In industrial boiler systems, heat exchangers are the core components for achieving energy conversion. Their function is to efficiently transfer the heat energy generated by fuel combustion to the working fluid, usually water or steam, to provide power or heat source for industrial production. Whether it is a coal-fired, gas-fired or oil-fired boiler, the efficiency of heat exchange directly determines the effectiveness of energy conversion - high efficiency means less fuel consumption to meet production needs, and vice versa, it will cause a lot of energy waste.
From an economic point of view, heat exchange efficiency is closely related to the operating costs of enterprises. According to industry data, a decrease in heat exchange efficiency will lead to an increase in boiler fuel consumption, and the increase ratio is related to the current operating efficiency of the boiler. For example, for a boiler with an initial efficiency of 85%, the fuel consumption increases by about 1.2% for every 1% decrease in efficiency; and for a boiler with an initial efficiency of 75%, the fuel consumption increases by about 1.3% for every 1% decrease in efficiency. The greater the efficiency reduction or the lower the initial efficiency, the higher the percentage of fuel consumption increase. For industrial boilers operating at high loads, this may generate hundreds of thousands of yuan in additional expenses each year. At the same time, inefficient operation will cause the equipment to be in a high-load state for a long time, accelerate the aging and wear of components, significantly shorten the service life of the boiler, and increase the cost of equipment replacement and maintenance.
This article will focus on the core issue of improving the efficiency of industrial boiler heat exchangers, systematically explain the key factors affecting heat exchange efficiency and practical optimization solutions, and provide industrial enterprises with a practical energy efficiency improvement path.
The contamination of the heat transfer surface of industrial boilers is the primary factor leading to reduced efficiency, and prevention and control measures need to be taken from both the fire side and the water side.
Ash and carbon deposition on the fire side will significantly increase thermal resistance. Carbon particles produced by incomplete combustion of fuel and minerals in fly ash will form a dense coating on the heating surface, reducing the heat transfer efficiency by 15%-30%. It is recommended to use mechanical cleaning or chemical cleaning to perform a comprehensive cleaning at least once a month. For boilers burning high-sulfur coal, it needs to be shortened to once every two weeks to prevent the deposition of corrosive substances such as ammonium bisulfate.
Scaling, corrosion and biofilm problems on the water side should not be ignored either. The scale formed by calcium and magnesium ions in water at high temperatures has a thermal conductivity of only 1/50 of that of metal. 0.5mm thick scale can reduce thermal efficiency by more than 10%. The hardness should be controlled below 0.03mmol/L through water pretreatment, and scale inhibitors and corrosion inhibitors should be added. For open circulation systems, biocides should be added regularly to prevent microbial growth. It is recommended to conduct an endoscopic inspection once a quarter and use high-pressure water jets or chemical pickling for thorough cleaning every year. The cleaning cycle can be dynamically adjusted according to the water quality monitoring results.
As an efficient waste heat recovery device, the economizer can reduce the boiler exhaust temperature from 250-350℃ to 150-200℃, and recover this part of the waste heat to preheat the boiler feed water, thereby reducing the heat load of the main heat exchanger. Its energy-saving principle is based on the reverse heat exchange between flue gas and feed water. For every 10℃ reduction in flue gas temperature, the boiler efficiency can be increased by 0.7%-1%.
For industrial boilers with an evaporation capacity greater than 4t/h, the investment payback period for installing an economizer is usually 1-2 years. In natural gas boilers, economizers can reduce fuel consumption by 8%-12%; in coal-fired boilers, installing economizers can usually increase boiler efficiency by 3%-6%. When selecting a model, it is necessary to match the appropriate heating area according to the exhaust volume, temperature and feed water parameters, while considering the corrosiveness of flue gas.
The operating conditions of the burner directly affect the heat exchange efficiency. Incomplete combustion not only wastes fuel, but also produces more pollutants and pollutes the heating surface. Ideal combustion requires precise control of the ratio of air to fuel - too high an excess air coefficient will take away too much heat, and too low an excess air coefficient will lead to incomplete combustion. The optimal range is usually 1.05-1.15 or 1.1-1.2.
It is recommended to conduct a routine inspection of the burner every month, including the flame shape should be a stable blue cone, the flue gas component O₂ content should be controlled at 3%-5%, and the CO concentration should be less than 100ppm. The flue gas analyzer can be used for real-time monitoring, and the damper opening can be adjusted dynamically in combination with the automatic adjustment system. For oil-fired boilers, the fuel injector nozzles should also be cleaned regularly to ensure that the diameter of the atomized particles is within the range of 50-80μm to ensure full mixing with the air.
Traditional fixed speed pumps and fans have the energy waste phenomenon of "big horse pulling a small cart" under partial load. Installing a variable frequency drive can achieve precise flow control. When the boiler load drops from 100% to 50%, the energy consumption of the water pump using VFD can be reduced by about 75%.
In actual applications, the VFD is linked with the boiler load sensor to achieve stepless speed regulation of the water pump and fan. For example, when the steam demand decreases, the feed pump speed is automatically reduced to reduce excess flow; at the same time, the induced draft fan speed is adjusted to maintain the furnace negative pressure stable. Data shows that the system equipped with VFD can reduce the energy consumption of pump equipment by 30%-40% on average, and the investment payback period is about 8-12 months.
