MEETING OP THE BURTON-ON-TRENT SECTION, HELD AT THE QUEEN’S
HOTEL, BURTON-ON-TRENT, ON WEDNESDAY, FEBRUARY 19th, 1930.
The following Paper was read and discussed :—
THE BOILING OF WORTS
by R. Worssam, M.I.Mech.E.
Discussions on the relative advantages of boiling worts by steam heating apparatus in the boiling vessel, or by direct fire heating beneath it, have long attracted attention. One of the main difficulties in dealing with this subject lies in the fact that brewers hold divergent views upon what constitutes a satisfactory boil, and no brewer appears to be able to define, with any precise exactitude, his requirements and the reasons for them. Generally, however, the points upon which he lays stress are :— 1. A central and rousing boil. 2. A satisfactorily high temperature. 3. Satisfactory evaporation.
All these requirements can be perfectly well met in a properly constructed fire heated copper in a well designed setting, and were such questions as economy and control and other points of no importance, probably the steam heated copper would never have been introduced.
It may be well to consider, one by one, the points which toll in favour of one type or the other.
(1). Initial Capital Outlay.—The difference in the first cost of a fire copper and a steam heated copper of the same capacity is not sufficient to be of material importance so far as concerns the actual boiling vessels themselves, but whereas the steam heated vessel merely needs steelwork to carry it and lagging for the outside, and can be fixed in any position in the building, the fire heated copper requires an expensive brick work setting with grate fittings, damper doors, etc., and a powerful chimney, to obtain the necessary draught in the furnace.
The annual depreciation of a steam heated copper due to wear and tear is extremely small, and many steam heated coppers have been in use for more than 30 years, and have required practically no repairs. This is not the case with fire coppers and settings, as both the vessels themselves and the sittings in particular undergo rapid deterioration owing to the high temperatures to which they are subjected. In addition there is always the chance at any time of burning the bottom of the fire copper, whereas a properly constructed steam copper is entirely free from risks of this nature, for steam can be turned on to the empty vessel without fear of doing harm to any part of the apparatus. With the fire copper additional coal bunkerage has to be found to that required in the boiler house, and dust and dirt are thereby introduced into the brewery.
But it is in the matter of economy in coal consumption that the steam heated vessel has such an enormous advantage over the one heated by direct fire. The steam heated vessel derives its heat from the steam boiler, and the dimensions of a steam boiler can be and are determined by the makers with the object of obtaining the highest possible economy in working, with long grates and flues, to give considerable heating surface and a high velocity to the heated gases, and consequently quite a high degree of efficiency is obtainable in any modern steam boiler.
On the other hand, the shape of the fire-heated copper does not lend itself to the adoption of a setting having such a high degree of efficiency as the steam boiler, owing to the grate and the flues being so much shorter. It is quite easy to keep the boiler setting tight where it comes in contact with the vessel, and free from leaks, but this is almost impossible in a fire-heated copper, and usually, in many places in the setting, currents of cold air are drawn in which cause a considerable loss of heat and still further reduce the economic factor. Again, in installations of steam boilers of any magnitude, steam economisers, super-heaters, etc., can often be fixed in the flues and a still higher degree of economy in coal consumption attained, whereas the setting of a fire-heated copper does not permit of this.
Thus it will be seen that if it is possible to obtain a satisfactory product with a steam-heated copper the brewer will be employing an apparatus which is economical in working. In breweries of moderate size which do not require special batteries of boilers for heating the coppers, but where the existing boilers will serve, there is already supplied the heat required to heat up and maintain in temperature the brickwork of the setting and the chimney of the boiler—and this un-productive heat has to be supplied every time a fire-heated copper is used.
One of the earliest forms of steam coppers in general use was the steam jacketed vessel. In this the whole of the bottom of the vessel has a jacket fixed on it, and into the annular space thus formed, steam is introduced. This type of vessel has several serious draw backs. In the first place only in small vessels is it possible to get sufficient heating surface, as this is determined by the surface area of the pan of the vessel. As the temperature is the same all over the bottom of the vessel, the heated liquid and the steam formed rise evenly over the whole area, meeting the cooler liquid as this descends; thus a central boil is not attained nor does the violent upheaval of the worts occur which is so essential for circulation and aeration. There is, however, another reason why these vessels—except in the smallest sizes—have now almost gone out of use. The jacket was made of iron or steel, and the inner pan of copper, and however carefully the joint between the two was made the different expansion coefficients of the two metals almost invariably led to a leak in a comparatively short time, and once this occurred a satisfactory repair could only be made by dismantling the whole vessel and remaking the entire joint—a costly job which throws the vessel out of use for some considerable time. In a fire-heated copper it is only possible to get a central boil by designing the setting in such a way that the maximum supply of heat occurs at the centre of the bottom of the vessel (a state of affairs not too easy to obtain) but the modern heater is placed in the centre of the vessel, thus assuring a central boil and a violent circulation of the liquid throughout the copper.
