Application of Tube-fin Heat Exchangers in Vacuum Drying Equipment

The vacuum drying chamber operates under negative pressure, resulting in low-temperature vaporization of moisture in the material. Two types of tube-fin heat exchangers are used in conjunction with the vacuum chamber:

1. Heating-type tube-fin heat exchanger (hot air circulation heating): Heat transfer oil/high-temperature water/steam flows through the tubes, while the fins contact the circulating carrier gas (nitrogen/dry air), heating the circulating gas in the vacuum chamber and thus heating the material at low temperatures, increasing the moisture vaporization rate.

During vacuum, the gas density is low and convective heat transfer is weak. The fins significantly expand the heat exchange area, compensating for the extremely low heat transfer coefficient on the gas side.

2. Cooling-condensing type tube-fin heat exchanger (water vapor recovery + vacuum protection): Cooling water/chilled water flows through the tubes. The high-temperature, humid vacuum airflow passes over the fin surface, causing water vapor to condense into liquid water upon cooling. This prevents large amounts of water vapor from entering the vacuum pump, protecting the pump body and reducing the vacuum load. It is the core auxiliary heat exchanger for vacuum drying.

Special Requirements for Tube-Fin Heat Exchangers under Vacuum Drying Conditions

1. Sealing (Most critical, distinguishing it from atmospheric pressure heat exchangers)

The entire equipment must be connected to the vacuum chamber. The shell, tube sheet, and finned tube bundle must be leak-free:

- The tube bundle and tube sheet use a composite expansion welding process to eliminate negative pressure leakage;

- The shell has a fully welded structure with minimal flanges, and vacuum sealing gaskets are used at flange locations;

- Helium vacuum leak testing is required at the factory to meet high vacuum leak rate standards.

2. Temperature Resistance and Media Compatibility

- Heating side: Thermal oil up to 280℃, steam 180℃, hot water 95℃;

- Cooling side: Cold water 10~30℃, ethylene glycol chilled water below 0℃;

- Drying volatiles: Organic solvents, acidic water vapor, and salt mist require matching corrosion-resistant materials.

3. Low Flow Resistance for Vacuum Circulating Fans

In a vacuum, the gas is thin, and the fan pressure head is limited:

- Largest fin spacing (6~12mm) is preferred to prevent water vapor crystallization and material dust blockage;

- Straight fins are optional, offering lower resistance compared to corrugated/serrated fins, reducing fan power consumption.

4. Anti-Scale and Easy to Clean

Drying generates dust, condensate scale, and organic matter adhesion:

- Large fin spacing facilitates high-pressure air blowing and spray cleaning;

- A drain port is added to the condensate side for periodic discharge of condensate wastewater.

Heating vs. Condensing Tube-Fin Heat Exchanger Design Differences

1. Heating Heat Exchanger (Circulating Hot Air Heating)

- High fin height and large heat exchange area enhance gas heat absorption;

- High-temperature medium flows inside the tubes, pressure-bearing design;

- Moderate fin spacing of 5~8mm, balancing heat exchange and preventing clogging;

- Equipped with a circulating fan, installed within the vacuum chamber duct.

2. Condensing Heat Exchanger (Moisture Capture)

- Larger fin spacing of 8~12mm prevents moisture icing and dust bridging;

- Water collection tank and drain valve at the bottom of the shell for continuous condensate drainage;

- Insulation layer can be added to prevent condensation on the outer wall from affecting the workshop vacuum environment;

- Located near the front of the vacuum pump, serving as a pre-cooling trap to replace some expensive plate cold traps.

Advantages and Disadvantages of Tube-Fin Heat Exchangers for Vacuum Drying

Advantages

1. Adaptable to rarefied vacuum gas heat exchange: Fins expand the heat exchange area, solving the problem of poor heat exchange in low-pressure gases;

2. Wide temperature and pressure resistance range: Suitable for both high-temperature heat transfer oil heating and low-temperature chilled water condensation;

3. Fully welded vacuum-sealed structure, low leakage rate, and stable maintenance of vacuum within the chamber;

4. Modular structure, allowing for the addition or removal of tube rows according to the vacuum chamber's airflow and heat load;

5. Smaller size compared to shell-and-tube heat exchangers, adaptable to compact air duct layouts in dryers;

6. Simple maintenance: Large-pitch fins allow for online purging, eliminating the need for frequent disassembly.

Disadvantages:

1. Thin fins: In dry, highly corrosive media, ordinary aluminum fins are easily corroded and rendered unusable, necessitating the use of all stainless steel/titanium materials, increasing costs;

2. Fine fin gaps: Long-term accumulation of ultrafine dust can clog air ducts, requiring regular purging;

3. Gas-side heat exchange resistance is higher than that of bare tubes, requiring appropriately increased power for the selected vacuum fan;

4. Complex vacuum leak detection process, resulting in higher factory costs than ordinary atmospheric pressure tube-fin heat exchangers.