The thing that is important in the operation of a turbine or any other kind of heat engine,
then, is the temperature range through which the heat energy falls in its passage through
the engine. The thermal efficiency of the engine depends upon this temperature range.
This can be explained very simply by imagining a perfect engine – one in which there
are no heat or friction losses of any kind. Of course, such a machine could never be built.
Assuming that we had such an engine, however, let us connect it to a source of steam
having a temperature of 400°F. Also, assume that the engine exhausts against atmospheric
pressure. The exhaust steam then would have a temperature of 212°F.