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D.C. POWER STATIONS - Boiler Economisers
By the time the Bath Electric Tramways power station was constructed, a large number of economy devices were available to prevent energy wastage.
The cold feed water to the boilers was first warmed by heat which would otherwise have been wasted up the chimney. Between the boilers and the chimney was a device known as an economiser. (The Walcot power station used a 'Clay Cross' economiser, named after the limekilns where it was first used). This comprised a brick box in which the hot flue gasses passed through a labyrinth of iron pipes contaning the feed water.
The snag with many of these economisers was the heavy deposit of soot which built up on the pipes, impeding heat transfer. In the Clay Cross Economiser, automatic scrapers were arranged to slowly and continually run up and down the surface of the pipes, scraping off the soot and allowing maximum efficiency to be maintained. They were driven by chains which could be powered from an auxiliary engine, line shafting or an electric motor.
After the steam had been produced, it passed through a superheater before leaving the boiler. The superheater is a grid of pipes, suspended in the hot furnace gasses. The temperature of the steam is raised which makes it capable of greater expansion in the cylinders of a steam engine which has been designed to take advantage of this property.
In the field of steam locomotive design, a great deal of controversy attended the introduction of superheating as it involved extra weight of pipework in the locomotive boiler. With a stationary installation, the weight of the extra pipework is of no consequence and the increased efficiency more than compensates for the extra costs of the superheating apparatus.
Many economies were made in the steam engine, these will be dealt with under a separate heading.
Working to Vacuum
Instead of the steam being exhausted from the engines into the atmosphere, it was connected to an evacuated condenser. This effectively removed atmospheric pressure from one side of the engine piston, increasing the force available from the steam on the opposite side without the need for greater steam pressure.
The cooling surface of the condenser was kept cool by the circulation of cold water and condensed the exhaust steam promptly. A small pump worked continuously to maintain the vacuum and remove any air which may have been carried in with the steam. Large quantities of cooling water were needed and these were often drawn from a nearby river (the Bath Electric Tramways Walcot power station was built beside the river Avon, just upstream of Pulteney Weir, which maintained a good depth of water).
The extra efficiency gained by the station easily made up for the small extra power drawn by the cooling pump and air pump. If these failed, or if the river level fell below the intake pipe, the steam could be allowed to escape to atmosphere, but this was noisy and inefficient.
The condensed water was led away before it could become chilled, so that it was already warm when it came to be circulated back to the Clay Cross economiser and begin the cycle again. This not only reduced the energy needed to re-heat it, but reduced the consumption of fresh water which would have either needed treatment to remove dissolved minerals or would risk them fouling the pipework and boiler tubes.
During a period of coal shortage following the 1914-18 war, the Chief Engineer of the BET system, W.E. Hardy, developed a number of other economy measures for which he received Government recognition. What these measures were has not yet come to light.
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