Exhaust Nozzles. It has been determined by experiment that the most efficient form of exhaust nozzle is that which keeps the jet in the densest and most compact form. Tests indicate that the nozzle giving the jet the least spread is the most efficient. Of the three forms of exhaust nozzles shown in Fig. 52, the spread of the jet is least for a and most for c. Nozzle a ends in a plain cylindrical portion 2 inches in length. Nozzle c is contracted in the form of a plain cylinder ending in an abrupt cylindrical contraction. It has been common practice, in cases where engines refuse to steam properly, to put across the exhaust nozzles round or knife-shaped bridges as indicated in Fig. 53. The use of bridges accomplishes the desired result but experiments have shown that this method materially affects the efficiency of the engine because of the increase of back pressure in the cylinders. It is, therefore, best not to split up the jet by using a bridge in cases where the draft is unsatisfactory, as the desired results may be obtained by reducing the diameter of the exhaust nozzle.
As previously stated, draft or petticoat pipes are used for the purpose of increasing the draft or vacuum in the front end and in the tubes. A great many tests have been made under the supervision of the Master Mechanics' Association to determine the proper proportions of the petticoat pipes and their best relative position with reference to the stack and exhaust nozzle.
The report of the committee of the Master Mechanics' Association with reference to single draft pipes states "that for the best results, the presence of a draft pipe requires a smaller stack than would be used without it but that no best combination of single draft pipe and stack could be found which gave a better draft than could be obtained by the use of a properly proportioned stack without the draft pipe. While the presence of a draft pipe will improve the draft when the stack is small it will not do so when the stack is sufficiently large to serve without it. The best proportion and adjustment of a single draft pipe and stack are shown in Fig. 54."
The finding of the same committee with reference to the use of the double draft pipes is as follows: "Double draft pipes of various diameters and lengths and having many different positions within the front ends all in combination with stacks of different diameters, were included in the experiments with results which justify a conclusion similar to that reached with reference to single draft pipes. Double draft pipes make a small stack workable. They cannot serve to give a draft equal to that which may be obtained without them provided the plain stack is suitably proportioned. The arrangements and proportions giving the best results are illustrated in Fig. 55."
Table of Contents; Page 54; Page 58; Index