FIRE-RESISTANT TRANSFORMERS FOOD PROCESSING FOR INSTALLATIONS

PYROCLOR-FILLED UNITS SAVE AND SPACE AS WELL AS CABLE INSTALLATION COSTS

Issued by Engineering in Britain Information Services, Published by The Certificated Engineer May 1967

In a modern food factory, where electric power may be required for boilers, ovens, dryers, refrigerators and furnaces as well as for mixing, grinding, sorting, and packaging machinery of all kinds, the cost of L.T. distribution cable from power sources to the various load centres can be very high unless the transformers used are sited close to the equipment they are to serve.

This normally means that fire-resistant transformers will be required as, for example, in the South African Railways pre-cooling sheds for citrus fruits at Table Bay Docks, where 6 x 750 kVA units have been installed. For the new works at Banbury, England, where Maxwell House coffee is manufactured by Alfred Bird & Sons Ltd., six transformers-three 1000 kVA and three 1 750 kVA-have been installed. Three of these are sited inside the building; the other three, owing to space limitations, are outside but close to the factory wall.

In all cases significant economies have resulted from the choice of transformers filled with Pyroclor. Their use in place of oil-filled units has made elaborate fire precautions such as firewalls and oil-retaining sumps or pebble drains unnecessary, with a consequent saving in space as well as direct cost. Nor did special fire-protection equipment have to be provided.

Significant economies

The new ￡7 million factory at Banbury replaces Bird's scattered premises in Birmingham. The company is very conscious of the need for fire precautions and appointed the Oxfordshire County Fire Service as advisors to the architects and engineers for the new factory before the detailed planning began. Consequently, fire precautions have been taken into consideration in every stage of the project, from the layout of the site roads, to give easy access for fire fighting vehicles, to the structure of the building and the equipment used.

Wherever there was space for a transformer to be installed close to the equipment it would supply, further savings resulted from the reduced lengths of L.T. feeder cable. Economies which can be made in this way are considerable. For example, nearly 1 ton of copper supply cable can be saved in reducing by 400 feet the distance between the service point and the low voltage side of a three-phase, 1000 kVA 13 500/380-volt transformer. At the same time power losses are reduced by some 18 kW, while the fact that no fire precautions are needed can make the total installed cost of a transfer containing Pyroc1or as much as 22 per cent lower than that of an oil-filled equivalent.

Askarel fluids

Pyroclor is an askarel-defined as a dielectric fluid which is both non-flammable and incapable of evolving explosive gas mixtures, even when decomposed by an electric arc. It is a mixture of 60 per cent hexa-chlorodiphenyl and 40 per cent trichlorobenzene by weight, with the addition of 0.125 per cent phenoxy propane oxide as a gas scavenger. Potentially Pyroclor is useful in any equipment which requires a fire-resistant cooling fluid with good electrical properties in the range -30°C to + 120°C. It has been widely used, not only in transformers but also in induction voltage regulators and is now finding its way commercially into transformer/rectifier assemblies.

Pyroclor has a dielectric strength of 40 kV minimum at 25°C, as tested by B.S. 148:1959, and suffers no thermal decomposition below 175°C. If the material is raised above 175°C-for example, by local overheating or transient arcing-small quantities of hydrogen chloride are produced; but these are absorbed by the phenoxy propane oxide gas scavenger and so prevented from attacking the iron core or cellulose insulation of the transformers. The fluid suffers no significant change below about 300°C. Above this temperature it breaks down into carbon and hydrogen chloride, the latter forming 97 per cent of all gases evolved.

The economics of Pyroclor

Apart from its fire resistance, a particular advantage of Pyroclor over mineral oil is that it does not hydrolise, nor is it subject to oxidation, sludging or other chemical degradation. Consequently no replenishment or maintenance is necessary, which leads to significant economies, especially where unskilled or newly trained staff are concerned. Pyroclor transformers can be permanently sealed, the initial charge of fluid lasting for at least 30 years.

But the main savings to be achieved are lower installation costs (in spite of the higher cost of the Pyroclor transformer itself) and reduced electricity consumption where transformers can be sited close to the load centre.