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Although their end uses sometimes vary from the manufacturers' original intentions (certainly no one would recommend that their airflow become blocked by wet clothing), it is very important that classroom unit ventilators operate as one would expect.
Having performance standards for this equipment gives HVAC contractors a basis from which to make equipment selections.
According to Mary Coleman, Marketing-Communications team leader, Trane, "It is important that designers of these systems have accurate information to make system decisions.
"That is why the industry has developed performance standards and certification programs, which ensure that the equipment information provided to the design community is correct and comparable across all the different manufacturers," she said.
Air-Conditioning and Refrigeration Institute (ARI) Standard 840 "ensures that performance data is the most useful to the customer," Coleman said. It offers a consistent method to rate the design performance of the equipment. Moreover, "ARI-840 certification sets a minimal acceptable level of economizer performance."
Certification ensures that the equipment performs up to today's classroom requirements - for comfort, economical operation, and good IAQ. Moreover, "Classroom unit ventilators can be a single-source unit for the multi-functioning school," Coleman said.
"Many schools choose classroom unit ventilators because of their single-source equipment design." ("Single source" in this case means one unit per classroom provides climate conditioning to students and staff.) "By installing one unit - a complete HVAC package - in the classroom, cross contamination between classrooms can be minimized, and with proper filtration, cross contamination can also be eliminated," Coleman said. Floor-mounted models deliver treated air into the classroom without ductwork.
Of primary importance is the health of students and staff at the facility. A key to that is classroom IAQ, which can be greatly affected by the amount of fresh air brought into a space.
Health Conditions"IAQ should be considered a top priority in the school environment because children are still developing physically and are more likely to suffer the consequences of indoor pollutants," Coleman pointed out. "Proper conditioning of the indoor air is more than a quality issue; it encompasses the safety and stewardship of our investment in the students, staff, and facility."
Some buildings rely only on natural ventilation through an open window, she continued. For rooms with high cooling/ventilation demands (i.e., computer suites, restrooms, auditoriums, and kitchens), natural ventilation may not meet their requirements.
"Providing proper ventilated air is part of a viable air quality strategy to help architects, engineers, and contractors design better IAQ into the mechanical system," Coleman said. Therefore, using a design standard in conjunction with ARI 840-certified equipment is the best strategy.
American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 62-2001 sets a criterion for proper airflow requirements in schools. It considers various types of environments within a facility to help determine appropriate proportions of airflow per person based on the room demand.
Economical PerformanceTo achieve ARI certification, the unit ventilator must be capable of providing a minimum of 80 percent of its ventilation airflow through the outside air economizer function, Coleman explained. "This measurement ensures that the expected energy savings by the economizer are realized in actual operation."
In addition, some newer classroom unit ventilators are able to support integrated economizing, she said. "But only ARI 840-certified equipment has been independently tested for compliance to the minimum requirement of 80 percent airflow through outside air."
The economizer functions by opening an outside air damper, bringing cooler outside air into a space with a unit fan, she explained. "The economizer cycle is controlled by a modulating damper motor that opens at specified increments dependent upon readings from discharged air or outside air sensors."
A return air damper closes as the outside air damper opens. Depending on room requirements, the modulating damper motor may mix return air with the outside air to provide maximum energy efficiencies without sacrificing comfort, Coleman said.
"When the room thermostat calls for cooling, the economizer control provides the right mix of outside and return air to cool the classroom. The equipment's airflow is generated from both fan energy and the economizing dampers. This design supports optimum ventilation and provides the greatest energy savings.
"As the outside air temperature rises (typically above 55Â°F), the outside damper fully opens, activating the second cooling stage on the room thermostat - the cooling-generating device (compressor, water pump, chiller, cooling tower)," Coleman said. "The return air and outside air dampers modulate to support the discharged air temperature. Dampers working together with the cooling coil is called integrated economizing, which allows a unit ventilator to mix outside air with return air."
AcousticsAcoustics can also be an issue within the classroom, especially when equipment is located so close to the staff and students, as is the case of floor-mounted unit ventilators. "While designing a quiet classroom may be challenging," Coleman said, "it is possible to develop a sound solution by varying the source of the noise: the airflow."
Some manufacturers provide a two-speed fan control that delivers airflow to fit space needs. When fewer cfm are needed to meet the needs of the classroom (typically 75 to 80 percent of the time, Coleman said), the equipment operates at low speed. In addition to saving energy, this helps keep sound levels to a minimum.
If the room temperature rises to the setpoint, the controller will switch to high speed to support the space needs. Ventilation is also considered within this strategy, Coleman explained. "The controller must also vary the outside airflow to ensure that the minimum outside air cfm is being met. This setup allows the unit ventilator to vary the airflow to meet the space cfm requirement."
For sound-sensitive rooms such as library, band, or choral rooms, ceiling-hung, ducted, horizontal unit ventilators may be the best solution. The equipment is placed above the ceiling and away from direct contact with students. It may also be located in a corridor or mezzanine, then ducted into the classroom, she said. Supply- and return-air ducts help attenuate air noise, which would typically have a direct path to the students in a vertical floor-mounted application.
"Although this option may be more expensive, the ducted unit is generally a better choice for sound-sensitive applications," Coleman said.
Of the many design enhancements available on unit ventilators, one of the most important is the certification of equipment capabilities from an impartial organization such as ARI.
For more information, visit www.trane.com, www.ari.org, and www.ashrae.org.
Sidebar: Trane Unit Ventilators Pass ARI TestsWACO, Texas - Trane recently announced 100-percent compliance with the Air-Conditioning and Refrigeration Institute (ARI) industry standard for measuring fresh airflow and unit ventilator performance. This is the second year in a row that the company achieved total compliance.
ARI 840 is the benchmark by which HVAC manufacturers build and test their unit ventilators. Architects, engineers, and contractors are utilizing ARI 840 as a requirement when designing mechanical systems within a school.
Trane says its unit ventilators "are up to the challenge by providing the most test data and superior efficiency and acoustical performance." By certifying its unit ventilators, the company helps ensure that each classroom system performs as intended. Coupled with a design that includes a noncorrosive, dual-sloped drain pan to decrease microbial growth, the unit ventilators are built with IAQ in mind.
The units also feature "quiet speed" options, factory-mounted controls, and easy service access, the company states.
Publication date: 11/03/2003