Ventilation as a dynamical problem : a paper read before the Annual Meeting of the Medical Officers of Schools Association, on February 6th, 1902 / by W.N. Shaw ; with the subsequent discussion.

  • Shaw, Napier, 1854-1945.
Date:
1902
Catalogue details

Licence: In copyright

Credit: Ventilation as a dynamical problem : a paper read before the Annual Meeting of the Medical Officers of Schools Association, on February 6th, 1902 / by W.N. Shaw ; with the subsequent discussion. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    om))loyo(l ; witli rotating fans it is ])roportional to the Sijuaro of ih(* velocity of the tips of the vanes. What 1 wish now specially to point out with re^'anl to this matter is that, if the head he compared with the flow it j>ro- diices, it will he fouiul that it is numerically j)roportional to the stpiare of the flow for any sim])le vtmtilation circuit. d’he i-atio is j)roportional to what ma}’ he called the comhined resist- ance of th(‘ inlets and outlets, depending upon the shaj>e 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 he generated, corners have to he turne<l, hends rounded. It ai)i)ears, from ex})eriments into which I will not now enter, that all these demands upon the energy reipiiretl foi- the j)assage of a single cubic foot of air are j)roportional to the S(]uare of the rate of travel of the cubic foot, and not to the simj)le current. In fact, the only item in the expemliture of energy which is simply ))roportional to the velocity is tin* friction in a long tube, and this, generally speaking, contributc*s only a small jiart to the whole resistance of the circuit. 1 have verified hy careful ex})criments upon a chimney that the law of proportion of head to the sipiare of flow is very closelj’ accurate. It follows—and this is the ])oint at which I want to arrive—that the })ower retpiired for a certain flow is l)roportional to the cube of the flow, and the conclusion to which I wish to draw s})ecial attention is that, under these cir- cumstances, it is the resistatice of the circuit, dei)ending upon the magnitude of inlets and outlets, that is the critical factor in ventilation. It is useless to attempt to compensate for smal 1 inlets and outlets by more energetic machinery. Let me give some examples. Su})pose you have an arrangement of inlets and outlets which, with a certain horse-])Ower, affords ventila- tion for 100 ])ersons, and you desire to accommodate 2fM) ])ersons. To do so without altering the inlets and outlets wouhl involve (‘ightfold expenditure of hurse-])(nver. Or, again, a room with a fire in it will accommotlate five persons comfort- ably, the fire sup])lying motive ]>ower for the necessiiry ven- filation. If you ))ut ten j)ersons into the room and h*ave the inlets and outlets as they were, you ought to double the flow to accommodate the additional ])ersons. It might be tlmught that you could get the extra flow by making uj) a more vigorous fire. It would reijuire eight times the fire to secure the object, and ])erhaj)s I cannot more conclusively illusfratt* the tlmninant influence of the size of oj»enings uj)on ventilation than by asking you to imagine the i-esult of attemj)ting to increase a fire to eight times its original size for the sake of accommodating five additional ])ersons. 1 have been asked fo say something with special reference to the pi’ovision of ventilation in buihlings already existing ;