Recent Developments in Tools .sx and Equipment .sx Automatic Reversal of Open-hearth Furnaces .sx IT is well known that preheating gas or air , or both , before combining to form a gas mixture for producing a high flame temperature increases its heating efficiency .sx The gain due to preheating is , of course , specially important if it can be undertaken without any contribution of heat beyond that developed at the production and combustion of gas itself , and in practice this is generally realised .sx The problem of making use of the heat carried in the products of combustion was solved by Siemens , in 1857 , by means of a regenerator , which , in principle , is applied to modern open-hearth steel furnaces .sx which operates automatically on a temperature-difference principle .sx It is the result of a thorough investigation by engineers of Leeds and Northrup Company , the principal object of which was to establish a reversal control based upon the actual conditions occurring during operation of the furnace , in order ( 1 ) to return to the furnace a greater amount of heat ; ( 2 ) to establish and maintain a balanced thermal condition between the two sides of the furnace ; ( 3 ) to provide absolute freedom for the operator in developing or building-up the heat head during the progress of each heat ; and ( 4 ) to provide a safeguard against early burning-out of the regenerative chambers .sx The method employed in heating the air and gas in the ordinary Siemens furnace is by passing each through separate chambers , built of refractory bricks , so arranged that the surface area exposed is considerable .sx For this purpose the brickwork in the chambers is checkered .sx Two pairs of chambers or checkers are used , so that air and gas are introduced from one pair at a time , the waste gases passing through the opposite chambers , giving off considerable heat , which is absorbed by the checker brick-work .sx After an interval , which varies , the process is reversed by operating valves , which shut off air and gas from one side and introduce them from the other .sx In this way the air and gas are preheated before they enter the furnace chamber .sx There are two modifications of the ordinary Siemens furnace :sx one in which the gas producer and furnace are combined , and which have air regenerating chambers only , the gas being supplied direct from the producer ; the other , in which both gas and air pass through regenerating .sx chambers .sx Although there are many kinds of open-hearth furnaces , they differ in constructive details only , the principles of the process remaining the same .sx The change-over valves to effect reversal are a very important part of the whole regenerator construction , but , in addition to these , it is necessary to have regulators for varying the quantities of air and gas as well as the chimney draft .sx In open-hearth furnaces it is customary to place regulating dampers in all the four conduits between the change-over valves and the chambers .sx To obtain the highest efficiency from the regenerators , however , the time of reversal should be accurately determined and consistently maintained .sx For long the reversal of a regenerative furnace has been based on an empirical time-cycle , but recently a more efficient system has been developed .sx Many different methods of reversal both from temperature and from time , some automatic , some semi-automatic , and some entirely manual were tested in actual commercial operation .sx Heat absorption , heat loss , thermocouple location , valve operation , fuels in brief , the factors affecting operation were carefully investigated and tabulated under varying conditions .sx In actual commercial operation on a number of different furnaces , the automatic temperature-difference method has proved to be the superior method for open-hearth furnace reversal .sx The weakness of the manual reversal is the uncertainty of the operator's response to the demands of the furnace .sx Even when the operator has pyrometric guidance , he will rarely reverse the furnace at the proper instant ; whereas , with this new automatic reversal method , the furnace itself may be said to decide when it should be reversed .sx Heat conditions within an open-hearth furnace change rapidly .sx As the checker system on the side which is heating up approaches a certain temperature its so-called " point of saturation " the rate of heat absorption decreases rapidly .sx This means , of course , that as this point is neared , increasing quantities of heat go out of the stack .sx Thus , there is one correct point for reversal , a point which is variable not only with every furnace , but also with each reversal during any one heat , throughout the campaign of a furnace .sx To delay reversals beyond this point would cause a waste of heat which could and should have been turned back into the furnace .sx The correct point for each reversal can be established in practical operation by means of an automatic temperature-difference reversal controller .sx Regardless of furnace design , age or efficiency , this controller will act at the point at which the furnace should be reversed for greatest effectiveness .sx This automatic reversal equipment consists of a con-troller and a two-point recorder for showing air-flue or .sx checker-chamber temperatures .sx Two thermo-couples for the controller are installed in the flues or checker chambers as suggested in the diagrammatic illustration , Fig. 1 .sx Couples for the potentiometer recorder are located directly adjacent to the control couples , usually in the same protecting tubes .sx The controller may be located wherever it is most convenient .sx Usually it is mounted on a panel with the recorder and located on the charging floor , opposite the furnace .sx When the temperature of one couple exceeds the temperature of the other by a predetermined number of degrees , the controller reverses the furnace .sx The scale on the controller , shown in Fig. 2 , is calibrated to show the total difference setting on each side of the zero point , and it is a simple operation to adjust the contact points along the scale to the temperature difference at which the control is set .sx Thus , reversal may be made as frequently as desired , without disturbing the heat balance , simply by narrowing the temperature-difference limits set on the controller .sx The Leeds and Northrup automatic temperature-difference reversal method has been installed