FLUES AND CHIMNEYS.

[Stanley]

FLUES AND CHIMNEYS.

Ask anyone to describe a factory chimney and they will know exactly what you mean. There are still enough of them left for them to be an instantly recognisable part of our landscape. However, if you were to then ask what they were for I suspect that many people would start to struggle, they would know that ‘smoke’ comes out of them but that would be the extent of most people’s knowledge. Perhaps a gentle guide to chimneys and flues might be in order.

If I was asked for a definition of the word chimney I would say that it was any structure designed to promote a draught in order to get rid of gases at a high level so that they could dissipate. My reason for this phrasing is that whilst the vast majority of chimneys were built to serve steam raising boilers many had a different purpose. Before we go into that I’d better explain exactly how a chimney functions.

In effect a chimney operating on natural draught is a tube constructed high enough to contain a column of gas at a higher temperature than ambient in order to obtain a pressure differential at its base. I know this sounds over-complicated but until you grasp exactly how a chimney works you can never understand how it can be useful. The opening into the base of the chimney is subject to normal atmospheric pressure. The column of gas inside the chimney weighs less than a corresponding column of cold air and so the pressure in the chimney is lower than that at the base. This small pressure differential is enough to allow atmospheric pressure to force air in at the base thus creating what we call a draught up the chimney. The higher the temperature of the gas the lighter it is and the bigger the differential. The height of the chimney also has an effect, the higher it is the more draught. In essence, the chimney isn’t ‘drawing’ on the base, it is creating a circumstance where atmospheric pressure forces air in.

Right, back to the ‘gases’, these can be anything from the products of combustion in a furnace to fumes from a chemical process or even sewer gas. Anything that needs to be extracted and dissipated at high level. We shall be concentrating on products of combustion but it’s as well that we recognise that the other uses exist.

The most common use of a chimney was to produce a draught on the furnace in a steam boiler in order to promote fierce combustion. We are interested mainly in coal-fired Lancashire boilers but it’s as well to recognise that they also served oil and gas fired plants. In many cases these used forced or induced draught produced by fans to supplement the natural draught and this system could be used for coal but we are going to concentrate on natural draught.

The classic set up was a Lancashire boiler which is a shell boiler; a large cylindrical tank of riveted construction with two internal iron furnace tubes which each had a grate at the front. Coal is burnt on these grates using air admitted from the front of the boiler. The products of combustion, flaming gases, are drawn along the furnace tubes to the back of the boiler heating the water in the shell as they pass. At the back of the boiler these hot gases are diverted downwards in the ‘downtake’ and back underneath the boiler to the front. They travel through the ‘flame bed’ or ‘sole flue’ heating the bottom of the boiler as they go. At the front of the boiler the gas flow splits in two and travels back along the ‘side flues’ heating the sides of the boiler. At the rear of the boiler the gases pass through the side flue dampers which are metal shutters which can be used to control the amount of draught on the furnaces. Once out of the side flues the gases are in the main flue and go directly to the chimney either via an economiser which heats the boiler feed water or direct to the chimney bottom up the ‘by-pass’ flue. So, the whole of the water bearing area of the shell of the boiler is bathed in hot flue gas and this is the point of transfer of the heat from combustion to the water in the boiler. Once the correct temperature is reached the boiler will start to produce steam. The pressure of the steam is governed by the temperature of the water and the safe levels imposed by the strength of the construction of the boiler. Most Lancashire boilers ran at about 150 pounds per square inch. Approximately 12 times normal atmospheric pressure. This steam could be used for heating, process steam or in our case, driving a steam engine in order to produce rotative power on the shafting to drive machinery.

Right, you’ve got the general picture I think. In passing it’s interesting to note that a really good Lancashire boiler, properly installed and fired could attain about 70% thermal efficiency. Allowing for friction losses in the drive train this could equate to about 35 to 40% thermal efficiency at the point of use. The most efficient electric motor drive from a modern power station is about 15% efficient at the point of use. Now there’s a fact to ponder on……..

So, if you’ve been following me you now know that the chimney is only part of the plant, the flues are just as important. In a Lancashire boiler plant these flues are made of brick (apart from the furnace tubes in the boiler itself). The construction is ordinary high quality brick faced with a layer of what most people call ‘firebrick’. Firebrick is actually a very low grade of ‘refractory brick’, that is a brick made to withstand very high temperatures. Ordinary brick breaks down at the temperatures reached on the internal faces of a flue. The nearer it gets to the boiler the hotter it is and in the downtake and flame bed it can be white hot. The best material is ‘Number one grade refractories’ which are very accurately made and can be laid with a very close joint which makes them last far longer. This lining in the flues extends into the chimney in order to protect the brick construction. The temperature in the chimney bottom of a hard-fired boiler can reach a point where it will melt lead, 622F. I mention this because it was one way the old engineers read the temperature, they would poke a piece of lead wire in to see if it melted. The chimney liner was sized to suit the installation, a 150 ft high chimney would have about a fifty foot liner.

