Application of High-Temperature Heat Pipe Hot Air Furnaces in Silica Gel Drying

I. Introduction

A heat pipe is a heat transfer element with extremely high thermal conductivity. Heat exchangers composed of heat pipes offer advantages such as high heat transfer efficiency, compact structure, and low fluid resistance. They are currently widely used in industries such as metallurgy, chemical engineering, oil refining, boilers, ceramics, light industry and textiles, and machinery, and have yielded significant economic benefits.

II. Heat Pipe-Type Hot Air Furnaces

A hot air furnace is a piece of equipment that burns fuel to heat air and produce hot air. With the advancement of heat pipe technology, heat pipes are now being used as heat transfer elements in hot air furnaces. Compared to other types of hot air furnaces, heat pipe-type hot air furnaces offer the following advantages:

(1) The ratio of the lengths of the cooling and heating sections, the fin density, and the heat transfer areas of both sections can be adjusted to enhance heat exchange at the cooling end and reduce wall temperature;

(2) Heat pipes with different structures and working fluids can be used in the high-temperature and low-temperature zones, respectively; this avoids the need to use expensive materials throughout the entire unit solely to withstand high temperatures;

(3) The tubes are removable, making installation convenient; Both ends of the heat pipe can expand freely, eliminating issues related to thermal expansion and thermal stress;

(4) Heat exchange on the flue gas side occurs on the exterior of the tubes, making it relatively easy to remove ash deposits;

(5) The use of finned tubes to enhance heat transfer results in a relatively compact overall equipment structure.

III. Spray Drying

The thermal efficiency of spray drying equipment can generally be expressed as:

Overall efficiency η: η = (t1 – t2) / (t1 – t0) × 100%

Where: t1 – t2 — Temperature difference of the hot air during the drying process (i.e., inlet air temperature – exhaust air temperature) [°C]

t0 —— Atmospheric temperature [°C]

As can be seen from the above equation, increasing the temperature of the hot air is key to improving thermal efficiency. Therefore, in actual operation, provided that the quality of the material being dried is not compromised, maximizing the hot air inlet temperature and minimizing the exhaust air temperature are of great significance for energy conservation.

In the case of silica spray drying, if mass transfer and heat exchange are carried out with the hot air and material flowing in the same direction, even hot air at 550°C will not affect product quality. Therefore, to improve drying thermal efficiency and reduce the operating costs of silica production, the inlet temperature of the hot air should be increased as much as possible; however, it is difficult to generate hot air above 400°C using a conventional hot air furnace. 

IV. Applications

Depending on the different temperature zones, the hot air furnace employs high-temperature heat pipes, medium-temperature heat pipes, and standard water-carbon steel heat pipes. This design enables the hot air temperature in this furnace to reach 550°C. The basic parameters of this hot air furnace are as follows:

  Thermal load: 880 kW (750,000 kcal/h)

  Flue gas inlet temperature: 850°C

  Flue gas outlet temperature: 180°C

  Hot air outlet temperature: 550°C

  Fuel: Coal

The high-temperature heat pipe hot air furnace has been in operation at a factory in Fujian to date. The hot air is clean, and all performance indicators have met design requirements, fully satisfying the production process needs of the factory. The original project was designed for an annual production capacity of 2,000 metric tons. Since the installation of this equipment, which has eliminated production “bottlenecks,” annual output has reached 3,000 metric tons. Compared to traditional hot-air furnaces, the hot-air temperature has increased from 350°C to 550°C, and drying efficiency has improved from 60% to 81%. The comprehensive annual economic benefit amounts to 700,000 yuan.

V. Conclusion

As a new type of hot-air furnace, the heat pipe hot-air furnace can generate clean hot air at temperatures exceeding 500°C, meeting the needs of various hot-air equipment. Furthermore, due to its low flue gas exhaust temperature, it offers high thermal efficiency and fuel savings, making it a highly efficient and energy-saving hot-air device. It is not only suitable for the spray drying of silica but is equally suitable for the drying of many food products, pigments, and other chemical products.