Selection suggestions for coaxial heat exchanger Air Conditioning Systems in the Textile Industry

Source: 　 Time: 2025-12-31 08:38:28　Hit:

The core objective of the air conditioning system in the textile industry is to precisely control the temperature and humidity in the workshop (typically 20 to 28 degrees Celsius and a relative humidity of 50% to 75%), to ensure the quality of fiber processing (spinning, weaving, and dyeing) (such as reducing static electricity, breakage rate, and color difference), while also lowering energy consumption (air conditioning systems account for 30% to 50% of the total energy consumption in textile factories). The coaxial heat exchanger (tubular type) can play a key role in the pre-cooling/preheating, dehumidification/humidification, waste heat recovery, secondary heat exchange and other links of textile air conditioning due to its advantages such as compact structure, high heat exchange efficiency, good isolation of cold and hot media, and high pressure resistance. The following are specific selection suggestions from five dimensions: application scenarios, medium characteristics, structural design, performance parameters, and maintenance energy conservation:

I. Clarify the core application scenarios of the textile air conditioning system

Textile air conditioning systems are typically composed of fresh air processing units, return air mixing units, temperature and humidity regulation units (cooling/heating/humidification), and supply air units. The selection of a coaxial heat exchanger needs to first match the specific application scenario. The medium characteristics, temperature range, and load characteristics of different links vary significantly:

Application scenarios, functions, medium types, temperature ranges, core requirements

The pre-cooling/preheating unit initially cools or heats the fresh air/return air (reducing the cooling/heating load of the main unit). Primary medium (cold water/hot water/steam) + secondary medium (air/water) : cold water 5~15℃, hot water 40~60℃. Efficient heat exchange, low resistance, and anti-scaling

The dehumidification/humidification coupling system cools and dehumidifies before reheating (to avoid over-cooling), or recovers the waste heat to heat the humidification water. It features low-temperature water (7-12 ℃) + high-temperature water (50-70 ℃), with low-temperature water ranging from 5 to 15℃ and high-temperature water from 40 to 80℃. It offers precise temperature control, frost resistance (in winter), and corrosion resistance

The waste heat recovery unit recovers the exhaust air and process waste heat from the workshop (such as the waste heat from printing and dyeing wastewater), preheats the fresh air with high-temperature exhaust gas/wastewater (40~80℃) + fresh air/make-up water (10~25℃). The high-temperature side is 40~80℃ and the low-temperature side is 10~25℃. It is high-temperature resistant, anti-pollution, and compact integrated

The secondary heat exchange unit connects the chiller to the terminal air conditioning box (isolating the water quality on the primary side and the secondary side). Primary side chilled water (5~12℃) + secondary side circulating water (7~15℃). Primary side 5~12℃, secondary side 7~15℃. Leak-proof, high-pressure resistant, and easy to clean

Ii. Medium Characteristics and Material Selection: "Corrosion-resistant, Anti-scaling, and Suitable for Water quality"

The medium of textile air conditioning mainly consists of water (cold water, hot water, circulating water), air (fresh air/return air), and steam. Special attention should be paid to water quality (scaling/corrosion), temperature (heat resistance), and cleanliness (anti-clogging). The selection of materials should follow the principle of "economy + durability" balance

1. Conventional water quality (tap water/softened water) : Stainless steel material is preferred

• Medium characteristics: pH=6.5-8.5, hardness < 300mg/L (calculated as CaCO₃), no obvious corrosive ions (such as Cl⁻ < 200mg/L), suitable for most textile workshops (non-printing and dyeing areas).

• Recommended materials:

• Inner/Outer tubes: 304 stainless steel (06Cr19Ni10) : Low cost (about 60% of 316L), resistant to general corrosion (neutral water), suitable for cold/hot water heat exchange at temperatures < 80℃;

• Key parts (tube sheet, flange) : 316L stainless steel (022Cr17Ni12Mo2) : Contains Mo (2% - 3%), resistant to slight chloride ion corrosion (Cl⁻ < 500mg/L), suitable for circulating water systems (may contain a small amount of residual biocides).

