STEAM Factor of Evaporation.
The factor of evaporation for any given feed water temperature and boiler pressure is calculated by dividing the total heat above F. in one pound of steam at the given pressure minus the total heat in one pound of the feed water above F. by the latent heat of steam at F., which is 970.4 British Thermal Units. (See Marks and Davis, New Steam Tables.) Example.
A boiler evaporates 5,00o pounds of water at F. into steam at 91.3 lbs. gauge pressure every hour. What is the boiler horse power? Steam at 91.3 lbs. gauge pressure contains '187.4 B.T.U. per pound (above F.); water at F. contains 77-32 = 45 - 7.4 45 B.T.U. per pound (above F.) 118 = 1.'77 970.4 factor of evaporation. 5,000 pounds of feed water per hour multiplied by 1.177 = 5,885 lbs. of water which would have been evaporated into steam with the same heat used to evaporate 5,000 lbs. from 77 ° F. into steam at 91.3 lbs. if the feed water temperature had been F. and the boiler pressure o lbs. gauge. 5,885 lbs. is the equivalent evaporation from and at 2 I F. and divided by 34.5 lbs. gives 170.6 boiler horse power.
Similarly for any other feed water temperature and boiler pressure.
British Thermal Unit: A British Thermal Unit (B.T.U.) is a standard unit for the measurement of heat, and is defined as the amount of heat imparted to one pound of pure water of a temperature of about 39° F. to raise its tem perature one degree Fahrenheit. Thus to raise the temperature of one pound of water from F. to F. requires one B.T.U. and to raise the temperature of
one pound of boiler feed water from say F. to 212° F. requires approximately 212 ---- 67 = 145 B.T.U.'s.
The Heat of Vaporization of Steam.
In raising the temperature of one pound of water from F. to F., 145 B.T.U.'s are required, but after a temperature of F. is reached we can impart heat to the water until it is all changed into steam with no increase in temperature, 970.4 B.T.U.'s being required for this change of state in converting one pound of water at 212° F. into one pound of steam at atmospheric pressure. This value 970.4 B.T.U.'s, is known as the latent heat of steam or "The Heat of Vaporization of Steam" at F.
Efficiency of the Boiler and Grate.
The efficiency of the boiler and grate is the amount of heat imparted to the water and steam per pound of fuel as fired, divided by the heat contained in one pound of the fuel. Thus, assume an equivalent evap oration from and at F. of 7 pounds of water per one pound of coal containing 12,000 B.T.U.'s per pound.
7 times 970.4 = 6793 B.T.U.'s imparted to the water per one pound of coal, then we obtained in the boiler 6793 divided by 12000, or 56.6 % of the energy in the coal.
Similarly, suppose an equivalent evaporation of 15 pounds of water per one pound of oil containing 19,191 B.T.U. per pound.
15 X 970.4 = 14,556 19,191 19,191 = 75.8% of the energy in the oil is obtained in the boiler.