10 replies to this topic

[kangas]

I know that blowers are normally rated by the flow rate they can achieve. Can I somehow find out the pressure outlet of a blower so I can be able to compare it with the pressure drop across my duct?

 

Thanks

[Bobby Strain]

It usually works the other way. You size the duct to match with blower options. Search a bit to find the size you need (flow) and available discharge pressure.

 

Bobby

[Art Montemayor]

Kangari:

 

There are blowers - and then there are more blowers.  You should know that there are all types of blowers  - dynamic and positive displacement.  If you are a student dealing with an academic problem, at least tell us the TYPE of blower.  All blowers have performance curves.  These curves tell you the capacities of the blower at differing discharge pressures, assuming your suction pressure is approximately atmospheric.

 

As Bobby states, the answer to your query is the other way around:  the discharge pressure is set by the resistance to flow that the blower meets in the discharge piping, fittings, valves, and other constraints.  All blowers have a discharge pressure set by the discharge characteristics of the system.  First, you calculate or find the pressure drop the blower has to overcome and this will lead to the blower's capacity - especially if the blower is a dynamic type (centrifugal).  If it is a positive displacement type ("Roots", pallet, lobe, etc.) then you have slippage and the performance curves will depict this.

 

Why don't you tell us ALL of your basic data?

[kangas]

Hello,

I am actually working on a volunteering project and not for academic.

 

I am using a centrifugal fan. I managed to find the fan curves online so this is helpful for me now.

 

I just have a question. If the blower cannot sustain the pressure drop does this mean that atmospheric air will be forced inside the duct. If yes, what happens to the blower?

 

Regards

 

Andreas

[Art Montemayor]

Andreas:

 

You are writing in generalities.  You don’t tell us WHAT pressure drop you are referring to.  I am going to assume you mean the pressure drop developed in the blower’s discharge piping.  If this is correct, than you must mean that you don’t know what will happen when the blower reaches a maximum (possibly “dead-head”) pressure due to excessive pressure drop in its discharge line (possibly a plug up, or a throttled valve, or another restriction).

 

If that should happen, then a conventional centrifugal blower will just continue to spin with a minimum or fluctuating flow - depending on the performance curve characteristics and the amount of constriction in the discharge line.  That is why performance curves are generated.  If you look at your curve for the blower, I would guess that the blower’s delivery will approximate zero as the discharge pressure reaches a maximum.  Sometimes some blowers will go into a “surge”-like characteristic (fluctuating flow, back-and-forth) depending on the design.  The mechanical energy inputted by the driver will convert to compression heat build-up and the blower should be shut down.  Some turbo-blowers will tolerate a little of this kind of abuse, but I wouldn’t.   Refer to the attached document.

 

I see no way that atmospheric air (at 0 psig) could infiltrate the discharge duct (which would be under positive gauge pressure).  Can you please explain your concern better?

 

 Understanding Fan Performance Curves.pdf   279.51KB   72 downloads

 

 

[kangas]

 

Hello Art, 

 

Very good explanation. Now everything is clear to me.

 

This is my fan performance curve.

One of the heaters (including baffles) that I have designed give a pressure drop of 859Pa meaning that my fan won´t work with that. The heater without baffles gives a pressure drop of 30Pa meaning that fan will be able to sustain that pressure drop.

 

Are the number displayed on each line, the noise level?

 

Thanks

 

 

[Art Montemayor]

Andreas:

 

I don't know.  Every fan manufacturer has his/her own way of expressing information on their Performance Curves.  I suggest you contact the fan manufacturer for complete details.

[kangas]

Thanks a lot for your guidance and explanation regarding fans.

Edited by Kangari, 26 July 2018 - 01:01 PM.

[gegio1960]

Andreas,

looking at the curves, the figures in circles are the noise levels in dB(A).

good luck!

[kangas]

Carrying out some experiments this week, I have realised that once I connect the blower to a heater (thus applying a pressure drop), the air flow rate slightly increases. The same thing happens to an extended 6m straight duct. Do you think this happens because blower power rating increases and fan speed increases too to sustain the pressure drop? 

[tarang.suthar]

How to find out the Blower Static pressure??