Industrial heat exchangers in furnace heating cycles

Technical sources on industrial fuel furnaces show that the furnaces are operated with rather low thermal efficiency not exceeding 20-30 %, i.e. 3-4 times lower than, for example, efficiency of the modern boiler plants.

The mentioned fact is generally a reason of great heat losses of exhaust gases, reaching 50 – 65 % of total heat amount supplied into the furnace. To increase the furnace thermal efficiency the recuperators are used to supply heated air to a burner. When air is heated in a recuperator, the amount of air introduced into the furnace increases proportionally to its heating temperature, i.e. in the incoming part of the heat balance of the furnace, the proportion (or percentage) of heat supplied with air increases, and, accordingly, fuel consumption for the furnace decreases. The efficiency of the combustion unit as a whole increases due to a decrease in the level of heat losses with the exhaust flue gases.

Note that the caloric unit entered with the preheated air supplied for fuel combustion has more value than the caloric unit generated by fuel combustion. In this case the part of the caloric unit generated by fuel combustion in the furnace is used in its working space, as the other part is discharged with the flue gases. The heat containing in the preheated air (fuel gas) is fully used in the working space, as the exhaust gas volume does not rise in this case.

Consider next which heating-performance parameters are influenced by the preheated air supplied to the furnace burner.

The preheated air facilitates flame temperature increase, as its calorimetric temperature rises. According to the technical specifications at excess ratio factor a=1.0 and its temperature 20°С the calorimetric temperature of natural gas burning is 2050°С, at the same a= 1.0 and at the preheated air temperature up to 400°С the calorimetric temperature is 2320°С.
As a result, the calorimetric temperature rises causes increase of the radiation constituent during heat transfer from heating agent to the heating units in the furnace working area. Increase of the radiation constituent results from the fact that radioactive heat transfer depends to the forth power from the flame temperature.

  • 1. Preheated air and fuel mixturein the burners provides fast heating of the fuel improving combustion procedure effectiveness and reducing incomplete combustion.
  • 2. At heat exchangers installation the flue gas temperature decreases at heat exchangers outlet and, as consequence, actual gas volume also decreases followed by decrease of the stack flue air pressure. Therefore the power costs for the flue gas fan motor operation are reduced.
  • 3. During air heating and therefore due to increased fuel combustion efficiency the most part of toxic components are burned and it allows a customer to reduce the payments for the emissions of toxic elements.

Here we specify the general heating-performance aspects of application of heat exchangers in industrial furnaces, however, together with the abovementioned effective heating-performance processes it is necessary to point out that there are a number of other parameters of the heat exchangers designed by Termo Nord Stream considered as decisive factors for application feasibility of one or another industrial heat exchanger design. The article of Termo Nord Stream specialists regarding «Comparative analysis of design parameters of metal heat exchangers with different design with the heat exchangers designed by Termo Nord Stream», considers in depth the performance and parameter analysis for different types of heat exchangers.

Hot air supply to the burner for solid fuel combustion speeds up fuel ignition and intensifies its burning process reducing heat losses arising from chemical and mechanical incomplete burning. The recommended air preheating at in-chamber combustion of different fuel types in furnace units is given in the following table.

Fuel Temperature of the preheated air
Brown coals, milled peat, wood wastes 350°C
Bituminous and T grade coal 300°C
Oil and gas 250-300°C