Cool shapes
Why are power station cooling towers shaped the way they are, with a wide circular top and an even wider circular base connected by an inward-curving middle?
鈥� Natural-draught cooling towers are built in a rotated hyperbolic shape for structural reasons. A cylindrical tower would be equally effective in getting rid of waste heat, but it would need to be built from much thicker concrete to withstand high winds. There are minor variations on the shape which change the position of the 鈥渨aistline鈥� of the tower, but these are not significant to the thermal performance of the tower.
A cooling tower is built as a thin wall of steel-reinforced concrete only 100 to 200 millimetres thick. There is very little inside the tower apart from a layer of packing near the bottom, around 3 metres deep, which acts as a heat exchanger. As warm water from the turbines trickles down through the packing it transfers its heat to the cold air that is rising through it. The remainder of the tower is an empty chimney through which the warm, moist air rises due to its buoyancy, pulling cooler air in under the skirt.
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The only emission from a cooling tower is water vapour 鈥� its appearance depends on the weather conditions. When there is a temperature inversion, as there often is in winter in this part of Yorkshire, cooling-tower plumes rise vertically for several thousand metres. At dawn or dusk the plumes can appear red, prompting calls to the station enquiring if anything is being discharged.
Dafydd Wynn
Performance Manager
Ferrybridge C power station
Knottingley, West Yorkshire, UK
鈥� Essentially, the cooling tower is just a large chimney. The base is larger than the top to give it stability. The structure of a cooling tower can be understood by carrying out the following experiment. Cut out two cardboard discs of different sizes, mark out the same number of equally spaced holes on the perimeter of each and join them up with equal lengths of string. Holding the strings taut, twist one disc relative to the other. The result is the shape of the hyperbolic natural- draught cooling tower.
Cooling towers are built in this shape because it allows the vertical stresses resulting from the weight of the tower itself to be transmitted in straight diagonal lines down to the foundations. It is a very strong and stable shape: a piece of string stretched from the bottom skirt to the top lip along one of these diagonals should touch the shell at all points. The supporting pillars at the base of the tower are normally angled \/\/\/\/\/\/ in both directions to transmit the dead load in straight compression (see Diagram). If diagonal reinforcing bars are used they can be straight, although circular horizontals would still be necessary to give the shell hoop strength, against wind load, for example.
The shape owes everything to the simplicity of the civil engineering design and has no bearing on the efficiency of the cooling towers. In some parts of Germany it is still possible to see older, steel-structured and sheeted cooling towers, which are essentially conic with multifaceted, straight-sectored sides, and no concave shape at all. They work just as well.
Tony Finn, FIChemE
Hull, East Yorkshire, UK
鈥� Cooling towers have curving hyperboloid walls. A hyperboloid is the surface that is described by the rotation of a straight line that is in a different plane from the axis of rotation; the result is a cone or a cylinder if the line is in the same plane as the axis. The walls are built to this shape for structural, thermodynamic and aerodynamic reasons.
The shell of the hyperboloid is curved in two directions, while a cylinder is curved in only one direction. This gives it great structural stability. Such a shell can be economically constructed of reinforced concrete and is surprisingly thin for its size, typically only 200 millimetres.
Radko Istenic
Ljubljana, Slovenia
鈥� A model of a hyperboloid can be seen at
Robert Hart
Nottingham, UK
Shades of brown
My newborn son produced a vast amount of tarry black faeces in the days following his birth. Then he produced a yellow/orange deposit and I鈥檓 told it will eventually become a more normal brown colour. Why the multicolour display?
鈥� The tarry substance passed by your newborn is meconium, the substance which packs the unborn baby鈥檚 bowel. The yellow/orange stools are perfectly normal for a breast-fed baby. If you bottle feed your son, the stools will look more 鈥渘ormal鈥� and brown.
Once the baby begins to take solid food, the contents of his nappy will vary, depending on what he has eaten. One of my sons ate a turquoise wax crayon, with startling results. Beetroot can give quite a fright 鈥� it tends to look like serious bleeding.
S. Haxby
Christchurch, Dorset, UK
This week鈥� question
Reading matter
I wonder if reading difficulties such as dyslexia arise from the fact that we scan from left to right. If so, readers of Arabic and Hebrew should not experience the same problems. Do they, and if they don鈥檛, do they have other problems? And what about people who read Chinese, which does not use phonetic symbols?
F. Murphy
Bateman, Western Australia