THE DESIGN OF INDUSTRIAL CHIMNEYS.

[Stanley]

THE DESIGN OF INDUSTRIAL CHIMNEYS.

Over the years, one of the most common questions I have been asked about chimneys is ‘Why do they have an ornate top?’ One very simple answer is that they look better, I don’t know why this is but if you look at classical architecture every column had a finial before it joined whatever it was supporting. There is a similar rule which says that long straight lines have to curve up slightly in the middle because otherwise the eye tells they are sagging. Something to do with the physiology of the eye and the way our brains process information.

Most classic industrial chimneys have a flange or oversiller near the top and this, apart from being pleasing to the eye make the chimney more efficient. I suppose the next question is ‘efficient at what?’ so before we look at the oversiller we’d better have the short explanation of what a chimney’s function is.

In the era of the great brick and masonry chimneys, every one of them was erected to provide draught in steam boilers. The steam may have been for an engine, process steam or heating. This word ‘draught’ is always used and most people are under the impression that by some mysterious property inherent in its design, the chimney sucks air through the boiler furnace and makes the fires burn hotter. This is incorrect and before you can understand the finer points of a chimney you need to know exactly what is happening.

The chimney is a way of enclosing the hot products of combustion from the furnace and conducting them to a height so as to produce a column of heated gas. Recognise that atmospheric pressure exists; everything on the surface of the earth is exposed to the weight of the atmosphere which, broadly speaking can be measured as sufficient to hold up a column of mercury 30 inches high (mercury gauge) or 30 feet of water (water Gauge). This is the reason why meteorologists express atmospheric pressure in terms of either inches or millimetres of mercury. The second fact to appreciate is that hot flue gases are lighter than air, they are less dense and the hotter they are, the greater the differential.

So, a chimney is a column of light gas surrounded by the atmosphere which is heavier. This is the key property of the chimney because, as the column of hot gas is completely enclosed right the way down to the furnace, whatever conditions prevail in the chimney are transferred to the furnace. Because the gas in the flue is lighter there is a differential in the pressure between that existing in the flue gas and atmospheric pressure. A chimney 200 feet high with flue gas at 650 degrees Fahrenheit will give a differential of approximately one and a half inches of water gauge. The front of the furnace is open to atmospheric pressure and it is this differential that forces air into the fire and aids combustion. So the key fact here is that the chimney isn’t ‘sucking’, it is creating the conditions which allow the atmospheric pressure to force its way into the furnace, to ‘blow’ the fire.

If we understand this clearly we can now appreciate some basic facts of chimney design. The greater the area of the fires in the furnace of the boiler (or boilers) the greater the volume of flue gas produced and the larger the diameter of the chimney. This is completely analogous with any pipe conducting a fluid, at a given pressure, the larger the pipe, the more volume it can accommodate. The general rule used by the old engineers was that the internal area of the chimney should be an eighth of the grate area in the furnaces with an allowance for friction in the flues. In practice, the smaller the grate area, the higher the allowance.

Most engineers also agree that under normal conditions three quarters of an inch of water gauge is sufficient to run the average Lancashire boiler. Bancroft chimney was 40 yards and gave about this amount of draught at the furnace but was only just adequate for heavy loads. Ellenroad at 70 yards was adequate for five boilers and wonderfully efficient on one boiler as we ran it. The truth is that the more draught an engineer has the better he can run his boiler because you can always cut the draught back with dampers in the flues but you can’t do anything to increase an inadequate pull. The governing factor in the end was the cost of the chimney and I have never seen a boiler installation that couldn’t have been improved by more draught.

Let’s digress just a little and look at the reason why I make this statement because I think that when we rebuilt all the boiler flues and main flue at Ellenroad I discovered something which I have never seen mentioned in any of the literature on flues.

Ellenroad had been very heavily loaded during its life and by the time I got there in 1984 the whole of the boiler settings and flues were in very bad condition. Settlement, attrition of the brickwork and the destructive forces of expansion and contraction had brought the structure to a sorry state. I made the decision very early to reset the boiler completely and renew all the brickwork as far as the chimney bottom. The consequence was that when we had finished we had a boiler with completely airtight flues, the lowest friction attainable because of smooth surfaces and a chimney originally designed for five times the load. I always knew that this was all to the good but even I was surprised the first time we fired the boiler on the new flues because it only needed one and a half tons to get the boiler from dead cold to 120psi pressure. As a comparison, the boiler at Bancroft took five tons to achieve the same pressure.

My old mate Newton Pickles was with me when we did this and he was amazed, he said that in all his experience he had never seen a boiler make steam with so little coal. I had to think about this for a long time before I finally realised what the reason was. I ran the theory past Newton and he agreed.

What normally happens with a cold boiler is that when you first light the fires you are relying on the natural draught in the chimney; being enclosed it is slightly warmer than the atmosphere and so has a very small pull even when cold. To maximise this draught you have to open the dampers in the side flues as wide as possible. These ‘dampers’ are sheets of steel which slide down in grooves set into the flue wall and can be controlled from the front of the boiler to regulate the fire. Because Ellenroad had perfect draught on account of the new flues and the size of the chimney, as soon as the fires were established you could close the side flue dampers down to within four inches of the floor.

