Snail Fan Motor High Current Issue

In summary, the fan blades need to get hot before the fan can operate at its full power. If the fan blades are too heavy, it can take several days for them to get hot, during which time there is a risk that the engine will overheat and burn out. By changing the fan pulley size, the motor can be limited to a lower speed to avoid overloading the fan.
  • #1
We producing industrial steam boilers. We also produce snail fans for our boilers. The problem is our fan blades are a little bit heavy for our motors until blades get warm. "We can't use more powerfull motors cause of energy saving." We control our fan motors via drivers and our fan blades takes several days to get warm. During this period, there is a risk that the engine may burn out(cause of high current). Our motor’s pulley diameter is 30cm and fan’s pulley diameter is 30cm. If we change our fan’s pulley diameter like 25–27cm, is 110 kw motor will high current or will it be same ? Also if we change the fan's pulley as 25cm, we'll limit motor speed to avoid fan's over rpm. So, can we avoid high current with decreasing fan's pully diameter ?
 
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  • #2
Engineergn said:
snail fans
"Squirrel cages?"
 
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  • #3
First, I need to make sure that I properly understand your question. If I understand correctly, you have an induced draft fan. The fan is a centrifugal blower. An induced draft fan handles hot flue gases from a boiler. You are driving the fan with a 110 kW (150 hp) electric motor through a belt drive. The belt drive is probably a V-belt drive. Both the motor pulley and the fan pulley are 30 cm diameter. You are concerned that the motor is overloaded, so want to reduce the load on the motor. The motor runs at one speed - the speed on the motor nameplate.

Is all of the above correct? If it is, then you reduce the motor load by:

1) Smaller pulley on the motor, or
2) Larger pulley on the fan, or
3) Run the motor at lower speed using a variable frequency drive

You can check the motor load by having an electrician use a clamp meter to measure the motor current, after which you compare the measured motor current to the motor nameplate data. It is good practice to measure the fan inlet temperature, pressure difference between fan inlet and discharge, and motor current. Then compare to the fan curve and boiler flue gas flow specification.

I recommend that you study the fan laws (search the term). It will help you to properly size any pulley changes.
 
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  • #4
Additionally, reducing power means reducing air flow. How much flow reduction is acceptable?
 
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  • #5
Also, I notice you say the fan is powered from a VFD. If so, why not adjust fan rpm programmatically?
 
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  • #6
Welcome, @Engineergn !
Could you explain this part a little more?:
“The problem is our fan blades are a little bit heavy for our motors until blades get warm.”
 
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  • #7
Lnewqban said:
Could you explain this part a little more?:
“The problem is our fan blades are a little bit heavy for our motors until blades get warm.”
I was about to ask the same thing! :smile:
 
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  • #8
Engineergn said:
The problem is our fan blades are a little bit heavy for our motors until blades get warm.
Maybe the temperature of the fan blades is measured and displayed, and that has become a proxy, used by the operators to assess the density of the flue gas. Once hot, the fan operates in less dense gas and so requires less power to maintain RPM.

I assumed that it takes a day or so for the chimney to come up to temperature and begin to draw properly. The fan in the flue has more work to do with the cooler, and so a more dense column of flue gas in the chimney. Later, when the chimney is up to temperature and drawing properly, the column of gas in the chimney is less dense, so there is less work required by the fan.
 
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  • #9
Engineergn said:
So, can we avoid high current with decreasing fan's pully diameter ?
You know how much volume of air and what pressure is needed from the fan at operating temperature.
you then calculate the required motor size to drive the fan at startup conditions, which would be at a lower temperature.
see @Baluncore above,
also
https://www.engineeringtoolbox.com/air-density-temperature-fan-capacity-d_144.html

What could be worse than having a system in place that you want to run but keep burning out motors and motor control? Clients would want a functioning system, yes.
 
  • #10
Engineergn said:
We control our fan motors via drivers and our fan blades takes several days to get warm. During this period, there is a risk that the engine may burn out(cause of high current). Our motor’s pulley diameter is 30cm and fan’s pulley diameter is 30cm. If we change our fan’s pulley diameter like 25–27cm, is 110 kw motor will high current or will it be same ? Also if we change the fan's pulley as 25cm, we'll limit motor speed to avoid fan's over rpm. So, can we avoid high current with decreasing fan's pully diameter ?
The strategy will depend on how the density of the flue gasses vary during the starting phase of operation. We do not know the range of fan blade or gas temperatures expected, so cannot predict the reduction in gas density.
It could be as simple as making the fan motor speed, some function of blade temperature. There should be no need to juggle pulley diameters if they have been optimised for normal hot operation.
 
  • #11
Isn't air mass flow rate a function of the combustion O2 requirements? I expect that temperature and motor load are secondary to O2. If that's so, the motor must be sized for the worst case condition.
 
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  • #12
We do not know the fuel being used, nor the furnace geometry. If the fuel was gas or oil, then I expect a proportional air draft would be metered by a forced draft blower, into the furnace to satisfy the combustion requirements. If this fan followed an air pre-heater, it could be that forced draft blower for the furnace.

I suspect that this is an induced draft fan, after the boiler. It may compensate for a following air or water preheater, and then possibly a bag filter. An induced draft fan would be subjected to higher gas temperatures, maybe up to about 900°C, with a proportionally lower gas density and higher fan speed sensitivity.

Without further information from the OP we can only guess.
 
  • #14
Even on smaller boilers this is an issue. The induced draught fan must be run at reduced speed or with a damper partially closed until the flue gas temp gets to acceptable levels. In our case this will be about 160-220 degrees.
Trying to run the I'd fan on full output when temps are low results in overcurrent trips. This is normal. Just run the vfd as reduced speed or if you have a damper just close it partially. Don't mess with you pulley ratios, this will reduce the air you can pull through the boiler at maximum steam and effectively derate steam generating capacity.
 
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