Centrifugal Pumps & Blowers: Why Prime Pumps not Blowers?

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Discussion Overview

The discussion revolves around the operational differences between centrifugal pumps and blowers, specifically addressing why centrifugal pumps require priming while blowers do not. The scope includes theoretical explanations and conceptual clarifications related to fluid dynamics and the behavior of liquids versus gases.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants propose that pumps deal with liquids while blowers handle gases, which influences their operational requirements.
  • It is suggested that a centrifugal pump cannot operate effectively without being primed because it needs liquid to create sufficient pressure; if it contains air, it cannot pump anything.
  • Others argue that a blower is always "primed" because it contains air, which is necessary for its operation, contrasting with the need for liquid in pumps.
  • One participant notes that the pressure created by a centrifugal pump is significantly lower when air is present compared to when it is filled with liquid, affecting its ability to generate flow.
  • Another point raised is that the pressure reduction in a pump filled with air is insufficient to initiate lifting, while a liquid-filled pump can achieve greater pressure differences.
  • There is a mention of the differences in cohesion between liquids and gases, which may contribute to the necessity of priming for pumps.

Areas of Agreement / Disagreement

Participants express differing views on the operational principles of pumps and blowers, with no consensus reached on the exact reasons for the need to prime pumps versus the self-priming nature of blowers.

Contextual Notes

The discussion includes assumptions about fluid properties and operational mechanics that may not be universally applicable, such as the density of liquids versus gases and the implications for pressure generation.

anshul144
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centrifugal pumps and blowers work on the same principal then why centrifugal pumps are needed to be primed but blowers are not?
 
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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 said:
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?
 
Hi JaredJames.
JaredJames said:
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:
JaredJames said:
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.
 
Cheers.
 
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.
 
JaredJames said:
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.
 
Last edited:
It's all to do with the cohesion in liquids compared with no cohesion in gases.
 

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