Consider the conditions in the side flues, as the furnace gases passed through on their way to the chimney the hottest gas rose to the top of the flue and the coldest crept along the floor. In a normal installation with the dampers wide open, all this gas passed through the dampers and into the chimney. With the conditions prevailing in the new flues at Ellenroad, the dampers were within four inches of the floor and only passed the coolest gas. This meant higher gas temperatures in the side flues and consequently better heat transfer to the boiler. Apart from the fact that the flues were completely airtight, this was the only other factor. Bear in mind that this condition prevailed the whole of the time the boiler was running. At Bancroft, on heavy load, the only way we could burn coal fast enough was to have the side flue dampers completely open so it is reasonable to assume that a similar advantage could have been gained during normal running. There is no way of proving it but this is the reason why I think the old engineers had it wrong. Building higher chimneys would have been a greater initial expense but over the life of the plant it could have effected great savings.

Right, let’s get back to the chimney. We’ve looked at the factors that govern the amount of draught and the ability of the chimney to cope with large volumes of gas. As is always the case, it’s not quite as simple as this. There are many other factors that can influence the draught a chimney can provide. Think about atmospheric pressure, the higher the pressure the more draught the engineer has at his furnaces. Boilers on natural draught were always easier to run in fine weather with high pressure, a hard frost was ideal. The worst day was a dank wet day with low pressure. Those of us reared in the days of open fires in the home can well remember the phrase ‘That fire’s burning frost.’ The colder it was outside, the whiter the fire burned and the more heat you got off it.

The location of the chimney was important. Consider the condition at the top of the chimney. The products of combustion are moving out and up but have only a very small differential pressure driving them out. Suppose you could arrange a draught of air downwards onto the chimney top, it wouldn’t have to be very strong to reduce or entirely overcome the advantage of the hot gas. To go back to house flues, this condition is known as downdraught and it was not unusual, with a bad flue, to get smoke puffing back into the room during windy weather. Exactly the same thing can occur with an industrial chimney, say in a valley bottom, where the wind is blowing down on the chimney. For this reason, many flues in these situation are conducted up the hillside underground to stubby chimneys built high up on the hillside. These are the cheaper equivalent of a very high chimney in the valley bottom.

Another factor that interferes with the draught is air turbulence round the stack. We spend all our lives at ground level. The air that passes us, the wind, has been slowed down by friction on the ground and local features like buildings or trees. 200 feet up in the air the same wind is probably moving twice as fast. Anything that obstructs the passage of this wind causes turbulence, vortices form as the wind is diverted round the chimney and in a steady wind you would find, if you could measure it, that there was lower pressure on the leeward side of the chimney than on the windward side or above the stack. The effect of this is to capture the gas as it escapes from the stack and drag it down the back side of the chimney. This reduces draught in the flue and also damages the ability of the chimney to fulfil one of its lesser functions, to get the gases away from the factory at high level so as not to constitute a nuisance. This is where the design of the chimney head can have an effect beyond purely aesthetic considerations.

We know far more about aerodynamics today than the old chimney builders did. However, they worked on experience and over the years the better men had noticed that certain constructions improved the performance of the chimney under adverse conditions. They noticed that string courses and ornamented brickwork, for instance the incorporation of recessed panels, improved the air flow on the head. We know now that what they were doing was guiding and breaking up the vortices and reducing their ability to produce a downdraught on the head. They soon found that by far the best device was to build a cill, a fin or as it became known an oversiller, just below the chimney head. They didn’t know it but this was acting as an aerofoil, smoothing and directing the air flow and improving the performance of the chimney head. These could be very ornate and gave an opportunity to make stacks very individual. In later years, Howarth’s steeplejacks at Colne specialised in rebuilding chimney tops with multiple oversillers or fins. I think the last survivor was the chimney on the laundry at the traffic lights at the junction of North Valley Road and Skipton Road in Colne.

In the interests of brevity I made an untrue statement earlier; ‘In the era of the great brick and masonry chimneys, every one of them was erected to provide draught in steam boilers.’ There were other reasons for building chimneys. Chimneys were used to ventilate large sewer systems in towns. Waste disposal was another role, many councils installed incinerators for household waste and used chimneys to provide the draught. I don’t know whether it still exists but there used to be a chimney at Falmouth in Cornwall which was called ‘The King’s Pipe’ which was used exclusively for burning contraband tobacco. The famous Wainhouse tower at Halifax is an incredibly ornate chimney built by a manufacturer but never used. Tradition has it that one of the design considerations was that it should cast a shadow over a rival’s garden!

Finally, there is one more consideration which, whilst it was a consideration in the early days, is now a strict design element. There is an old adage; ‘What goes up must come down’. This is as true of combustion gases as it is of a stone but the process is slower. Modern power station chimneys are built to ensure that the ‘plume’, the products of combustion, fall in the place where they will cause the least problems. The higher they can be injected into the atmosphere, the greater the chance of dispersal and if, as in the case of power stations in the Eastern counties of the UK, you can arrange matters so that the plume-fall is in the North Sea, so much the better. This is the reason why the giant stack at Drax power station was built to a height of 850 feet.

If I have proved anything in this essay it’s probably that chimneys fascinate me. I have spent a lot of time looking at them, repairing them, using them and climbing up them. My good friend Robert once described them as ‘The lonely sentinels of the Industrial revolution’ and went on to buy many and preserve them for posterity. I have to say I thoroughly approve of this. They are immensely complicated and very beautiful structures and we should preserve as many as we can. One final thought for anyone who watched the acres of archive film that was screened in 1995 on the fiftieth anniversary of the end of WWII; did you notice that in the panoramic views of Hiroshima and Nagasaki the only structure left undamaged were the brick industrial chimneys? These slender and improbable structures are evidently far stronger than their appearance suggests. Beauty, utility and strength, what more could you ask of a building?

SCG/14 March 2006