Practical sheet metal duct construction

The modern demand for economy and efficiency in methods of production have given a strong impetus to specialization in the various branches of sheet metal work. In the larger cities are found very many shops that contract exclusively for heating and ventilating, exhaust pipes and ducts, etc. Only the well equipped, well-manned plant and only the skillful operator thoroughly familiar with his special calling, can readily take advantage of the opportunities presented in this large field.

Many excellent books have been prepared for the guidance of operators in various distinct branches of sheet metal work but none has heretofore been designed to cover the subject of heating and ventilating duct construction. The plan of the present work is to take up each operation, and by means of descriptions, with in nearly all cases accompanying illustrations, to show all operations incident to the construction and erection of heating and ventilating ducts. The reader may thus be equipped with a complete series of practical methods, available almost at a glance.

The material has been compiled from the columns of SHEET METAL where is was presented serially, with the definite purpose of its subsequent publication in book form. The illustrations, mainly from carefully made pen and ink drawings, are a special feature.

It should be stated that the operator whose business it becomes to install heating and ventilating pipes should thoroughly understand reading architectural plans such as are used in building construction; that is, he should be familiar with the methods by which the various means used in heating and ventilation are indicated upon plans, elevations and Sections, as the furnaces, blowers, steam pipes and coils, air ducts, registers, ventilators, smoke flues, etc., as well as with the usual locations of the same, so that he may trace the courses of ducts and pipes from basement up through the several floors, locating bends, risers, junctions, etc. He should also understand the methods of calculating capacities of ducts and amount of heating surface of pipes and coils and the requirements of the same, as well as areas of wall and glass surfaces with reference to the heat units required. It may be said that the best way of learning how to read plans is to learn how plans, sections, etc., are made. This is a subject which is taught in all technical schools and in courses given by correspondence. Any attempt to illustrate this subject as it should be done, would involve the presentation of at least one complete set of heating engineers' plans, while it is to be doubted if one set, as designed for any particular case, would include everything usually indicated upon plans made for this purpose.

Those systems of heating which require the use of sheet metal work are termed the warm air furnace, the indirect steam or hot water, the direct indirect steam or hot water and the forced blast systems.

In the warm air furnace system, galvanized sheet iron casings are placed around the furnaces, and the cold air ducts, as well as those for warm air, are all made of sheet metal. Sometimes furnaces for burning wood are used, as shown in Fig, 27, the entire casings being double and made portable. The cold air Inlet is shown at A and the warm air outlets are indicated by a, etc. The construction of these cold and warm air pipes and casings will be taken up in a later part of this treatise.

Sheet metal work  employed in the indirect steam or hot water heating system in the construction of a special form of heater which is placed below the ceiling and encased with No. 23 galvanized sheet iron, as shown by AA, etc., in Fig, 28. A cold air box or flue is connected to the bottom of the heater, as shown, and the warm air pipes at the top are connected to the registers placed in the walls or floors.

In the direct indirect heating system, the fresh air is admitted through an opening in the outside wall, as shown at A in Fig. 39, passes through the fresh air duct B, and is heated in passing up the flues between the radiator. By raising the treadle C, the fresh air duct is closed, while the damper at the foot of the radiator opens, thus allowing the cold air on the floor to pass up and become heated. The fresh air duct through the wall can be made of heavy sheet copper to prevent corrosion, and sometimes the space between the beams a and b are lined with sheet metal, and the top of the fresh air duct is placed on a level of Heating with the bottom of the beam and connections made to the cold air box, thence through the floor to the radiator.

In the application of the forced blast for warming of factories, schools, theatres, etc., where good fresh warm air is desired, the cold fresh air is forced through a heater of special design and discharged by a blower into ducts made of No. 22 galvanized iron, which lead to the rooms to be warmed, as shown in Fig. 30, which represents the interior of a factory. In schools, halls, etc., these ducts are placed in the walls or partitions, and discharge through registers.

Various systems of ventilation also require the use of sheet metal work in their construction. They are termed natural ventilation, heated flue or stack ventilation, heated drum ventilation and forced ventilation, consisting of the plenum and vacuum or exhaust systems.

In the natural system of ventilation, the drafts in the flues or ducts are caused by the difference in density of the air in the flue or duct and that in the outer atmosphere. The higher the temperature of the air in the ducts, the greater will the draft be. The ducts are run to the attic floor, as shown by a a, etc., in Fig. 31, and the foul air is carried to the outside by means of an ordinary ventilator. Stationary louvres may be placed instead in the gable at each end of the building if desired.

In the heated flue or stack ventilation system the smoke pipe of the furnace is connected to the smoke flue, around which a sheet metal stack is placed as shown in Fig. 32. This stack runs above the roof and is capped by a hood to keep out the rain. Ventilating registers are connected to the stack and the heat in the flue raises the temperature of the air in the stack, which, rising, creates a suction, thus drawing out the foul air through the registers V. If desired, the registers when placed away from the stack can be connected by means of a flue or duct placed between the beams, as indicated by a b.