In dealing with what may be regarded us a satisfactorily high temperature the difference between the meaning of the terms “Heat” and “Temperature”, should be appreciated. They are frequently used as though they were synonymous, and this leads to a considerable confusion of thought. Temperature is the measure of degree of heat, and not a measure of quantity. The writer has frequently been told by brewers that they are desirous of increasing the heating capacity in their coppers because they wish to obtain a higher temperature, or that, having fixed a steam heater, they do not obtain so high a temperature in their vessel as that obtained when the copper was heated by direct fire. It is a well-known physical law that the difference of the temperatures of the boiling points of liquids is constant under all variations of pressure. That is to say, a liquid boils at a definite temperature, which varies only with the pressure to which the liquid is subjected. That temperature remains constant, so long as the liquid continues to boil. By raising the temperature of the heating medium or by increasing the heating surface, if this is already sufficient to boil the vessel, so as to transmit a greater quantity of heat into the liquid, only the rate of evaporation and consequently also the circulation is increased, but not the temperature. The temperature that will be reached and maintained in a copper when the wort is boiling can be calculated regardless of the method used for heating the copper, so long as the barometric pressure is known.
In recent years many brewers have resorted to pressure boiling. About 1½ to 2 1b. per sq. in. is a common pressure adopted, although pressures up to 20 lb. per. sq. in. have been used with apparent success.
When a vessel is heated by fire, a thin film of gas forms upon both surfaces of the plate, and as a result the burning gases do not come into direct contact with the vessel. This film has a high degree of resistance to the transmission of heat—very much higher than that of the metal plate itself. Consequently increasing the thickness of the plates of which boilers or coppers are made only affects the transmission of the heat to a small degree, for it is passing the heat from the fire into the plate and then out again into the liquid that absorbs a greater proportion of energy. For this reason the bottom of a fire copper is not at nearly so high a temperature in operation as it appears to be to the casual observer.
A number of brewers have adopted super heaters for raising the temperature of the steam supply to their coppers. Heating apparatus constructed originally for saturated steam will usually stand a moderate degree of super-heating though this must not be carried to excess. As pointed out, however, increasing the temperature of the heating medium does not affect the maximum temperature attained in the copper, and unless the super-heater draws its heat from sources which otherwise would be lost, it is difficult to see that its application has any economic value.
The heat which passes away through the boiler chimney has its uses, and only when its temperature is excessive is the fixing of a super-heater or economiser at the back of the boiler of practical value. Otherwise it will be found that the draught is considerably interfered with owing to the consequent reduction of the temperature of the flue gases.
Whilst super-heated steam is undoubtedly a great advantage in prime movers where it is essential that dry steam is supplied to apparatus which makes use of the expansive properties of the steam in heating or boiling liquids, it is the heat units contained in the steam that are required to be removed and employed, and a large proportion of the total heat units contained are liberated and made available for use when the steam is condensed into water, for the latent heat of vaporisation of water is 966 B.T.U.’s, whilst the equivalent quantity of heat required to raise the steam up to a super heated temperature of 100°F. is about 52 B.T.U. In other words, by adding 100° of super-heat to steam, the total available heat is only increased by less than 6 per cent., and this super-heated condition of the steam disappears immediately the steam enters the heater and comes into contact with the heating surface, for super-heated steam and saturated steam cannot exist in the heater together.
Some advantages may be gained from superheating the steam used for boiling purposes, especially when the boiling vessel is some distance away from the boilers themselves. It is an advantage to convey dry steam through the mains up to the heating apparatus, but where superheated steam is used it is well to ensure that steam is never turned on except when the copper is full of wort or water, as the high temperature has a bad effect upon copper.
