Static pressure is a crucial parameter in the performance and selection of centrifugal fans. It represents the pressure difference between the fan inlet and outlet, excluding the influence of velocity pressure. In other words, static pressure is the force per unit area that a fan can generate to overcome the resistance to airflow in a system.
What Is Static Pressure
Static pressure is the pressure exerted by a fluid at rest, or in the case of a centrifugal fan, the pressure difference between the fan inlet and outlet when the velocity pressure is zero. It is a measure of the fan’s ability to overcome resistance to airflow in a system, such as ductwork, filters, dampers, and other components.
Static pressure is typically expressed in units of inches of water gauge (in. WG) or Pascal (Pa). A higher static pressure indicates that the fan can overcome greater resistance to airflow, while a lower static pressure suggests that the fan is suitable for systems with less resistance.
Pressure-Airflow Relationship
The relationship between static pressure and airflow in a centrifugal fan is typically represented by a fan curve. A fan curve is a graphical representation of the fan’s performance, plotting static pressure on the vertical axis and airflow on the horizontal axis.
As the airflow increases, the static pressure generally decreases. This is because the fan must work harder to move a larger volume of air, and the increased velocity pressure results in a decrease in static pressure. Conversely, as the airflow decreases, the static pressure increases, as the fan can more easily overcome the resistance to airflow in the system.
Factors Affecting Static Pressure in Centrifugal Fans
Fan Speed
The rotational speed of the fan impeller directly affects the static pressure. As the fan speed increases, the static pressure generated by the fan also increases. This is because a higher fan speed imparts more energy to the air, allowing it to overcome greater resistance to airflow.
Blade Design and Geometry
The design and geometry of the fan blades significantly impact the static pressure generated by a centrifugal fan. The blade shape, angle, and number of blades all contribute to the fan’s performance.
Forward-curved blades are typically used in applications requiring high airflow and low static pressure, while backward-inclined and airfoil blades are used in applications demanding higher static pressures and improved energy efficiency.
The number of blades also affects static pressure. Generally, a fan with more blades will generate higher static pressure than a fan with fewer blades, as there is more surface area to interact with the air and impart energy.
Housing and Scroll Design
The housing and scroll design of a centrifugal fan also play a role in determining the static pressure. The housing is the enclosure that surrounds the impeller, while the scroll is the gradually expanding section that guides the air from the impeller to the fan outlet.
A well-designed housing and scroll can help to minimize air turbulence and losses, resulting in improved static pressure and overall fan efficiency. Factors such as the scroll shape, cutoff angle, and tongue position can all be optimized to enhance fan performance.
System Resistance
The resistance of the system in which the centrifugal fan is installed has a direct impact on the static pressure. As the system resistance increases, the fan must generate higher static pressure to maintain the desired airflow.
System resistance can be caused by various factors, such as ductwork length and configuration, filters, dampers, and heat exchangers.
In some cases, it may be necessary to modify the system design to reduce resistance, such as increasing duct sizes, minimizing bends, or selecting lower-pressure-drop components. By reducing system resistance, the fan can operate more efficiently, consuming less energy and potentially allowing for a smaller, less expensive fan to be used.
