Feedwater Heating

Economizer and Feedwater Heating

Feedwater Heating

The temperature of feedwater entering the boiler should be as high as possible in order to maximize steam production and to minimize thermal stresses in the boiler structure

Feedwater heating can be achieved in many ways.  The “steam injectors” commonly used on First Generation locomotives provides a degree of preheating, however they only work at relatively low water temperatures.  Where high temperature feedwater heating is used, the water must be pumped through the heater.  In the case of the Red Devil, Wardale provided a single feed pump but retained the original injector as an independent standy, the injector-fed water bypassing the feedwater heater.

Feedwater temperature should be as high as possible to maximise energy savings.  Ideally it should be 100oC or above which necessitates the use of a “closed” or pressurized system.  A typical closed system takes the form of a tube-and-shell heat exchanger in which feedwater passes through the tubes while exhaust steam is fed into the outer shell and condenses on the tube surfaces.  The Red Devil’s heat exchanger was of this sort, being in the form of a ø550mm cylindrical casing with a tube plate at each end (one fixed, one floating) with six banks of 23 x ø20mm copper tubes through which feedwater passed in a sequence designed to maximise heat transferby exposing (as far as possible) the hottest water to the hottest steam and vice versa.

Two similar but smaller heat exchangers were proposed for 5AT each exchanger containing 6 banks of 15 x ø20mm copper tubes.  Wardale’s sketch below illustrates the counterflow principle:

The “economiser” (described below) is another type of feedwater heater which may be used on its own or in conjunction with an exhaust steam heater.  Porta combined both in his more advanced designs.

Advantages of Feedwater Heating:

Improved performance and fuel economy:

As Wardale says on page 156 of his book, “Normally a feedwater heater was viewed as a device for saving fuel and water, savings of 10-15% in both being generally claimed. However its importance, like that of having the best possible exhaust system, seems to have been rarely appreciated [by FGS designers].  The significance of both becomes fully apparent only when locomotive performance in terms of the power generated from a given mass of hardware was pushed to its limit”.

A calculation is presented under the heading of Specific Enthalpy which demonstrates how a 10% saving in energy requirement can be expected by preheating feedwater to 100oC, however Wardale shows how feedwater heating can produce a very much greater saving in fuel consumption when a locomotive is operating at or close to its grate limit.  He illustrates the point in the diagram below:

Fig 41 - Effect of Feedwater Heating on Boiler Output

Water saving through Recyling of Condensed Steam

Wardale also points out that exhaust steam feedwater heaters deliver significant water savings through the recycling of condensd steam.  In the case of The Red Devil, he noted that “it was estimated that some 13.5% of the cylinder exhaust steam would pass through the heater, equivalent to approximately 12.5% of the total evaporation, this latter figure representing the water saving due solely to the feedwater heater system.  This was in itself a significant figure contributing to a greater operating range between water stops.”

Cautionary notes:

  • The application of feedwater heating to an existing design will result in reduced superheat temperature as mentioned on page 161 of “Wardale’s book“.  This is because less heat needs to be generated in the firebox to boil preheated water, and therefore there will be less heat available for superheating the steam that is generated”.
  • From Wardale’s FDCs for the 5AT:  “It is essential that the piping from the cylinder exhaust passages to the heaters allows [the required amount of steam to be delivered to the heat exchanger].  As the exhaust steam flow to the heaters is in parallel with that to the blast nozzles (and the combustion air preheater) this becomes more difficult as exhaust design is improved and the blast nozzle size increases.  With a free exhaust, exhaust steam escapes more easily from the blast nozzles and correspondingly less will go to the feedwater heaters if the piping to them is restrictive.  At the detail design stage analysis must be made to ensure that the flow resistance of the pipes concerned relative to that of the exhaust passages to the blast nozzle tips will allow [the required proportion of exhausted steam to be reached].  An approximate comparison of the flow resistances involved suggests that the exhaust steam pipes to the heaters, each of length ≈ 4~5 m, must be absolutely not less than 100 mm bore diameter and should preferably be larger.  The take-off for the feedwater heater steam should be just downstream of the steam chest exhaust chambers: a suitable arrangement is to have a take-off in each exhaust passage just inside the frame portion of the cylinder block, the two pipes from each cylinder joining at a Kordina-type joint.

Economizer

An “economizer” is a supplementary water heater consisting of the front part of the boiler which is partitioned off from the rear section by a thin steel plate baffle, closely-fitted inside and around (but not rigidly attached to) the boiler barrel, tubes and flues. Thus, the pressure on both sides of the baffle is the same – the baffle simply serving to retain the colder incoming water to give it time to gain temperature before it “overflows” into the evaporative (rear) section of the boiler.

Keeping the colder water at the front of the boiler serves two purposes:

  • it maximises the temperature differentials across the tubes – i.e. between combustion gases and boiler water – allowing the cooler water in the front of the boiler to extract heat from the gases that have lost most of their energy evaporating the water at the back of the boiler.
  • segregating “the cold” from the hot water, minimizes thermal stresses within the boiler shell.

Economizers were not commonly used in the past, being introduced (invented?) by Chapelon in his experimental 2-12-0 freight locomotive 160A-1 designed in 1936.

[Note: the Franko-Crosti boiler used on some of the BR 9F 2-10-0s was a singularly unsuccessful form of Economizer.]