DISINFECTION BY HEAT.

Heat may be applied in various ways: (1) Prolonged boiling in water of materials which are not spoilt by this means. (2) Destruction by fire of infected articles. (3) Dry hot air. (4) Steam.

Boiling kills most pathogenic microbes. The cholera vibrio is killed in four minutes at a temperature of 52° C. (126° F.); the typhoid bacillus at 59°.4 C. (138°.8 F.) in ten minutes. If boiling be continued for five minutes, the spores of pathogenic microbes are killed. The addition of one to two per cent. of washing soda to the water hastens this effect. For infected linen nothing beyond this is required.

Destruction by Fire is to be recommended for comparatively worthless articles, such as toys, straw from beds, rags, old clothing and bedding.

Dry Hot Air has been largely used in the past in ovens, for the disinfection of bulky bedding. It is now entirely superseded by steam. Its disadvantages are that (a) heat penetrates very slowly into the interior of bedding. Disinfection in test experiments was not accomplished in the interior of small bundles of clothes in three or four hours. (b) Scorching of articles often occurs. The sole advantage of this method is that bound books and leather goods are less liable to be damaged by it than by steam. If no other apparatus is available a baker’s oven will serve to kill the non-sporiferous microbes of cholera, enteric fever, and diphtheria, as well as animal vermin. If, however, we accept the proper test proposed by Buchanan of the efficacy of disinfection, the “destruction of the most stable known infective matter,” dry heat is unsatisfactory.

Steam may be employed as a disinfectant either (a) superheated, or (b) saturated, i.e. close to the temperature at which condensation occurs. This temperature depends upon the pressure under which the water has been boiled. At ordinary atmospheric pressure it is 100° C. (212° F.). The temperature of boiling is raised by subjecting the water to pressure. Consequently boiling water and the steam produced from it may be at any temperature. Thus steam may be

Fig. 57.
Equifex Saturated Steam Disinfector.

A—Disinfection chamber. B—Partition wall separating infected from disinfected side. C—Door on disinfected side. D—Door on infected side. EE—Safety-locking bolt for securing door. FF—Stiffening rings on doors. G—Steam inlet from boiler. H—Steam separator for arresting water condensed in G. I—Valve controlling admission of steam to disinfecting chamber. K—Valve controlling admission of steam to coils. LM—Safety valves regulating steam pressure in chamber and coils respectively. NU—Pressure guages indicating steam pressure in chambers and coils respectively. OO—Objects after disinfection. P—Wheeled carriage and cradle for OO. Q—Hinged rails on which P runs. R—Exhaust pipe for steam and air on first admission. S—Thermometer showing rise of temperature (to control complete air evacuation). T—Valve for closing exhaust pipe R when air is completely evacuated. V—Sluice valve to cause sudden escape of steam. W—Cock to admit steam to ejector. X—Exhaust pipe fitted with ejector for escape of steam before and of air during drying. Y—Valve for admission of air for drying under suction of ejector. [In some types of this machine this valve is placed on the lower part of C.]

Steam when admitted into a disinfecting stove comes into contact with cold objects. If the steam is saturated, immediate condensation to 1 ∕ 1600 part of its original volume occurs. Its latent heat is at the same time evolved. The condensation causes enormous shrinkage in bulk. More steam is thus insucked into the partial vacuum produced, and this is repeated, until in every part of the mattress or other material undergoing disinfection equality of temperature is reached, when condensation of steam will cease, and disinfection is complete. If the steam is superheated and no condensation allowed, disinfection occurs by the relatively slow method occurring with dry heat. In practice at the early stage cooling causes some conversion of superheated into saturated steam, though subsequently the much slower process of disinfection by conduction of heat goes on. Hence superheated steam is a less efficient disinfecting agent than saturated steam.

Superheating is produced in disinfecting stores in two ways: (1) By a jacket around the stove, which is kept at about double the pressure and about 20° to 30° F. hotter than the interior of the stove; as in the older patterns of the Washington Lyon stove. (2) By having a jacket containing a solution of calcium chloride, which is heated by a furnace under the stove. This solution is kept at a constant strength by an automatic supply from a cistern. The temperature of the boiling water is thus raised without pressure to 225° F. This is the principle of Thresh’s stove. The object of superheating steam is to assist in rapidly drying materials; but this object can equally well be secured by periodically allowing the sudden escape of the steam confined under pressure, in pressure disinfectors. This last method is the best, as it can not only be utilised at the last stage of the disinfection for drying the articles; but at the earlier stage for sweeping the air out of the stove, and thus removing what, owing to its low conductivity for heat is one of the most serious obstacles to rapid and efficient disinfection.

In the above description it has been assumed that the steam, whether saturated or superheated, is confined, except when the exhaust is employed for drying purposes. Steam may also be employed as current steam. Current steam disinfectors are initially cheap, but more steam, and therefore more fuel, is required in their use; and unless pressure is used by impeding the escape of the steam a temperature of only 212° F. can be secured. Accepting Buchanan’s dictum, a stove supplying saturated steam under pressure at a temperature in the interior of the stove of 234° F. is to be preferred. This temperature with saturated steam destroys the spores of the most resistant known microbe (that of symptomatic anthrax). With superheated steam or hot air stoves on the same basis a temperature of 280° F. would be required, which is damaging to most textiles, except horsehair.