CFC-free
I read that Britain has a mountain of discarded refrigerators because there is a lack of facilities to destroy the CFCs in the foam insulation. I would have expected the CFC gases in the foam to have long since diffused out and been replaced by air. If not, why not?
鈥� The practice of recovering blowing agent 鈥� the gas used to create the foam 鈥� from a refrigerator at the end of its life is not new. It has been carried out in Germany, Sweden, Japan and elsewhere for some years, and at least 250 grams of blowing agent are recovered at the decommissioning stage. This compares favourably with the 100 grams of refrigerant also removed from the unit at that time.
The original decision to target this blowing agent was prompted by studies of old refrigerators by appliance manufacturers, at the request of the United Nations Environment Programme鈥檚 Technical Options Committee on Foams. The committee is an adviser to the parties to the Montreal Protocol on environmental damage. The studies suggested that more than 90 per cent of the original blowing agent still remains in the cabinet after 15 years or more. So much is retained because the foam is totally encapsulated by steel and a plastic liner.
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European Regulation 2037/2000 calls for the recovery of these blowing agents by member states. Initially there was confusion in Britain about whether the text of the regulation referred to foam-blowing agents as well as refrigerants, and the apparent slowness of the European Commission in clarifying this point led to some of the hold-ups we have seen in Britain in the past year. But the technical case for recovery has not been in doubt for some time.
The cost-benefit of such processes can always be questioned and the regulation makes provision for this by defining some of the CFCs not covered by existing legislation that may be recovered 鈥渋f practicable鈥�. The word 鈥減racticable鈥� has technical and economic dimensions since, although the recovery of blowing agents from foam in refrigerators is technically feasible, it might not be viewed as economically viable. For this reason the British government at first felt that recovering blowing agent from foam should fall into the 鈥渋f practicable鈥� category. The European Commission finally ruled otherwise and the rest is history.
Paul Ashford
Co-chair of the UNEP Technical Options Committee on Foams
Bristol, UK
Shapely curves
Why are the walls of a barrel curved? Why not save the trouble of bending the staves and make it perfectly cylindrical?
鈥� My grandmother, whose maiden name was Cooper, tells me that it was to allow the restraining iron hoops around wooden barrels to be pounded on tight at each end over the dry, fitted stave wood. The shape also allowed the filled cask or barrel to be rolled and steered easily.
A barrel can be thought of as two wooden buckets, fitted top totop. If you make a wooden bucket, similar to half a barrel, you must make it with a tapered shape so that the iron hoops can be pounded down on the taper to pull the wooden staves together while dry. Then, when the bucket is filled with water and the wood swells, the bucket is watertight.
Often, objects that were originally produced in wood retain their shape when new materials such as galvanised steel are introduced. A galvanised steel bucket has the same tapered shape as a wooden bucket, even though a cylindrical bucket would hold more, as would cylindrical metal beer kegs.
There is another, final, benefit to the shape, however. Large plastic buckets have a design that is just slightly more barrel-like than a cylinder to allow them to be stacked, yet hold a maximum amount in a minimum of floor space. The metal barrels used for petroleum products are also cylindrical because they are not moved with muscle power, but with a forklift.
Rosemary Antel
Seattle, Washington, US
鈥� The traditional practice of storing barrels of beer and wines in cellars below pubs and bars means they are lowered from a street-level trapdoor by rolling them down a pair of sloping parallel rails. If barrels were cylindrical and rolled down even slightly off-line they could fly off the rails with spectacular results. But if the containers narrow towards each end, they will self-correct as they roll down.
On a level floor, the barrel shape also makes manhandling by turning and swivelling easier. Cylindrical barrels are hard to steer.
John Schapel
Maslin Beach, South Australia
鈥� Real beer is not filtered (or pasteurised) before being drawn off into casks, so fermentation continues while it is being stored in the brewery鈥檚 cellar. When the cask is horizontal the curved shape allows the flocculated solid particulates 鈥� dregs to you and me 鈥� to settle in the belly of the cask, below the level of the tap. This means that, in effect, the beer is being decanted when it is pulled into the pipe up to the bar.
David Wright
Dursley, Gloucestershire, UK
鈥� On a related point, consider the much larger cousin of the barrel, the wooden vat used for maturing alcoholic beverages and, until recently, for chemical manufacture. These can hold up to 450,000 litres and are not designed to be moved. They are therefore made with straight staves and have a slightly conical appearance.
Because the diameter at the top of the vat is smaller than that at the bottom, the hoops can be driven on against the taper, as before. But the modern way of hooping a vat uses threaded bars that can be tightened with a nut, doing away with the need for a taper and resulting in a cylindrical vat, the design suggested by your questioner.
Richard Foskett,
Yateley, Hampshire, UK
鈥� The largest wooden barrel in the world, 8.5 metres wide by 7 metres high, can be found in Heidelberg Castle in Germany. It was made in 1751 from 130 oak trees, and can hold more than 220,000 litres.
Davide De Focatiis
Engineering Department University of Oxford, UK
This week鈥檚 question
Quick sand, slow sand
The sand on beaches in South Carolina is so hard you can ride a bike on it. By contrast, the sand on the central Oregon coast is so soft that even walking fast over it is difficult. This difference is maintained whether the sand is wet or dry. Why?
Marsha Veit
Chappaqua, New York, US