Heat loss is the invisible killer of low efficiency of industrial boilers. Good insulation can keep the surface temperature within the ambient temperature + 25℃. The boiler body should use high-density aluminum silicate fiber felt with a thickness of not less than 100mm, covered with stainless steel plate for protection; centrifugal glass wool or polyurethane foam should be used for pipe insulation, and the thickness of the insulation layer of pipes above DN100 should not be less than 50mm.
Pay special attention to valves, flanges and other parts prone to heat leakage, and use removable insulation sleeves to ensure that the heat loss rate is less than 5%. For high-temperature steam pipes, it is recommended to conduct thermal imaging inspection every 3 years and repair insulation damage points in time. After the transformation, not only can the heat dissipation loss be reduced by 5%-10%, but also the ambient temperature of the boiler room can be reduced, the working conditions can be improved and the risk of fire can be reduced.
The quality of boiler feed water directly affects the state of the water side heating surface. In addition to conventional softening treatment, dissolved oxygen and pH value must be controlled to prevent corrosion. Continuous blowdown can remove high-concentration salts, and regular blowdown can remove sediments. The blowdown rate should be controlled between 2%-5%.
The installation of a blowdown heat recovery device can recover the heat of this part of high-temperature sewage for preheating cold water or supplementing softened water. A standard blowdown expansion tank system can recover 50%-70% of the blowdown heat, which increases the overall efficiency of the boiler by 1%-2%. For a boiler with a daily blowdown volume of 100 tons, it can save about 50,000 yuan in fuel costs each year.
When the excess air coefficient exceeds the design value by 10%, the exhaust heat loss will increase by 1%-1.5%. Precise control is required by optimizing the air-coal ratio and improving the damper adjustment accuracy. For positive pressure combustion systems, a furnace wall structure with good sealing performance should be used to prevent cold air from leaking in; negative pressure systems should avoid excessive air caused by excessive ventilation.
Steam entrainment will cause heat and water loss. The entrainment loss can be controlled within 0.5% by installing high-efficiency steam-water separators, controlling the water level of the drum, and reasonably designing the steam outlet structure. Regularly check the separation efficiency of the steam-water separator and replace damaged corrugated plates or shutter components in time.
The heat exchange process of industrial boilers follows the basic law of heat transfer: heat is transferred from high-temperature fluid, that is, flue gas, to low-temperature fluid, that is, water or steam, through the solid wall, and the two fluids complete energy exchange without direct mixing. This process includes three key stages:
First, there is convection heat transfer and radiation heat transfer on the flue gas side. The high-temperature flue gas transfers heat to the metal heating surface through convection and radiation. The heat transfer efficiency at this stage depends on the flue gas flow rate, temperature and the absorptivity of the heating surface. Secondly, there is heat conduction on the metal wall. The heat is transferred from the high-temperature side, that is, the flue gas side, to the low-temperature side, that is, the water side, through the tube wall. The conduction efficiency is closely related to the thermal conductivity of the material and the wall thickness. For example, the thermal conductivity of copper (about 398W/m・K) is significantly higher than that of ordinary carbon steel (about 45W/m・K), which is about 9 times that of the latter. Finally, there is convection heat transfer on the water side. The heat from the tube wall is transferred to the working fluid to form steam or hot water. The efficiency at this stage is affected by the water flow rate, turbulence and phase change state.
The physical basis for improving thermal efficiency lies in strengthening the heat transfer process in these three stages: increasing the heat transfer temperature difference, expanding the heat transfer area, optimizing the fluid flow state, and reducing thermal resistance. Understanding these principles helps to select efficiency improvement solutions in a targeted manner.
The types of heat exchangers commonly used in industrial boilers have their own applicable scenarios: Shell and tube heat exchangers are resistant to high temperature and high pressure, suitable for large boilers with an evaporation capacity of more than 10t/h, but they are large in size; the heat transfer coefficient of plate heat exchangers is 2-3 times that of shell and tube heat exchangers, suitable for medium and low pressure conditions, and widely used in waste heat recovery systems, but not resistant to high pressure; spiral plate heat exchangers have the advantages of high heat transfer efficiency and compact structure, suitable for viscous or particle-containing fluids, but difficult to clean.
The heat transfer surface material needs to comprehensively consider thermal conductivity, corrosion resistance and cost: copper alloy has excellent thermal conductivity, but poor corrosion resistance, and is suitable for low-pressure systems with good water quality; stainless steel has strong corrosion resistance and can be used for sulfur-containing flue gas or seawater conditions, but has a low thermal conductivity; carbon steel is low in cost and is the mainstream choice for conventional boilers, but it needs to be treated with anti-corrosion to extend its life.
Counterflow arrangement can form the largest average heat transfer temperature difference, which is 20%-30% higher than the heat transfer efficiency of parallel flow arrangement, and is the preferred method for boiler heat exchangers. Crossflow is suitable for occasions that require a compact structure. Its average temperature difference is between counterflow and parallel flow, and it can approach the counterflow effect through multi-flow design.
