Flow and Pressure drop in reduction pipes

[MedicineEng]

Daer All:
Yesterday I already put this thread but for any reason, it disappeared.
Well my problem (big to me, small for you experts for shure), is the following:
I have a blower that feeds combustion air to an incinerator. The admission of this blower is DN200 (the actual pipe has around 2 meters long). Now, I have to consider to reduce this diameter to DN80 and include a straight pipe around 1.5 meters long. The original pipe will be maintained. As such, the final setting will be a 2 eters long DN200+1.5meter long DN80
I would like to know how much my flow and pressure will be influentiated in these new conditions.

Thanks a lot to all.

PR

 

[TD2K]

Are you saying these two (the old pipe and new one) will be in parallel to each other? Or are they going to be in series (the DN 80 connected to the end of the DN200)?

To determine the effect, you need the curve for your fan/blower. Assume a pressure out of the blower and from the curve you read off the flow. Then, you calculate the flow through the downstream piping. Depending how well the two agree, you pick a new pressure and repeat the exercise until the flow from the fan at the assumed pressure equals the flow through the downstream pipe or pipes. I'm assuming you know how to do an air flow calculation through a pipe.

You do this for the existing and new setup and see what the effect on flow and pressure is.

IF you are going to connect the DN80 pipe to the end of your existing DN200 pipe (in series) I wouldn't be at all surprised to see a significant reduction in flow since the DN80 pipe has a much lower flow capacity than the DN200 pipe but it really depends on the flow through the system. There's no way to quantify it without more data.

 

[MedicineEng]

TD2K:
Thansk for your input.
The pipes will be connected in serie. My problem is that I don't have the curve of the blower since this is an old piece of equipment. The question is that I would like to estimate the approximate flow reduction to see if this application is feasible or not before I do the modification.

PR

 

[quark]

Is it a centrifugal blower or a positive displacement blower? A PD blower can take care of it at the expense of some extra energy and your motor should be capable of doing extra work.

Incase of a centrifugal fan, measure initial flow rate and blower discharge pressure. Based on this flow rate calculate the extra pressure drop across the DN80 pipe and add it to the initial blower discharge pressure. Now throttle the discharge valve to get the final pressure and measure the corresponding flow rate.

Your final flowrate and pressure will be inbetween the initial measured value and the final measured value. (because the frictional resistance may not be equal to that of calculated value as the flow rate gets reduced if you increase resistance)

If you tell us what is your present flow rate then it will be easy to calculate the pressure drop.

Regards,

 

[friartuck]

If my maths is right (after brushing off a few cobwebs), the velocity increase from reducing the dia from 200 to 80 will be a ration of the dia squared.

200x200/80/80=6.25

So the increase in velocity from 200 to 80mm will be 6.25x.

PD is proportional to velocity squared, so there ewill be a large increase in pressure drop in your system. i.e. 6.25x6.25= 39.06. (if your old PD was for instance 10Pa through this section, the new loss would be 39x higher if the same flow rate was required)

This figure excludes the shock loss in the reduction, which will also be quite high, plus the exit loss at rhe 80mm outlet end.

So unless your blower has a lot to spare, it may stall.

Backward curved bladed blowers are more resilient to pressure drop but if you have a forward curved, you might struggle.

What are you moving, what flow rate, and why do you have such a sharp reduction. It would be better if the reduction was gradual or at least from 200 to 100 and not to 80mm.



Friar Tuck of Sherwood