Direct-fired units utilize air that will be sent to the heated space for combustion without use of a heat exchanger. The products of combustion are mixed directly with large volumes of outdoor air. Such mixing is considered safe because of the high dilution ratio. Buffalo Blower Co. makes various types of commercial gas fired air makeup units for industrial building ventilation. In many dryer applications, the static pressure requirement is between 0.5 and 3" 0f 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). Divide density and moisture values by altitude correction factors to determine actual density and moisture.