Ventilation as a dynamical problem / [Mr. Shaw].

  • Shaw, Mr.
Date:
[1902]
Catalogue details

Licence: In copyright

Credit: Ventilation as a dynamical problem / [Mr. Shaw]. Source: Wellcome Collection.

    2/12 (page 2)
    9 frj employed ; with rotating fans it is proportional to the square of the velocity of the tips of the vanes. What I wish now specially to point out with regard to this matter is that, if the head be compared with the flow it pro¬ duces, it will be found that it is numerically proportional to the square of the flow for any simple ventilation circuit. The ratio is proportional to what may be called the combined resist¬ ance of the inlets and outlets, depending upon the shape and size of those areas. Work is spent in getting a cubic foot of air into a room and out of it again in many different ways ; velocity has to be generated, corners have to be turned, bends rounded. It appears, from experiments into which I will not now enter, that all these demands upon the energy required for the passage of a single cubic foot of air are proportional to the square of the rate of travel of the cubic foot, and not to the simple current. In fact, the only item in the expenditure of energy which is simply proportional to the velocity is the friction in a long tube, and this, generally speaking, contributes only a small part to the whole resistance of the circuit. I have verified by careful experiments upon a chimney that the law of proportion of head to the square of flow is very closely accurate. It follows—and this is the point at which I want to arrive—that the power required for a certain flow is proportional to the cube of the flow, and the conclusion to which I wish to draw special attention is that, under these cir¬ cumstances, it is the resistance of the circuit, depending upon the magnitude of inlets and outlets, that is the critical factor in ventilation. It is useless to attempt to compensate for small inlets and outlets by more energetic machinery. Let me give some examples. Suppose you have an arrangement of inlets and outlets which, with a certain horse-power, affords ventila¬ tion for 100 persons, and you desire to accommodate 200 persons. To do so without altering the inlets and outlets would involve eightfold expenditure of horse-power. Or, again, a room with a lire in it will accommodate five persons comfort¬ ably, the fire supplying motive power for the necessary ven¬ tilation. If you put ten persons into the room and leave the inlets and outlets as they were, you ought to double the flow to accommodate the additional persons. It might be thought that you could get the extra flow by making up a more vigorous fire. It would require eight times the fire to secure the object, and perhaps I cannot more conclusively illustrate the dominant influence of the size of openings upon ventilation than by asking you to imagine the result of attempting to increase a fire to eight times its original size for the sake of accommodating five additional persons. I have been asked to say something with special reference to the provision of ventilation in buildings already existing ;