Heat exchange tubes are the core components for heat transfer in shell-and-tube heat exchangers. As the heat transfer boundary between two fluids, their performance directly determines the efficiency, safety, and economy of the equipment. This article will systematically introduce their core characteristics, material selection, structural forms, and key application considerations.

 

Heat exchange tubes are tube bundles installed in shell-and-tube heat exchangers, usually made of metal or non-metallic materials. During operation, one fluid flows inside the tubes (tube side), and the other flows outside the tubes (shell side). Heat is transferred from the high-temperature fluid to the low-temperature fluid through the tube wall. Their core role is to provide an efficient heat transfer surface area while strictly isolating the two fluids to prevent mixing.

 

1.  Dimension Specifications: Common outer diameters are Φ19mm, Φ25mm, etc. Small-diameter tubes can increase unit heat transfer area but are prone to clogging; wall thickness is usually expressed by Birmingham Wire Gauge (BWG), which affects pressure bearing and heat transfer; standard lengths are 6m, 9m, etc., which determine the heat transfer area and equipment size.

2.  Material Selection: Comprehensive evaluation based on fluid corrosiveness, temperature, pressure, and cost:

     Carbon steel (e.g., 20# steel) for non-corrosive media (water, oil);

     Stainless steel (304/316) for corrosion resistance, suitable for chemicals or seawater;

     Copper alloys (e.g., cupronickel) with excellent thermal conductivity and high cost;

     Titanium alloys resistant to chloride ion corrosion, used in seawater and extreme chemical working conditions;

     Nickel-based alloys (e.g., Hastelloy) for high-temperature, high-pressure, and strong corrosion environments;

     Non-metallic materials (graphite, PTFE) for strong acid and alkali environments.

3.  Structural Forms:

     Smooth tubes: Low cost, widely used, but relatively low heat transfer efficiency;

     Finned tubes: Increase heat transfer area through external or internal fins, suitable for gas-liquid heat exchange;

     Threaded/corrugated tubes: Enhance heat transfer by disrupting the fluid boundary layer and have anti-fouling capabilities.

 

1.  Tube End Connection Technologies:

     Expansion joint: Suitable for low-pressure and low-temperature scenarios;

     Welding: Ensures sealing and strength under high temperature and pressure;

     Expansion-welding combination: Integrates anti-leakage and anti-crevice corrosion, preferred for harsh working conditions.

2.  Tube Bundle Arrangement:

     Triangular arrangement: High heat transfer efficiency but difficult to clean;

     Square arrangement: Facilitates mechanical cleaning and has strong anti-fouling properties.

3.  Common Failures and Protection:

     Corrosion: Including pitting, stress corrosion cracking, etc., which need to be avoided through material compatibility design and protective coatings;

     Fouling: Fouling layers reduce heat transfer efficiency, requiring regular cleaning or selection of anti-fouling tube types;

     Vibration fracture: Avoid resonance by optimizing support structures and fluid flow rates;

     Leakage: Mostly occurs at tube sheet connections, requiring prevention through process quality control.

 

Selection requires comprehensive consideration of:

1.  Process conditions: Temperature, pressure, flow rate, and heat transfer requirements;

2.  Fluid properties: Corrosiveness, fouling tendency, particulate content, and phase change (boiling/condensation);

3.  Economy and maintenance: Balance initial investment and long-term maintenance costs; for severe fouling scenarios, prefer cleanable designs (e.g., square-arranged smooth tubes).

Although heat exchange tubes are a single component, they integrate multiple technologies of materials science, heat transfer, and mechanics. Their rational design and material selection are key to ensuring the long-term safe operation of heat exchange devices.