Figure 1: A mysterious chimney-like structure in the middle of a road near the south coast of England, UK. At top left, some smaller chimneys gaze down knowingly at the structure.
Most of us probably see hundreds of chimneys each day but pay them absolutely no heed unless one is collapsing or on fire. But at a certain time of year, not too far from now, the chimney in a house becomes the focus of attention, mainly for the younger occupants. Because during a certain night in December, a portly, bearded gentleman called Santa might need to use the chimney to gain ingress to that home.
Some chimneys are hidden in plain sight. Not far from your writer’s humble hovel, sticking up in the middle of a residential road, is the structure in Figure 1. I have met many local people who have not a clue what this is.
The purpose of the structure (and an idea of its age) can be gleaned from an Ordnance Survey map dated 1888 – 1913 (Figure 2). It is marked as an air shaft and it descends to a railway tunnel directly beneath. I imagine it would originally have allowed the exhaust smoke and steam from locomotives to dissipate. The tunnel is around 1.2 km (0.75 miles) long and a comparison of the heights of the road and the emerging track to the west shows that the whole chimney structure must be around 33 m (108 ft) in depth, including the bit you see and allowing for the tunnel clearance. This is more than twice the height of the building in the left side of Figure 1.
Figure 2: Historical map showing the ventilation shaft (arrowed red) above an underground section of rail track and the section of track emerging at the western end of the tunnel (arrowed green).
With this article we will be giving chimneys some of the attention that I think they deserve. There will be no extensive list of the parts of a chimney (the internet is full of these), but I couldn’t help but wonder if ‘chimney cricket’ ( a kind of mini‑roof diverting water around the uphill side of a chimney) was any kind of inspiration for the naming of the Jiminy Cricket character in Disney’s Pinocchio. A quick and non‑comprehensive search of the internet yields no evidence for this whim, so I will consider the matter a simple coincidence. Our ruminations today will be more on the functional aspects of chimneys, the whats and the hows, if you will.
Figure 3: The late Fred Dibnah (left) starting to ascend a 53 m (175 ft) chimney at the Cambridge Museum of Technology (right) in England, UK.
So why do we construct a chimney and why are some so tall? Most chimneys exhaust stuff that we do not want to be inhaling or getting in our eyes and on our skin. The most basic function of a chimney is to put distance between people and the release site of the exhaust gases. Sometimes this distance is considerable and I would encourage anyone who has not just eaten a greasy breakfast to search for videos of the late Fred Dibnah lashing ladders by hand to chimney megastructures then ascending to the top, even past overhangs, without feeling the need to clip on to anything (Figure 3).
Air pressure decreases with height. Air pressure is simply the weight of the overlying air and the higher up you are the less air there is above you. The air at the bottom of an unused chimney is at higher pressure than the air at the top, giving it a natural tendency to want to flow upwards. In lighting a fire in a hearth, before it becomes a significant source of heat, a chimney of sufficient height will ensure that the relatively cool smoke goes upwards instead of into the room. In normal use, however, many chimneys are venting hot exhaust gases. Hot gases tend to have lower density than cold gases and will rise due to buoyancy (like releasing a football under water). The tendency of gases to rise in an enclosed channel is known as the stack effect and can be exploited to help naturally ventilate buildings.
Combustion processes generally generate exhaust gases. The extra volume of these exhaust gases introduced at the base of a chimney help push the gases above towards the top of the chimney. Even if the chemical equation for the combustion process shows no net increase of gases on the product side compared with the reactant side, remember that the reactants are supplied outside of the chimney, so the chimney volume has a net input of exhaust gases.
Pressure and buoyancy differences in fluids tend to want to even themselves out and this will happen through lateral or vertical flow or anything between the two. Constraining the gases within a chimney structure helps ensure the flow is one way and that should be upwards.
I will now shoehorn in a brief consideration of cooling towers; you can argue among yourselves whether these count as chimneys or not. Most of us only get brief glimpses from a train or motorway of distant cooling towers venting white plumes (Figure 4). They are usually smooth concrete towers that flare slightly at their base and their top. Their function is to cool hot water from, for example, the turbines of a power station. The cooled water can then be recycled or discharged back to the environment.
Figure 4: Twelve cooling towers beside Drax, a large power station near York in the north of England, UK, that once burnt coal but now burns biomass. Its main chimney (centre) is 259 m (851 ft) tall, 26 m (85 ft) in diameter and weighs 44 000 tonnes (48 500 tons).
I never gave cooling towers a great deal of thought, but I assumed the base contained a pool of hot water that was left to passively cool in its own time. The truth is very different: the cooling process is highly engineered and the towers contain a number of components and technologies to drive the process. The basic principle involves hot water droplets falling under gravity while cool air flows around them. The airflow enhances evaporation which, by definition, cools the water. The white plumes you can see are simply water condensate from this evaporation process. Within the concrete walls you might find:
- Nozzles to spray the hot water
- Filler material below the nozzles on which the water‑air interaction is enhanced
- Drift eliminator material above the nozzles that allows upward gas flow but restricts water loss
- Louvres at the top to restrict sunlight that might encourage algal growth
- A fan to draw the moist air upwards (the towers of Figure 4 are natural draft so have no fan)
- A basin to catch the falling water
At the base of the towers, seldom seen from a distance, are air intake structures (Figure 5).
Figure 5: the air intake structure at the base of a cooling tower at Richborough power station in the southeast of England, UK. These towers have now been demolished despite being a popular local landmark and a useful navigation reference for mariners near the mouth of the English Channel (La Manche, to the French).
My favourite thing about cooling towers concerns their shape. The shape is called a hyperboloid and is functional: the waist accelerates the rising air through a venturi effect, while the flared base allows a larger capacity basin and more air intake area. You might assume that curved steel reinforcing rods are required to achieve this. Not so. If you arrange a circle of vertical straight steel rods to make a cylinder shape then collectively rotate just the tips, you make an exquisite hyperboloid (Figure 6). Search your favourite craft outlet for ‘hyperboloid’ and you will likely find a number of domestic examples of this structure.
Science over the past few decades has made most of us acutely aware that what we pump into Earth’s atmosphere can have effects regardless of where we discharge it. Chimneys are likely to play an increasingly important role in controlling the content of the exhaust gases they channel, whether this is carbon capture or desulfurization. The industrial chimney will be with us for a long time to come, even if many modern homes no longer have a chimney.
Figure 6: A curved hyperboloid constructed from straight components, modelled here using Mecway finite element analysis software.
What if you are one of the lucky people who live in a modern home with no chimney? You might be worried that come the early hours of Christmas Day, there will be no way for presents to be delivered. Worry not, dear reader. A quick internet search will reassure you that Santa has an extensive armoury of tricks to get him where he needs to be. These include magic keys, shrinking himself down to fit through the keyhole and teleportation.
That settled, I will finish by wishing all of my readers a wonderful Christmas break.
Sources of images
- Google maps
- National library of Scotland
- Cambridge Museum of Technology and Dayhoot
- Wikipedia
- Wikipedia
- Me (Dr David Stupple)
