Meanwhile it had been found that still higher efficiencies were obtainable from lower steam velocities, pointing to the use of more stages and smaller diameters. This required tandem cylinder construction like that of the special 2400-r.p.m. machines made for North Tees in 1917, and machines of 10,000 kW output at 3000 r.p.m. were built in 1923. As well as the better efficiency, tandem construction gave improved operating characteristics, easy starting and stopping, and smooth running on load, and outputs went up progressively. A 41,000-kW 1500-r.p.m. set put in hand in 1926 for extensions at Barton had four-stage feed heating to 300°F and was the prototype of many turbines installed over a long period. By 1926 also the first test had taken place on the new turbine test bed, which comprised nine berths providing for turbines from 500 kW to 50,000 kW.

Developments in condensing plant and the like were taking place round about 1923. The use of surface condensers instead of the jet type was spreading from the larger sets to industrial plants, and the steam operated surface-cooled air ejector invented by Leblanc began to displace the Westinghouse-Leblanc rotary air extraction plant. The early large condensers, which could only show a low rate of heat transmission, were replaced by the central flow design which ensured also a high temperature and effective de-aeration of the condensate.

Power plant continued to increase steadily in size. Sets of 51,250 kW and 67,200 kW at 1500 r.p.m., the latter (for the new Battersea station) having a three cylinder turbine, were put in hand in 1928, and a 32,500-kW set ordered for the Victoria Falls Company in 1929 was notable as having the largest output then obtained from a normal turbine running at 3000 r.p.m. Following experience obtained at North Tees a 38,500-kW reheater turbine for Japan was provided with effective protection against overspeeding on a sudden decrease in load (due to expansion of steam in the system beyond the control of the governor). Signs of a tendency towards higher operating conditions with their consequent economies appeared in 1927, when three 12,500-kW back pressure turbines were made for initial steam conditions of 630 p.s.i.g. 833°F.

Machine foundations began to be designed on scientific lines after the war with the object of preventing resonant vibration. In a few years the usual combination of rolled steel joists and concrete was superseded by steelwork alone for the largest sets (today ferroconcrete would be used).