Document Y9dQRQvmMQzK4p93Yg7qbd50

Heating Ventilating Air Conditioning Guide 1939 6 How may the pollution of the atmosphere be lessened? By compelling industrial plants to install dust catching and smoke controlling devices. In many cities the domestic heating plant is one of the most serious offenders, but these plants are too small to justify the installation of dust catchers. Public education in improved firing methods would be of considerable help in this field. 7 What size particles are detrimental to health? While fairly large particles may enter the upper air passages, those found in the lungs are seldom more than 10 microns in size, and comparatively few of them are more than 5 microns. It is agreed that particles between H and 2 microns may be harmful; some authorities place the upper limit at about 5 microns, and some incline to extend the lower limit to 0.1 of a micron. 8 Is the shape of the particle of any significance? Hard particles with sharp corners or edges have a cutting effect on the delicate mucous membranes of the upper respiratory tract which may lower the resistance of the nose and throat to acute infections. This is aggravated by the irritating effects of some chemical compounds which may be taken in with the air and which act to reduce resistance. 9 What are the principal meteorological effects of smoke and dust? a. The reduction in the amount of light received. Measurements have shown that visible light may be as much as 50 per cent less intense in a smoky section of a city than in a section that is free from smoke. Ultra-violet light is reduced as much or more, and in some cases is cut out entirely for a time. b. Smoke and dust aid in the formation and prolongation of fogs. City fogs accumulate smoke and become darker in color and very objectionable. The sun requires a longer time to disperse them, and when the water is evaporated, there is a rain of smoke and soot particles that have been entrained. 10 O Why has not smoke abatement been more effective? L Because communities have not been made sufficiently aware of the possibilities of burning high volatile fuels smokelessly and of separating cinder and ash from the stack gases to a degree that will prevent a nuisance. 11 Is the abatement of dust and cinders important? Yes. Only a small percentage of the solid emission from stacks is smoke, in the accepted popular sense; the remainder is fiy-ash and cinders. While black smoke is disagreeable and its tarry matter and carbon particles soil anything with which they come in contact, the cinders and some of the ash are hard and destructive. They also, together with dusts from industrial processes, make up the irritating, air-borne solids that are breathed by individuals not working in a dusty mill or factory. 12 Are air-borne impurities causative factors in hay fever, bronchial asthma, and allergic disorders? ' Yes. Recent medical investigations indicate that 90 per cent of seasonal hay fever and 40 per cent of bronchial asthma are caused by air-borne pollens, tree dusts, and other allergic irritants. 13 Name some essential requirements for the smokeless combustion of fuels. Time, temperature, and turbulence.. A study of these factors is usually of value in overcoming a smoke nuisance. 14 What is the Ringelmann Chart Method of comparing smoke; densities? See Chapter 44. The Ringelmann Chart consists of four cards ruled with lines having different degrees of blackness. These cards, together with a white card and a.black one, are hung in a horizontal row 50 ft from the observer. At this distance the lines become invisible and the cards appear to be different'shadesof gray, ranging from white t<y black. The observer, by matching the cards;agains the shades of smoke coming from a Stack, is able to estimate the blackness of the smoke as compared with the chart. Chapter 5 HEAT TRANSMISSION COEFFICIENTS AND TABLES Methods of Heat Transfer, Coefficients, Conductivity of Homogeneous Materials, Surface Conductance Coefficients, Air Space Conductance, Practical Coefficients, Table of Con ductivities and Conductances, Tables of Over-all Coefficients of Heat Transfer for Typical Building Construction, Combined Coefficients of Transmission IN order to maintain comfortable living temperatures within a building it is necessary to supply heat at the same.rate that it is lost from the building. The loss of heat occurs in two ways, by direct transmission through the various parts of the structure and by air leakage or filtration between the inside and outside of the building: The purpose of this chapter is to show methods of calculation and to give practical trans mission coefficients which may be applied to various structures to deter mine the heat loss by direct transmission. The. amount lost by air filtration is determined by different methods, as outlined in Chapter 6, and must be added to that lost by direct transmission to obtain the total heating plant requirements. METHODS OF HEAT TRANSFER Heat transmission between the air on the two sides of a structure takes place by three methods, namely, radiation, convection and conduction. In a simple wall built up of two layers of homogeneous materials separated to give an air space between them, heat will be received from the high temperature surface by radiation, convection and conduction. It will then be conducted through the homogeneous interior section by con duction and carried across to the opposite surface of the air space by radiation, conduction and convection. From here it will be carried, by conduction through to the outer surface arid leave the outer surface, by radiation, convection and conduction; ' The' process of heat' transfer through a built-up wall section is complicated in theory, but in practice it is simplified by dividing a wall into its component parts and considering the transmission through each part separately. Thus the average wall may be divided into external surfaces, homogeneous materials and interior air spaces. Practical heat transmission coefficients may be derived which will give the total heat transferred by radiation, conduction and convec tion through any of these component parts and if the selection arid method of applying these individual, coefficients is thoroughly understood it is usually a comparatively simple matter to calculate the over-all heat transmission coefficient for any combination of materials. 87