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In fan selection, airflow rate and static pressure values are the two most critical parameters for system performance and energy efficiency. These values determine how much air the fan can move in a given environment and the resistance it can overcome while doing so. Incorrectly specified airflow and pressure values can lead to insufficient ventilation, excessive energy consumption, and system failures. Therefore, fan selection should be based not only on physical dimensions but also on engineering calculations and technical data.
Flow rate refers to the amount of fluid passing through a specific cross-section within a unit of time. It is typically 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 the efficient operation of a system and guarantees the desired flow. Particularly in HVAC, irrigation, and process control systems, flow measurement is crucial for both performance and energy efficiency.
Determining the correct flow rate values in fan selection is critically important for ensuring efficient system operation and providing the necessary air circulation for the environment.
Airflow Based on Room Volume
Calculating airflow based on room volume is the most basic method to determine the amount of air a fan should supply. This calculation is done 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 would be 650 m³/h. This value serves as the basis for meeting the general ventilation needs of the space.
This method is particularly used for areas with fixed volumes and is refined by adjusting the air change rates according to the purpose of the room. Different spaces such as offices, restaurants, and laboratories have varying air change rates. Therefore, proper room classification should be done, and the correct coefficient should be selected to accurately 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 generally accepted as: 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 in the space to determine the total air requirement.
This approach yields more accurate results for spaces such as offices, classrooms, gyms, and meeting areas. Even if the room volume is sufficient, a high number of occupants may create a need for additional ventilation. Therefore, calculating airflow per person should not be overlooked for comfort and air quality.
Airflow from Processes and Equipment
In industrial facilities, machines, furnaces, laboratory equipment, and other devices release additional heat, gas, or vapor into the environment. As a result, airflow requirements based on equipment should also be considered in addition to room volume and occupancy. Manufacturer specifications provide recommended airflow values per device, which are used to calculate the total requirement.
This calculation is crucial for high-emission equipment such as paint booths, welding machines, or chemical processes. In these cases, air change rates can rise to values as high as 25-50. Thus, process-based airflow calculation is essential for ensuring safe and efficient system operation.
Duct Design and Air Velocity
The air velocity inside ducts directly impacts the airflow delivered by the fan. Higher air velocity increases airflow but also raises noise levels and friction losses. In comfort zones, air velocities of 2-5 m/s are preferred, while industrial spaces often use 5-10 m/s.
Duct diameter and length also play a significant role in this calculation. Narrower and longer ducts create more resistance, requiring the fan to operate at higher pressures. Therefore, both air velocity and airflow requirements must be evaluated together during duct design.
Balancing Exhaust and Fresh Air
The amount of exhaust air must be balanced with an equal amount of fresh air supply. In kitchens, laboratories, and chemical processing areas, exhaust fan airflow should be matched with fresh air fans to avoid creating negative pressure in the space. This balance is critical to maintaining healthy air circulation.
Unbalanced systems may result in insufficient air supply, causing vacuum effects that make doors harder to open, odors to accumulate, or equipment to operate inefficiently. For this reason, exhaust and fresh air flow rates should be planned together, and fan selection should be based on this balance.
Fan pressure value refers to a fan's ability to overcome resistance while maintaining airflow. This value is typically measured as static pressure (Pa or mmWC) and is necessary for the fan to compensate for pressure losses across system components such as air ducts, filters, and grilles. Static pressure varies depending on factors like friction and constrictions within the system the fan directs airflow through.
Fan pressure is categorized as total pressure and static pressure, based on system design. Total pressure represents the entire pressure difference between the fan's inlet and outlet, while static pressure refers only to the resistance against airflow. In fan selection, these values are considered alongside 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 for ensuring that the system can deliver airflow to the desired points and maintain energy efficiency.
Static Pressure (Pa or mmWC)
Static pressure is the fan's ability to overcome resistance against airflow. This resistance comes from system components such as air ducts, filters, grilles, heat exchangers, and elbows. It is typically expressed in Pascals (Pa) or millimeters of water column (mmWC). Axial fans are suitable for low static pressure applications, while centrifugal (radial) fans are preferred for systems requiring higher pressure.
Insufficient static pressure can prevent the fan from delivering airflow to the intended location, which is common in long duct systems or filter-based applications. Excessive static pressure, on the other hand, can strain the fan motor, increase energy consumption, and shorten the motor's lifespan. Therefore, total resistance losses of the system should 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 airflow. It is especially 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 of airflow velocity. This value indicates how quickly the fan can deliver airflow.
High velocity pressure allows air to be delivered over longer distances but may also increase noise levels and friction losses. For this reason, low velocity pressure is preferred in comfort zones, while higher values may be required in industrial exhaust systems. Fan selection should consider both velocity pressure and static pressure together to determine total pressure requirements.
If airflow and static pressure values are not properly considered during fan selection, system performance can significantly decrease, leading to both technical and economic issues. Insufficient airflow prevents adequate air circulation in the environment, resulting in temperature rise, moisture buildup, poor air quality, and reduced occupant comfort. In production facilities, this can negatively impact product quality and workplace safety.
Similarly, an incorrectly specified pressure value may prevent the fan from delivering airflow to its intended destination. In systems with long duct runs, filters, or grilles that create resistance, a low-pressure fan cannot overcome these obstacles, causing the system to operate inefficiently. Conversely, excessively high pressure can strain the fan motor, increase energy consumption, raise noise levels, and shorten equipment lifespan. Ultimately, incorrect airflow and pressure values bring risks such as high maintenance costs, system failures, and regulatory non-compliance.
Airflow and pressure are the two fundamental parameters of a ventilation system and are directly related to each other. While airflow represents the amount of air moved per unit of time, pressure indicates the force required to overcome resistance in the system. Generally, low-pressure systems can deliver high airflow, whereas high-pressure systems may limit airflow. This relationship varies depending on system design, duct structure, and fan type. For example, axial fans are ideal for high airflow but low-pressure applications, whereas centrifugal fans are preferred for higher-pressure scenarios.
If balance between these two values is not achieved, the system operates inefficiently. Insufficient pressure prevents air from reaching its intended point, while insufficient airflow results in inadequate air exchange. Applying excessive pressure also increases energy consumption and can shorten equipment lifespan. Therefore, both airflow and pressure values should be evaluated together in fan selection to ensure a solution that meets the specific needs of the environment. This relationship becomes even more critical in systems with long duct runs or filtration. For seamless fan selection, contact ERF Group today.
