The heat balance is a comparison between the useful heat used to generate steam or hot water, heat losses and the total amount of heat entering the furnace.
Types of heat balance of boilers
1. The direct balance equation establishes the relationship between the fuel consumption and the heating capacity of the boiler.
In this case, the parameters and the amount of steam or water produced are necessarily measured.
2. The inverse heat balance equation establishes the relationship between the boiler efficiency and heat losses (values are expressed as a percentage).
The heat balance is compiled to analyze the processes occurring in the boiler furnace during fuel combustion, in order to: determine the reasons for the decrease in boiler / unit performance; develop measures necessary to improve efficiency.
Heat balance terms
The heat balance of the boiler can be written as the equality Q = Q1 + Q2 + Q3 + Q4 + Q5, where Q is the total amount of heat supplied to the furnace. It consists of the heat of combustion of the fuel, its physical heat, as well as the heat supplied to the furnace with steam and air supplied for combustion: Q = Qn + Qf.t + Qf.w + Qpair.
Qн - the lowest heat of combustion of fuel, which is released during complete combustion without taking into account the heat of condensation of water vapor.
Qf.t - the physical heat of the fuel, taken into account if the fuel is heated before being fed into the furnace.
Qf.v - heat of the air introduced into the furnace is taken into account when air heaters are installed in the boiler room.
Qsteam - heat of steam supplied to the furnace.
The right side of the equation is the sum of heat consumed to produce steam or water (Q1) and heat losses (Q2 + Q3 + Q4 + Q5)
Q1 - useful heat used for the production of steam or hot water.
Q2 - heat losses with flue gases (the most significant in value, reaching 4-10% for modern boilers. Their value depends on the type of fuel used, the load of the unit / unit, temperature and volume of flue gases, and significantly increases with an increase in the amount of air supplied for combustion).
Q3 - heat losses from chemical incompleteness of fuel combustion (increase with a decrease in air supply for combustion, in addition, depend on the type of fuel burned, the method of its combustion, the design of the furnace and other factors).
Q4 - heat losses from physical incompleteness of fuel combustion (taken into account only when operating on solid fuel).
Q5 - heat loss to the environment (depends on the quality and thickness of the boiler lining, on the thermal conductivity coefficient of its material, on the outside air temperature, area, etc.). Calculated using approximate formulas.
The heat balance is compiled at steady-state boiler operation, expressed in kJ / kg (kJ / m3) and usually refers to 1m3 of gas or 1 kg of solid and liquid fuel at T = 0 ° C and P = 760 mm Hg. Art. (0.1 MPa).
Reverse balance equation
It is mainly used for testing boilers. In this case, the value of heat losses is calculated and the gross efficiency of the boiler is determined from the known heat of combustion of the fuel: ηbr = 100 - (Q2 + Q3 + Q5).
Errors in determining heat losses are lower than when calculating fuel consumption, therefore, the method for determining the efficiency from the inverse balance is more accurate.
