CHAPTER V

EVAPORATION TESTS

Evaporation tests, object of-Preparing for a test-Suggestions as to apparatus needed-Taking the temperature of the feed water-Precautions necessary to obtain accurate resultsDuration of test-Feeding a boiler during a test-How to proceed if the boiler is fed by an injector-Determining the percentage of moisture in the steam-Moisture in the coalChimney draft-Draft gauge-Rules for determining the results of a test-"Equivalent evaporation," how to compute-Factors of evaporation-Boiler horse power.

Evaporation Tests. The object of making evaporation tests of steam boilers is primarily to ascertain how many pounds of water the boilers are evaporating per pound of coal burned; but these tests can and should be made to determine several other important points with reference to the operation of the boilers, as for instance: 1. The efficiency of the boiler and furnace as an apparatus for the consumption of fuel and the evaporation of water; whether this apparatus is performing its guaranteed duty in this respect, and how it compares with a known standard. 2. To determine the relative economy of different varieties of coal, also to determine the relative value of fuels other than coal, such as oil, gas, etc. 3. To ascertain whether or not the boilers as they are operated under ordinary every day conditions are being run as economically as they should be. 4. In case the boilers, owing to an increased demand for steam, fail to supply a sufficient quantity without forcing the fires, whether or not additional boilers are needed, or whether the trouble could

be overcome by a change of conditions in operating them.

As was stated in the chapter on boiler setting, every steam plant can and should be equipped with the necessary apparatus for making evaporation tests, and every engineer should take pride in making a good showing in the economical use of coal, and, be it said to their credit, the majority of engineers do this, although many of them are working under conditions. that prevent them from doing all that they might desire along this line. Too many engineers are compelled to look after work entirely outside of and foreign to their vocation as engineers, often having to go to some distant part of the building to make repairs to some part of the machinery, leaving their boiler and engine to care for themselves for the time being, thus not only endangering the safety of property, but of life as well. But conditions are gradually changing for the better in this respect, and employers and owners of steam plants are coming more and more to recognize the fact that the engineer is something more than a mere handy man about a factory, in fact, that he has a distinct and responsible vocation to be fulfilled, viz., the safe and economical operation of the plant where the power comes from.

The author proposes to present in as brief a manner as possible a few simple suggestions and rules for the benefit of engineers who desire to make evaporation tests with a view of determining one or more of the points mentioned at the beginning of this chapter.

Tests for the last three purposes named can be made by the regular engineering force of the plant, but in case a controversy should arise between the maker of the boiler and the purchaser regarding the first men

tioned point, viz., the guaranteed efficiency of the boiler or the furnace, the services of experts in boiler testing may be resorted to.

Preparing for a Test. All testing apparatus should be kept in such shape that it will not take three or four days to get it ready for making a test. On the contrary, it can be and should be always kept in condition. ready for use, so that the preparations for making a test will occupy but a short time. A small platform scale sufficiently large for weighing a wheel-barrow load of coal should also be provided in addition to the apparatus referred to in Chapter II.

The capacity of each of the two tanks therein mentioned, and which are illustrated in Fig. 10, can be determined in two ways. either by measuring the cubical contents of each or by placing them one at a time on the scales, filling them with water to within a few inches of the top, and note the weight. Also make a permanent mark on the inside at the water level. The water should then be permitted to run out until within an inch or so of the outlet pipe near the bottom, where another plain mark should be made, after which the empty tank should be again weighed, then by subtracting the last weight from the first the exact number of pounds of water that the tank will contain between the top and bottom marks can be determined and a note made of it.

It is much more convenient to have each tank contain the same quantity of water, although not absolutely necessary. The tanks should also be numbered I and 2 respectively in order to prevent confusion in keeping a record of the number of tanks full of water used during the test. Care should be exercised to have the water with which the tanks are filled while on

the scale, at or near the same temperature as that at which it is to be fed into the boiler during the test. Otherwise there is liability of error owing to the variation in the weight of water at different temperatures. In order to guard against this, the capacity in cubic feet of each tank between the top and bottom marks should be ascertained by measuring the distance between the marks, also the diameter, or, if the tanks. be square, the length of one side, after which the cubical contents can be easily figured and noted down. By knowing the capacity in cubic feet of each tank all possibility of error in the weight of feed water will be eliminated.

The scales for weighing the coal can be fitted with a temporary wooden platform large enough to accommodate a wheel-barrow, and after it has been balanced with the empty barrow on it, the record of weight of coal burned during the test can be easily kept.

The same barrow should be used throughout the test, and to save complications in estimating the weight, the same number of pounds of coal should be filled in the barrow each load. The coal passer will learn in a short time to fill the barrow to within a few pounds of the same weight each load by counting the shovelsful and the difference can easily be adjusted by having a small box of coal near the scale from which to take a few lumps to balance the load, or if there is too much coal in the barrow some of it can be thrown into the box.

At least two separate tally sheets should be provided, marked respectively coal and water, and the one for coal placed near the scale, and care should be taken that each load is tallied as soon as it is weighed. The tally sheet for water should be near the measuring

tanks and as soon as a tank is emptied it should be tallied. The temperature of the feed water should be taken at least every thirty minutes, or oftener if possible, from a thermometer placed in the feed pipe near the check valve, as described in Chapter II. The readings should be noted and, at the expiration of the test, the average taken.

If the object of the test is to ascertain the efficiency of the boiler and furnace it is absolutely necessary that the boiler and all its appurtenances be put in good condition, by cleaning the heating surface both inside. the boiler and outside, scraping and blowing the soot out of the tubes if it be a return tubular boiler, and blowing the soot and ashes from between the tubes if it is a water tube boiler. All dust, soot and ashes should be removed from the outside of the shell and also from the combustion chamber and smoke connections. The grate bars and sides of the furnace should be cleared of all clinker, and all air leaks made as close as possible. The boiler and all its water connections should be free from leaks, especially the blow-off valve or cock. If any doubt exists as to the latter it should be plugged or a blind flange put on it. It is very essential that there should be no way for the water to leak out of the boiler, neither should any water be allowed to get into the boiler during the test except that which is measured by passing through the tanks.

The engineer making the test should know the num⚫ber of square feet of grate surface and also the area of the heating surface in square feet. Rules for computing the latter are given in Chapter II. If the boiler is a water tube boiler the outside diameter of the tubes must be used in estimating the heating