Extractor fan ducting - flat channel vs round efficiency

[MasterMark]

@Alec_t - that's good to know. Thanks! Once I've managed to get it done, I'll report back here to let people know if there's been a noticeable improvement. A bit niche, but who knows, it might help someone else.

 

[Carnmore]

@ericmark - technically, yes, it's possible, but a bit of a faff and then becomes a cost issue as well (creating new hole in external wall etc).

Re mould - I keep the fan running a decent length of time after finishing showering, so in the 18 months since installing with the 250 power fan, no mould issues. But to reduce risk of this, and also super fast steaming up of the shower room (eg mirrors totally steamed up within 3 or 4 minutes), thought I'd upgrade to a more powerful fan and replace ducting. The fan is also currently unsightly (resting above a bedroom wardrobe), so will need to be boxed in (though still accessible for maintenance). But I really want to do the job just once to the best possible level without overkill.

Hello MasterMark, I have a similar set-up in a mid-floor apartment with flat channel ducting and a long duct span (6-7 metres) to the vent. I'm renovating the bathroom and have removed the original centrifugal fan c.25 years old) and need to decide on a replacement.

Inline fans seem to be the most effective but there is no room above ceiling level to install it. I note you mentioned your fan is "resting above a bedroom wardrobe"-does this mean you have installed the inline fan below ceiling level with the flat channel duct above ceiling level?

Also, how does the flat channel ducting connect to the inline fan-they all seem to have circular connections/spigots.

Thank you in advance.

 

[JohnD]

IIRC the TD350 is supplied with a reduction snout to fit 100mm duct, but is capable of fitting bigger.

I will look it up

I don't suppose you have access to a loft where the duct could run out of sight?

Edit
Ah, it fits 5" duct (125mm) round

Your existing duct is much too small.

Performance charts, and accessories such as non-return flaps and duct adaptors, are here
https://www.tlc-direct.co.uk/Technical/DataSheets/SolerPalou/TD_Silent.pdf

5"/125mm In-Line Duct Extractor Fan - Silent 350m3/hr | Soler_&_Palau (5211360400)

 

[morqthana]

A = pi * R²

pi = 3.14

3.14 * 50 * 50 = 7854

110 * 54 = 5940

So 100 mm round will have approx 50% larger cross sctional area than the 110 * 54 rectangular.

Avoid flexible corrigated ducting as it invariable results in turbulant air flow which significant reduces the effective cross sectional area of the duct.

TD350 is a 125mm fan

 

[morqthana]

the builders installed low profile/flat channel ducting of 110mmx54mm.

Why that rather than 125mm round?

 

[Carnmore]

Why that rather than 125mm round?

.

 

[JohnW2]

If nothing else changes then the 50% increase in cross sectional area will allow the fan to push approx 50 % more air than is being moved through the existing duct.

potentially quite appreciably "approx", I suspect ...

... you are assuming that the fan acts as a 'constant pressure generator', such that the air flow is determined by the resistance to outflow (e.g. due to a duct) - in which case, of course, the flow would approach 'infinitely high' if the resistance flow was negligible (e.g. 'no duct').

In practice, the design of a fan will determine the maximum amount of air it can shift, even in the absence of any appreciable resistance to outflow (which is essentially what happens with those 'open to the air' fans we use in hot weather).

If one progressively increases the size of the duct, I would therefore imagine that although, initially flow would probably increase roughly pro-rata to the decrease in resistance (hence roughly pro-rata to the increase in duct CSA), the point would eventually come when increasing duct size further would not result in an increase in flow ('cos the fan was already shifting as much as it ever could do) - which, more generally, probably means that as one gradually increases duct size, the resultant increase in air flow with each increase will gradually get smaller.

Kind Regards, John

 

[bernardgreen]

In practice, the design of a fan will determine the maximum amount of air it can shift, even in the absence of any appreciable resistance to outflow (which is essentially what happens with those 'open to the air' fans we use in hot weather).

Totally agree with that, The design of cooling systems for racks of electronic equipment was ( and probably still is ) a mixture of black art and the science of fluidics.

 

[ebee]

Agreed . Change of shape therefore change of geometry gives some higher resistance and also any reduction in volume does the same but any maximum push is limited by the fan outlet push anyway.
The ones i have seen have been kitchen extractor hoods in the the main which make a nice looking flat channel riding on top of wall units but i imagine that it is greatly affected by both of these features. Off course tge other one seen is elbow joints to change direction and of course even the plain old round duct has its friction increased as length increases too

 

[bernardgreen]

but any maximum push is limited by the fan outlet push anyway.

It may be limited as well by the fan's ability to suck air into the system. The density of the air will affect the maximum suck rate and the density of the air is affected by the temperature of the air. I recall that blowing cold air into equipment was more effective than sucking warm air out of equipment.

Obviously it is not practical to blow cold air into a kitchen in order to move the warm air and cooking fumes through a passive ( no fan ) exhaust duct.