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

 

[katmar]

As you have discovered, you cannot separate flow rate from pressure drop. This is also true of the fat filters. At what flow rate are they supposed to give 200 Pa back pressure? Since you cannot deliver 200 Pa, the flowrate is obviously down considerably (as proven by the lack of smoke extraction).

I can't see whether the flow from each fan goes through its own pipe or not. If you have 300 m3/h per duct the pressure drop would be only 7 Pa, or 26 Pa if the fans combine to put 600 m3/h down the same pipe. This doesn't seem excessive. If you can give the pressure vs flow data for the filters then it is possible to estimate the actual flow rate.

Are the fat filters clean? How well do the fans work if you run without the filters for a short while? Do not be intimidated into buying new fans until you have eliminated all the other potential problems. Check even the basic things like are the fans turning in the right direction.

A good indicator would be the fan motor powers. What power were the motors on the old fans, and what are the new ones? This will give you a good idea of whether you have gone down in capacity.

Katmar Software - Uconeer 3.0

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[DRWeig]

But in what application could a 300m3/h fan be used at 70Pa pressure?

It would make a good toilet exhauster, or shop exhauster, or would work in many applications that have 70 Pa (1/4" W.C.) maximum external pressure drop, such as a window air conditioner of 0.5T capacity.

Good on ya,

Goober Dave

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[zdas04]

But probably nothing with a filter.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[ryanmcarthy]

Hi,

Thank you for the replies.

I've been told the type of filters give the 200Pa back pressure, but I am awaiting feedback from the manufacturer on this, as nothing is proven yet. Hopefully I will get the data sheet on these this week.

Each fan does indeed have it's own pipe. Imagine a large 'box' fryer, with a pipe at each end, with it's own fan, filter and jet cap on top.
Would you mind explaining how you get 7Pa please per pipe? As I would like to try to gain some knowledge from this error I've made.

I bought brand new fat filters for this project (2-off). But I am going to try running without the filters installed, as in theory this would eliminate a lot of the back pressure. I will also remove the jet cap too. I need to make up some covers prior to removing the filters to 'plug the gap'.

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.




Thanks a lot

 

[ryanmcarthy]

DRWeig, apologies I cannot see how to quote.

But the fans are quite large and wouldn't fit in a toilet etc. They are approx 8500mm long and 2500mm wide.

 

[ryanmcarthy]

850mm by 250mm not 8500x2500, hehe.

 

[katmar]

The pressure drop is calculated using the Darcy-Weisbach formula. You can take the air as incompressible in this case (i.e. density unchanged over the length of the pipe) which makes it easier. In your situation most of the pressure drop comes from simply getting the air moving (usually called exit losses), and very little (about 2 Pa) is the friction in the pipe.

But after reading the replies from the two Davids, I must concede that 70 Pa is very little if you want any sort of filter on it.

Katmar Software - Uconeer 3.0

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[ryanmcarthy]

Thanks for the reply. Yes, 70Pa does seem very little, but if I remove the filters, the fans should still extract to allow decent working conditions I think.....for the short term anyway!

So for Darcy-Weisbach formula, Could you help me fill in the blanks please?

f = friction factor = ?

L = length of pipe work = 5

D = inner diameter of pipe work = 0.2

v = velocity of fluid = 1.2?

g = acceleration due to gravity = ?



Thanks a lot

 

[vpl]

Metric units? g = 9.8 m/sec the last time I looked.

Doing a quick glance at my Crane Flow of Fluids Technical Paper 410 metric edition, f isn't addressed for stainless. Commercial steel has a friction factor of 0.014 for a 200 mm pipe, assuming complete turbulence. That might be a bit high.



Patricia Lougheed

******

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[Latexman]

Ryan,

Are you an engineer? Most engineers know this stuff, especially mechanical engineers.

Good luck,
Latexman

 

[katmar]

I am a bit reluctant to fill in the blanks because the calculation is a bit more than that. You should try to get your hands on the Crane manual recommended by Patricia to get some background. Seeing that I started you down the path of Darcy-Weisbach I will try to help a bit further - but I cannot compress a full semester course into 2 paragraphs!

