The subject matter of the paper is divided under the following headings:-Introduction.Thermal efficiency.Operation costs.Reliability.Factors in design affecting reliability and operation.Flue gas, dust and sulphurous fume extraction.Arrangement of boiler plant.Acknowledgements.References.Introduction.—In the Introduction a brief survey is made of the present position of boiler plant design in this country and America. The outstanding thermal performance of modern American generating stations is commented upon, and it is shown that these results are in no small measure the effect of American boiler plant development. The greatest difference in the trend of design between the two countries is in the wider adoption of higher operating steam pressures for American generating stations, the increase in individual boiler unit capacity, and the extended use of pulverized fuel firing, particularly on the unit system.Thermal efficiency.—Under this heading present-day thermal results of British boiler plant are discussed, and examples are given of such results for both stoker-fired and pulverized-fuel-fired boiler plants.Operation costs.—In this part of the paper the importance of repairs and maintenance costs is stressed. Examples are given of these costs for two large pulverized-fuel installations and one large stoker installation.Reliability.—The reliability of modern boiler plant is discussed under this heading, and the reliability of a large pulverized-fuel-fired British plant is given in analytical tabulated form and also graphically, showing the hours of availability lost as a result of outage, and the cause of such outage. Three examples are given on a somewhat similar basis of the reliability of American plants. The various factors affecting reliability in general are discussed.Factors in design affecting reliability and operation.—This section of the paper is subdivided under the following headings:—(a) Water-heating and evaporative parts (boiler and economizer).(b) Superheater.(c) Combustion chamber.(d) Combustion equipment.Under (a) the author discusses the trend towards higher ratings and the results accruing from such practice. He suggests that the conventional present-day boiler form is unsuitable for development for high rates of heat transfer, and that such rates can only be effected by radiant heat, necessitating very radical changes in boiler unit form. He suggests, in general, that for the commonly accepted boiler-unit forms the average rate of evaporation per square foot of total water-heating surface at normal load should not exceed 5.5 lb. per hour. The need for further development of soot-blowing apparatus is discussed under this sub-heading.Under (b) the trend in superheater design is discussed, and the vulnerability of the modern superheater location isemphasized. It is suggested that with the demand for still higher superheats, development in design is necessary to provide a superheater location where definite and separate control of superheat temperature can be effected. Superheater troubles within the experience of the author are given, and these include records of temperatures which superheater tube materials have to withstand in everyday practice.Under (c) the author discusses and emphasizes the importance of combustion chamber design, and suggests that the boiler parts proper should be designed and arranged in dependent relation to the combustion chamber. The development of the water-cooled combustion chamber is briefly outlined, and the various advantages and limitations are discussed. The form of the combustion chamber and factors affecting reliability of refractory and water-wall construction are discussed.Under (d) the author discusses the relative merits of stoker and pulverized-fuel combustion equipment, and suggests that pulverized-fuel firing has many advantages over stoker plant. These advantages are discussed, but the suggestion is qualified by the necessity for designers, constructors and operators being thoroughly conversant with the subject of pulverized-fuel plant design and operation.This section is followed by a discussion of the relative merits of the unit and bin-and-feeder systems of pulverized-fuel plant, and the author suggests that the greater reliability and other advantages of a form of bin-and-feeder system cannot be ignored.It is suggested that the adoption of boiler units of greater capacity for the modern large generating station has many advantages without sacrificing reliability or incurring greater costs, and a further suggestion is made and briefly discussed in favour of pulverized-fuel-fired units of not less than 500 000 lb. of steam per hour.Air-heater equipment is discussed in this section of the paper, and the author criticizes and discusses modern design. He suggests simplicity in form and construction and greater attention to the laws governing the flow of air and gases. A brief survey of one phase of the author's experience in the application of high-temperature air to stoker practice is given and conclusions are reached. These latter were principally the necessity for cooling the grate surface to at least 350° F. and limiting the air temperature to this value. This section concludes with a survey of the author's experience with a large pulverized-fuel installation.Flue gas, dust and sulphurous fume extraction.—The author has the honour to be a member of the Chimney Emissions Committee set up by The Electricity Commission, and also a member of the Technical Sub-Committee, and therefore he cannot freely discuss dust and sulphurous fume extraction in the present paper.Arrangement of boiler plant.—In this final section of the paper the relative merits of the accepted forms of boiler-plant lay-out are considered.