Classification and Functions of Heat Exchangers in Industrial Indirect Hot Air Furnaces [Selection Guide]

Introduction: Heat Exchanger - The "Core Heat Transfer Heart" of Industrial Indirect Hot Air Furnaces

In industrial production, indirect hot air furnaces are widely used in food drying, metallurgical heating, chemical raw material preheating, grain storage and other fields due to their advantage of "clean and pollution-free hot air". As the core component of the hot air furnace, the heat exchanger directly determines the equipment's thermal efficiency, hot air temperature stability and operating costs.

Many enterprises, when purchasing or maintaining indirect hot air furnaces, often have problems such as low equipment thermal efficiency, high energy consumption and even impact on product quality because they do not understand the classification, characteristics and applicable scenarios of heat exchangers. This article will detail the classification standards, core characteristics and functions of various types of heat exchangers in industrial indirect hot air furnaces, and provide selection suggestions to help enterprises accurately match their needs and reduce production costs.

I. Core Classification of Heat Exchangers in Industrial Indirect Hot Air Furnaces: Classified by Heat Transfer Structure

In industrial scenarios, the structure of the heat exchanger directly affects the heat transfer efficiency and application range, and is the core basis for selection. The following is a detailed explanation of 5 types of mainstream structural heat exchangers:

1. Shell-and-Tube Heat Exchanger: Industrial Universal Heat Transfer Equipment

(1) Structural Characteristics

A shell-and-tube heat exchanger consists of a "tube bundle (heat exchange tubes)" and a "shell". High-temperature flue gas flows in the shell side (the gap between the shell and the tube bundle), while cold air flows in the tube side (inside the tube bundle), and indirect heat transfer is achieved through the tube wall. Some designs add baffle plates (to avoid flue gas short-circuiting and extend the heat transfer path) or fins (to enhance the heat transfer effect outside the tubes) to adapt to different heat load requirements.

(2) Applicable Scenarios

• ①Scenarios with medium flue gas dust content (≤10g/m³) and temperature ≤600℃;
• ②Small and medium-sized drying equipment (such as grain drying, Chinese medicinal material drying) and light industry heating (such as paper drying);
• ③Industrial environments requiring equipment pressure-bearing capacity (such as high-pressure flue gas working conditions).

(3) Core Advantages and Functions

• ①Strong stability: The tube side and shell side are completely isolated, preventing flue gas dust and impurities from contaminating cold air and ensuring the cleanliness of hot air;
• ②Good pressure-bearing performance: The shell is made of carbon steel or stainless steel, which can withstand certain fluctuations in flue gas pressure and is suitable for long-term industrial operation;
• ③Convenient maintenance: The tube bundle can be disassembled and cleaned separately, with low later maintenance costs, suitable for general needs of multiple industries.

2. Plate Heat Exchanger: High-Efficiency and Energy-Saving Heat Transfer Equipment

(1) Structural Characteristics

It is composed of multiple stacked metal corrugated plates. Adjacent plates form two independent flow channels - high-temperature flue gas and cold air flow in opposite directions in alternating flow channels (maximizing the temperature difference to improve heat transfer efficiency), and rapid heat transfer is achieved through the corrugated plate walls. The number of plates can be increased or decreased according to the heat load, with strong flexibility, and the plates are isolated by gaskets to prevent medium mixing.

(2) Applicable Scenarios

• ①Scenarios with high flue gas cleanliness (dust content ≤5g/m³) and temperature ≤400℃;
• ②Food processing (such as hot air heating before baking, candy drying) and clean hot air supply in the electronics industry;
• ③Supporting gas-fired hot air furnaces (with clean flue gas and no dust pollution) and production lines with limited installation space.

(3) Core Advantages and Functions

• ①High thermal efficiency: The corrugated plate design increases the heat transfer area, and the heat transfer coefficient is 2-3 times higher than that of shell-and-tube heat exchangers. The thermal efficiency can reach 85%-92%, significantly reducing fuel consumption;
• ②Compact size: With the same heat transfer area, its volume is only 1/3 of that of shell-and-tube heat exchangers, saving workshop installation space;
• ③Precise temperature control: The counter-flow channel design ensures small hot air temperature fluctuation (within ±3℃), meeting the needs of industries such as food and electronics that have high requirements for temperature accuracy.

