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Laboratories typically require 100% outside air at high ventilation rates-between 6 and 12 air changes per hour-primarily for safety reasons. The heating and cooling energy needed to condition this air, as well as the fan energy needed to move it, often is five times greater than the amount of energy used in most offices for these same purposes. Energy recovery can substantially reduce the mechanical heating and cooling requirements associated with conditioning ventilation air in most laboratories. As a result of the lower peak heating and cooling requirements with energy recovery, heating and cooling systems can be downsized.
Many opportunities exist for energy recovery in laboratories. Energy recovery can occur between any two media or processes that differ in energy content. The focus of this article is on airto-air energy recovery-using enthalpy wheels (Figure 1), heat pipes, or runaround loops in new construction.
Technology Overview
Air-to-air energy recovery devices exchange energy from one stream of air to another. Most commonly, energy is recovered from exhaust air and used to precondition supply air. National Fire Protection Association's NFPA 45: Standard on Fire Protection for Laboratories Using Chemicals, 2004 edition, states that, if there is a chance of cross contamination between airstreams, air-to-air energy recovery only can be used on general exhaust. The air contains sensible (heat) and latent (water vapor) energy. Both types of energy can be recovered.
However, not all recovery devices exchange both types of energy. The effectiveness of an energy recovery device is the ratio of actual energy recovered to theoretical energy that could be recovered. Most devices have a rating for sensible effectiveness. Some also have a rating for latent effectiveness and total effectiveness.
Energy recovery devices increase the pressure drop across the supply and exhaust fans. Enthalpy wheels generally have a lower pressure drop than heat pipes and runaround loops, although the pressure drop depends on the design. If exhaust filtration is required to protect the energy recovery device, filter pressure drop also must be considered. An additional pressure drop of no more than 1 in. w.g. (249 Pa) in the supply and exhaust airstreams is a reasonable design goal, and will minimize the increase in fan energy.
For laboratory applications, the design face velocity of devices in air-handling units is...