What makes your industrial centrifugal fan roar to play its role? It's the motor that connects to your fan in one of two ways: Direct Driven Fan or Belt Driven. In the former, the motor is attached to the fan wheel with nothing in between. In the latter, a belt (sometimes more than one) connects the motor to move the fan.
Let's take a look at the differences to give you a standard baseline for choosing a drive in your application. Deciding between a Direct Driven Fan and a Belt Driven Fan is fairly simple once you understand these things.
Belt Driven Fan
In a Belt Driven configuration, the motor exists independently of the fan blades, and at least one belt (and sometimes more) connects the motor to the fan's moving parts.
Direct Driven Fan
In a Direct Driven Fan configuration, the fan motor that controls the movement of the fan blades is connected to the shaft or fan shaft. Therefore, the fan blades will spin at the same speed as the motor spins.
Advantages of Belt Driven Fan:
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More flexible
Belt Driven fans offer greater flexibility in RPM speed. Generally speaking, this type of fan is less expensive than a similarly sized Direct Driven Fan.
By definition, the impeller of a Direct Driven fan is attached to the motor shaft. Before replacing the motor, the impeller must be removed from the motor shaft. This requires disassembly of the fan, which means airflow contact.
There are inherent critical access and timing issues with replacing the motor on a Direct Driven Fan. Some potential implications and questions are: Is there enough room for disassembly? Where will the disassembly take place? Will the demolition take place on the roof or inside the building? Does the fan have to be removed from the system/building?
Less flexibility compared to Belt Driven fans. It can also be more expensive when comparing similar Direct Driven Fans and Belt Driven fans side by side. Direct Driven Fan units are also louder than Belt Driven units.
Belt Driven Fan fan motor replacement is usually easier. Of course, you must have access to the fans. Then you have to remove the guard, release the belt tension, remove the motor and belt, and check tension and alignment. However, unlike Direct Driven Fan fans, you don't have to remove the fan from the system, there is no airflow contact, and you don't need to disassemble the fan.
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Speed change
Because of the ease of configuring the Belt Driven components (pulleys and belts), Belt Driven fans offer a wide range of flexibility over the fan speed range when operating with a fixed speed motor Using the Belt Driven Fan and various pulley sizes or diameters allows you to vary the fan speed . This is especially useful in material handling applications where initial estimates may be incorrect or velocity specifications may change over time.
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Reduce wear
Sometimes you don't want the motor to run as close to the fan as it needs Direct Driven Fan. Instead, you'll probably want a belt buffer between the motor and fan to absorb some of the wear that the fan can't avoid but the motor can. However, there is more friction between moving parts, which can lead to more maintenance and higher energy costs.
Advantages of Direct Driven Fan
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High efficiency
Higher efficiency is the biggest advantage of Direct Driven fan configuration.
In a Direct Driven fan, as the name suggests, the impeller is attached directly to the motor shaft. There are no power transmission losses - the power generated by the electric motor is transferred directly to the impeller, where it enters the airflow and pressure develops.
On a Belt Driven Fan there are power transfer losses associated with the drive from the motor to the fan shaft. When properly selected and installed, Belt Driven Fan losses can vary from 20% for small motor driven fans to 3% to 4% for larger motor driven fans.
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Lower maintenance
Direct Driven fans require fewer parts and therefore generally require less maintenance. You have a fan, a motor, and maybe a coupling. Compared to Belt Driven fans which require belts, pulleys and bushings, Direct Driven Fans are simpler and maintenance personnel prefer Direct Driven Fans for routine maintenance.
Direct Driven Fans with re-greasable motor bearings must be lubricated according to the fan manufacturer's recommendations for grease type and frequency. Belt Driven Fans have two motor bearings (similar to Direct Driven Fans) and two shaft bearings. These motor and shaft bearings need to be lubricated according to the fan manufacturer's recommendations.
Belt Driven Fans need to maintain belt alignment and tension. Usually, this is not a difficult task - it is important to understand that the plant's belts will stretch during the first few days of operation. Belts are properly aligned and tensioned by the fan manufacturer. Once the fan is in service, it should be allowed to run for 2 to 4 weeks, depending on how many hours of the day the fan is running. Then, the fans need to be locked and the belt tension adjusted (as they are stretched).
While tensioning the belt, the alignment should also be checked. After the break-in period, the belt will not continue to stretch. Once the belts are retensioned after the initial 2 to 4 weeks of break-in, they should provide long-term quality service.
When lubricating motor and shaft bearings, maintenance personnel should watch for loose or worn belts, which can be caused by insufficient tension or misalignment. (This becomes apparent due to the formation of belt residual dust.)
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More compact footprint
This type of fan is arranged more compactly, so it can usually fit in smaller spaces, while the impeller is either mounted on or attached directly to the motor shaft. We often see this in constrained applications that require a compact design to fit the space.
More difference
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Safety
Unless the motor is in the fan's airflow, it must have access to ambient air circulation to cool the motor. Moving parts such as belts, pulleys and shaft coolers must be protected according to OSHA and/or other local requirements. Motor replacement on a Direct Driven Fan requires disassembly of the fan. This could be a safety issue as removal would expose personnel to airflow components.
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Reliability
When properly selected, installed, aligned and tensioned, a belt should provide many years of quality service. On both Direct Driven Fans and Belt Driven fans, the motor has two bearings. Belt Driven Fans have two additional bearings. The two bearings on Direct Driven Fan motors of horsepower and greater can usually be re-greased.
Direct Driven Fans should be designed with acceptable bearing life. This is done by fan designers and manufacturers to ensure that axial, radial and thrust loads are fully/properly considered in the design.
Conclusion
There are many variables to consider when choosing between Belt Driven and Direct Driven Fans. The final decision should always be based on weighting the variables that are most important for a particular application. Seek advice from your engineer if applicable.
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