2. Poor water quality (hard water/printing and dyeing wastewater) : Enhanced anti-scaling and corrosion resistance

• Medium characteristics: The circulating water in the printing and dyeing workshop may contain dyes and auxiliaries (pH=4-9), with a hardness greater than 500mg/L (prone to scaling), or contain Cl⁻ greater than 1000mg/L (enhanced corrosiveness). In some areas, the hardness of tap water is high (for example, in the north, it is greater than 450mg/L).

• Recommended materials:

• Inner tube (for media prone to scaling) : 316L stainless steel + surface modification: The inner wall of the inner tube is sprayed with polytetrafluoroethylene (PTFE) coating (thickness 50-100 μm) or nano-ceramic coating (such as Al₂O₃-TiO₂) to reduce the scaling rate (scaling coefficient < 0.1m²·K/kW);

• Outer tube (for corrosive media) : Duplex stainless steel (2205/2507) : PREN value (pitting resistance equivalent) > 35 (PREN=35 for 2205 and PREN=43 for 2507), resistant to Cl⁻ pitting and stress corrosion cracking (SCC), suitable for high Cl⁻ or acidic wastewater;

• Non-metallic auxiliary: CPVC/PVDF sleeve: If the medium temperature is less than 90℃ and the pressure is less than 1.0MPa, CPVC (chlorinated polyvinyl chloride) inner pipe can be selected. It is completely resistant to acid and alkali corrosion (such as printing and dyeing wastewater), but attention should be paid to the pressure limit.

3. Steam/high-temperature water medium: Resistant to high temperatures and thermal stress

• Medium characteristics: Steam temperature 110~180℃ (saturated steam), high-temperature water (such as on the waste heat recovery side) 80~120℃. The material needs to be resistant to high-temperature creep and oxidation.

• Recommended materials:

• Inner tube (steam passage) : 20# carbon steel (GB/T 8163) + Anti-corrosion coating: Carbon steel has a low cost, and the outer surface is sprayed with inorganic zinc-rich primer (temperature resistance 200℃), suitable for short-term high temperatures (< 150℃); For long-term high temperatures (> 150℃), it is recommended to use 15CrMo alloy steel (with a temperature resistance of 550℃).

• Outer tube (for the heated medium) : 304 stainless steel: Temperature resistance < 450℃, meeting the high-temperature water requirements of textile air conditioning.

Iii. Structural Design: Optimized for "anti-scaling, low resistance, and easy maintenance"

The textile air conditioning system needs to operate continuously and stably (24 hours a day), and the medium may contain a small amount of fiber dust or impurities. The structural design should focus on solving three major problems: scaling and blockage, excessive resistance, and difficult maintenance.

1. Flow channel design: Enhance heat transfer + inhibit scaling

• Spiral groove/corrugated inner tube: The outer wall of the inner tube is processed with shallow spiral grooves (depth 0.3-0.8mm, pitch 8-15mm) or microwave patterns (wave height 0.5-1mm) to enhance fluid turbulence (Reynolds number Re > 10000), increase the heat transfer coefficient by 20% - 30% (compared with smooth tubes), and at the same time reduce fouling deposition (turbulent erosion inhibits scaling).

• Variable cross-section flow channel: On the high-temperature side (such as waste heat recovery), a "gradually shrinking flow channel" (inlet cross-sectional area > outlet) is adopted to increase the flow rate (1.5-2.5m/s) and prevent the sedimentation of suspended solids. On the low-temperature side (such as cold water), a "gradually expanding flow channel" is adopted to reduce the flow rate (1.0 to 1.5m/s) and decrease the pressure drop.

• Multi-flow diversion: When the heat exchange capacity is large (such as > 100kW), the "inner pipe multi-flow series + outer pipe single-flow parallel" approach is adopted to balance the flow distribution on the cold and hot sides and prevent local overheating/overcooling.

2. Anti-clogging and easy-to-clean structure

• Detachable end cover design: Quick-release flanges (with silicone rubber sealing rings) are installed at both ends of the outer pipe. When the machine stops, they can be opened to clean the scale (such as fiber dust and water scale) between the pipes.

