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When selecting a fan, airflow (air volume) and static pressure are the two most critical parameters in terms of system performance and energy efficiency. These values determine how much air the fan can move in a given environment and against what resistances it can push this air. Incorrectly specified airflow and pressure values may lead to insufficient ventilation, excessive energy consumption, and system failures. Therefore, fan selection should be based not only on physical dimensions but also on technical data derived from engineering calculations.
Flow rate refers to the amount of fluid passing through a given cross-section per unit of time. It is usually measured in units such as m³/h (cubic meters per hour), L/min (liters per minute), or gpm (gallons per minute). Flow rate is a critical parameter in many engineering applications, from pump systems to ventilation ducts. The correct flow rate ensures efficient system operation and guarantees the desired flow. Particularly in HVAC, irrigation, and process control systems, flow rate measurement plays a key role in performance and energy efficiency.
Selecting the correct airflow rate in fan selection is critically important for ensuring efficient system operation and providing the required air circulation in the environment.
Airflow Based on Room Volume
Calculating airflow based on room volume is the most fundamental method of determining the amount of air a fan should deliver. This calculation is made by multiplying the room volume by the air change rate. For example, if an office has a volume of 130 m³ and the air change rate is set to 5, the required airflow is 650 m³/h. This value serves as the basis for meeting the general ventilation needs of the space.
This method is particularly used in constant-volume spaces and can be refined using different air change rates depending on the function of the room. Offices, restaurants, and laboratories all require different coefficients. Therefore, proper room classification must be made, and the appropriate coefficient selected to calculate the required airflow.
Air Requirement per Person
In fan selection, the air requirement per person is a critical parameter, especially in areas with high occupancy. Depending on activity levels, these values are typically set at 45 m³/h for light activity, 29 m³/h for normal activity, and 60 m³/h for heavy activity. These values are multiplied by the number of people present to calculate the total air requirement.
This approach provides more accurate results in offices, classrooms, gyms, and meeting rooms. Even if the room volume is sufficient, high occupancy may create additional ventilation needs. Therefore, person-based airflow calculations should not be neglected to ensure comfort and air quality.
Process and Equipment-Based Airflow
In industrial facilities, equipment such as machines, furnaces, and laboratory devices generate additional heat, gases, or vapors. For this reason, airflow requirements should not only be based on room volume and occupancy but also on equipment load. According to manufacturer data, the recommended airflow per device should be added to the total requirement.
This is especially vital for equipment such as paint booths, welding machines, or chemical processes, which produce high emissions. In such cases, the air change rate can be as high as 25-50. Therefore, process-based airflow calculation is a critical step to ensure safe and efficient system operation.
Duct Design and Air Velocity
Air velocity inside the ducts directly affects the airflow that the fan must deliver. Higher air velocity provides more airflow but also increases noise levels and friction losses. For comfort spaces, an air velocity of 2-5 m/s is typically preferred, while industrial areas use 5-10 m/s.
Duct diameter and length also play an important role. Narrow and long ducts create more resistance, requiring the fan to operate at higher pressure. Therefore, duct design should always consider both airflow and velocity requirements.
Balancing Exhaust and Fresh Air
The amount of exhaust air must be balanced with fresh air intake. This is especially important in kitchens, laboratories, and chemical processes. Exhaust fan capacity must be matched with supply fans to prevent negative pressure in the space. This balance is essential for maintaining healthy air circulation.
In unbalanced systems, inadequate fresh air intake may occur, leading to issues such as difficulty opening doors due to vacuum effect, odor accumulation, or inefficient operation of equipment. For this reason, both exhaust and supply airflows should be planned together, and fan selection should be made accordingly.
Fan pressure value refers to a fan's capacity to overcome the resistances encountered while providing airflow. This value is usually measured in terms of static pressure (Pa or mmWG) and is required for the fan to compensate for pressure losses across system components such as ducts, filters, and grilles. Static pressure varies depending on factors such as friction and constrictions within the system where the fan directs the airflow.
Fan pressure is classified into total pressure and static pressure depending on the system design. Total pressure covers the overall pressure difference between the inlet and outlet of the fan, while static pressure refers only to the resistances opposing airflow. In fan selection, these values are evaluated together with parameters such as duct length, filter type, and air velocity to determine the correct fan type.
Determining the correct static pressure values in fan selection is critically important to ensure that the system can deliver airflow to the desired point and maintain energy efficiency.
Static Pressure (Pa or mmWG)
Static pressure is the fan's capacity to overcome resistances against airflow. These resistances are caused by system components such as ducts, filters, grilles, heat exchangers, and bends. It is generally expressed in Pascals (Pa) or millimeters of water gauge (mmWG). Axial fans are suitable for low static pressure applications, while radial fans are preferred in systems requiring high pressure.
Insufficient static pressure prevents the fan from delivering airflow to the target point. This problem is common in long duct systems or applications with filters. Excessive pressure, on the other hand, can strain the fan motor, increase energy consumption, and shorten its lifespan. Therefore, the total resistance losses of the system must be accurately calculated, and the fan should be selected accordingly.
Velocity Pressure (Dynamic Pressure)
Velocity pressure refers to the kinetic energy generated by the speed of the airflow. It is particularly important in exhaust systems or applications where high air velocity is required. Velocity pressure is measured at the fan outlet or duct opening and is proportional to the square root of air velocity. This value indicates how fast the fan can deliver airflow.
High velocity pressure allows airflow to travel longer distances, but it may also increase noise levels and friction losses. For this reason, lower velocity pressure is preferred in comfort areas, whereas higher values may be necessary in industrial exhaust systems. In fan selection, velocity pressure should be evaluated together with static pressure to determine the total pressure requirement.
If airflow and static pressure values are not properly considered in fan selection, system performance decreases significantly, leading to both technical and economic problems. Insufficient airflow results in inadequate air circulation, which can cause temperature rise, moisture accumulation, poor air quality, and reduced occupant comfort. In industrial facilities, this situation may negatively affect product quality and occupational safety.
Similarly, an incorrectly specified pressure value prevents the fan from delivering airflow to the target point. In systems with resistance components such as long ducts, filters, or grilles, a low-pressure fan cannot overcome these obstacles and the system operates inefficiently. On the other hand, excessively high pressure puts strain on the fan motor, increases energy consumption, raises noise levels, and shortens equipment lifespan.
As a result, incorrect airflow and pressure values can lead to higher maintenance costs, system failures, and risks such as regulatory non-compliance.
Airflow and pressure are two fundamental parameters of a ventilation system and are directly related to each other. While airflow represents the volume of air transported per unit of time, pressure refers to the force required for this air to overcome resistances within the system. In general, high airflow can be achieved in low-pressure systems, whereas in high-pressure systems, airflow may be limited. This relationship varies depending on system design, duct structure, and the type of fan used. For example, axial fans are ideal for high airflow but low-pressure applications, while radial fans are preferred for higher-pressure applications.
If a balance between these two values is not established, the system operates inefficiently. Insufficient pressure prevents airflow from reaching the target point, while insufficient airflow results in inadequate air exchange in the environment. Moreover, applying excessive pressure increases energy consumption and shortens equipment lifespan. For this reason, both airflow and pressure values must be evaluated together in fan selection to determine a solution suited to the environment's needs. This relationship becomes even more critical in long duct systems or applications with filters.
If you want a smooth process in fan selection, you can contact ERF Group right away.
