Monthly Archives: August 2013

Mercury and aeroplanes

One of the many items you are not permitted to bring aboard aeroplanes is a mercury thermometer (or more accurately “mercurial thermometers and barometers”). Why?


Aeroplanes are made largely of aluminium, as it has one of the best available strength-to-weight ratios.* When aluminium is exposed to air it forms a tough coating of aluminium oxide that doesn’t flake away like iron oxide (rust) does and which prevents chemicals from reacting with the aluminium.

But if the raw elemental aluminium is exposed (e.g. by a scratch) and comes into contact with mercury it forms an amalgam, tearing away at the aluminium and causing it to lose its structural integrity. As the aluminium is eaten away it combines with the air to form aluminium oxide and falls away (as seen in this video). This allows the mercury to reach fresh aluminium and the process then repeats, so a small amount of mercury can do a large amount of damage. If a mercury thermometer were to leak aboard an aeroplane the aeroplane would need to be taken out of service and disassembled to assess the damage the mercury might cause. There have been at least two incidents in which aircraft exposed to mercury have been written off by their insurers.

The forty second timelapse video above shows the effect of a small amount of mercury on an aluminium I-beam over the course of two hours.

* Titanium has a better strength-to-weight ratio (288 kNm/kg compared with aluminium’s 214 kNm/kg) but it costs about five-and-a-half times more. Titanium is used in jet engines, where strength-to-weight ratio is key.

Cooling towers


A pair of metal cooling towers at a power station in the Ukraine.

Cooling towers are an essential part of many power stations. Although they are most commonly associated with nuclear power stations, they are found in any power station that uses heat from burning fuel to generate electricity. (Drax, a coal-fired power station that is the largest in Western Europe at 3960 megawatts has twelve cooling towers.)

An aerial view of Drax coal-fired power station.

The purpose of a cooling tower is to extract waste heat from the power station so that the working fluid (water) can be reused. This waste heat passes into cold water, heating it in the process, and this warm water is then fed into the cooling tower where it cools and is then reused. The separation of the working fluid (purple and blue) and the waste heat extraction (grey pipes) is obvious in the diagram below.


The space below the water inlet is filled with a material designed to increase the surface area of the water in contact with the air, helping to speed up the process of heat loss.


Fill plates mounted underneath the outlet nozzles.

The hyperboloid shape of cooling towers ensures continuous non-turbulent airflow, maximising the amount of air per unit time moving through the tower. Air enters at the bottom of the tower and accelerates up the tower as it is heated by the warm water, drawing fresh cool air in at the bottom due to the stack effect. In the photograph of a disused cooling tower below, the spaces at the bottom of the tower through which air enters are clearly visible.


Source: Tom Blackwell