This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
In St. Thomas's Hospital, Lambeth, each ward contains central fireplaces facing the end of the room. The fresh air is admitted at the floor level, after passing through a flue open at one end to the external air, and warmed by passing through a hot-air chamber behind the fire. The vitiated air escapes into an up-cast flue through a grating at the level of the ceiling, from whence it is drawn into an iron flue enclosing the smoke flue of each fireplace, the heat of the latter being considered sufficient to create the required suction for its extraction.
A better arrangement, in the author's opinion, is that adopted at Guy's Hospital, London, where advantage is taken of the girders carrying the floor for ventilating the wards. The fresh air is drawn down 2 lofty shafts, one on each side of the main entrance, into a compartment in the basement, where it is heated by hot-water pipes before passing through the air-ducts into the wards, entering them through gratings on the floor level. The upper flue is embedded in the concrete of the floor, while the lower flue is below the ceiling of the ward. After passing through the wards, the hot air is extracted through apertures near the ceiling into a series of independent flues communicating with a shaft placed near the centre of the building, so as not to interfere with the action of the down shafts, and carried up outside the roof to a greater height than the other shafts. The velocity of an escaping current will be proportional to the square root of the excess of the temperature of the heated air in a flue over the air outside the flue, an 1 also to the square root of the height of the flue or chimney, and the volume of air extracted is consequently proportional in addition to the sectional area of the flue.
Harding's ventilators are better known in the north of England than the south. They are recommended by Pridgin Teale, surgeon to the General Infirmary at Leeds, as a means of securing freshness of atmosphere without draught, and free from all mixture of dust, soot, or fog. The outside air is conducted through a grate and aperture in the wall about 7 ft. 6 in. above the floor level, where it is made to pass through a series of small tubes fixed at an angle of about 30° with the wall. The currents of air are said to be compressed while passing through the tubes, but to expand and diffuse in all directions as soon as they are liberated into the apartment. In all filtering arrangements it must be remembered that if air is to pass through a screen or filter without retarding the current entering the room through a tube, the area of the screen must be greater than the area of section of the tube. This can be effected by placing the screen diagonally within the tube which admits the air. In some buildings the filter is dispensed with, and the apparatus is used simply to diffuse the air as it enters the room. An outlet for the vitiated air is provided by the chimney flue, either through the fireplace or by a mica valve placed in the flue near the ceiling.
In rooms where flues do not exist an air extractor is provided, consisting of 2 perforated cones and a central tube. The external air impinging upon the perforated cones is deflected, creating an induced current up the vertical tube, drawing the foul air from the interior of the room, and expelling it through the perforations. In fixing the extractor, a wooden base or frame is placed on the ridge and covered with lead to make it watertight; the extractor is then placed over this and fixed in the ordinary manner. A small inner cone is provided simply to prevent rain from getting into the tube. Harding's extractors are so designed that they may be easily fixed inside an ornamental turret without in any way affecting their action. They can be obtained in London from Strode and Co., at prices varying from 15s. to 6l. and upwards. Their action is illustrated in Fig. 1389: a, wall; b, grating outside; c, filter.
Another system for admitting fresh air into a room, free from fog and other impurities, is that recommended by the Sanitary Engineering and Ventilating Co., 115, Victoria Street, Westminster. They provide for the introduction of fresh air in vertical currents by means of a suitable number and disposition of vertical tubes, varying in size, section, and weight according to each special case. The current can be regulated in amount by throttle valves, and the heated or vitiated air is removed by means of exhaust ventilators, placed directly over the roof or in connection with air flues and shafts. The exhaust ventilator is thus described by the makers: There are no working parts to get out of order, and no attention is required to ensure its constant action. In this respect, a groat improvement is claimed over the numerous forms of revolving cowls, which require occasional lubrication, otherwise the working parts become corroded and the cowl ceases to act. They are made of circular or rectangular section, or other shapes to suit special circumstances. One great merit of the system is the element of length which is introduced by means of the tube arrangement, and thus a current is continually passing which diffuses itself over the room.
