The exhaust system, dynamically connecting the boiler and cylinders, is thermodynamically the heart of the locomotive and must therefore be as good as possible, within practical limitations. That the exhaust entrains sufficient combustion air to sustain the combustion rate necessary to match the steam demand throughout the boiler's evaporative range is a cardinal point for good performance from any steam locomotive, and that it does this with the minimum of exhaust steam energy is the key to optimum performance. This point is especially important on the 5AT as the locomotive is to operate mostly at high speed with full throttle and low cut-off, giving high heat conversion to mechanical work in the cylinders and therefore limiting the amount of energy available for draughting work in the exhaust steam (it is common for locomotives to steam adequately at long to medium cut-offs but not at short, for this reason), and this is compounded on the 5AT by the use of piston valves with exhaust lap, delaying release.
The following note on the subject of triple exhaust is also instructive:
Triple exhaust. The number of chimneys is a compromise between maximising the length : diameter ratio of each individual chimney in order to increase ejector efficiency, which multiple chimneys achieve by reducing individual chimney diameter within the available height restriction, and minimising complication within the smokebox. If therefore a triple exhaust gave sufficient increase in blast nozzle tip area, its complication could be justified. The available length would allow triple chimneys 'in line' to be fitted for oil firing only (i.e. no self-cleaning plates or spark arrestor, and allowing for the rearmost part of the back chimney bell mouth and diffuser being set behind the front of the superheater header). The foregoing calculations have therefore been exactly repeated for a triple exhaust, the total primary, secondary and mixture mass flow rates being divided by 3 to get the figures per chimney, and the preliminary data being based on the optimum values calculated for the double exhaust. Note that as the diffuser outlet diameter is smaller for each chimney in a triple exhaust, and also because the chimneys can be set lower down, their length being ~1 800 mm. However the calculated total blast nozzle tip area ~164 cm2, which is only some 2% more than for a double Lempor. This small increase is insufficient to justify the extra complexity of a triple exhaust, which is therefore not considered further, a double exhaust being sufficient for the prevailing conditions.