In modern HVAC systems, air handling units (AHUs) are responsible for supplying, filtering, heating, cooling, and circulating air throughout the buildings. AHUs are the core of the building, and the performance of AHUs depends heavily on the fan. The system needs fans that keep air moving through filters, coils, dampers, and the connected duct system. Each of these components adds resistance to the airflow path; the fan must provide not only the required airflow volume but also enough pressure. Due to the higher static pressure and higher airflow volume characteristics, the centrifugal fan is particularly suitable for such applications.
Selecting the right AHU centrifugal fan requires several performance parameters, including airflow, static pressure, fan efficiency, speed, noise, and operating conditions. This blog explains the main specifications of an AHU centrifugal fan and shows how these performance parameters affect the actual operating point of an air handling system.
What is an AHU Centrifugal Fan?
An AHU is a component of an HVAC system that conditions and distributes air. The centrifugal fan provides the needed airflow to keep the process moving. Its working principle is drawing the air in the direction parallel to the shaft and accelerating the air outward by roughly 90 degrees. The centrifugal fan’s scroll-shaped housing then helps convert part of the velocity into static pressure.
Compared with other types of fans, centrifugal fans are generally better suited to AHU applications. This is because the AHU fan must overcome pressure losses from multiple components that add resistance to the airflow.
AHU Centrifugal Fan Types
According to the impeller design, AHU centrifugal fans can be classified into several types:
- Forward Curved Centrifugal Fan
- Backward-Curved Centrifugal Fan
- Radial Centrifugal Fan
- Airfoil Centrifugal Fan
AHU centrifugal fans are available in several configurations, including housed centrifugal fans, plug fans, plenum fans, and fan arrays. The appropriate design depends on the required airflow, static pressure, efficiency, installation space, noise limit, and control method.
Key Performance Parameters for an AHU Centrifugal Fan
Note: To make key performance parameters easier to understand, the following section uses the LONGWELL LWBE3G450-102NS-19 centrifugal fan as a practical example.
|
LWBE3G450-102NS-19 Parameter |
Specification |
|---|---|
|
Voltage |
230 V |
|
Input Power |
700 W |
|
Current |
3.1 A |
|
Maximum Air Volume |
7,468 m³/h |
|
Maximum Static Pressure |
1,030 Pa |
|
Fan Speed |
1,450 RPM |
|
Noise |
70 dB(A) |
|
Protection Class |
IP55 |
|
Insulation Class |
F |
|
Operating Temperature |
-25°C to +60°C |
|
Control |
0–10 VDC / PWM |
Airflow
Airflow is the amount of air the fan moves in a set time, usually in one minutes or one hour. Engineers measure it in cubic feet per minute (CFM) or cubic meters per hour (m³/h). The space’s cooling load sets how much airflow is needed.
The rated airflow of a centrifugal fan should not be treated as a fixed value. As air passes through filters, coils, dampers, and ducts, these components create resistance.
The LONGWELL LWBE3G450-102NS-19 centrifugal fan has a maximum airflow of 7,468 m³/h at 1,450 RPM. According to its P-Q curve, this value occurs under a high-airflow, low-resistance condition. As system resistance increases, the required static pressure rises and the airflow the fan can actually deliver decreases.
Static Pressure
Static pressure measures the fan’s ability to overcome resistance in the airflow path and is commonly expressed in wg or Pa. As system resistance increases, the centrifugal fan has to produce more pressure to keep air moving. This is why static pressure cannot be considered separately from airflow. On a centrifugal fan P-Q curve, airflow generally has a negative correlation with static pressure.
The selected centrifugal fan has a maximum static pressure of 1,030 Pa at 1,450 RPM. This value occurs near the zero-airflow end of the P-Q curve. It should therefore be treated as the fan’s pressure limit rather than the pressure available at its maximum airflow of 7,468 m³/h.
Total Pressure
Total pressure is the sum of static pressure and velocity pressure:
Total Pressure = Static Pressure + Dynamic Pressure
Static pressure reflects the pressure available to overcome resistance in the system. Dynamic pressure is derived from the velocity of the moving air. Added together, represents the total energy carried by the airflow and becomes total pressure. As air velocity increases, dynamic pressure also increases, so total pressure can be noticeably different from static pressure at higher air speeds.
For AHU fan selection, it is important to distinguish between static and total pressure. Many fan datasheets and performance curves are given in static pressure versus airflow, while some specifications may use total pressure. Comparing these values directly without checking which pressure is being used can lead to incorrect fan selection.
Power and Motor Input
Power indicates the electrical energy required and is usually expressed in W or kW. Due to energy loss, the electrical input power is different from the useful power transferred to the airflow. Some centrifugal fan suppliers consider actual electrical input power as the power required by the fan system at a specific airflow and pressure duty point.
