Fossil fuel boiler designs are more similar, so shrink-swell varies with the density difference between the water and steam. In higher pressure boilers, the density difference is less than in lower pressure boilers. Therefore, boilers operating at higher pressures generally exhibit less shrink-swell than lower pressure ones.

Ссылка на статью.
http://www.controlglobal.com/articles/2006/148.html

Второе выражение
heating size

Выражение встречается в следующем контексте:

For a particular heat exchanger we assumed that the product of its heating size and coefficient of thermal transmittance Z=FU is a constant and characteristic value of this heat exchanger.
......
Fig. 3. shows the optimal total characteristic value of the steam boiler for different mass flow rate at the steam boiler exit and different type of coal. A greater mass flow at the steam boiler exit
causes a higher total characteristic value of the steam boiler Ztot = 6z., which means a
greater heating size of the steam boiler. The highest value Ztot exists when the steam boiler’s fuel is lignite, and the lowest value exists when the fuel is stone coal. Reductions of total heating size of the steam boiler, in relation to the case when coal is lignite, are up to 18% for dark coal and 23% for stone coal. Total costs are reduced up to 25% for dark coal and 30% for stone coal.

Ссылка на статью
http://facta.junis.ni.ac.rs/me/me2003/me2003-13.pdf

Заранее спасибо