Two Types of Cabinet-Type Centrifugal Smoke Exhaust Fans

Two Types of Cabinet-Type Centrifugal Smoke Exhaust FansTwo Types of Cabinet-Type Centrifugal Smoke Exhaust Fans: (1) Forward-curved cabinet-type centrifugal smoke exhaust fan. Cabinet-type centrifugal smoke exhaust fans with forward-curved blades operate at relatively low speeds and are typically suitable for applications with high airflow and low static pressure. The normal flow rate range is 30–80 (at full opening), with a static pressure efficiency of 60–68, typically displayed on the right side of the static pressure curve. Because the performance curve tends to rise, these fans are prone to overload. This design offers the advantages of low cost, low speed, a slender shaft, small bearings, and a wide operating range. The disadvantage is that the shape of the performance curve may run parallel to the resistance curve of the ductwork, and a drop in system static pressure may cause the motor to overload. Additionally, the blades have low structural strength and cannot operate at high speeds. (2) Backward-curved cabinet-type centrifugal smoke exhaust fan. The operating speed of a cabinet-type centrifugal smoke exhaust fan is approximately twice that of a forward-curved blade fan. The effective operating range for typical applications is 40–85, with a static pressure efficiency of approximately 80. Generally speaking, under specific operating conditions, larger cabinet-type centrifugal smoke exhaust fans are more efficient. Cabinet-type centrifugal smoke exhaust fans with backward-curved blades exhibit greater variability than those with forward-curved blades. The advantage of cabinet-type centrifugal smoke exhaust fans with backward-curved blades is their high efficiency, which does not impose excessive loads on the output curve. Their power curve typically reaches a specific value at the center of the operating range, so they generally do not overload. The blades and impellers are structurally robust, allowing them to be used in systems with higher static pressure. The disadvantage of cabinet-type centrifugal smoke exhaust fans with backward-curved blades is that, due to the high-speed rotation of the impeller, the shaft must be thicker, the bearings must be larger, and the fan must be carefully balanced. Furthermore, operating conditions are prone to fluctuate due to variations in static pressure. An improvement for backward-curved blade fans involves replacing the blades with constant-section blades. With this modification, the fan’s static pressure efficiency increases to approximately 86, and noise levels are correspondingly reduced. Cabinet-type centrifugal smoke exhaust fans with properly designed airfoil blades may achieve a noise ratio comparable to—or even lower than—that of cabinet-type centrifugal smoke exhaust fans with forward-curved blades. The advantages of cabinet-type centrifugal smoke exhaust fans are increasingly recognized by users, particularly manufacturers of HVAC terminal equipment, and they are gaining popularity in applications where low noise levels and high performance are required.