all induced draught furnaces have a negative pressure. Forced draught: draught is obtained by forcing air into the furnace by means of a fan (FD fan) and duct-work. Air is often passed through an air heater; which, as the name suggests, heats the air going into the furnace in order to increase the overall efficiency of the boiler. Dampers are used to control the quantity of air admitted to the furnace. Forced draught furnaces usually have a positive pressure. Balanced draught: Balanced draught is obtained through use of both induced and forced draft. This is more common with larger boilers where the flue gases have to travel a long distance through many boiler passes. The induced draft fan works in conjunction with the forced draft fan allowing the furnace pressure to be maintained slightly below atmospheric. Firetube boilers The next stage in the process is to boil water and make steam. The goal is to make the heat flow as completely as possible from the heat source to the water. The water is confined in a restricted space heated by the fire. The steam produced has lower density than the water and therefore will accumulate at the highest level in the vessel; its temperature will remain at boiling point and will only increase as pressure increases. Steam in this state (in equilibrium with the liquid water which is being evaporated within the boiler) is named "saturated steam". For example, saturated steam at atmospheric pressure boils at . Saturated steam taken from the boiler may contain entrained water droplets, however a well designed boiler will supply virtually "dry" saturated steam, with very little entrained water. Continued heating of the saturated steam will bring the steam to a "superheated" state, where the steam is heated to a temperature above the saturation temperature, and no liquid water can exist under this condition. Most reciprocating steam engines of the 19th century used saturated steam, however modern steam power plants universally use superheated steam which allows higher steam cycle efficiency. Superheaters L.D. Porta gives the following equation determining the efficiency of a steam locomotive, applicable to steam engines of all kinds: power (kW) = steam Production (kg h−1)/Specific steam consumption (kg/kW h). A greater quantity of steam can be generated from a given quantity of water by superheating it. As the fire is burning at a much higher temperature than the saturated steam it produces,