The height and diameter of the chimney was dictated by how much gas it had to deal with and in some cases the physical location of the stack. A chimney in a valley bottom might have to be higher to avoid downdraughts on the head because of the surrounding hills. A common ploy in a situation like this was to lead the main flue up the hillside underground and have a shorter chimney on the crest of the hill. This explains to small detached chimneys you will see in places like the Todmorden Valley which don’t seem to be connected to anything. There were many formulas for sizing stacks. A common one for the area of the chimney at the top in square inches was 180 x area of grate in square feet divided by the square root of the height of the chimney. The one thing they all had in common was that the chimneys were too small, no doubt the expense had something to do with this but it was a big mistake. I have only ever seen one boiler that had all the draught it needed, this was at Ellenroad where the chimney built to service five boilers had only one on it. This gave a draught of over one inch water gauge at the furnace and resulted in a very economical installation.

There were other factors which determined the draught available at the furnace. Properly designed and built short flues were an advantage as there is friction between the gases and the face of the flues. Another obvious factor but not well recognised is the ambient atmospheric pressure. If the temperature of the gas in the stack is constant it stands to reason that the higher the atmospheric pressure the greater the draught. Chimneys always performed best in dry frosty weather with high pressure and worst on damp foggy low pressure days. It makes a significant difference. Another factor is the temperature of the brickwork under the boiler. In winter it was noticeable at Bancroft that the settings took until Wednesday to get fully hot after the weekend break. Monday morning with full load on the boiler was always bad news. Three days into the week and we could play with the same load.

Perhaps the most important factor was the amount of attention and maintenance given to the flues. All old boiler settings developed leaks, they would suck in cold air from outside. There were two good ways of dealing with this, the first was the ‘magic wand’. This was a piece of half inch iron water pipe with a rope threaded through it. This was kept in a bucket of paraffin in the cellar and so was always soaked with oil. If you lit this and went over the outside face of the brickwork the flame was sucked into any fissures and you could block these with fireclay. The second line of attack was to whitewash the outside of the settings with thick whitewash; hydrated lime and water. Apart from brightening things up this stopped up a lot of tiny leaks which could be missed with the wand. Because of expansion and contraction the brickwork was always developing cracks and once a year, at the annual summer shut down all these were repaired.

If you look into a coal-fired furnace with a good bright fire burning you will see that the power of the draught pulls sparks into the furnace tube. These are small pieces of burning coal light enough to be carried away into the flues. The harder the boiler is fired, the more sparks and as these burn out and cool down they settle in the flues. This is not ‘soot’ such as you get in a domestic fire which is unburnt carbon, it is ‘flue-dust’ or ‘fly-ash’ and is completely burnt.

Over time, this flue dust accumulates in the flues and if it wasn’t cleared out it would eventually block the flues completely. We used to ‘flue’ the boiler at Bancroft four times a year at holiday times when we could shut the boiler down and cool the flues sufficiently to allow entry. I should mention here that a Lancashire Boiler is always kept in steam. Each night it is ‘banked up’. The grates are cleaned and about twenty five shovelfuls of coal put in each furnace, ten each side and five down the middle, with just enough draught on to carry the smoke away up the flue. This banked fire burned all night and kept the temperature of the boiler and settings, and therefore the pressure, up all night. When you came in in the morning the pressure was about 140psi. All you had to do was waken the fires up and you were ready for a start. The usual flueing times were Christmas, Easter, Wakes Week and September holidays.

There is only one way to get the flue dust out of the flues and settings of a Lancashire boiler, you have to cool the flues down enough to allow men in and then someone has to get ‘rugged up’ and go in and scrape and shovel the flue dust out. At Bancroft the only way out was at the front of the boiler either through the furnace tubes or via small access holes at the front of each side flue into the pit at the front of the boiler. From there it was shovelled and barrowed out and piled in the yard. There was never any trouble disposing of the flue dust because local builders and flaggers took it away free. Unlike sand, it has very sharp edges and if used as a bed for stone flags it will never move once the flag is dropped on it.

Rugging up consisted of at least one boiler suit and swathing the head and face with cotton fents so that only the eyes showed, a pair of goggles and some knee pads made out of old carpet completed the outfit. I can’t remember ever seeing a fluer wearing gloves. There were no face masks as such, the fents filtered the dust out before you breathed it in. The tools were scrapers, fire shovels, ordinary shovels and hand-brushes.

Our regular flue chap was Charlie Sutton and the firm was ‘Weldone’ of Brierfield. They swept domestic chimneys as well as industrial boilers and their busy times were always in the holidays. He had a regular mate, Jack, a hard lad, and Charlie and Jack were the ‘inside men’. There was at least one labourer to get the dust away from the boiler front but they never went in the flues.