There are several forms of D-W. For now take it as

Presure drop = (f x L / D)(rho x v² / 2)

where Pressure drop is in Pascal
L is length in meter = 5
D is diameter in meter = 0.2
rho is density in kg/m³ = 1.15 (approx)
v is velocity in m/s = 2.6
f is Moody friction factor = 0.023

i.e. Pressure drop is about 2.2 Pascal

Note that the friction factor suggested by Patricia is for complete turbulence, but you are not there because of the relatively low velocity and density. Note also that you only need a value for g if you are using the form of D-W that gives the pressure drop in terms of head of flowing fluid. That is the traditional form, but I prefer to work directly in pressure terms.

Katmar Software - Uconeer 3.0

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[zdas04]

I happened to have that program open.
[ul]
[li]Velocity (assuming sea level): 2.65 m/s[/li]
[li]Reynolds No: 7.07E4 (turbulent) [/li]
[li]Absolute roughness: 0.046mm[/li]
[li]Relative roughness: 2.286E-4[/li]
[li]Moody friction factor: 0.02[/li]
[li]Fanning friction factor: 0.00508[/li]
[/ul]

Darcy Weisbach calls for Moody friction factor which has been shown by any number of researchers to be too high for gases. Most gas equations use Fanning. I've had ok luck using Fanning in Darcy Weisbach, but I generally have better luck with the gas-specific equations (although pressure is too low here for compressibility to be an issue).

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[ryanmcarthy]

VPL, thanks for that.

Latexman, no unfortunately I'm not an engineer, I'm more of a fitter but need to try to do everything at work to keep us going. I wish I was more knowledgeable with regards this stuff, as I wouldn't have made such an error not specifying a higher pressure rating

katmar, thank you for your continued help. I'll confess I am still not too sure about the Pressure thing, as like you said it would be a whole unit in itself on a Uni course I imagine, but it is good for my knowledge to have an idea of what is going on with our pipe work and fans.

zdas04, thanks for your input too with the figures. Not all of them go in to the equation katmar posted though do they?

Overall I'm hopeful that if I remove the filters tomorrow, and the jet cap the fans, the new fans with a rating of 300m3/h and 70Pa should manage to extract the smoke if the pressure drop is around 2.2Pa. The temperatures are also high, which will have an affect of the flow too, but hopefully not too much of a negative affect



Thanks all.

 

[zdas04]

We're back to "a semester in two paragraphs". A couple of them are required to calculate the others.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[ryanmcarthy]

I see, thanks zdas04.

Another thing I cannot calculate, would be the velocity of the air will be different I presume, as the air will be about 100 Deg C, where as I think the 2.6m/s is calculated at 20 Deg C?



Thanks

 

[Latexman]

If the filters are removed, is flow adequate? I.e. no smoke accumulation.

Good luck,
Latexman

 

[ryanmcarthy]

Hi,
There is still some smoke to be honest! I thought it would improve matters a lot, but it hasn't made too much difference which I don't really understand.

I asked the company what pressure drop over the pipe work they had calculated, and they tell me 5Pa per meter? Which is more than 2.2Pa?

They have now offered to change the motor and impeller for us FOC, which would give us 600m3/h and 280Pa per fan, but they still won't guarantee that it will extract the smoke for us, but if we accept the parts they are washing their hands of us so to speak.

It really is beyond my knowledge now, and the company doesn't seem to be able to give me any scientific calculations etc to say how or if it will extract correctly.



Thanks

 

[sspeare]

You didn't allow any pressure loss for the HOOD that's over your fryers. For hoods that ventilate fryers, kettles, grilles, institutional automatic dishwashers, etc., I use at least 0.75 inches wc. The 0.75 inches wc is required to cover these 2 items:

1) The Friction Loss in the hood; and the friction loss in the transition/neck-down from hood-to-duct.

2) The Energy reqd to accelerate the air from rest, up to the Velocity Pressure created by the air that's moving at V = ? in the conveying ductwork (say, V = 1800 feet/minute?).

The 70 Pa fan, as others have pointed out, equates to only 0.25 "wc (I get 0.28 "wc - but whatever .....). You'll never get this thing to work unless you provide the energy the HOOD ITSELF requires.

Frankly, an Engineer competent in exhaust duct design should have done the fan selection, duct sizing, etc. The competent Engineer would understand what really happens inside that hood.

 

[Artisi]

Seems the fan company are very lax (read - wrong) in not asking what pressure was required when they took the order, this is probably why they are now offering motors and impellers FOC. They are right in not accepting responsibilty without a specific number to work to.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)