With regard to the question of evaporation, if it is merely to make sure that the worts have reached their maximum temperature, and have remained there for the necessary period, quite a low percentage of evaporation will ensure this, and if it be carried further, it is at a considerable cost in fuel consumption. Some brewers will not have their coppers domed because they claim that, having evaporated off a certain proportion of their copper contents, they do not wish to condense it, and return it to the vessel again, whilst others will deliberately boil under a pressure and decrease their evaporation.
If steam heating apparatus is to work satisfactorily, it must be served by u steam trap of suitable design for removing the condense. The large majority of steam traps on the market are unsuitable for use in a brewery, because in steam heating, as carried out in the brewery, a much larger volume of condense water is formed than in cases where steam is used in other industries and as it is extremely important that this water should be removed as quickly as possible after it is formed, the steam trap should have a fullway passage through it. This is not provided in the design of the ordinary steam trap, and many steam heating apparatus have worked unsatisfactorily merely because they have been connected to steam traps, nominally of sufficient size, but which were never designed to perform the functions required.
Some brewers claim that boiling by fire imparts a palate to the beer, which is not obtainable by steam boiling. Others make the same claim for super-heated steam. It is not possible to trace any material difference between liquids boiled by the different methods, and the writer has not met a brewer who can differentiate between beers produced by the different systems of heating.
Some brewers consider that caramelisation occurs in a fire copper. At the worts in a copper contain matter in suspension, it may be possible that some of this matter at times may settle at the bottom of the fire-heated copper, and so permits of the metal rising to a higher temperature than would otherwise be the case, but it is difficult to believe that this can occur to any appreciable extent without causing considerable damage by overheating of the bottom of the vessel itself.
In concluding these remarks on the boiling of worts, it may be thought that the writer has put the case in favour of boiling by steam rather strongly. It should not be forgotten that formerly a large and lucrative business was done in repairing fire coppers; urgent calls for assistance came in almost daily from all parts of the country. These have been reduced almost to vanishing point by the adoption of steam boiling.
Discussion.
Mr. G. C. Matthews said there appeared to be very little difference between fire-boiling and steam-boiling (by saturated steam under atmospheric pressure) so far as the effect on the finished beer was concerned. He was in favour of a quiet boil. Violent movement promoted by some types of fountain and chains in the copper had an unfavourable effect in over-extracting the hops and particularly in extracting oil from the hop seeds, which had been known to produce harshness and haze. The same objection applied to boiling under high pressure, and even in boiling at atmospheric pressure with superheated steam. It was obvious that steam boiling was more convenient and far more economical than fire-boiling. It appeared also that little was to be gained by using superheated steam from an economic point of view; indeed, it seemed to him that boiling by superheated steam might have disadvantages in promoting too violent a boil.
Mr. A. Barrington remarked that he believed it had been definitely proved that there was no difference in flavour in the beer whether the copper was fire or steam heated. There was no question that the steam copper was more economical. It was also convenient to have the coppers wherever they were wanted, and that could be done with steam coppers. Moreover, with steam heating a central boil could be obtained. Another advantage was the absence of loose steam in the brewery.
Mr. F. Milner said that too much could be extracted from the hops, under pressure boiling the oil from the seeds caused trouble with the fining process. He considered that pale ales 2 lb. pressure per sq. in. was ample for sterilisation.
Mr. G. Younie said he had had experience of the three methods of boiling worts mentioned by the Author, and he (the speaker) was still convinced that the best results were obtained with fire boiling. The other two systems, although more economical were not so satisfactory as the tendency was to break up the hops too much and the over extraction of resinous matter, which was consequently carried over to the finished product and in many cases led to trouble in fining down. Caramelisation was also a factor to be considered and the results he thought were better with fire boiling.
Mr. F. E. P. Forster asked the Author if a fan had ever been tried in a domed copper to assist in the evaporation. He felt doubtful if the agitation in the copper did disintegrate the hop seeds.
Mr. G. T. Peard said that steam boiling was more economical than fire boiling when ordinary coal was utilized but was it the same when using pulverised fuel or mechanical stokers? Oil firing had been discontinued except in cases where great difficulty was experienced in obtaining coal. The Author had referred to the rapid fall of temperature at the surfaces of the plate through which heat was being transmitted. Was there any detectable difference between the temperature at the “liquid” surface of the plate when the source of heat was steam or fire? When a brewer complained that his worts were agitated too much by steam boiling surely it was tin indication that the heating surface was too large and if it were reduced the trouble would disappear. Conversely the use of superheated steam was only advantageous when the heating surface was insufficient.
