In many dryer applications, the static pressure requirement is between 0.5 and 3" of water column. For these pressure requirements, a duct vaneaxial fan often is the best selection. The duct vaneaxial fan offers a broad performance range and good pressure capability at high static efficiency. It also offers the benefit of placing the bearings, shaft and drives on the negative pressure side of the propeller. This setup draws ambient air across those components during operation and discharges it into the hot moist air. This fan also can be provided with a shaft seal to further isolate the components or high temperature construction for operation up to 500 deg. F (260 deg C).

For higher pressure ranges, a backward curved centrifugal fan often is selected. This design offers pressures up to 20" water gauge. They also offer the benefit of having the shaft and bearings outside the fan, and they can be mounted on the roof with a weather cover.

Divide density and moisture values by altitude correction factors to determine actual density and moisture.

Fan performance tables are based on standard air. Selecting a fan using standard tables will provide a fan capable of moving the required flow and developing the required pressure. As the temperature increases and humidity decreases, system resistance and the pressure developed by the fan will decrease at the same rate, and a constant flow rate is maintained. Altitude reduces the air pressure and density along with the amount of moisture the air can carry. The density and moisture values in table 1 must be divided by the correction factors in table 4 to determine the actual density and mois-ture content per cubic foot.

The fan efficiency is defined as the ratio of air power (output) to shaft power requirement (input). An optimally designed fan impeller with improved aerodynamics requires less shaft input power for desired delivery of air volume with total pressure rise as per the system requirement. With reduced shaft power requirement, there is a commensurate reduction in energy consumption.

Thus the fan efficiency is directly contingent upon selection of fan and the operating point relevant to the system resistance.

As with all fan systems, it is important to remember that only available air can be exhausted. Fans cannot operate in a vacuum. If the exhausted air cannot be replaced in the facility, fan performance will drop off and production will suffer. The production facility must have makeup air for every cubic foot that is exhausted from the facility. The makeup air can be provided through general ventilation systems or as part of a heating, ventilating and air conditioning system.

With the exhaust requirements calculated and fan system sized to handle the required volume at the required pressure, the drying process will run smoothly. Production results are consistent and the production facility's environment also is improved by removing contaminated air. A good understanding of exhaust requirements allows better control of the drying process.