The gas is drawn into the burette through the U tube H, which is filled with spun glass, or similar ma terial, to clean the gas. To discharge any air or gas in the apparatus, the cock G is opened to the air and the bottle F is raised until the water in the burette reaches the 100 cubic centimeters mark. The cock G is then turned so as to close the air opening and allow gas to be drawn through H, the bottle F being lowered for this purpose. The gas is drawn into the burette to a point below the zero mark, the cock G then being opened to the air and the excess gas expelled until the level of the water in F and in A are at the zero mark. This operation is necessary in order to obtain the zero reading at atmosphere reading pressure.
The apparatus should be carefully tested for leak age, as should also all connections leading thereto.
Simple tests can be made, for example, if, after the cock G is closed, the bottle F is placed on the top of the frame for a short time and again brought to the zero mark, if the level of the water in A is above the zero mark, a leak is indicated.
Before taking a final sample for analysis, the burette A should be filled with gas and emptied once or twice to make sure that all the apparatus is filled with the new gas. The cock G is then closed and the cock I in the pipette B is opened, and the gas driven over into B by raising the bottle F. The gas is drawn back into A by lowering F and when the solution in B has reached the mark in the capillary tube, the cock I is closed and a reading is taken on the burette, the level of the water in the bottle F being brought to the same level as the water in A. The operation is repeated until a constant reading is obtained, the number of cubic centimeters being the percentage of CO. in the flue gases.
The gas is then driven over into the pipette C and a similar operation is carried out. The differ ence between the resulting reading and the first read ing gives the percentage of oxygen in the flue gases. The next operation is to drive the gas into the pipette D, the gas being given a final wash in E, and then passed into the pipette C to neutralize any hydro chloric acid fumes which may have been given off, thus increasing the volume of the gases and making the reading on the burettes less than the true amount. The process must be carried out in the order as the pyrogallol solution will also absorb carbon dioxide, while the cuprous chloride solution will also absorb oxygen.
As the pressure of the gases in the flue is less than atmospheric, they will not of themselves flow through the pipe to the burette. For rapid work a rubber bulb aspirator connected with the air outlet of the cock G will enable a new supply of gas to be drawn into the pipe, the apparatus then being filled as already described.
Another form of aspirator draws the gas from the flue in a constant stream, thus insuring a fresh supply for each sample.
The analysis made by the Orsat apparatus is vol umetric; if the analysis by weight is required, it can he found from the volumetric analysis as follows : Multiply the percentages by volume by either the densities or the molecular weight of each gas, and di vide the products by the sum of all the products ; the quotients will be the percentages by weight. For most work sufficient accuracy is secured by using the even values of the molecular weights, given below : Combustion may be confined by the rapid chem ical combustion of oxygen with carbon, hydrogen and sulphur, accompanied by the diffusion of heat and light. That portion of the substance thus combined with the oxygen is called combustible. The princi cipal combustible in fuel oil being carbon, hydrogen and sulphur. Carbon being the most abundant as is shown by analysis. Comparative analysis of various crude oils may be found at the front of this book.
The proportionate amount of air required for com bustion is a most important question. The theoretical amount of air required to supply the oxygen for com bustion can be worked out, but in practice it is impos sible to obtain perfect combustion with the theoretical amount of air. Nitrogen is an inert gas and does not aid the combustion. It passes through the furnace without any change, other than absorbing the heat of the furnace, thus reducing the temperature. It is one of the unavoidable sources of heat losses in fur naces. The reason for excess air, over the theoretical amount required, is that it is impossible to bring out each particle of oxygen in the air into intimate con tact with the particles of the fuel that are to be oxi dized, due not only to the dilution of the oxygen in the air by the nitrogen, but, also to the fact that the fire is too dense to allow the air to pass through it.