ERV Cross Leakage Defined And Under Control
In ERVs, the two airstreams are brought side by side with each other, explained Aaron Gotham, general manager, ERV and makeup air products at Greenheck, Schofield, Wis.
"In rotary ERVs, the wheel rotates between these two airstreams, and some crossover occurs."
While plate and heatpipe-style ERVs greatly reduce the potential for leakage, the possibility still exists, he said.
The crossover could be considered contamination, but that depends on what is in the air that crosses over from one airstream to the other, Gotham explained.
"If you have a lab fume exhaust application with toxic chemicals in the exhaust air, it's cross contamination. It's leakage in all cases. Only in some cases is it contamination."
Perhaps more importantly, "HVAC system designers now have clear-cut parameters for allowable ERV cross leakage, sometimes referred to as cross contamination," Gotham said.
Addendum y of ASHRAE Standard 62.1-2004, "Ventilation for Acceptable Indoor Air Quality," answers the common question, "How much ERV cross leakage is acceptable when the exhaust air is from a restroom?"
Leakage Rates"The Air-Conditioning and Refrigeration Institute (ARI) has published certified ratings for program 1060 for air-to-air energy recovery ventilators since January 2001," Gotham said.
It rates product effectiveness and cross leakage.
"The critical ARI-certified rating with respect to leakage is Exhaust Air Transfer Ratio (EATR)," he continued. "EATR identifies the percentage of exhaust air that is transferred to the outdoor airstream during energy recovery. When recovering energy from a restroom or other Class 2 airstream, the proper interpretation of addendum y is that the EATR value shall not exceed 10 percent."
Cross leakage could increase from its original rating due to neglect or poor maintenance, he noted. "Manufacturers typically recommend an annual inspection. If it's neglected over years, cross leakage could increase." How much cross leakage is typical? That depends on the type of device used.
"The energy wheel-type device Greenheck uses has a 2- to 5-percent leakage range for the wheels." Wheel design is critical to minimize leakage, he pointed out.
Air ClassificationTo figure out whether a certain amount of leakage is appropriate, you need to classify the cleanliness of the air you will be moving. Addendum y spells out four classes of air:
Class 1 - This air has a low contaminant concentration, inoffensive odor, and low sensory-irritation intensity. "It is suitable for recirculation or transfer to any space," Gotham said. Sources of Class 1 air include office spaces, classrooms, assembly rooms, church facilities, and corridors.
Class 2 - This air has "moderate contaminant concentration, mildly offensive odors, and/or sensory-irritation intensity," Gotham said. It is suitable for recirculation or transfer to any space with Class 2 or Class 3 air that is used for the same or similar purpose and involves the same or similar pollutant sources.
"Class 2 air is not suitable for recirculation or transfer to spaces with Class 1 air, or dissimilar spaces with Class 2 or Class 3 air. Examples of Class 2 spaces include restrooms, swimming pools, dining rooms, locker rooms, warehouses."
Class 3 - This air has significant contaminant concentration, significantly offensive odor, or sensory-irritation intensity. It is suitable for recirculation with the same space, but is not suitable for recirculation or transfer to any other space. Examples of Class 3 spaces include kitchens, dry cleaners, beauty salons, laboratories, and pet shops.
Class 4 - This air has "highly objectionable fumes or gases, or potentially containing dangerous particles, bioaerosols, or gases at a concentration high enough to be considered harmful," said Gotham. It is not suitable for recirculation or transfer to any other space. Examples include paint spray booths, lab fume exhaust, and kitchen grease exhaust.
Energy RecoveryNow comes the tricky part.
According to the addendum, "Class 2 air may be redesignated as Class 1 air in the process of recovering energy when it is diluted with outdoor air such that no more than 10 percent of the resulting airstream is Class 2 air. Class 3 air may be redesignated as Class 1 air in the process of recovering energy when it is diluted with outdoor air such that no more than 5 percent of the resulting airstream is Class 3 air."
There are some common sense points to consider. Gotham explained, "When applying an ERV to recovery energy from the exhaust air from a heavy-use smoking lounge, it's recommended to bring the outdoor air back into the smoking lounge area because the 5 percent leakage of the ERV may be objectionable to nonsmokers.
"When you think about the real world and how these are likely to be applied," he continued, "a common application is a commercial building that requires a minimum of 1,000 cfm of restroom exhaust and 3,000 cfm of fresh outdoor air, based on the ventilation code.
"With an ERV, you would size the outside air side for 3,000 cfm and bump the exhaust side up to 2,500 or 2,700 cfm (maintaining a positive building pressure) to optimize your energy recovery. The result is a much fresher smelling restroom because you're more than doubling the code required exhaust air as you balance the building airflow."
Putting it another way: "You may recover energy from restroom exhaust (Class 2) air to precondition outdoor air for a Class 1 space (i.e., classroom or office space) as long as the energy recovery cross leakage is no greater than 10 percent. The same logic applies to recovering energy from Class 3 air, with the cross leakage limit reduced to 5 percent."
More Efficienct Design"The language in addendum y is extremely important," Gotham said, "because it enables specifying engineers to be comfortable designing higher-efficiency systems. ASHRAE's official endorsement of the practice of â€˜recirculating' a small amount of Class 2 air encourages the practice of recovering energy from restroom exhaust air.
"Now, total enthalpy energy recovery wheels with ARI-certified cross leakage ratings may confidently be specified for restroom applications, maximizing total energy recovery while keeping code officials satisfied."
Another energy benefit of the addendum's language, he said, is that "energy wheel purge sections are not necessary for Class 2 air, and may not be necessary for Class 3 air. This is important because a wheel purge option typically increases the ERV horsepower 50 to 75 percent.
"With good energy recovery wheel designs limiting cross leakage to less than 5 percent without a purge option, engineers can have very low cross leakage and a highly efficient ERV system."
For more information, visit www.greenheck.com, www.ari.org, or www.ashrae.org.
Publication date: 04/25/2005