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Diffuser, Fume-hood Retrofit Saves Lab an Expensive A/C Unit Replacement

HVAC design/build contractor uses textile air diffusers and new fume hoods to solve lab's cross-draft, high-velocity ventilation challenges.

Faced with temperature and relative humidity swings of 10°F and 30% in its laboratory area, a Corpus Christi, TX-based international chemical processor was ready to invest hundreds of thousands in new air-conditioning equipment to resolve the challenge.

Instead, Superior Laboratory Services Inc., a Pasadena, TX-based laboratory and IAQ company specializing in certification, design-build and HVAC contracting services, suggested a laboratory ventilation retrofit of replacing ceiling diffusers and fume hoods at a fraction of the cost for an estimated 100 tons of new A/C equipment.

Other engineering bids proposed costly cooling-tonnage increases and accompanying larger sheet-metal supply duct and registers to accommodate the increased A/C capacity of the end user. However, Superior President Rick Meyer's diagnosis revealed the temperature and humidity problems were not due to insufficient A/C capacity, but from ineffective cross-drafting among the 30 existing supply-air diffusers and 18 original draft hoods that were 35 years old.

The current rooftop air-conditioning system was properly sized for the 4,000-sq-ft area of five labs. Adding more A/C tonnage might have worsened the cross-drafts, according to Meyer. Instead, his ventilation modification strategy now provides tighter temperature and humidity tolerances of ± 3°F and ± 5% RH, which improves quality control in the lab's polypropylene research and development functions while also enhancing IAQ and employee comfort.

Curbing excess airflow velocity and turbulence
The temperature and humidity variances related to airflow velocity and turbulence also skewed sensitive scale readings for weights and dimensions. The extreme air velocities were generated by the conventional 2 x 2-ft, four-way high-throw metal diffusers in the drop-ceiling design. Even smoke tests, a standard air-dispersion test procedure, were difficult to perform in the lab's pre-retrofit environment.

"We had difficulty performing standard smoke tests within the lab because the air velocities were so intense and there was no continuity to its flow direction, even after blocking off one or two sides of most diffusers," said Meyer.

The first modification was changing out the 2 x 2-ft diffusers with 2 x 4-ft LabSox textile diffusers by DuctSox Corp. The LabSox Series, which uses a highly permeable, non-shedding, partially post-recycled “green” polyester filament with an anti-microbial treatment, was specifically designed to provide the lab industry with quiet and uniform low-velocity airflow dispersion patterns. The uniform Surround Flow series that provides minimum airflow turbulence throughout the air distribution area is most common. For areas requiring directional flow, the fabric faces can be customized with factory-engineered vent patterns for one, two, three or four-way directional flow of the more precise Select Flow.

Instead of the open-louvered, 100-fpm or more design of metal diffusers, the supply air now flows through the textile facing at a rate near 30 fpm. For each supply-air diffuser and hood replacement's correct sizing and output, Meyer calculated the lab area's total cfm, with replacement fume-hood velocities, supply-air cfm, number of supply outlets, distances between diffusers and hoods, ceiling height and other factors helping maintain the negative pressure required in lab ventilation.

Instead of a drop-in 2 x 2-ft textile replacement, Meyer chose a 2 x 4-ft model with an aluminum-face frame that universally installed easily to the lab's suspended T-Bar ceiling configurations. The increased size increased the textile surface area to ensure a gentle flow that minimizes cross-drafts, scale and balance-skewing high-airflow velocities. Meyer also chose the MetalPan model, allowing a flexible metal-to-flex connection adaptor from the HVAC supply to accommodate the lab's tight ceiling clearances.

"The anti-microbial treatment and the fact a fabric diffuser actually acts as a secondary air filter is also an additional advantage in that it minimizes microbial and particulate contamination that can affect lab tests," said Meyer.

To maintain capture in the draft environments, fume hoods were set to operate with sash velocities of 120–130 fpm. This additional airflow within the space created summertime and wintertime temperatures that were too warm and cool, as the hoods exhausted more than the air-conditioning system could supply. As this negative pressure pulled more airflow through the make-up air systems, humidity control became challenging as added airflow prevented coils from reaching preferred condensing temperatures.

Meyer replaced the dated hoods with bench-top low-flow hoods with 80-fpm drafting by Labconco. The hoods also incorporate technological advancements such as perforated baffles and airfoils, and a hand-operated airfoil type of sash that helps reduce the face velocity to 80 fpm.

Once installed, Meyer typically runs several smoke tests using different methods such as a high-volume theatrical smoke generation or low-volume smoke tubes that release small, targeted amounts. Another smoke test analyzes a hood's drawing power with the sash up and down to determine containment efficacy. Rooftop exams determine if any flue stacks are causing re-entrainment into the building. Meyer also tests hood drafting with dry ice, which mimics hazardous heavier-than-air gases such as hydrogen sulfite, methanol, propane and propylene.

The labs were also outfitted with AFA-1000 Series airflow monitors by Temperature Electronics Ltd. and air balancing blowers by Central Blower Co.

Superior guarantees the retrofit for five years, which includes an annual inspection using testing instruments such as a thermal anemometer by TSI, and certifies the lab under various code compliances that include:

  • ASHRAE Standard-110 Method of Testing Performance of Laboratory Fume Hoods;
  • American Industrial Hygiene Association;
  • ANSI Z-9.5;
  • NFPA Standard-45; and
  • Scientific Equipment Furniture Association.

Included in the performance guarantee is an annual LabSox cleaning for optimum IAQ, which requires approximately two hours to disassemble, launder and re-install.

Meyer's ventilation retrofit strategy was successful resulting in no temperature or humidity complaints. Furthermore, the lab can now expand to additional bench hoods using the same A/C equipment.

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