immersion heater vs steam generator

immersion heater

noun
  • A device for supplying domestic hot water in which a thermostatically-controlled heating element is immersed within a tank of water 

steam generator

noun
  • a type of boiler fitted in a diesel locomotive or another rail vehicle, which provided steam heating in steam-heated passenger trains that had previously been hauled by steam locomotives. 

How often have the words immersion heater and steam generator occurred in a corpus of books? (source: Google Ngram Viewer )