Could house fires be prevented by increasing dryer vent speed?

Question

In the US, clothes dryers are one of the leading causes of house fires:

In 1998, CPSC estimates that there were approximately 15,600 clothes dryer fires resulting in 20 deaths, 370 injuries and $75.4 million in property damage.

Or more recent numbers:

In 2010, an estimated 16,800 reported U.S. non-confined or confined home structure fires involving clothes dryers or washing machines resulted in 51 civilian deaths, 380 civilian injuries and $236 million in direct property damage. Clothes dryers accounted for 92% of the fires; washing machines 4%, and washer and dryer combinations accounted for 4%.

A fairly comprehensive study about lint ignition was completed by the Consumer Protection Safety Commission (CPSC) in 2003. This study showed that the dryer's internal temperature greatly increased once the vent pipe's area was reduced by 50%. In homes, this reduction is due to lint gathering in the pipe.

This manual for a random dryer mentions that minimum air flow through the 4" (10 cm) diameter vent pipe is 1,200 feet per minute (20 ft/s or 6.1 m/s).

There is a concept of minimum velocity in drainage pipes and culverts to ensure that they are self cleaning, i.e. debris does not build up.

Obviously air and lint are different than water and sediment. I haven't been able to find any particle properties for lint.

Is the solution to lint clogging dryer vents just to increase the velocity of the air in the vent?

Answer 1

Taking the data you supplied at face value, a relatively simple solution is for the dryer to measure either the exhaust air flow or back pressure, and refuse to operate when out of spec. Sensing these parameters can be done cheaply with today's electronics, especially considering such dryers already have a microcontroller in them to do the sequencing, run the user interface, etc.

The real question is whether consumers would put up with a nanny-dryer, and whether they'd be willing to pay the small extra cost. The cost delta would be quite small, but in such a high volume highly competitive market where consumers buy largely on price, this might be a problem. I expect manufacturers would strongly resist attempts to mandate this.

Answer 2

One of the issues of having an exhaust velocity that is too high is the increased possibility of injuring someone who might go past, particularly and eye or ear canal injury. A cover on the outlet would reduce air velocity and increase the chance of lint build up near the exhaust outlet.

The other issue with increased air velocity is increased noise from the outlet and vibration of the duct. The owners and the neighbours could be upset. Additionally, increased air velocity may also increase power consumption.

With the ducts being made of metal, lint build up due to static charge will be less of a problem that if than if the ducts were made of plastic.

As @Air intimates, the problem might lie with the installation and alignment of the duct. More bends and kinks can reduce cross-sectional area, increase shock losses, reduce air velocity and the carrying power of the air to carry the lint.

Screws or rivets joining duct segments that protrude into the internal duct space would be issue as they would act as collection points for lint.

My suspicions are that a “set and forget” attitude and a total unawareness of the necessity for periodic maintenance and cleaning of the duct are a significant factor.

I like the idea of a back pressure sensor that @Olin Lantrop discusses in his answer.

Other solution would be to alter the airflow system of driers to give a high speed short timed pulse of air before each drying cycle and at the end of each drying cycle to flush out accumulated lint or debris in the duct.

Answer 3

There is a product on the market today that is specifically aimed to address this issue - it is the dryer booster fan.

I installed one of these on my dryer (I was seeing excessive lint buildup in my ducts which were borderline for size and length) and it made a big difference: not only do the ducts stay cleaner - but the clothes dry significantly more quickly.

Based on this experience, it does seem to me that the built in fan in most dryers is underpowered - a variable speed fan that measures the flow rate (and maintains it constant) would clearly solve this problem without some of the drawbacks mentioned in other answers. If you added an alarm when back pressure exceeds a certain value it would be even better.

So the answer is almost certainly "yes". But the appliance business is cut-throat: it would be hard to justify the additional cost (unless insurance companies cotton on and provide a premium discount for this safety feature). I have no objective measurement of the cost savings due to faster drying times - just anecdotal evidence.

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disclosure the only relationship I have with the linked site is that I bought one of their fans to solve the problem described in my house

Answer 4

Just for shiggles I'm imagining a perfect dryer duct with no kinks that fits recommended installation parameters.

It seems to me that influencing the vent speed would:

• Force slightly more particles through the lint trap
• Simply cause the lint clog to collect further from the dryer

Preventative maintenance aside (annually cleaning your ducts), I think a better solution would be to design a lint trap with a larger surface area and a smaller mesh.

While this doesn't necessarily require an increase of fan speed, it would require an increase in pressure to overcome the resistance of the finer mesh.

A secondary option would be an additional lint trap that was ionized to catch the fine particles that sneak past the first trap. It seems like half the work is already done here, as dryers tend to cause static-y clothes.