• Large pipe diameter and small radius of curvature: It is recommended that the outer diameter of the inner pipe be ≥25mm (to avoid blockage), and the ratio of the inner diameter of the outer pipe to that of the inner pipe be controlled at 1.5 to 2.0 (for example, inner pipe Φ25mm→ outer pipe Φ50mm), to reduce fluid dead zones.

• Pre-filtration: A Y-type filter (mesh size 80-120) is installed at the medium inlet to intercept large particle impurities such as fiber debris and welding slag (especially suitable for the recovery of waste heat from printing and dyeing wastewater).

3. Compact and low-resistance design

• Maximize the heat exchange area per unit volume: Prioritize small-diameter tube combinations (such as inner tube Φ19mm+ outer tube Φ38mm), and the heat exchange area per unit volume can reach 150 to 200m²/m³ (compared with 80 to 120m²/m³ of shell and tube type), saving space in the air conditioning machine room.

• Low-resistance flow channel: A deflector cone (at an Angle of 30° to 45°) is installed at the inner pipe inlet to prevent the fluid from directly impacting the pipe wall. The annular gap flow velocity of the outer pipe is controlled at 1.0 to 2.0m/s (resistance significantly increases when exceeding 2.5m/s), and the pressure drop is less than 10kPa (meeting the residual pressure requirements of the fan).

Iv. Performance Parameters: "High Efficiency, Stable, and Adaptable to Load Fluctuations"

The load of textile air conditioning fluctuates greatly with the production process (such as the start and stop of the spinning machine) and the outdoor climate (seasonal changes) (load rate 50% to 120%). The performance parameters of the coaxial heat exchanger need to match the adaptability to variable loads:

1. Heat exchange efficiency: Precisely calculated based on logarithmic mean temperature difference (LMTD)

Selection formula: Heat exchange capacity Q = K/cdot A/cdot Delta T_m, where K is the overall heat transfer coefficient (W/m²·K), A is the heat exchange area (m²), and Delta T_m is the logarithmic mean temperature difference (℃).

• Key parameters:

• Overall heat transfer coefficient K: When water-water heat exchange occurs, for smooth tubes, K≈800-1200 W/m²·K, and for spiral groove tubes, K≈1200-1800 W/m²·K (subject to correction based on actual flow patterns);

• Temperature difference ΔT_m: Textile air conditioners mostly adopt "small temperature difference heat exchange" (such as cold water from 5℃ to 10℃, and the cooled medium from 25℃ to 20℃), with ΔT_m≈5 to 8℃. It is necessary to avoid choosing a model with a temperature difference that is too small, which may lead to an overly large area (increased cost).

2. Temperature and pressure resistance grade: Reserve a 20% safety margin

• Design temperature: Based on the maximum medium temperature +20℃ (for example, cold water with a maximum temperature of 15℃→ design temperature of 35℃; steam with a maximum temperature of 180℃→ design temperature of 200℃).

• Design pressure: Based on the maximum working pressure of the system +0.2MPa (for example, the pressure of the air conditioning water system is 0.6MPa→ design pressure 0.8MPa; the pressure of the steam system is 0.3MPa→ design pressure 0.5MPa).

Note: The high-temperature wastewater (80℃) in the printing and dyeing workshop may be accompanied by pressure fluctuations. Therefore, a model with a pressure resistance of ≥1.0MPa should be selected.

3. Variable load adaptability: Supports flow/temperature regulation

• Flow regulation: Electric control valves (such as V-type ball valves) are installed at the inlet and outlet of the inner and outer pipes to adjust the flow rate according to load changes (such as the increase or decrease in the number of people in the workshop) (regulation ratio 1:3).

• Temperature control: In conjunction with a temperature sensor (PT100) and a PLC system, precise control of "outlet water temperature ±1℃" is achieved (for example, the outlet water temperature of the pre-cooling unit is stabilized at 12±1℃).

