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 much 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.
The 5AT design incorporated an economizer section at the front end of its boiler, described in the introduction to FDC 9 “Feedwater Heating” as follows:
“A Chapelon-type economizer at the front of the boiler barrel. This is a partition at the front of the barrel formed by an intermediate tubeplate and in free communication (by an overflow hole(s)) with the rest of the boiler. This free communication means zero pressure difference between the two and there may therefore be no need for pressure-tight joints where the boiler tubes pass through the intermediate tubeplate – a good fit of the tubes in the holes to stop gross flow of water between the two sections may suffice, e.g., for parallel tubes, to be achieved by drilling all tubeplates to the same template and aligning them up with some dummy tubes in place when welding to the barrel. The boiler clack valves are situated in the economizer section, all incoming feedwater entering the boiler at the front where the combustion gasses are at their lowest temperature, and therefore acting to give the highest temperature difference, and consequently highest heat transfer, in this part of the boiler, improving its absorption efficiency.”
[Note: the Franko-Crosti boiler used on some of the BR 9F 2-10-0s was a singularly unsuccessful form of Economizer.]