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Diffusers in turbomachinery are used to transfer some of the kinetic energy of the fluid to pressure energy and to direct the fluid to the ducting/second stage/... I understand this and the mechanism behind it (geometry of diffuser changes the mass flux which creates a pressure gradient which decelerates the fluid) but I just cannot find any information how diffusers create more suction at the inlet of the pump/compressor. In laymans terms, how does the diffuser make the pump better for the end user? Is it more efficient?

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This website puts it pretty well: http://bellgossett.com/hydronic-plumbing-accessories/pump-accessories/suction-diffuser/

Without a suction diffuser, the pump inlet flow is subject to recirculation zones that produce pressure losses and a non-uniform velocity inlet. These recirculation zones can be the result of pipe bends and fittings. A suction diffuser "straightens" the flow and removes these flow structures. An uneven inlet pressure distribution and velocity profile inflicts uneven stresses onto the pump impeller. In absence of these stresses,the pump can operate more efficiently. Pressure losses are also detrimental to pumps since the inlet suction head is reduced. This can cause cavitation of the pump. Does that all make sense?

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  • $\begingroup$ Thank you for your perspective, I was not aware of a such design in the pump industry. I think something similar is used the vacuum cleaner suction unit design (youtube.com/watch?v=-ZdQVkwt_rw)and as a so-called inducer in the centrifugal compressors (youtube.com/watch?v=moYvcO-i574) to also create a more uniform inlet velocity. The other advantage is to prevent recirculation inside the impeller at off design conditions (outside of the optimal point). But my original question was directed at the diffusers at the outlet of the impeller. I understand that --- $\endgroup$ – user2882635 Oct 10 at 6:24
  • $\begingroup$ ---- (continued) serve a similar purpose, to minimize the back pressure and to create a more uniform outlet velocity profile.There are also drawbacks such as the impeller-(vaned) diffuser interaction, but let's not focus on that. My question is, how does an outlet diffuser improve the stage performance by turning some of the kinetic energy into pressure energy? $\endgroup$ – user2882635 Oct 10 at 6:29
  • $\begingroup$ My apologies for misunderstanding your question the first time around. You've hit the nail on the head by stating the diffuser converts kinetic energy into pressure energy. Since the goal of the pump/compressor is to increase the pressure of the fluid, any leftover kinetic energy is viewed as a penalty to achieving a high pressure ratio. The impeller increases both the pressure and kinetic energy of the fluid, but the diffuser converts this kinetic energy into pressure energy. If you consider there is an expander of some sort downstream, a high inlet pressure is most desirable for performance. $\endgroup$ – mechcad Oct 10 at 16:51
  • $\begingroup$ No problem. I understand the need for high pressure ration in a compressor as that is what we want to achieve with it, compressors are mostly needed to supply high pressure air. I suppose a high pressure ratio in a pump is needed to supply the water to higher elevations or to overcome pressure drops in the piping. What I don't understand is how a larger pressure at the outlet of the pump results to larger pressure drop at the inlet. Would it be possible to use a piston inside a sealed cylinder for an analogy; when pressure in front of piston increases, pressure behind it drops? $\endgroup$ – user2882635 Oct 11 at 6:32
  • $\begingroup$ One more question: what do you mean by an expander? Is this just a part with large volume increase? By high inlet pressure; high inlet at the expander inlet? $\endgroup$ – user2882635 Oct 11 at 6:41

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