Axial fans are used for relatively high flow rates and low pressures with flow parallel to the axis of fan.
Several intrinsically different fan types are on the market, and each offers different performance characteristics. Consequently, if an engineer wants to achieve specific air movement patterns or has certain control requirements, it is vital for the system designer to understand the different fan types and control options that are available.
Knowing whether your application requires high volumes of airflow with low pressure, or high pressure airflow with low volumes is just one of the questions to be answered in order to determine the correct fan shape selection. This article will take a look at what a designer needs to know to make an informed decision.
Centrifugal fans offer low flow rates and high pressures with flow perpendicular to blower axis. Typical applications include air-handling units, process heating and cooling, electronic cooling and boiler combustion air.
The most common fan types are axial flow, tangential, centrifugal and mixed flow.
Axial Flow. This type has various blade shapes such as aerofoil, sickle, paddle and variable pitch. Axial fans are used for relatively high flow rates and low pressures with flow parallel to the axis of fan. They also have low power input.
Axial flow fans generally are selected for simple extraction or cooling applications with very low system resistance. Examples include moving air from one large space to another (e.g., from factory to outside), an everyday desk fan and condenser cooling in refrigeration.
Tangential. These barrel-shaped fans have curved forward blades that are generally many times longer than any given diameter. Tangential fans tend to scoop the air up and deliver it at relatively low pressure over a long length. Ideal for cabinet cooling or process air coverage, tangential fans have relatively low power required.
The main advantage of tangential fans over axial fans is that they provide extremely smooth, laminar airflow over the whole frame width. This makes them suitable for applications such as air conditioning systems, air curtains, computer systems, instrument racks, convector fans, ovens, dehumidifiers and ventilators.
Centrifugal. This type of fan typically has an impeller blade construction such as paddle, radial, backward curved and forward curved. Characteristics are low flow rates and high pressures with flow perpendicular to blower axis. Air enters around center of the fan and exits around the outside.
Centrifugal fans with paddle impellers are used when debris will pass through the system because this configuration prevents any clogging.
Fans with backward curved blades produce less air volume than axial units, but they generate considerably more pressure and are the least power-hungry in the centrifugal range. They also can be produced as multi-stage units to give even higher pressures. By comparison, centrifugal fans with forward curved blades develop the best airflow and pressure requirement in size-by-size comparison, although they will require extra motor power.
Typical applications for centrifugal fans include air-handling units, process heating and cooling, electronic cooling and boiler combustion air.
Mixed Flow Fans. These designs combine the features of axial and centrifugal fans by employing the air movement techniques of both. Air enters the inlet side of the fans and exhausts in an axial route from the outlet side. The pressure development of the mixed flow is greater that an axial fan and is more akin to the centrifugal fan design.
The most common fan types are axial flow, tangential, centrifugal and mixed flow. This diagram illustrates the differences in airflow among the different types.
Designers must also take into consideration application-specific factors. These include:
- The density of the air to be moved.
- The air temperature.
- The pressure required to achieve flow across a filter or pressure drop due to ducting.
All ducting will have an effect on the air passing through a system, and this will nearly always take the form of a restriction to the flow. Therefore, all ducting will have an influence on pressure requirements. At the earliest stage in the design process, the ducting should be evaluated carefully and made as efficient as possible. This will reflect beneficially on the final design in the overall cost, the compactness of the fan and the running costs.
In reality, there are many factors that impact upon fan specification. Applications that demand particular air movement characteristics for cooling, exhausting, aerating, ventilating and drying should undergo a “system resistance” assessment. Only by evaluating all the impacting factors - flow resistance, power units, size and space envelope - is it possible to ensure maximum efficiency and reliability of both the fan and the equipment involved.
Speed Controllers. Systems also can benefit from recent advances in variable-speed drive technology. These range from simple step-by-step to infinitely variable speed control coupled with feedback facilities offering constant flow against variable resistance conditions.
Single-phase supply can offer speed control, from simple manual thyristor phase control or automatic from a 0 to 10 V DC supply that could also incorporate sensors. Main single-phase input also will power units up to 550 W, which are brushless DC electronically commutated and can be infinitely controlled with a 0 to 10 V DC supply or PWM signal through a special control module that can incorporate a pressure sensor to give constant volume conditions.
Three-phase supply availability will offer control capabilities ranging from simple step-by-step transformer speed control through to sophisticated systems based upon inverter drive units that will offer full control parameter integration.
To help ensure that all aspects of fan sizing have been considered, the following checklist may be useful.
Volume Flow Rate
- Velocity required.
- Does this take into account the worst case open area?
Density of the air being moved (temperature or different gas).
- Pressure required to generate desired flow through open area.
- Pressure drop across filter.
Pressure drop due to ducting.
Other Parameters to Consider
- Any special hazards (flammable gas, corrosive agents, etc.).
- Noise level restrictions.
- Inlet and discharge configuration.
- Electrical supply. Is speed control required?
- Physical size constraints.
- Even flow across aperture. (If it is a large area then poor distribution could occur.)
- Dwell time. Is the velocity across the filter giving desired dwell times?
When selecting a fan, it is always worth talking to an industrial fan manufacturer who will advise you correctly. They might have encountered your problem before and can help you solve it. Fan manufacturers also can recommend another type of fan for your application and suggest possible suppliers, or work with you to reach a solution to solve your air movement problem. Specialist fan manufacturers can offer advice and, where necessary and when economies of scale permit, develop fans and blowers tailored to suit a unique application.