(9.1) The overall feedwater heating system on the 5AT is as follows:
- A 'hot well' and auxiliary heater in the tender. The hot well is a compartment in the tender water tank, at the front end of the well between the bogies if this is fitted, otherwise at the front of the body of the tank, in free communication with the rest of the tank and from which the feed pipe to the boiler feedwater pump is run, see Fig. 9.1. Exhaust steam condensate from the feedwater heater is piped back to the tender and mixes with tender water in the auxiliary heater at the entrance to the hot well. By this means condensate is effectively mixed with the boiler feed when the feed pump is working and the hot well is continually replenished by tender water passing through the auxiliary heater, but if the engine steams for short periods without the pump operating and uncondensed exhaust steam reaches the tender it avoids undue rise in the water temperature in the hot well. Any exhaust steam from auxiliaries which is similarly recovered (see [1.3.(134]) may be piped back directly to the hot well. By mixing tender water and condensate / exhaust steam immediately upstream of the feed pipe, recovery of heat energy in the condensate / exhaust steam is accomplished together with the recovery of water.
- A 'closed' or 'surface' type (shell & tube) heat exchanger ('heater') situated between the feedwater pump and the boiler: feedwater under pressure is pumped through the tubes and is heated by exhaust steam condensing on the outside of the tubes in the heater shell.
- 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.
(9.2) The importance of feedwater heating in general, and the advantages of the surface type heater over mixing types (which include exhaust steam injectors) are given in Ref. . To summarize, feedwater heating becomes ever more essential as the power : weight ratio of steam locomotives is pushed towards its limit, as is the case with the 5AT, and obtaining the highest feedwater temperature entirely from exhaust steam heat mandates the surface type heater.