(4) Precautions

• ①When the flue gas dust content is high, the flow channels are prone to blockage, so it is necessary to be equipped with pre-dust removal equipment (such as bag filters);
• ②The temperature resistance of gaskets is limited (conventional rubber gaskets ≤200℃, high-temperature resistant gaskets ≤400℃), so regular inspection and replacement are required to avoid leakage.

3. Finned Tube Heat Exchanger: High-Temperature Enhanced Heat Transfer Equipment

(1) Structural Characteristics

Fins (such as spiral fins, straight fins, corrugated fins) are installed on the outer or inner surface of ordinary heat exchange tubes (smooth tubes) through welding or rolling processes. The heat transfer area can be 3-10 times that of smooth tubes. High-temperature flue gas usually flows on the fin side (outside the tubes), while cold air flows inside the tubes. Fins are used to enhance heat transfer on the flue gas side (solving the problem of weak heat transfer caused by high flue gas viscosity and slow flow rate).

(2) Applicable Scenarios

• ①Scenarios with relatively high flue gas temperature (200-800℃) and requiring enhanced heat transfer;
• ②Metallurgical industry (steel rolling drying, steel preheating), chemical industry (raw material heating, solvent recovery);
• ③Supporting coal-fired/biomass hot air furnaces (with relatively high flue gas dust content and not easy to block fins) and waste heat recovery systems.

(3) Core Advantages and Functions

• ①Strong temperature resistance: Fins are mostly made of stainless steel (304/316L) or carbon steel, which can withstand temperatures above 800℃ and adapt to high-load heat transfer requirements;
• ②Significant energy saving: It greatly increases the heat transfer area, and the waste heat recovery rate of high-temperature flue gas can reach more than 80%, reducing the fuel cost of the hot air furnace;
• ③Wide adaptability: Different fin spacings can be selected according to the flue gas dust content (wide-spacing fins for high dust content, dense-spacing fins for low dust content) to reduce the risk of blockage.

4. Regenerative Heat Exchanger: Special Equipment for High-Temperature Waste Heat Recovery

(1) Structural Characteristics

With a regenerator (such as ceramic honeycomb, metal filler, refractory brick) as the core, heat transfer is achieved through "alternating heat absorption and heat release":
① Heat absorption stage: High-temperature flue gas passes through the regenerator, transferring heat to the regenerator and increasing its temperature;
② Heat release stage: Cold air flows through the regenerator in the reverse direction, absorbing the heat stored in the regenerator and converting into high-temperature hot air;
③ The direction of flue gas and cold air is automatically switched through a reversing valve to achieve continuous heat transfer.

(2) Applicable Scenarios

• ①Heavy industry scenarios with extremely high flue gas temperature (800-1200℃) and large heat load;
• ②Iron and steel industry (sinter cooling hot air recovery, blast furnace gas waste heat utilization), glass industry (kiln flue gas waste heat heating);
• ③Waste incineration hot air furnaces and biomass high-temperature combustion equipment (flue gas containing corrosive components).

(3) Core Advantages and Functions

• ①Highest waste heat recovery rate: The waste heat recovery rate of high-temperature flue gas can reach more than 90%, which can heat cold air to above 600℃, significantly reducing fuel consumption (saving 30%-50% energy compared with traditional heat exchangers);
• ②Strong weather resistance: Regenerators are mostly made of silicon carbide and cordierite ceramic materials, which are resistant to strong acid, strong alkali corrosion and wear, and adapt to high-dust and high-corrosion flue gas environments;
• ③Adapt to high-temperature processes: Meet the needs of industries such as metallurgy and glass for high-temperature hot air (500-1000℃), replacing some electric heating equipment and reducing energy consumption.

(4) Precautions

• ①There is "reversing loss" (a small amount of flue gas mixes with cold air during switching), so it is necessary to be equipped with a precise reversing control system (such as PLC automatic control);
• ②The regenerator is prone to dust accumulation, so regular purging with compressed air or high-pressure water cleaning is required to avoid increased thermal resistance.

