Vertical, laminar flow, air recirculation systems are typically used in laboratory cleanrooms to achieve the required cleanliness. This air flow may bathe only the product and manufacturing equipment or be provided throughout the cleanroom. Energy savings are realized when the rate of air flow is reduced throughout the cleanroom and maintained over the product and manufacturing equipment only. [Naughton, "HVAC Systems… Part 1," 1990]
Three air recirculation configurations should be evaluated for the cleanroom: the fan can be mounted locally over the cleanroom in an AHU; the fan can be installed remotely in a fan "tower" configuration; or a group of small, local fans with filter units (FFUs) attached can be mounted directly in the ceiling grid. [Hunt et al., 1990]
AHUs with vane-axial fans have high individual efficiencies but do not necessarily yield the highest system efficiency. Pressure losses from dynamic system effects must be considered. These include air-flow velocity conversions at the fan entrance and exit as well as ductwork sound attenuation and vibration isolation pressure drops. [Hunt et al., 1990]
The fan tower configuration operational cost is higher because of system pressure drops associated with the longer duct distribution system. This configuration is used when separation is required between rotating equipment and the area directly over the cleanroom. Vane-axial fans or centrifugal fans can be used in a fan tower configuration. [Hunt et al., 1990]
Small, local fan filter units (FFUs) for cleanroom recirculation should be evaluated in contrast to the use of larger, more efficient fans, mentioned above. When localized FFUs are used to reduce a system's static pressure, there is a tradeoff between decreasing efficiency and decreasing static pressure. [Naughton, "HVAC Systems… Part 1," 1990]
Recirculation and make-up air combined
Recirculation system acoustics and vibration