Dr. A. Slator pointed out that the copper had to carry out two different processes, the one cooking of the wort and the extraction of the hops and the other the evaporation of the wort. There might be an advantage in using a separate plant for each process. A digester of some kind would be suitable for the one process and an evaporator such as was used in sugar factories for the other.
Mr. C. Robertson said that with the steam copper a properly constructed heater must be provided that was capable of evaporating a definite volume in a given time under specified conditions. In conjunction with the heater there should also be an appropriate trap system. Many traps did not work properly and the heaters became water logged and the efficiency of the heater was thereby greatly reduced. He would like to ask the Author what the equivalent of evaporation was of a copper boiling open to the atmosphere and the same quantity of wort boiled in a pressure copper under 2 lb. pressure? Boiling under excessive pressure increased the boiling point, and it was at these higher boiling heats that certain extraction from the hops took place which produced haze. An economy could be effected if use were made of the exhaust steam from the steam copper heaters.
Mr. H. E. Dryden doubted if it was a question of steam or fire coppers. Primarily, he thought it was a case of new structures. If a brewer had open coppers already erected it did not seem worth while installing steam heaters at great expense when the fire copper would do the work just as efficiently. If a new brewery was being built the question became more serious. Boiling in an open copper implied a certain amount of aeration, which did not occur in a closed copper under steam pressure. In closed coppers with fire or steam the boil was in absence of oxygen, and it seemed to him that the soft resins of the hops were less likely to be oxidized. The preservative properties of the hops would probably undergo less change by a slightly increased pressure in the absence of air. There was also a possibility that a little more of the hop aroma was retained in the worts boiled in closed coppers. It would be of interest to know how often the coils in the coppers required scaling, and if such scale was difficult to remove.
The Author, in reply, said that in a fire heated copper the heating surface was limited to the bottom and the fire course of the vessel, and it was easy to obtain in a heater a much greater heating surface than was possible in a fire copper. Also, in the latter the whole of the bottom was heated, but the greatest heating effect occurred in the centre of the bottom. In a steam heater the whole of the heat effect was localised in the heater, which was placed in the centre of the vessel and, therefore, in the fire copper there was not such a violent circulation as in a steam copper, but the latter could be easily controlled by the manipulation of the steam valve. A copper could be boiled quite satisfactorily by the use of steam coils, but the latter must be of considerable dimensions to have sufficient heating surface. They were difficult to clean and were apt to impede the egress of the hops and prevent the proper circulation of the hops when the vessel was boiling.
It was not a correct statement that for every pound pressure put on to a copper the temperature rose 3°. Boiling a copper under an internal pressure of 1 lb. to the sq.in. resulted in a temperature of 215° F. and of 2 lb. 219° F. on the assumption that there was no hydraulic pressure on the liquid in the vessel due to the depth of the wort in the copper. A drop of 1 in. in the barometric pressure would reduce the boiling point by nearly 2°.
The advantage of a domed copper was that the heat was retained and the amount of steam required to boil a copper was practically equivalent to the amount of evaporation and, therefore, doubling the rate of evaporation would double the amount of coal consumed. The same applied to a pressure or open copper.
Attaching a fan to a chimney of a boiling copper would be of no advantage in assisting evaporation unless the fan was powerful enough to reduce the pressure in the dome of the copper below the atmospheric pressure, in which case a reduction in temperature would ensue.
He did not think pulverised fuel and mechanical stokers and oil fuel would be of advantage in a fire copper. They would certainly not reduce the coal consumption of a fire copper to the level of that of a steam copper. It was quite possible, and usual, in a brewery to utilise waste heat from a steam copper by returning the condense to the boiler. Some breweries also economised in heat by condensing the steam which had been given off from the wort, either by using the condense for other purposes or by using the heat contained in it for heating water.
He knew of no instance of a copper being heated by gas. Heaters should be cleaned if possible after every brew, and it was not a difficult matter to prevent them from scaling up. A wire brush could be used with advantage. If scale was allowed to form, difficulties would be experienced in cleaning.
Crescent City Brew Talk 