5. Double Pipe Heat Exchanger: Special Heat Transfer Equipment for Small Flow Rates

(1) Structural Characteristics

It consists of two concentric pipes (inner pipe + outer pipe). High-temperature flue gas flows in the inner pipe or the annular gap of the outer pipe, while cold air flows in the other flow channel, and indirect heat transfer is achieved through the pipe wall. Multiple sets of double pipes can be used in parallel/series to adapt to different small-flow heat load requirements, with a simple structure and low manufacturing cost.

(2) Applicable Scenarios

• ①Scenarios with small flow rates and large temperature differences (such as small laboratory hot air equipment, local process heating);
• ②Low-output industrial drying (such as small-scale feed drying, small-batch drying of Chinese medicinal materials);
• ③Pipeline heat tracing (such as anti-freezing heating of chemical pipelines in winter).

(3) Core Advantages and Functions

• ①Simple structure: No complex components, low manufacturing cost, suitable for small enterprises or laboratories;
• ②Easy cleaning: The double pipes can be disassembled, facilitating the cleaning of viscous impurities in the flow channel (such as dust adhesion after feed drying);
• ③Stable heat transfer: High flow rate in the flow channel, no "dead zone" easily occurs, and stable heat transfer efficiency can be maintained under small flow rates.

II. Classification of Heat Exchangers in Industrial Indirect Hot Air Furnaces: Classified by Material (Affecting Temperature and Corrosion Resistance)

In addition to the structure, the material of the heat exchanger directly determines its temperature resistance, corrosion resistance and service life, and should be selected according to the flue gas composition and temperature:

1. Carbon Steel Heat Exchanger

• ①Temperature resistance range: ≤600℃;
• ②Core characteristics: Low cost, high strength, easy processing, suitable for non-corrosive flue gas environments;
• ③Applicable scenarios: Clean combustion flue gas of coal and gas (without sulfur and chlorine components), ordinary industrial drying (such as wood drying);
• ④Precautions: Prone to oxidation and rust, so regular coating with high-temperature resistant anti-corrosion coatings (such as organic silicon high-temperature resistant paint) is required to extend the service life.

2. Stainless Steel Heat Exchanger (304/316L)

• ①304 stainless steel: Temperature resistance ≤800℃, resistant to weak corrosion (such as flue gas containing a small amount of water vapor), suitable for the food and pharmaceutical industries (requiring clean hot air);
• ②316L stainless steel: Temperature resistance ≤900℃, resistant to strong corrosion (such as flue gas containing sulfur and chlorine components, such as biomass combustion flue gas and chemical tail gas), suitable for the chemical and metallurgical industries;
• ③Core advantages: High hot air cleanliness, no rust shedding pollution, suitable for scenarios requiring high medium purity.

3. Ceramic Heat Exchanger (Silicon Carbide, Cordierite)

• ①Temperature resistance range: ≥1200℃;
• ②Core characteristics: Resistant to strong acid, strong alkali corrosion, wear and high temperature, suitable for the harshest flue gas environments;
• ③Applicable scenarios: Waste incineration hot air furnaces, metallurgical smelting flue gas (containing high concentrations of SO₂, Cl⁻), high-temperature flue gas of glass kilns;
• ④Precautions: High brittleness, so collision should be avoided during installation; the thermal conductivity is lower than that of metals, so structural optimization (such as honeycomb regenerators) is required to enhance heat transfer.

4. Non-Ferrous Metal Heat Exchanger (Copper, Aluminum)

• ①Copper heat exchanger: Temperature resistance ≤300℃, high thermal conductivity (twice that of carbon steel), suitable for low-temperature hot air heating (such as PCB board drying in the electronics industry);
• ②Aluminum heat exchanger: Temperature resistance ≤200℃, light weight (1/3 of carbon steel), suitable for small equipment (such as small household hot air furnaces);
• ③Precautions: Poor corrosion resistance (easily corroded by sulfides in flue gas), only used in clean and low-temperature scenarios, and rarely used in industrial fields.

