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Suction diffusers on pumps
3

Suction diffusers on pumps

Suction diffusers on pumps

(OP)
On chilled water applications (HVAC), do pumps really need suction diffusers? When to use or not to use them?

RE: Suction diffusers on pumps

If I'm right diffusers are also called "straightening vanes" inducers to improve suction characteristics of the circulating chilled water centrifugal pumps.

In a closed, well-designed, clean system where no air pockets or bubbles may appear, nor suction filters or strainers needed, such as in most HVAC package units, the installation of diffusers may seem superfluous.

What's the opinion of HVAC makers and experts in this forum ?  

RE: Suction diffusers on pumps

25362 - I am curious about that diffuser term also.  I have no HVAC experience so I was laying back.
But, diffuser is normally a speed reduction device that like you say, can also be used change or eliminate a direction of flow.
However, being in a picky mood, I would never call a diffuser anything intended just to straighten flow out because technically speaking, a diffuser changes flow from velocity to diffusion by speed reduction, thus the name diffuser.

I am thinking like you that perhaps vmonceller is talking about a flow straightener?

If so, then they are best to be avoided, no practical way to specify one easily and be sure of the results.

Usually scumbag architect never checked with pump guys nor does the architect want to leave enough room for the dog gone pump intake which always seems superfluous to everyone, no need to allow for that, just jam that 'ol pump in the corner somewhere.  Then we go get a flow straightener which does not work half the time, so they just replace bearings more often, but hey, we all have to work and there is no real problem says the architect that moved on down the road telling people what a great guy he was, works with engineers real well, knows how to handle 'em.

OK, I’m a little screwy today.  But hey, what’s a forum for anyway, just want to blow off steam a little.  I’ve tried to cooperate with architects often.  Job this year overseas left bad taste AGAIN.  Every time I figure out a correct set of pumps and intakes, scumbag architect changed it and then attempted to tell me that it should not be a problem, then they tell me how to fix it, then I just kept my mouth shut while I imagine evil things I would like to do to him, then tried to stick my hand through the phone cord to get a grip on the little rat’s throat while he arrogantly tells me, “Sorry, but that is where the pumps are going, and this is the route for your intake lines”, and now my suction speed is …, hopeless!!!  

After that loser architect and his buddies finish the roof will leak, paint will peal, sidewalks will be under water, site drainage will be screwy, glass will pop out of the frames, everything will be wrong with the building.  S then that filthy rat scumbag will tell the owner, you gotta watch those engineers and the contractors, If I was not there to help you just imagine how it would have turned out.

May the fleas of a thousand camels infest his armpits.

Ahhhhhhh, feel better now.

PUMPDESIGNER

RE: Suction diffusers on pumps

(OP)
I'm a new member and this is my first posted question. Thanks, guys, for your response.

As far as I know, suction diffuser has three functions: 1) Replaces a 90 deg elbow and saves space 2) It functions as a strainer 3) It stabilizes flow at suction.

Even with these in mind, questions and/or discussions still persist. Only experienced HVAC professionals could answer and share their ideas. I'm from oil and gas industry and have not seen one before. I tend to agree with 25362.

RE: Suction diffusers on pumps


The word diffuse means to spread out - expand - so to me, the term is being incorrectly used when being applied to suction flow staighteners on the approach to a pump suction, unless it is expanding in diameter as its length increases.
A diffuser should be used to describe something that allows for expansion after an event, ie, in an eductor (ejector) pump, the area after the mixing chamber where velocity is reconverted to pressure would be correctly known as a diffuser.

RE: Suction diffusers on pumps

When used in reference to pumps, a diffuser definitely refers to a device that reduces velocity causing an energy transformation from velocity to pressure.
The result of the velocity reduction is diffusion flow, thus the name.

And yes, Artisi is correct that diffusion flow is the "spreadying out" of the fluid, which is a definition of diffusion, moving from higher concentration to lower concentration.

PUMPDESIGNER

RE: Suction diffusers on pumps

   I've never heard of a suction diffuser upstream of a centrifugal pump but vmonceller's description of its functions sounds like it's an expansion elbow with a smaller upstream diameter and a larger downstream diameter.
This seems to represent the converse of the recommended practice of making the inlet piping larger than the pump suction nozzle with the presumed intent of stabilizing the flow into the impeller eye by converging the flow. Diverging (diffusing) flow upstream of an impeller is an invitation to serious flow instabilities particularly at off-design flowrates. How the so-called "suction diffuser" could act as a flow strainer baffles me unless its got some kind of syphon to drain off particles in the streamtubes that are centrifuged to the outside wall of the bend. If its got a filtering screen installed, then thats even more bad news for the impeller.

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