plenums;
. • Air should flow into, but not out of, the exhaust area, which
may require louvered panels in doors or walls to provide an
un¬obstructed pathway for replacement air;
. • The integrity of walls and ceilings of rooms to be exhausted
must be well main¬tained to prevent contaminated air from
escaping into the return air plenum; and
. • Provisions must be made for replacing all air exhausted out
of the building with make-up outside air.
Return Air Plenum
. • Space above the ceiling tiles is often used as a return air
plenum.
. • Strictly follow code which restricts ma¬terial and supplies
in the plenum to pre¬vent contamination and insure that airflow
is not interrupted. Remove all dirt and debris from construction
activity.
. • All exhaust systems passing through theplenum must be
rigorously maintained to prevent leaks, and no exhaust should be
released into the plenum.
. • Avoid condensation on pipes in plenum area. Moisture creates
a habitat for microbial growth. ❑ |
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