In Feb 2017, Chris Newman wrote to David Wardale to ask if there was any justification in the suggestion that the predicted evaporation rate for the 5AT of 17,000 kg/h might be unrealstically high, particularly with coal-firing.

He pointed out that in lines [197] and [198] of FDC 1.3F a maximum specific burning rate for the 5AT is given as 709 kg/m^{2}/h, or some 11% higher than the maximum sustained rate achieved with the Red Devil. However when comparing evaporation rates, the figure of 6,376 kg/m^{2}/h for the 5AT is some 36% higher than that for the Red Devil (4,658 kg/m^{2}/h). Similarly, the evaporation per unit of coal burned appears to be about 21% higher for the 5AT than for the Red Devil.

Wardale replied as follows:

“The following figures are taken directly or calculated from the FDCs or *The Red Devil … *Consider:

- Fuel
*lower*calorific values for 5AT taken as: gas oil / diesel fuel = 42.9 MJ/kg, coal = 30 MJ/kg. - Average
*upper*calorific value of coals used for 3450 tests = 27.5 MJ/kg. The higher values for the 5AT fuels will increase the evaporation rate per m^{2}of grate area compared to 3450. - The 5AT has a combustion air preheater for a combustion air temperature of 100 °C.
- 3450 has no combustion air preheater.
- The provision of a combustion air preheater on the 5AT will increase the evaporation rate per m
^{2}of grate area compared to 3450. - The feedwater heater heat transfer area of the 5AT per m
^{2}of grate = 18.27 m^{2}/ 2.67 m^{2}= 6.84 m^{2}/ m^{2}. - The feedwater heater heat transfer area of 3450 per m
^{2}of grate = 13.30 m^{2}/ 6.44 m^{2}= 2.065 m^{2}/ m^{2}. - The feedwater heater heat transfer area per m
^{2}of grate of the 5AT is thus 3.3 times larger than for 3450. - The temperature of the feedwater entering the boiler of the 5AT will therefore be significantly higher than on 3450 (calculated as 110.5 °C with clean tubes, 3450
*best*measured value = 102 °C with clean tubes, therefore 5AT calculated value seems conservative.) - The higher feedwater temperature on the 5AT will increase the evaporation rate per m
^{2}of grate area compared to 3450. - The boiler evaporative heating surface area per m
^{2}of grate of the 5AT is 56.25 m^{2}/ m^{2}. - The boiler evaporative heating surface area per m
^{2}of grate of 3450 is 44.78 m^{2}/ m^{2}. - The 5AT thus has 26% more evaporative heating surface area per m
^{2}of grate than 3450. - The higher boiler evaporative heating surface area per m
^{2}of grate of the 5AT will increase the evaporation rate per m^{2}of grate area compared to 3450. - You have 4 reasons above why the 5AT would have a higher evaporation rate per m
^{2}of grate area compared to 3450. There are also more esoteric ones, but the above is enough I think. - All individual factors such as the above are insignificant compared to the fact that the integrated calculations predict what the evaporation will be taking all factors into account. Those who dispute the calculated figure should be asked to prove what they are saying by equally comprehensive calculations, or otherwise shut up.
- But there is a far easier way of showing the 5AT evaporation is an entirely realistic figure. The 1934 Chapelon rebuilds, the 240-700 Class, achieved a sustained evaporation of 23,500 kg/h with a narrow, deep grate area of 3.76 m
^{2}(Chapelon pages 75 – 76), i.e. 6,250 kg / m^{2}of grate per hour. This with coal, without the gas producer system, with hand firing, with no combustion air preheater, and a steam temperature not much lower than that of the 5AT. Your 5AT figure is 2% higher than this. Is this an “unrealistic” increase for over 80 years of technical progress? [Note: I have always expected that this figure would be increased in practice, i.e. the calculations are conservative.]”