Pressure Drop In Pipe Work? 1

[ryanmcarthy]

Hi,

I'm hoping someone will be able to help me out, as I'm really struggling to understand an issue at work.

We have a fryer with two extract fans which worked fine previously. But as they were very old we decided to overhaul all the ducting and fans, and also they were galv so we couldn't keep them too clean, and they leaked oil.

We replaced the pipe work, and made everything straight, and bought two new bifurcated fans, everything made from stainless steel.
I asked the manufacturer of the fryer what m3/h each new fan should be rated at, to which I was told 300m3/h per fan is what they normally install and is more than enough. So I ordered 2-off new fans at £900 each, and had everything fitted.

Unfortunately the fans do not extract what they should and it makes the room full of smoke after a short while. At the time of ordering the fans I didn't specify any Pressures, as I don't have a clue about what they should have been (or that I needed to), just the 300m3/h per fan. The company selling the fans didn't state or ask anything either!

The new fans have a Pressure rating of 70Pa, which from a bit of reading shows this is hardly anything! We have fat filters on the outlet of the ducting, which I believe have a pressure drop of 200Pa in them!

Can anyone please help me in estimating what the pressure rating of the fans at 300m3/h should be with the following:

- Fat filter: 200Pa (awaiting confirmation from manufacturer)
- 200mm diameter stainless steel pipe work, straight, total 5m
- Ducting cowling at the top (Jet Cap) - ?Pa (awaiting confirmation from manufacturer)

I think we are going to have to spend even more money buying new fans now, as the company said it was my fault for specifying 70Pa pressure for the fans, although I didn’t, but I accept I was inexperienced in purchasing new fans, thinking all I needed to ask for was stainless steel fans with a rating of 300m3/h.

But in what application could a 300m3/h fan be used at 70Pa pressure? As I think even short runs of pipe work would result in a higher pressure loss then 70Pa?


Thanks a lot
Ryan

 

[berkshire]

ryanmcarthy (Mechanical)
You said
""The old fans are the same speed and motor size. I think off the top of my head they are 0.37kW and 1400RPM. I did think a cheaper option than buying a whole new fan would be to install a 2-pole motor on the fans to double the speed of it, but I'm not sure how much the speed increase would improve things? The new fans are bifurcated, where as the old ones were Centrifugal.""

One thing you are missing here is that you say the old fans were centrifugal.
A centrifugal fan is much better at handling resistance on a line than an axial flow fan.
Look at the fan curve for the axial flow fan you are putting in, and compare it to the curve for centrifugal fan you pulled out.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[curtis2004]

I also have noticed that a centrifugal fan was replaced with an in-line one. DP generated by centrifugal fans are a several times are higher than in-line fans.
I think you do not have enough fan capasity to move air.

Regards,
Curtis

 

[ryanmcarthy]

Thanks for the replies guys.

I wish the fan company had really asked more questions, rather than just taking the order, but it is my own fault really from not researching things good enough.

berkshire & curtis, I wish I knew this at the time too really. It wouldn't have been an issue fitting a lot bigger in-line fan to produce the volume and pressure I require at the time of install, but it is a huge issue now due to pipe work, and other ancillaries fitted to the pipe work, and not affordable to change.

I fitted the new motors at the weekend, but the impellers didn't arrive unfortunately, so we will see how things go with the motor speed doubled. Although we will still fit the impellers when the arrive.


Thanks