Calculate Suction Needed to turn Blower Impeller

[goal_less]

Let's say the motor for a blower is turned off.

How do I calculate what suction pressure is needed to start to turn the impeller when the motor is off?

I imagine I have to calculate the torque and solve for pressure?

 

[1503-44]

First, "blowers" are for moving air and they have "fans" or "blades". Pumps are for moving liquids and have impellers. So which one do you want to talk about?

You need to find out what power, mass flow rate x pressure change, is required at design flow and select a motor with a matching power rating. Typical electric motors can produce up to 150% of their rated torque when starting up. Also when starting up, the fan, or impeller, turning at slow speed will not demand much initial inflow. So actually starting is not usually going to be the problem. If you can provide the design flowrate at the pump's required suction pressure stated by the manufacturer, that same pressure will normally be sufficient for start up purposes. As a pump speeds up, suction pressure normally will get lower, because of increasing friction losses associated with the increasing flow, so you usually see problems happening when flow is approaching design rates, rather than when pushing the start button.

Why do you think there will be a problem with starting?

 

[LittleInch]

I took the question to be at what flow / differential pressure does the fan / blower start to free wheel with no power?

It's not an easy thing to find out as it depends on the static friction of the fan bearings and motor bearings / inertia. Not many people both to find out what they are.

They then don't even bother finding out what differential pressure it takes to start rotating it.

Why do you want to know?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Im going to have to start taking these questions more literally!

For a fan, you can use mass moment of inertia of all rotating parts at low rpm to calculate torque.

For pumps its the same, but there can be more resistance to initial flow from downstream effects and the liquid mass and motion within the pump to consider.

 

[ashtree]

A blower could also be a positive displacement lobe type which would often be called a roots blower.

Regardless of type if the unit is fitted with a VSD you could check if the VSD will give you a power output reading at very low speed. Its rough but it will give you the power required to turn the motor and the blower at that low speed, as it is likely the blower will not be moving much air.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[rotw]

I assume when you mention blower this is referring to a fan type of equipment (thus although not compressible it is not literally a liquid being transferred) and when you mention impeller, I assume it is centrifugal design.

When you mention suction pressure value, are your referring to reverse flow and reverse rotation (instead of discharge pressure increasing it would be suction pressure lowering below equilibrium pressure)? is this the case?

For centrifugal compressors (by analogy to centrifugal blowers as the later are close from design and principle of operation stand point except lower pressure ratio are involved), assessing reverse rotation risk requires to perform a calculation which basically compares the total adiabatic energy (function of pressures and volumes) vs. inertia of rotating parts and this determines the probability of reverse flow.

Also for a centrifugal compressor equipped with hydrodynamic bearings (tilting pads) the static friction torque is about 15% of the rated torque. I would expect that there is a dependency on type of bearings (add to this what LittleInch also mentioned like the prime mover bearings).

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