Why do centrifugal pumps need priming but blowers don't?

[anshul144]

centrifugal pumps and blowers work on the same principal then why centrifugal pumps are needed to be primed but blowers are not?

 

[JaredJames]

Well I assume your view is a pump deals with liquid and blower with gas.

If there is no liquid in the pump (on the vanes) then it can't pump anything - it would just spin and do nothing. So you need to prime the pump by getting the fluid into it, so that once it is switched on it has something to pump.

 

[russ_watters]

A blower is always primed!

 

[JaredJames]

A blower is always primed!

Care to expand on that? What is the difference aside from one is primed and one isn't? What causes one to be always primed?

 

[Q_Goest]

Hi JaredJames.

Care to expand on that? What is the difference aside from one is primed and one isn't? What causes one to be always primed?

I think it's exactly as you already pointed out:

Well I assume your view is a pump deals with liquid and blower with gas.

Generally when people talk about pumps, they're talking about a machine that moves liquid. Similarly, when referring to blowers, they're talking about machines that move gas. Also, we would naturally assume the blower to have air in it before it starts up, hence it is already "primed", whereas a pump that has air in it is starting out with the wrong working fluid.

I think more fundamentally, the reason a centrifugal pump can't generally self prime is because of the way it creates pressure - by accelerating a given fluid. If the fluid is a liquid, it is almost certainly going to be much more dense than a gas and hence it will produce much more pressure accelerating a liquid. If a centrifugal liquid pump is spinning with air in it, the amount of pressure it creates is very small, so most systems they are placed in won't flow until there's enough pressure to get the fluid to move.

 

[JaredJames]

Cheers.

 

[sophiecentaur]

AS far as I can see it, the reason that you may need to prime a (liquid) pump is that there would not be enough pressure reduction in the expanding volume inside the pump to produce sufficient lift if it were just full of air. If it is full of liquid then the pressure reduction could be much more as the volume is increased - allowing the lifting process to start.
This argument also applies in a centrifugal pump in which air is not dense enough to produce a sufficient pressure difference as it is thrown out by the spinning vane.

 

[russ_watters]

Care to expand on that? What is the difference aside from one is primed and one isn't? What causes one to be always primed?

You need to prime a pump becuase it isn't full of liquid and isn't capable of generating enough vacuum to pull the liquid into itself. A blower/fan is always full of air unless it is in space.

 

[sophiecentaur]

It's all to do with the cohesion in liquids compared with no cohesion in gases.