with which the current of air would move through is where is, as above, the head producing the flow. This would be the velocity produced in by the difference in pressure were it not for the resistance to the flow caused by the friction of the air in passing through the radiator and ducts and past dampers, registers, etc. This resistance often reduces the velocity to less than half of the theoretical velocity. Mr. Alfred E. Wolff, however, recommends that 50 per cent. of the theoretical velocity be taken in the case of ventilating flues which depend on a heated column for their action.
The practical application of this theory is, however, one of considerable difficulty. In an indirect radiator in a given situation we do not know the temperature of the heated column, and, what is most important, we do not know the resistance of the air passages. What we do know is that we have a radiator of so many square feet surface located in a certain system of boxing, ducts, etc., and supplied with steam, or hot water, at a certain temperature. The temperature of the air outside being also known, or assumed for extreme conditions, the question is how much air will be delivered by this radiator to the room and also to what degree it will be heated.
The amount of heat given off to the air depends upon the velocity and upon the difference between the temperature of the steam in the radiator and the mean temperature of the air around it; and the velocity depends, again, upon the difference of temperature between the entering and out-going air as well as upon the air resistance as embodied in the arrangement of ducts, structure of radiator, etc. All of these make a complicated system of variables which it is impossible to apply in theoretical formulas and anticipate what the actual result will be. In practice, a given radiator in a given setting, and with given temperatures of steam and outside air, condition of wind being constant, will deliver a definite amount of air heated to a definite degree and the velocity and final temperature adjust themselves until there is an equality between the temperature head acquired and the velocity head plus the head necessary to overcome the resistance. But exactly how this combination will adjust itself it is wellnigh impossible to say beforehand, inasmuch as the air resistance is a quantity very difficult to predetermine, it being very greatly affected by a slight change in the arrangement.
