In a typical laboratory facility, more than half the total HVAC load can be latent. To optimize energy recovery, energy engineers should recover both sensible and latent energy from the exhaust air stream. In the past, use of the rotating energy recovery wheels has not been recommended because of the potential carryover of contaminants from the exhaust air stream to the supply air stream. Recently, a molecular sieve, desiccant-based heat wheel technology has been developed that actually improves indoor air quality. The molecular sieve heat wheel provides sensible and latent energy recovery with a very low level of cross contamination between the incoming outdoor air and the exhaust system discharge. Innovative arrangements of heat wheels are being applied including: a two-stage dehumidification device that uses heat rejected from its DX condenser to "dry" the heat wheel and a device that combines heat pipes with a heat wheel to provide free reheat to the unit's air flow. [Harimann, 2002] [Wier, 1983; Carroll, 1995; Diblasio, 1995]
Molecular sieve heat wheels have been installed in many laboratory facilities from a multiple-story medical research facility to an animal virology laboratory to hospitals. Cooling requirements have been reduced by half and heating and humidification requirements by more than two-thirds. Installed chiller and heating plant capacities have been reduced by as much as 50 percent, saving both money and space. These new heat wheels have provided enough first-cost savings to pay for themselves before they were put into operation. [Diblasio, 1995]
The mass of the molecular sieve heat wheel is a coated desiccant matrix. The wheel rotates slowly, typically at about 20 rpm, between the building exhaust and supply air streams. The desiccant medium transfers heat with an efficiency of 75 percent to 90 percent by adsorbing and transferring vapor from one air stream to the other. Because moisture is transferred in vapor form, there are no wet surfaces to support microbial growth or chemical byproducts associated with boiler steam humidification. [Diblasio, 1995]
Overcoming the major obstacle of exhaust contaminant cross-contamination into the supply air stream is accomplished with a three-angstrom (Å) molecular sieve. The wheel incorporates a sieve, which prevents the transfer of materials larger than 3Å. All molecules larger than 3Å are too large to enter the sieve and pass by the wheel without being absorbed into the underlying desiccant medium. Water vapor at 2.8Å passes through the sieve so that latent energy is recovered. With a heat recovery wheel and this selective adsorption feature, cross-contamination has been virtually eliminated. The molecular sieve heat wheel recovers latent energy and limits cross-contamination to less than 0.04 percent by particulate count. [Carroll, 1995]
