This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol3", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
A further example may be given in Fig. 76. This is the plan of one floor of an office building having several floors, each being treated as shown in this illustration. The fan is of the "exhauster" type (which has nothing to do with the exhaust system of ventilation) the air flowing through the heater to the fan and not being blown from the fan through the heater. From the fan (closely connected to it by a large duct) is a large vertical shaft built up in the building, and extending up to all the floors to be heated. In this shaft, at the ceiling level of each floor, is an opening from which a horizontal delivery duct is taken, beginning in full size, as the illustration shows, and then reducing as it is branched to the different rooms. The delivery at nearly all points is over door heads, but it could be made, of course, at any suitably good point. The main and branch ducts are of rectangular cross section, as flat as possible, and in the majority of instances these are not considered unsightly on the ceilings of the corridors. In cases, however, where the appearance is of first consideration a false ceiling is made extending across vol. III - 9 level with the bottoms of the ducts, and with high corridors this is in no way objectionable.
A final example may be shown by Fig. 77. This, again, is a quite different undertaking, the building being a manufactory. The fan is of the type which draws the air through the heater and delivers it to horizontal ducts as shown. These are branched into vertical pipes, each having outlets at four floors. The central pipe duct has four outlets at each floor, the idea being to discharge in a line with the walls each way, also at an angle of 45° in two directions. It was thought best to adopt this plan, owing to the building being of such open construction and having a large glass area. Ordinarily the pipe ducts would have been up one side wall, with outlets to discharge towards the other.
In working out the details of a Plenum ventilating and heating system very few engineers make, or even work out, the specification for, their own fans ; and few, even if they make the needed calculations for surface, construct their own heaters. These are all commonly obtained from a manufacturing firm who are specialists in these appliances, but the following particulars may be given. Many of the calculations which follow are based on those given by the Sturtevant Engineering Company, who make fans and heaters for every purpose.
The designing and manufacture of fans or air propellers is outside the province of this book, but it may be said that in choosing a fan a full size should be selected for low-velocity work, such as in schools, offices, institutions, etc. ; while a smaller fan, worked at a higher speed, is admissible in factory work where higher velocities are allowed, and are frequently necessary in order to force the air long distances. It must not be forgotten, however, that high speeds mean an increase in driving power, and loss by excessive friction in the ducts. There are so many considerations in the choice of a fan that it is best to let the size and speed be decided by the maker, who is, or should be, the best qualified to settle this important detail. Makers are prepared to do this, and guarantee results.
In arranging the delivery ducts from a fan the conditions must enter very largely into the scheme ; for this, after a building is erected, is a different matter to running hot-water pipes. Correctly speaking, a building should have the details of all ducts and flues in its plans prior to erection, in which case the whole might be of brickwork, which is best, at the least possible cost. Failing brickwork, the earthenware pipe can be pressed into use in many places, but it is galvanised sheet iron that has to be relied upon chiefly when brick ducts are impossible. The chief point to remember with this material is that the joints must be made as soundly as possible, and all curves must be of the fullest possible radius.
The area of a duct is, of course, based on the volume of air required to pass through it, and while there is a leaning towards small ducts with high velocities, with unsatisfactory results, the extreme opposite of this would require ducts of such a size as to be prohibitive in cost. The general rule is to allow fairly high velocities in the main ducts where they leave the fans, and to reduce as the air proceeds to the rooms. A velocity of 2000 feet per minute is considered correct where the air leaves the fan. From this the velocity should drop to 800 and even to 500 feet in branch ducts, while that in the delivery duct should never exceed 300 feet per minute for dwelling or schoolrooms. In factory work the delivery may rise to a speed of 2000 feet per minute, while the velocity at the fan, worked at high speed, may then be as high as 3500 feet. As will be understood, velocity, after the air leaves the fan, is reduced by increasing the areas of the ducts and the delivery openings. It will be understood that to maintain an excess velocity or pressure at the beginning or base of each branch duct tends to keep all equally well served, and none are neglected or have variable results occurring in them.
The two following tables, published by the Sturtevant Company, afford all particulars for calculating the areas of ducts :-
Number Of Square Inches Of Flue Area Required Per 1000 Cubic Feet Of Contents For Given Velocity And Air Change.