A Practical Manual On Sea Water Distillation
With A Description Of The Necessary Machinery For The Process

MULTIPLE DISTILLATION. 189 Dirty Surfaces—Economy Reduced. 33. The economy of an apparatus is very much affected by the condition in which the surfaces are in—i.e., as to the scale deposited on them—especially on the boiler heating surfaces, because, as such scale forms on the boiler heating surface, the heat from the fire is prevented from passing to the water, and instead, goes up the boiler chimney unused. 34. Scale accumulating on the evaporator surfaces, although it reduces the evaporative power (i.e., in point of time), does not affect the economy to anything like the extent that scale formed on the heating surfaces of the boiler does. The reason is that when scale forms on the evaporator surfaces, the evaporative power is reduced, and consequently less gained water produced, but then less primary steam is used in proportion, so that the economy is not very much affected, as the proportion of water yield to coal consumption is about the same. 35. To compensate for the boiler surfaces getting dirty, a greater pressure has to be got up. If, therefore, the boiler is started at its utmost capacity and pressure, the pressure cannot be increased as the surfaces get dirty, and the result is that the initial large production can only be kept up for a short time. It is sometimes said that it is better to lose heat by making short runs of work and then to clean the surfaces, than to continue working with dirty surfaces. This view may be advanced and supported by argument, but in practice it is found that the cost of working is less by running the apparatus as long as possible without stopping it for cleaning purposes.