Analysis of the Efficiency of Using the Heat of Natural Gas Combustion Products to Preheat Forced Air Supplied to the Boiler

Abstract

Preheating of forced air in a surface-type heat exchanger prior to supplying it to the air heater of the boiler by using flue gases while cooling them below the dew point is proposed. Athermal calculation of the condensing heat exchanger installed downstream of the boiler (model BKZ-420-140 NGM) was performed to determine the thermal capacity of the heat exchanger, flow rate of flue gases required to preheat forced air supplied to the boiler while cooling them below the dew point, flow rate of the water vapor condensate obtained from the gaseous fuel combustion products, temperature of flue gases at the outlet of the boiler unit, and fuel utilization factor of the boiler resulting from the use of the condensing heat exchanger. The water vapor condensate obtained from flue gases in the surface heat exchanger can be used as make-up water for the heat network. It was established that depending on the ambient air temperature, the percentage of flue gases directed to the heat exchanger for preheating forced air of the BKZ-420-140 NGM boiler assuming their cooldown below the dew point constitutes 3.9 to 20.7 and 7.6 to 24.4% of their total amount, when heating air to 20 and 30°C, respectively. The economic effect in the monetary terms due to installation of the condensing heat exchanger downstream of the BKZ-420-140 NGM boiler amounts to 12, 691.7 and 17,242.2 thousand RUB/year, when heating forced air to 20 and 30°C, respectively. Considering five power-plant boilers installed at the Samara CHP operated by “Samarsky” branch of the PJSC “T Plus,” the amount of savings should reach 63.5 and 86.2 million RUB/year, respectively.