in many open-hearth furnace plants in the United States , and the results indicate that the method effects economy and gives improved efficiency .sx In open-hearth furnaces fitted with this control fuel costs have been reduced more than was thought possible ; furnace production has increased , refractory costs have been reduced , and the over-all efficiency of the furnaces has been raised because of the more regular operation .sx Correct heat balance is maintained continuously on both sides of the furnace , as is shown by an actual size record made by the two-point recorder , illustrated in Fig. 3 , from which it can be assumed that , since the maximum temperature of preheat is maintained and the cycle of preheat is controlled , the combustion rate and flame temperature within the furnace are controlled .sx Further information regarding this automatic reversal method is obtainable from the Integra Co. , Ltd. , 183 , Broad Street , Birmingham , the sole British agents of the Leeds and Northrup Company .sx A NEW HEAT-TREATMENT FURNACE .sx THE selection of the most suitable heat-treatment furnace is frequently a difficult problem when such a form of equipment is required to be installed .sx So many factors are usually raised with the object of assisting in the determination of the type of equipment , particularly in regard to the form of heat energy to be used , that the main factors may easily be overlooked .sx After all these factors have been discussed , however , it will ultimately be appreciated that the measure of the utility of a furnace is dependent on the quality and overall cost of the finished product .sx Thus , the real problem , from a manufacturer's point of view , is that of manufacturing a product of high quality at low cost , and of using that type of furnace and that medium of heat energy which will give this result under his particular manufacturing conditions .sx The question as to whether fuel or electricity is most suitable is an important one , but it should be appreciated that solid , gaseous , liquid or pulverised fuels , as well as electricity , have each a distinct field of usefulness , and each has limitations ; it is therefore necessary to determine which has the most practical value in meeting the conditions under consideration .sx The electric heat-treatment furnace under review is designed by Messrs .sx August Muffle Furnaces , Ltd. As the detailed description will show , the aim has been to produce a robustly constructed unit for the specific range of work for which electrical heating is best .sx The design appears admirably to suit the policy of the firm's production scheme , which in turn covers a range of types and firing methods broad enough to provide the most suitable for the work in hand and the conditions of operation .sx It is well known that the characteristics of the electric furnace are distinct .sx The heat value of the k.w.h. , forinstance , is a constant value and not an average value , and the energy is converted without combustion ; thus , conditions inside the furnace are such as to be independent of individual control ; further , the use of electricity facilitates accurate control of energy .sx The cost of electric energy measured in B.th.u. is usually higher than that for fuel , but the indirect advantages associated with its use frequently make possible a lower production cost .sx Thus , in electric heating equipment , the products of combustion are eliminated , there is an absence of heat and odour .sx from spent gases , the atmosphere inside the furnace is under control , the temperature control is automatic , and generally its use tends to establish new standards of production .sx In the August furnace the current supply is 200 volts , 50 cycles , 3-phase , connected through a Brookhurst triple-pole straight-on contactor , complete with T.P. isolating switch and overload trips , and a 35-k .sx v.a. transformer .sx Cambridge automatic temperature control equipment is also fitted .sx The furnace is of the box-resistance type , constructed for batch heating , as shown in Fig. 1 .sx Inside the mild-steel shell is adequately lined with high-grade refractory insulating material , to reduce heat losses to a minimum , and to ensure high thermal efficiency .sx The charging door is in halves , and is of a special type .sx It is suspended in such a manner that the hot face can always be turned away from the operator .sx It sockets well into the door-opening , and can be opened to any desired extent .sx Special insulating refractory is used for the door blocks , asbestos mill-boarding forming a backing to reduce heat losses .sx A door " dead " switch is fitted which automatically cuts off the current when the door is opened .sx The heating chamber is rectangular in shape , 60 in .sx by 30 in .sx , with an arched roof , a nickel-chromium heat- and scale-resisting plate forming the hearth .sx The heating elements are of the open type , and are composed of " hair-pins " of quality nickel-chromium-alloy tape , of robust section .sx An interesting feature is the method of maintaining these elements in position .sx They are threaded through rings of similar material , each ring being securely anchored to the brickwork of the chamber and located sufficiently close to adjacent rings to preclude the possibility of the elements coming into contact with one another .sx The elements are distributed over the full length of the chamber , on the roof , side walls , and hearth , giving an even temperature throughout the chamber .sx Special nickel-chromium guards protect the side elements from mechanical damage with the minimum of radiation interference .sx The wide distribution of the elements tends to place less dependence upon indirect radiation , and increases the amount of space that can be profitably occupied by the charge .sx An ingenious arrangement of the elements at the back of the furnace renders them easy of access when the furnace is in operation .sx The " hairpins " are thickened at the ends and led out through the back wall to a neat terminal board , which is enclosed in the false back , as shown in Fig. 2 .sx The system of wiring the phases in three banks of electrodes , together with the method of element suspension and connection , facilitates the removal of a faulty hairpin whilst the furnace is hot , and operations can be safely and efficiently continued by merely shorting the hairpins adjacent to it on the terminal board , Fig. 2 .sx