In an AHU, power consumption should be considered together with airflow and pressure. A centrifugal fan that has to overcome higher system resistance or operate at a higher speed generally requires more power. This matters because AHU fans often run for long periods, so the electrical input at the required operating point has a direct effect on system energy use. For variable-speed fans, actual power consumption can also change as the fan speed and operating point change.
Fan Speed
Fan speed is one of the main variables that influences centrifugal fan performance. It usually uses RPM (revolutions per minute) to describe how fast the impeller rotates.
A relatively small increase in fan speed can produce a much larger increase in pressure and power demand. For the same fan diameter and air density, the fan laws show that airflow changes roughly in direct proportion to RPM, static pressure changes with the square of RPM, and power changes with the cube of RPM.
The LWBE3G450-102NS-19 model has a rated fan speed of 1,450 RPM. The fan also supports 0-10 VDC and PWM speed control, allowing the AHU controller to adjust fan output according to operating demand. The 1,450 RPM value should be viewed together with the fan’s P-Q curve, since changing the speed also changes the available airflow and static pressure.
Curves in AHU Ventilation System
P-Q Curve
A P-Q curve shows the relationship between airflow (Q) and the pressure(P). The pressure here usually refers to static pressure. There are some manufacturers that use total pressure as the y-axis, so the pressure type should always be checked before comparing different models.
For centrifugal fans, the curve generally slopes downward. This means that the centrifugal fan does not deliver its maximum airflow and maximum pressure at the same time. The P-Q curve describes the centrifugal fan’s performance, but it does not determine the actual airflow once the fan is installed. In an AHU ventilation system, the P-Q curve is used to understand how the fan will perform across different airflow and pressure conditions.
System Resistance Curve
A system resistance curve shows how much pressure the AHU centrifugal fan must overcome at different airflow rates. The real operating condition depends on the resistance created by the AHU and duct system. This resistance comes from components such as filters, heating and cooling coils, dampers, silencers, duct friction, or elbows. A core relationship here is: System pressure loss ∝ airflow².
The system curve can also shift during operation. The additional duct resistance increases the system resistance and moves the curve upward. Poor inlet or outlet conditions of a centrifugal fan can further increase the pressure required from AHU centrifugal fans.
Recommended Operating Range
The operating point should fall within a stable and efficient part of the centrifugal fan curve rather than near either extreme. Although an AHU centrifugal fan may technically operate across a wide range of airflow and pressure, performance can deteriorate when the duty point moves too far away from its recommended region.
On the low-airflow side of the curve, operating too close to shutoff can lead to unstable airflow, increased vibration, higher noise, and possible aerodynamic stall. At the opposite end, very high airflow may reduce efficiency and, depending on the centrifugal fan design, increase motor load.
The best efficiency point (BEP) provides a useful reference when selecting the operating range. A fan does not need to run exactly at the BEP, but the intended AHU operating point should normally remain reasonably close to the efficient and stable region around it. In practice, the actual system resistance can be higher than the calculated value. These additional losses can result in a corrected system curve with greater resistance than the calculated curve.
How to Select the Right LONGWELL AHU Centrifugal Fan?
Selecting an AHU centrifugal fan starts with the actual duty point. Confirm the required airflow and static pressure first, including pressure losses from filters, coils, dampers, and connected ductwork. Noise limits and expected changes in resistance, such as dirty-filter conditions, should also be considered so that the operating point remains within the recommended range of the fan curve.
For preliminary matching, engineers can also use the calculator to narrow down suitable models before confirming the final selection with technical support. LONGWELL provides direct-driven centrifugal fans, belt-driven DIDW forward-curved fans, and belt-driven DIDW backward-curved fans, etc. Control and system integration are another part of the selection. For variable-air-volume AHUs, EC fans with adjustable speed can follow changing airflow demand.
LONGWELL also supports customized AHU fan projects when standard models do not fully match the requirements. Its engineering support can include CFD simulation, 3D structural design, BMS integration, prototype validation, and a structured 90-day NPI process from specification lock to production release.
FAQ
How to calculate static pressure for AHU?
AHU static pressure is calculated by adding the pressure losses from all components in the airflow path. Engineers should also consider the expected dirty-filter or worst-case condition.
How do EC fans save energy in AHU applications?
EC fans combine a high-efficiency electronically commutated motor with integrated speed control. This allows the fan to operate at the required speed, avoiding energy waste at partial load.
Can I replace a belt-driven AHU centrifugal fan with a direct-drive unit?
Yes. Belt-driven AHU fans can often be retrofitted with direct-drive centrifugal or EC plug fans. The replacement should be selected from the current duty point but not simply matching the old fan size.
Choose the right centrifugal fan
Longwell manufactures 956 centrifugal fans in-house. Send airflow, static pressure and voltage — an engineer replies with a matching model, performance curve and price within one working day.
