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Monday, 28 October 2019

DIFFERENCE BETWEEN FANS AND BLOWERS

We were discussing various important topics based on the basics of centrifugal compressor and axial flow compressor construction and working principle in our previous posts. 

Today we will be interested here to understand a very important topic i.e. Fans and blowers with the help of this post. 

Fans and blowers 

If a hydraulic machine delivers the working fluid i.e. air with a high static pressure and relatively lower velocity then that hydraulic machine will be termed as compressor and that compressor might be centrifugal compressor or axial flow compressor. At the outlet of compressor, the working fluid will have high energy in the form of high static pressure and relatively lower velocity or kinetic energy. 

Suppose, if our requirement is to secure the air from a hydraulic machine with high velocity of flow or with high kinetic energy and lower static pressure, then we will use fans and blowers. 

Therefore, fans and blowers are those turbo-machines which will deliver the air at high flow velocity and with relatively much lower static pressure. 

Fans and blowers will also take energy from outside like compressors and working fluid flowing through fans and blowers will be delivered at the outlet of fans and blowers with high flow velocity and relatively much lower static pressure. 

Let us see here something very interesting about fans and blowers to make a note about the difference between the fans and the blowers 



The pressure rise across the fan will be extremely low and it will be in the order of few millimeter of water gauge. The maximum pressure rise across the fan will be in the order of 250 mm of water gauge and it will be needed in order to overcome the losses of pressure due to friction while air flowing through various duct pipes. 

As we know that atmospheric pressure is 10.33 meter of water gauge and therefore, we can see here that the pressure rise across the fan will be very small. 

Fans are used where we need to circulate less amount of air through duct pipe or we can say that fans are used for handling less mass flow rate of air. Therefore, there will be less frictional resistance during flow of air through duct pipe lines or we can also say that loss of pressure while air flowing through duct pipes will be less when we use fans. 

Therefore, there will be less static pressure required at the outlet of fans to overcome the frictional resistance while air flowing through duct pipes. 

The pressure rise across the blower will be relatively high and it will be more than 1000 millimeter of water gauge and it will be needed in order to overcome the losses of pressure due to friction while air flowing through various duct pipes. 

A blower might be designed in multistage for discharging the air at higher pressure. 

Blowers are used where we need to circulate large amount of air through duct pipe or we can say that blowers are used for handling higher mass flow rate of air. Therefore, there will be relatively higher frictional resistance during flow of air through long duct pipe lines or we can also say that loss of pressure while air flowing through duct pipes will be relatively higher when we use blowers. 

Therefore, there will be relatively higher static pressure required at the outlet of blowers to overcome the frictional resistance while air flowing through long duct pipes. 

Therefore, we have seen here the basic concepts of fans and blowers with the help of this post. We will see the basic concept of a centrifugal fan or blower with the help of our next post. 

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Further we will find out, in our next post, Velocity triangle for centrifugal fan and blower.   

Reference:  

Fluid Machines, By Prof S.K. Som 
Image courtesy: Google  

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