The heat transfer surface area is positively correlated with efficiency, but excessive increase in area will lead to increased costs and increased flow resistance. It is necessary to expand the effective heat transfer area in a limited space by optimizing the structure. Temperature difference is the driving force of heat transfer. Increasing the flue gas temperature or reducing the feed water temperature within the allowable range can significantly enhance the heat transfer effect. However, problems such as scaling and corrosion will increase thermal resistance and damage the heat transfer surface. They are the main negative factors affecting long-term performance and must be controlled through effective maintenance measures.
This is the most widely used type in industrial boilers. It consists of a shell, a tube bundle, a tube sheet, etc. The flue gas flows in the shell and the water flows in the tube. Its advantage lies in its sturdy structure, which can withstand pressures above 10MPa and temperatures above 400℃, and is suitable for the main heat exchanger system of large power plant boilers and industrial steam boilers. Typical applications include economizers, superheaters and reheaters of boilers, which account for more than 70% of the total capacity of heat exchangers in coal-fired boilers.
It is composed of a series of corrugated metal plates stacked together, with fine flow channels formed between the plates. When the fluid passes through the flow channels, strong turbulence is formed, and the heat transfer coefficient can reach 2000-5000W/m²・K. It performs well in the waste heat recovery system of industrial boilers, and is particularly suitable for medium-low temperature and low-pressure waste heat utilization scenarios. Its modular design facilitates expansion and maintenance, and the plates can be directly removed for cleaning, which is suitable for working conditions with poor water quality.
The concave plate heat exchanger enhances turbulence and increases the heat transfer area through the concave and convex structure on the plate, which is 15% more efficient than ordinary plate heat exchangers and is often used in oil-water heat exchange systems of oil-fired boilers. The plate spiral heat exchanger rolls the metal plate into a spiral flow channel to form a complete countercurrent heat exchange, which is suitable for heating high-viscosity fluids and is widely used in special boilers in the chemical industry.
When selecting a model, it is necessary to make a comprehensive judgment based on the working pressure, temperature range, fluid properties and space restrictions. For example, high pressure is preferred to shell and tube type, high temperature is selected for metal materials, particle-containing fluids avoid plate type, and compact occasions are selected for plate type. Heat transfer calculation and economic analysis should be carried out when necessary.
The decline in heat exchange efficiency will trigger a series of chain reactions. The first is a surge in energy consumption. If the efficiency of a 10t/h steam boiler drops from 85% to 75%, it will consume about 1.2 tons of standard coal per day, and the annual additional cost will exceed 100,000 yuan. Long-term inefficient operation will also lead to an increase in incomplete combustion products of fuel, accelerate corrosion and scaling of the heating surface, and form a vicious cycle.
Equipment maintenance costs will also rise accordingly. Local overheating caused by poor heat transfer will increase the temperature of the metal tube wall. When it exceeds the design limit, it may cause creep or tube burst accidents. A typical boiler tube leak repair requires 2-3 days of shutdown, with a direct repair cost of 50,000 to 100,000 yuan. Adding the loss of production suspension, the total loss can reach hundreds of thousands of yuan. When scaling is serious, offline chemical cleaning is required, and the cost of each cleaning is about 2%-3% of the original value of the equipment.
Safety hazards cannot be ignored. Excessive dust accumulation on the heat transfer surface may cause secondary combustion, especially for oil and gas boilers, leading to the risk of furnace explosion; thinning of the tube wall caused by water side corrosion may cause steam leakage, endangering the safety of operators. In addition, inefficient operation will increase the exhaust temperature, increase the thermal stress of the chimney and flue, shorten its service life, and aggravate the emission of pollutants such as nitrogen oxides, facing the risk of environmental protection penalties.
Improving the efficiency of industrial boiler heat exchangers is a systematic project, and a comprehensive plan needs to be formulated based on equipment characteristics, operating conditions and maintenance levels. The core is to reduce heat loss and strengthen the heat transfer process from the source by cleaning the heat transfer surface, recovering waste heat, optimizing combustion, and precise control. Practice shows that enterprises that adopt the methods described in this article can usually improve the thermal efficiency of boilers by 5%-15%, and the investment recovery period is generally 1-3 years.
It is crucial to establish a long-term energy efficiency management system: it is recommended to formulate a detailed maintenance plan such as monthly inspections, quarterly inspections, and annual overhauls, and to equip professional thermal inspection personnel to regularly monitor key parameters such as exhaust temperature, flue gas composition, and heat transfer temperature difference to form a data-based efficiency evaluation mechanism. At the same time, attention should be paid to technology upgrade opportunities, such as converting traditional burners to low-nitrogen and high-efficiency types, or introducing artificial intelligence control systems to achieve adaptive adjustment.
For complex industrial boiler systems, it is recommended to seek professional energy efficiency service agencies for diagnosis and customize optimization plans based on actual conditions. Through continuous improvement and technological innovation, not only can the operating costs be significantly reduced, but also the equipment life can be extended and production safety can be improved, laying a solid foundation for the sustainable development of enterprises.
