The ratio of the energy required to produce the artificial draught (expressed in metres head or J/kg of flue gas) to the mechanical equivalent of extra heat carried away per kg of flue gases due to natural draught, is known as

In a steam condenser, the partial pressure of steam and air are 0.06 bar and 0.007 bar respectively. The condenser pressure is

The difference between the atmospheric pressure and the absolute pressure is known as vacuum in a condenser.

The amount of water evaporated from and at 100° C into dry and saturated steam is called evaporative capacity of a boiler.

A condenser maintains a low pressure (below atmospheric) so as to obtain the maximum possible energy from steam and thus to secure a high efficiency.