Charlie and his gang were always booked for the day after we stopped the engine. We had to get ready for them as soon as the engine stopped. We used to allow the water to evaporate away for the last couple of hours as the engine was running. In fact John Plummer, the firebeater would be burning his fires off an hour before we stopped, one of the advantages of a Lancashire boiler is that because it holds so much water there is a great reserve of steam even though the fires are out. By the time the engine had stopped John had burned off and cleaned all the ash and clinker out of both furnaces and the ash pit below. What followed will make most engineers cringe if they haven’t worked with Lanky boilers, I used to go up on top of the boiler and lift the low water valve, this was a safety valve designed to operate in conjunction with the main dead-weight valve in conditions of high steam or low water. By lifting the valve and supporting the valve weight on a couple of bricks I opened the boiler up to the atmosphere and steam roared out up a three inch pipe at about 140psi. The sound was tremendous, like a jet taking off and could be heard all over Barlick. We would have a brew while most of the steam blew off and then open the blow-down valve at the front of the boiler. This allowed what superheated water that remained to run away down the drains, flashing off into steam as it went. Steam came up through cracks in the yard and in winter especially it looked like a scene out of Dante’s Inferno! When the boiler was empty and the steam had cleared we opened all the dampers wide and knocked the lids in on the fronts of the side flues, this allowed the draught to pull cold air through the flues and start to cool them down. If it was Wakes Week and we were doing the boiler inspection we would open the top lid of the boiler as well and the mud hole at the front. This allowed cold air to circulate through the boiler. By the following morning the temperature in the flues and the boiler shell was about 110F which was accepted by the fluers as comfortable! Even so, there would be odd pockets of hot ash and I’ve seen Charlie come out with his knee pads smouldering more than once.

The plan of campaign was always the same. Jack started in the front of the settings clearing the area round the access doors and then swept the side flues shovelling the ash out into the blow-down pit. Charlie went straight down the sole flue and up, through the downtake, into the three feet diameter furnace tubes where he shovelled all the dust forward and over the ash pit back wall into the furnace from where the outside man could drag it out. By the time Jack had finished the side flues Charlie had done in the tubes and they both attacked the sole flue together. Charlie had the worst job because due to the fact that the entry doors were open, there was no draught up the flues to get the dust away from him. Jack had the advantage that he had a draught blowing the dust away as he worked. When they were together in the other flues they always worked backwards, facing into the draught so that the dust was carried away and they could see what they were doing. By the way, the only lights they had were cap lamps. Once the side flues, sole flue and downtake were clear they usually came out for a break before tackling the dirtiest part of the boiler, the economiser pit where the flue dust was always black and sooty because of the cooling influence of the economiser tubes. Once the pit was clear they swept the by-pass flue and the chimney bottom.

I’m going to post some pictures that Daniel Meadows did in the flues of Charlie and Jack and one of the things that will surprise you is that the walls and the bottom of the boiler in the flame bed are white. Here and in the downtake the ash has been at such a high temperature that it burns as white as snow. In a clean boiler like Bancroft Charlie only swept this off once a year, at Wakes Week.

There were dangers in the flues of course, the biggest was heat exhaustion. It was hard work and very hot, it took special men to work efficiently in such conditions. There were other hazards as well, I know it takes some believing but one funny duty of a firebeater in the old days was to dispose of dead dogs and cats. People would bring them in and we’d burn them while we were burning the fires off each night. Charlie reckoned that the smell of burning dog took a fortnight to clear so we never did any cremations in the two weeks before flueing. Once when they were flueing the boiler at Victoria Hospital in Burnley they found a partially burned arm in the downtake. Many downtakes had explosion boxes. These were a cast iron frame at the top of the flue with a heavy lid which could blow out of there was a flue gas explosion. This was a handy place to pop anything you wanted to burn as it is a very hot shop when the boiler is firing hard. Charlie said that if he was working on a boiler with explosion boxes he always kept his eyes open because he had found gold nuggets in them in the past. He reckoned that many a nagging wife had ended up in there. (he swore this was true….) The gold was from the teeth.

Well there you have it, the craft of the fluechap, one of the unsung heroes of the Industrial Revolution. Without him the boilers couldn’t have functioned. Probably one of the worst jobs in the world and it certainly shortened the life expectancy of the practitioners. Charlie reckoned that oil-fired flues were the worst. The flue dust was very aggressive and anything cotton that you wore fell to pieces after you had been in them. I think one of the aggressive chemicals was Vanadium Pentoxide. Charlie reckoned that this was what had given him a small cancer in his lower colon…. He got a mate of his who was a doctor to implant a ‘Hisotope’ as he called it in the wall of his rectum and reckoned that this stopped the itching. He also said that it sent the colour TV in the pub haywire if he stood next to it.

One last Little Known Fact about flue dust. Germanium is a very rare metal that is used in the production of computer chips. A chemist told me that the highest concentrations of Germanium were to be found on the walls of coal-fired flues. The metal evaporates in the fire and condenses on the flue walls. He reckoned that there was money in reclaiming it…… If only Charlie had known this, he could have died a rich man.

SCG/30 December 2006

[Stanley]

Bumped. A useful compendium of facts about flues and chimneys.....