The system admits of a patent air-cleansing box being built into the •wall at the foot of the tube, fitted with special deflector plates and a tray to hold water or, when necessary, disinfectants. Where the arrangements of furniture or fittings in a room preclude the use of vertical tubes fixed near the ground, they recommend the substitution of a ventilating bracket fixed at 6-7 ft. above the floor. This bracket may contain an air purifying or cleansing box; if required, a valve is provided for regulating the admission of fresh air, and a 9 in. by 6 in. hinged air grating to cover the opening outside. The air-cleansing box is illustrated in Fig. 1390: a, inside of room; b, floor; c, trough or tray for holding water or disinfectant fluid; d, tube.
Boyle's patent self-acting air-pump ventilators are well known, and are found to answer well in their continuous action under all varieties of wind pressure; they are often adopted without any inquiry being made as to the scientific principles on which they are constructed. They consist of 4 sections, each acting independently of the other. The exterior curved baffle-plate prevents the wind blowing through the slits formed in the immediate interior plates, and tends to concentrate the current. These interior plates are curved outwards, so as to take the pressure off the vertical slits, which form a communication with the internal chambers, through which the air impinges on inner deflecting plate3, and is further directed by the radial plates. The external air impinging on the radial plates is deflected on to the side plates, and creates an induced current. In its passage it draws the air from the central vertical chambers, expelling it at the opposite opening. The vitiated air immediately rushes up the shaft connecting the ventilator with the apartment to be ventilated, extracting the air and producing a continuous upward current without the possibility of down draught.
The partitions separating the chambers prevent the external air being drawn through the slits upon which the wind is not directly acting. The whole arrangement being a fixture, with no mechanical movement, it is never liable to get out of order, and the apparatus can be easily fixed over a wood base or frame covered with zinc or lead to secure a good watertight connection. Where Boyle's ventilators are used the air is renewed imperceptibly, the vitiated air being extracted as rapidly as it is generated.
A somewhat similar arrangement to Boyle's ventilator is patented by Arnold W. Kershaw, of Lancaster, and consists of 3 rims of deflectors or plates with openings in each, so arranged that the openings in one rim are opposite the deflectors in the next inner or outer rim, the effect being that whatever the direction of the wind, it passes through the ventilator without being able to enter the central shaft, and in passing creates a partial vacuum, which induces an upward current in the upcast shaft without the possibility of down draughts. Both Boyle's and Kershaw's roof ventilators are suitable for fixing in ventilating towers or turrets. While Kershaw's is somewhat simpler in construction, Boyle's is said to possess the additional advantage of preventing the entrance of snow by the curve in which the inner plates are fixed. In the case of chimney flues where there is any obstruction that breaks the wind and produces a swirl, such as would be caused by close proximity to higher buildings or raised gables, a down draught may be prevented by the use of a properly-constructed chimney cowl. Kershaw's chimney cowl is a modification of his pneumatic ventilator, and consists of deflecting plates so arranged that there is no possibility of a down draught.
Boyle's chimney cowl is better known than Kershaw's, and is very effective. It consists of deflecting plates so fixed that if a body of air is forced in at the false top, instead of passing down the vent, it is split up by an inner diaphragm, deflected over the real top, and passed over at the side openings, thus checking the blow down and assisting the up draught. Kershaw's patent inlet and air diffuser consists of a tube connection between the outside and inside of an apartment rising vertically on the inside, the upper extremity having radiating plates, which diffuse the incoming current. Generally speaking, a sufficient amount of fresh air enters under the door to a room or between the window sashes or frames; but in apartments where doors and windows fit tightly, some arrangement for the admission of fresh air becomes indispensable. In this climate, during 7 months of the year, the external air is usually too cold to be admitted directly into the rocm. The plan of admitting fresh air to a space behind the grates, leading up the air through channels on each side of the fireplace, and ultimately passing it through perforated gratings within the wall or through perforations in the skirting board on each side of the fireplace cannot be commended, as the passages are apt to get choked up with dust, and the temperature of the air cannot be well regulated in its passage into the room.
The true object of a fire and chimney flue should not be to supply fresh air, but to extract it after it has done its work.