III. Core Functions of Heat Exchangers in Industrial Indirect Hot Air Furnaces (Applicable to All Types)

Regardless of the structure and material, the core goals of heat exchangers are centered on the three needs of "efficient heat transfer, clean hot air and stable temperature control". The specific functions are as follows:

1. Efficient Energy Recovery: Reduce Enterprise Energy Consumption Costs

The fuel combustion of industrial indirect hot air furnaces will generate high-temperature flue gas of 600-1200℃. If directly discharged, the heat loss rate can reach more than 40%. Through indirect heat transfer, the heat exchanger transfers the heat in the flue gas to cold air, with a thermal efficiency generally reaching 75%-95% (the highest for regenerative heat exchangers), which significantly reduces heat loss and fuel consumption (for example, coal-fired hot air furnaces can reduce coal consumption by 30%), directly reducing enterprise operating costs.

2. Clean and Pollution-Free Hot Air: Ensure Product Quality

The core advantage of indirect heat transfer is the "complete isolation of flue gas and cold air", which prevents dust and harmful gases (such as CO, SO₂, NOx) in the flue gas from mixing into the hot air. For industries such as food, medicine and electronics, clean hot air is the key to ensuring product quality - for example, if the hot air for food baking contains dust, it will cause product deterioration; if the hot air for electronic component heating contains corrosive gases, it will damage the components.

3. Stable and Controllable Temperature: Meet the Precision Requirements of Industrial Processes

Different industrial processes have different precision requirements for hot air temperature (such as 80-120℃±3℃ for food baking, 500-800℃±5℃ for metallurgical heating). Through structural optimization (such as baffle plates guiding flue gas flow, counter-flow channel design), the heat exchanger ensures uniform heating of cold air and small temperature fluctuation of the output hot air, avoiding product scrapping caused by temperature fluctuation (such as temperature deviation in ceramic firing will cause product cracking).

IV. Selection Guide for Heat Exchangers in Industrial Indirect Hot Air Furnaces (4 Key Factors)

When selecting a heat exchanger, enterprises need to accurately match their own working conditions to avoid "choosing the expensive one instead of the right one" or "too small a selection leading to low efficiency". The core reference points are the following 4 aspects:

1. Flue Gas Conditions: Determine the Heat Exchanger Structure and Material

• ①Flue gas temperature: ≤400℃, select plate/double pipe type; 400-800℃, select finned tube/shell-and-tube type; ≥800℃, select regenerative type;
• ②Flue gas dust content: ≤5g/m³, select plate type; 5-10g/m³, select finned tube/shell-and-tube type; ≥10g/m³, select regenerative type (ceramic regenerator is blockage-resistant);
• ③Flue gas corrosion: No corrosion, select carbon steel; weak corrosion, select 304 stainless steel; strong corrosion, select 316L stainless steel/ceramic.

2. Hot Air Requirements: Match the Core Requirements of the Process

• ①Hot air temperature: Low temperature (≤200℃), select plate/double pipe type; medium temperature (200-600℃), select finned tube/shell-and-tube type; high temperature (≥600℃), select regenerative type;
• ②Hot air cleanliness: For food and medicine industries, select stainless steel plate/shell-and-tube type; for industrial drying, select carbon steel finned tube/shell-and-tube type;
• ③Hot air flow rate: Large flow rate (≥10000m³/h), select shell-and-tube/regenerative type; small flow rate (≤5000m³/h), select plate/double pipe type.

3. Installation and Maintenance: Combine with the Actual Conditions of the Workshop

• ①Installation space: For narrow space, select plate/double pipe type; for sufficient space, select shell-and-tube/regenerative type;
• ②Maintenance frequency: For high-dust scenarios, select finned tube type (wide-spacing fins) that is easy to clean; for clean scenarios, select plate type (only regular gasket replacement is required).

4. Cost Budget: Balance Initial Investment and Long-Term Energy Consumption

• ①Economical choice: Carbon steel finned tube/shell-and-tube type (low initial investment, suitable for conventional industrial scenarios);
• ②High-efficiency and energy-saving choice: Stainless steel plate/regenerative type (high initial investment, but low long-term energy consumption, suitable for enterprises with high-load operation);
• ③Special demand choice: Ceramic regenerative type (suitable for high-temperature and high-corrosion scenarios, with high cost but long service life and high comprehensive cost-effectiveness).

Conclusion: Choose the Right Heat Exchanger to Make the Industrial Indirect Hot Air Furnace "Efficient and Energy-Saving"

Author:Hangzhou Thermal Engineering Technology Co., Ltd

Source: Open website