Some heating comes from direct injection of steam, but all lines into or leaving the tank should also be steamed. Temperature control comes from heat transfer with water or steam. For small to moderate sized fermenters, a jacket is best because it leaves the inside of the vessel with fewer surfaces. Although the inlet and outlet are shown near each other in the sketch, they should be separated to insure a good flow pattern. The outlet is high up on the jacket to allow any trapped air to escape.
Volume is measured in cubic units while area is measured in units squared.
As vessel volume increases, the area of the vessel becomes inadequate for heat transfer. Demo with numbers. Think a moment about possible alternatives to a jacket and then see what to do when a jacket is inadequate.