Roof-Mounted Centrifugal Fan

These fans are widely used in office and commercial buildings, oil and gas industries, hospitals, industrial factories, and similar settings. Compared to utility centrifugal fans, this type offers lower efficiency at high flow and pressure rates and is typically chosen solely due to its lower cost and the lack of sufficient space for installing a utility centrifugal fan. Roof-mounted centrifugal fans are only applicable as exhaust fans. They can also be used in explosive environments, high-temperature and high-humidity areas, and highly corrosive conditions, provided that special structural and technical modifications are applied. The impeller used in this fan type is backward-curved. The power transmission system in roof-mounted centrifugal fans is available in two configurations: direct drive and belt-driven (pulley and belt).

Specifications of Roof-Mounted Centrifugal Fans

Impeller Type: Backward-curved (inclined blades).

Housing and Base Material: Made of hot-dip galvanized steel sheet, with all connections using galvanized or appropriately coated bolts and nuts—welding is not permitted unless structural reinforcement necessitates it, in which case welding with proper preparation and electrostatic paint coating is acceptable. The fan must also feature a conical or suitably shaped inlet bell that extends a few millimeters into the impeller to facilitate smooth airflow and improve efficiency.

Impeller Material: The backward-curved impeller must be made of aluminum, with riveted connections (no welding).

Rain Hood: Made of aluminum, round for direct-drive and belt-driven models, and rectangular for belt-driven models in sizes 45 to 71. The hood must be larger than the main fan body and fully cover it to prevent rain or snow from entering the duct.=

Shaft: Must be made of stainless steel 304 to resist rust and corrosion.

Bearings: Must be genuine Japanese (FYH, YASAHI, or equivalent imported brands) and equipped with grease hoses.

Belts: Type A (up to size 45) and Type B (size 50 and above), manufactured in Korea or Germany.

Pulleys: Made of cast aluminum; for sizes 50 to 71, cast iron is also acceptable.

Mounting for Belt-Driven Models: Must include an adjustable motor base for easy belt replacement, adjustment, and motor alignment during maintenance.

Outlet Guard: The discharge opening must be fitted with a metal guard (electrostatically painted or epoxy-coated) to prevent bird entry.

Motor: Must be MotorGen or ElectroGen brand, 380V, IP54, Class F insulation (220V is not recommended due to higher noise, vibration, and energy consumption).

Balancing: Impellers must be statically and dynamically balanced using a portable balancer in accordance with ISO-1940, with a certificate issued.

Design Speed and Flow Rate: The impeller speed and design must ensure that at the required static pressure, the actual airflow (without damper adjustment) does not exceed ±5% tolerance.

Nameplate: All fans must bear a nameplate with technical specifications and serial number.

Warranty and After-Sales: The manufacturer must guarantee all moving parts (impeller, shaft, pulley) for 24 solar months from the date of manufacture or 30 solar months from delivery, and provide at least 15 years of after-sales service. Belts must be warranted for 2,000 operating hours and bearings for 6,000 hours. The manufacturer must also supply installation instructions, maintenance schedules, and dimensional drawings.

Adjustable Damper: All roof-mounted exhaust fans must include an adjustable damper for airflow control.

Factory Testing: After manufacturing, the site must provide conditions for technical tests, including airflow measurement (using a duct with pressure drop per AMCA 210–ASHRAE 51), vibration and dynamic balancing (per ISO-1940 G2.5 in fully assembled condition), noise level (dB), current draw, and other parameters.

Size Limitation: If the required airflow and pressure exceed a 63 cm impeller diameter, it is recommended to use a utility backward-curved centrifugal fan instead due to its higher efficiency.

Motor Power and Speed Limits: The minimum motor power and maximum fan speed must be determined per the table below, ensuring that blade tip speed does not exceed 3300 fpm.

Important Note:

Whenever possible, for impeller diameters of 400 mm and above—depending on the application and local climate—it is recommended to use backward-curved utility centrifugal fans. These offer better efficiency and prevent outside airflow from entering the fan, which can cause unpleasant noise and reduce performance.

If deemed appropriate by the consultant, and considering that 900 RPM motors are readily available, direct-drive fans up to 355 mm impeller size may be designed and proposed accordingly by the manufacturer.