In a steam locomotive, water from the tender is used to keep the water
level in the boiler constant. Both the engineer and firemen can control
the insertion of water from the tender into the boiler. The water in the
tender is at ambient temperature. The water in the boiler can be around
400 degrees Fahrenheit. Adding relatively cold water from the tender
to the boiler through the use of an injector does partially heat the
incoming water. However, even though this injector-heated water is around
200 degrees Fahrenheit, there is still roughly a 200 degree temperature
differential between the injector-heated water and the boiler temperature.
Furthermore, the energy used in heating the incoming water by the injector,
comes from the boiler. The 200 degree temperature differential causes both
stress on the boiler is a drain on the energy in the boiler. To improve
the thermodynamic efficiency of the locomotive, it would be beneficial to
preheat the water before it is fed into the boiler using energy that would
otherwise be wasted.
The steam exhausted from the cylinders of a steam locomotive is still
quite hot. Instead of releasing this steam to the atmosphere and wasting
the heat energy contained within it, feedwater heaters use this exhausted
steam to preheat the water from the tender before it is fed into the boiler.
The feedwater heater usually replaced the injector on the fireman's side
of the locomotive.
Given the benefits of a feedwater heater the obvious question is: "Why didn't
all steam locomotive have them?" The answer is: Because railroads had to
weigh the cost of higher maintenance against the price of fuel. Sometimes,
the cost of fuel was simply cheaper than the cost to maintain them.
There are two main types of feedwater heaters: open and closed.
Open Feedwater Heaters
In an open feedwater heater both the feedwater and steam are allowed to mix.
This required two pumps. One disadvantage of open feedwater heaters was
that the valve lubrication oil present in the exhausted steam was mixed
with the feedwater going into the boiler. A Worthington feedwater heater
is an example of an open feedwater heater.
Closed Feedwater Heaters
In a closed feedwater heater the feedwater is run through a small pipe
which is contained in a chamber of steam. Condensed steam is collected and
piped back into the tender. One disadvantage of closed feedwater heaters
was that the additional small copper piping was prone to leaks. The Elesco
and Coffin feedwater heaters are examples of this type.
The company name "Elesco" was taken from "Locomotive Superheater Company".
"LSCo" is pronounced "El" "Es" "Co" or "Elesco".
Feedwater Heater Identification
Feedwater heaters on steam locomotives are easily identified by their
location and shape. Elesco bundle type feedwater heaters were often
contained within a cylinder mounted horizontally above the smokebox.
They can also be partially inset within the top of a smokebox.
Coffin feedwater heaters were designed to be contained within the top
of a smokebox. However, when retrofitted, they were mounted in front
of the smokebox.
Worthington BL type feedwater heaters were often mounted on the side of a
locomotive and may look like an air pump to the untrained eye. Worthington SA
type feedwater heaters were inset into the top of the smokebox. By 1930
most steam locomotives were built with feedwater heaters and the Worthington
SA type was the most popular.