All About Engines

The Modern Reciprocating Engine T45 a pressure which can be detected. When the tem- perature reaches ioo° C. or 2120 F.» steam is vigorously produced. If, now, the temperature falls ever so little some of the steam is re-converted into water, and in the actual experiment the steam con- denses on the relatively cooler walls and trickles down the sides. A vapour in contact with the liquid from which it has been formed is said to be satu- rated, and no matter what the temperature is, cool- ing always results in partial condensation. But if the heating is continued until all the water is boiled away the steam continues to expand, and the tempera- ture, which has been constant during boiling, begins to rise. The steam is now said to be superheated, and cooling does not result in condensation until the temperature at which it all boiled away is reached. The use of superheated steam was not possible in the early days, because neither the packing nor the oil used for lubrication would stand the tempera- ture. Both of them would have charred. But the employment of metallic packing and the discovery of mineral oils capable of withstanding high tempera- tures enable steam to be heated to from ioo° F. to 300° F. above the temperature in the boiler. Such steam does not suffer condensation on admis- sion to the cylinder, and will undergo considerable expansion before condensation from this cause occurs. Unless, however, the boiler pressure is low or the range of expansion is very great, the exhaust steam will be at such a temperature that it is still cap- able of performing a considerable amount of work. K