Thanks for that thumbnail sketch.
What’s a ‘ subcritical design’?
You asked “What’s a ‘subcritical design’?” Good question — get ready...here’s more than you wanted to know!
“Subcritical” refers to water and steam below the “supercritical pressure.” For water and steam, the supercritical pressure about 3,200 psi. You are familiar with a pot of boiling water on the stove. At one atmosphere pressure and 212 deg F, you have two material “phases,” liquid water and gaseous steam. Water turns into steam.
When you get above that 3,200 psi point (and 705 deg F), you no longer have distinct liquid and gas phases. Water directly becomes steam without boiling! Technically, the term “boiler” is not used for a supercritical pressure steam generator as no “boiling” actually occurs.
It’s all mystical, weird thermodynamics.
Why is it done? To improve the thermal efficiency of a power plant, i.e., get more electricity out for a given amount of fuel in. The efficiency of the Rankine cycle (i.e, steam turbine) depends on the temperature of the steam going into the turbine. Higher pressure and temperature increase the efficiency of the thermal cycle and power plant.
Higher thermal efficiency has been the goal of power engineering since Edison built the first generating plant, the “Pearl Street Station,” in 1882. The goal long predated the fetish with burning less fuel to curtail global warming.
This consideration of reliability is the chief reason why the winter of 1881 saw Mr. Edison supervising the laying of the underground system. He snatched about two hours’ sleep in the twenty-four, and this usually on top of a pile of gas pipes in some dark basement room. He was fought tooth and nail by the gas companies, and then to add to his worries the Commissioner of Public Works sent for him. “You are laying tubes down there and the city will have to furnish inspection,” said the Commissioner. “Five men will report to you Monday, and you will pay each of them $5 a day.”Some things never change, do they? A no-show job in 1881 that paid a then-great wage ($5 a day in 1881 is about $123 a day in 2019). Edison had to pay $600 a day just to buy off the crooks.Still more remarkable was the dry statement of Mr. Edison to the interviewer: “None of those inspectors ever appeared to inspect, but they all appeared, however, at the once each Saturday to get their pay.” So, in spite of all the troubles and the presence of the existing pipes of the gas companies, the tubes were installed.