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Fan pressure and speed 1

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saeedplc

Electrical
Nov 27, 2021
126
Dear Experts,

Something in centrifugal pump or fan is vague for me so please correct my understanding:
In centrifugal pump or fan the vertical axis is the differential pressure across the pump or fan and horizontal axis is
flow and more flow means low pressure across the pump or fan and vice versa.
By measuring the pressure across the pump the flow can be determined by referring to its curve(by considering the pump or fan speed)

The questions that I don't know their answers so please let me know:

Will increasing the speed of pump or fan cause the inlet pressure to it get decreased?
Suppose there is an exhaust fan and a differential pressure transmitter connected to it's inlet . what happened to the pressure
indicated by the transmitter according to the fan speed change.

Regards,
 
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Well it depends on your system design.

Increasing speed will increase flow and differential pressure.

How that affects the inlet pressure depends on how that pressure reacts to increased flow.

Your second question, increased speed means increased differential pressure if nothing else changes.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
 
Hi,
[Q-1] Will increasing the speed of pump or fan cause the inlet pressure to it get decreased?
Ans. No. Inlet pressure will not get decreased. Theoretically, Centrifugal machines are constant head machines. Only because of system losses, we get to see a dip in head / differential pressure as flow increases. Also, in case of pump/fan, the fluid is considered incompressible. Hence, inlet pressure will remain the same, only the machine will deliver more flow. From a mathematical standpoint, contribution of the velocity component at the pump/fan inlet is insignificant as compared to the overall inlet pressure.

[Q-2] What happened to the pressure indicated by the transmitter according to the fan speed change?
Ans. With increase in speed, the differential pressure across the pump/fan will increase, as will be evidenced from an elevated transmitter reading. Recall affinity laws. Pressure / Head is proportional to the square of the speed.

Thanks.
 
Typically, the inlet pressure of the pump should always be about the same due to an expansion tank piped near the inlet.
 
If you increase the speed of the fan (or pump) then you are operating on a completely different curve that is above the original curve on the head vs. flow diagram. For instance if you increase the fan speed 2x then you increase the available head 4x. This is because the velocity of the fluid existing the impeller is proportional to the tip circumferential speed of the impeller. In a fan (or pump) the velocity is what is produced by the impeller. The velocity is basically kinetic energy (velocity square divided by 2g). Therefore increase in head is always proportional to increase in speed (RPM) squared. The velocity head exiting the impeller is converted into pressure head in the volute. Therefore by doubing the speed you increase the head output capabilities 4x So in this respect the operating point on the lower speed curve shifts up.

So say you are operating on the lower speed curve at a given flow and differential pressure. Then you double the speed. The output head/pressure is now higher at the same flow. However at the same flow the system pressure drop is still the same. Since the pump is capable of putting out more head than the system losses at the flowrate then the pump will run down on the higher RPM curve until the pump head equals the piping system loss at the same flow whatever that equillibrium flow turns out to be. Therefore the operating point will run further out (to the right) on the higher RPM curve at a higher flow and differential pressure. So in this respect you will measure a higher differential pressure because you will be producing more flow with the higher RPM curve given the same piping system. Now if you start throttling (closing) a downstream valve you can back the operating point back up the curve to the original flowrate but at a much higher pressure head.

Note also that at the higher RPM a larger motor is usually required since you are producing more differential pressure at a given flow. You can use the affinity laws to plot the new curve at the higher speed.
 
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