Heat balance is a comparison between the useful heat used to produce steam or hot water, the heat losses and the total amount of heat supplied to the furnace.
Types of boiler heat balance
1. The equation of direct balance establishes the relationship between the fuel consumption and the heating output of the boiler.
In this case, the parameters and amount of generated steam or water are necessarily measured.
2. The equation of the reverse heat balance establishes the relationship between the efficiency of the boiler and the heat losses (the values are expressed as a percentage).
The heat balance is compiled for analysisProcesses occurring in the furnace of the boiler during the combustion of fuel in order to: determine the causes of the decline in productivity, and / / - develop the measures necessary to improve the efficiency.
Heat balance components
The thermal balance of the boiler can be written in the formEquations Q = Q1 + Q2 + Q3 + Q4 + Q5, where Q is the total amount of heat supplied to the furnace. It is made up of the heat of combustion of the fuel, its physical heat, and also the heat supplied to the furnace with steam and supplied to the combustion with air: Q = Qn + Qf.t + Qf.v + Qpar.
Qн - the lowest heat of combustion of fuel, which is released at full combustion without taking into account the heat of condensation of water vapor.
Qf.t - the physical heat of the fuel, is taken into account when the fuel is preheated before being fed into the furnace.
Qf.v - the heat of the air introduced into the furnace is taken into account in the case when the boiler is equipped with air heaters.
Qpar - the heat of the steam entering the furnace.
The right side of the equation is the sum of the heat consumed to produce steam or water (Q1) and heat loss (Q2 + Q3 + Q4 + Q5)
Q1 is a useful heat used to produce steam or hot water.
Q2 - heat losses with outgoing gases (mostSignificant in magnitude, reaching 4-10% for modern boilers. Their value depends on the type of fuel used, the load of the unit, the temperature and volume of the exhaust gases, and increases significantly with increasing amount of air supplied for combustion).
Q3 - Heat loss from chemical incompletenessCombustion of fuel (increase with decreasing combustion air supply, in addition, depend on the type of fuel burned, the method of its combustion, the design of the furnace and other factors).
Q4 - loss of heat from physical incompleteness of combustion of fuel (only taken into account when working on solid fuel).
Q5 - loss of heat to the environment (depend onQuality and thickness of the installation of the boiler, the coefficient of thermal conductivity of its material, the temperature of the outside air, the area, etc.). Estimated by approximate formulas.
The heat balance is compiled with the establishedThe operation of the boiler is expressed in kJ / kg (kJ / m3) and usually refers to 1 m3 of gas or to 1 kg of solid and liquid fuel at T = 0 ° C and P = 760 mm Hg. Art. (0.1 MPa).
The inverse balance equation
It is used mainly in testing boilers. In this case, the value of heat losses is calculated and, based on the known heat of combustion of the fuel, the gross efficiency of the boiler is determined:? Bp = 100 - (Q2 + Q3 + Q5).
Errors in determining heat losses are lower than when calculating fuel consumption, so the method for determining the efficiency of the reverse balance is more accurate.