©2013 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 55, no. 9, September 2013.
By Barry Barnet, P.E.
About the Author
Barry Barnet, P.E., is a senior professional associate and senior mechanical engineer at HDR in Princeton, N.J.
Energy recovery in laboratories has some special concerns because of the possibility of cross contamination between the supply and exhaust streams. This article illustrates two methods of improving energy recovery in laboratory air-handling systems (with 100% outside air), where fume hoods and similar exhaust are considered to be hazardous (as defined by the International Mechanical Code [IMC]) and the energy recovery system is designed to preclude the possibility of cross-contamination between supply and exhaust. With both methods of recovery, the air-handling system supplies air at a neutral air temperature using dual energy recovery.
For the purpose of revealing the improvement in energy recovery with the neutral air concept, both methods of recovery are also compared to a more traditional air-handling unit supplying air at a temperature of 55°F (13°C).
The results show that with the neutral air approach it is possible to achieve efficient energy recovery even where the general exhaust used for recovery is only 40% of the total supply air, while still precluding the possibility of cross-contamination and complying with the IMC requirements for hazardous exhaust.
Recovery and Air-Handling Systems
The first energy recovery method uses an energy wheel. The second recovery method uses glycol runaround.
Under the first air-handling system, the traditional unit serves as a combined ventilating, makeup, humidity control, and cooling system (in a sense a “jack of all trades”), producing leaving air at a temperature of 55°F (13°C). Energy recovery in this case is single stage using only one energy wheel or a single glycol coil.
With the second air-handling system (neutral air), the unit provides only ventilation, makeup air, and humidity control using dual-energy recovery, and produces a leaving-air temperature in the range of approximately 63°F to 70°F (17°C to 21°C). This system is combined with air recirculating supplemental cooling devices: typically fan coil units, chilled beams, or other similar devices. In hood intensive rooms care should be exercised to avoid interference between these devices and proper hood performance. Dual energy recovery consists of two energy wheels, one enthalpy and one sensible, for the first recovery method. Under the second recovery method, two glycol coils are substituted for the wheels. The two glycol coils in this case can be described as a wraparound/runaround system. The glycol flows in series from the second coil, where the supply air is heated up to neutral conditions, then wraps around the chilled water coil (providing dehumidification) to the first glycol coil (serving to precondition the outside air).
Based on the previous variations, there are a total of four different combinations, with two energy recovery methods each applied to two different air-handling systems:
- Energy wheel method of recovery with a traditional system (Figure 1; with first wheel only);
- Energy wheel method of recovery with a neutral air system (Figure 1; with dual wheels);
- Glycol coil method of recovery with a traditional system (Figure 4; with first glycol coil only); and
- Glycol coil method of recovery with a neutral air system (Figure 4; with dual coils).
A method of recovery using heat pipes is not examined as this system is expected to have characteristics similar to glycol runaround but with slightly higher recovery effectiveness, due to the higher heat content of refrigerant verses a glycol solution.
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