logs the passages.
Cleaning such heaters is costly and the efficiency drops off rapidly as
scale forms. A closed heater is not advisable where the engines work
intermittently, as is the case with mine hoisting engines. In this class
of work the frequent coolings between operating periods and the sudden
heatings when operation commences will tend to loosen the tubes or even
pull them apart. For this reason, an open heater, or economizer, will
give more satisfactory service with intermittently operating apparatus.

Open heaters are best suited for waters containing scale-forming matter.
Much of the temporary hardness may be precipitated in the heater and the
sediment easily removed. Such heaters are frequently used with a reagent
for precipitating permanent hardness in the combined heat and chemical
treatment of feed water. The so-called live steam purifiers are open
heaters, the water being raised to the boiling temperature and the
carbonates and a portion of the sulphates being precipitated. The
disadvantage of this class of apparatus is that some of the sulphates
remain in solution to be precipitated as scale when concentrated in the
boiler. Sufficient concentration to have such an effect, however, may
often be prevented by frequent blowing down.

Economizers find their largest field where the design of the boiler is
such that the maximum possible amount of heat is not extracted from the
gases of combustion. The more wasteful the boiler, the greater the
saving effected by the use of the economizer, and it is sometimes
possible to raise the temperature of the feed water to that of high
pressure steam by the installation of such an apparatus, the saving
amounting in some cases to as much as 20 per cent. The fuel used bears
directly on the question of the advisability of an economizer
installation, for when oil is the fuel a boiler efficiency of 80 per
cent or over is frequently realized, an efficiency which would leave a
small opportunity for a commercial gain through the addition of an
economizer.

From the standpoint of space requirements, economizers are at a
disadvantage in that they are bulky and require a considerable increase
over space occupied by a heater of the exhaust type. They also require
additional brickwork or a metal casing, which increases the cost.
Sometimes, too, the frictional resistance of the gases through an
economizer make its adaptability questionable because of the draft
conditions. When fig