V. Maintenance and Energy Conservation: "Low-cost Operation and Maintenance + Waste Heat Recovery for Efficiency Enhancement"

Textile factories pursue "cost reduction and efficiency improvement". The selection of coaxial heat exchangers needs to take into account both low maintenance costs and energy-saving potential:

1. Easy-to-maintain design

• Online cleaning interface: A DN20 cleaning port is set at the bottom of the outer pipe. Citric acid solution (2% - 3%) or high-pressure water (pressure 5-8MPa) is regularly introduced to flush the scale between the pipes (it is recommended to do this once every quarter).

Corrosion monitoring points: Pre-embed corrosion test pieces (such as carbon steel, 316L) in easily corroded parts like welds and flanges. Take them out every six months to test for weight loss and predict the service life.

• Backup module design: For large air conditioning systems (heat exchange capacity > 500kW), "multi-tube parallel modules" can be adopted. When a single module fails, it can be isolated for maintenance without affecting the overall operation.

2. Energy conservation and waste heat recovery

• Priority for waste heat recovery: In high-temperature process links such as printing and dyeing and setting machines, the coaxial heat exchanger is used to recover the waste heat of wastewater (40-60 ℃) to preheat the fresh air (from 10℃ to 25℃), which can reduce steam consumption by 15% to 25%.

• Variable frequency drive matching: Linked with variable frequency water pumps/fans, it dynamically adjusts the medium flow rate according to the load (for example, the flow rate drops to 50% at low load at night), reducing the pump power consumption.

• Insulation reinforcement: The exposed part of the outer pipe is covered with rubber and plastic insulation cotton (thickness 30-50mm, thermal conductivity ≤0.038W/m·K) to reduce heat dissipation loss (especially on the hot water side in winter).

Vi. Typical Selection Case: Air Conditioning Pre-cooling System of a Certain Spinning Mill

• Operating conditions: Workshop area: 5,000 square meters, pre-cooled fresh air volume: 100,000 m³/h, fresh air temperature: 35℃→ target 25℃, cold water (7℃ supply water /12℃ return water) flow rate: 200m³/h.

• Selection plan:

• Material: Both the inner and outer tubes are made of 304 stainless steel (for softened water with a hardness of 200mg/L).

• Structure: Inner tube Φ25mm (spiral groove, pitch 10mm), outer tube Φ50mm, 6 sets of sleeves connected in parallel (total heat exchange area 120m²);

• Performance: The overall heat transfer coefficient K=1500W/m²·K, the logarithmic mean temperature difference ΔT_m=7.2℃, and the heat exchange capacity Q=1500×120×7.2=129.6kW (meeting the pre-cooling load requirement of 120kW);

• Maintenance: Equipped with quick-release flange +DN20 cleaning port, acid washing is carried out once every quarter.

• Effect: After pre-cooling, the fresh air temperature remains stable at 24 to 26℃, the load of the chiller is reduced by 20%, and approximately 80,000 kWh of electricity is saved annually.

Summary

The selection of the coaxial heat exchanger air conditioning system in the textile industry should closely adhere to the four core aspects of "scene adaptation, material anti-scaling, structure easy maintenance, and stable performance".

1. Select functions based on scenarios: Pre-cooling/preheating focuses on efficient heat exchange, waste heat recovery emphasizes high-temperature resistance and anti-pollution, and secondary heat exchange focuses on leak-proof and easy cleaning.

2. Select materials based on the medium: 304 stainless steel is used for conventional water quality, 316L/ duplex steel + coating for poor water quality, and carbon steel/alloy steel for high-temperature steam.

3. Select the structure based on maintenance: Spiral groove for enhanced heat transfer + quick-release end cover for easy cleaning + pre-filter to prevent clogging.

4. Select parameters based on energy conservation: match variable load regulation + priority for waste heat recovery + low resistance design.

Through the above selection strategies, the energy efficiency ratio (COP) and operational stability of the textile air conditioning system can be significantly improved, helping enterprises reduce energy consumption costs (it is estimated that the energy consumption of the air conditioning system will decrease by 15% to 30%), while ensuring product quality.

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