A Brief Analysis of Air-Warmth Gasifiers and Their Applications

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An air-temperature gasifier is designed by taking advantage of the property that liquefied gases such as liquefied petroleum gas vaporize by absorbing heat upon depressurization. Its production and application have become highly mature abroad, especially in Japan. In China, however, its emergence is a recent development, and it has not yet been widely used in production. The author provides a brief analysis here.

1. Application of Air-Temperature Gasifiers in China
The gasifiers used in China include electric heating type, electric water bath type, hot water circulation type, and air-temperature type. From the current usage situation in China, air-temperature gasifiers are still considered “new” for many users. Moreover, their performance is indeed greatly affected by temperature. However, the advantage of air-temperature gasifiers is that their operating costs are much lower than those of other gasifiers. For this reason, they are increasingly favored by users. Judging from the sales situation of several major air-temperature gasifier manufacturers in China, supply and demand are generally in balance. According to statistics, last year, the products produced by the six main manufacturers of air-temperature gasifiers were distributed across the country, with a concentration in Jiangsu, Zhejiang, and Guangdong. In economically developed areas such as Shenzhen, Zhuhai, Panyu, Zhongshan, Nanhai, and Shunde in the Pearl River Delta region of Guangdong, sales have increased several times in recent years. It is believed that driven by these regions, air-temperature gasifiers will be more widely adopted.

2. Production of Air-Temperature Gasifiers
Air-temperature gasifiers are new types of efficient, environmentally friendly, and energy-saving gasification devices that utilize the property of gases such as liquefied petroleum gas (LPG) and liquefied natural gas (LNG) to vaporize upon depressurization. The core component of such devices is the heat exchange unit, which is designed to extract maximum heat energy from the atmosphere within a minimal space. Currently, the heat exchange units in air-temperature gasifiers produced by Chinese manufacturers mostly use anti-corrosion aluminum alloy finned tubes. Due to lack of optimized design, these units generally suffer from the following drawbacks: large size, high cost, and insufficient flow rate. The author believes that, aside from human factors, the main reason lies in the heat exchange area. To achieve a certain flow rate, more fins must be added. Through numerous studies and experiments, various technical solutions have been developed to address this issue to the greatest extent possible. A brief explanation is provided below, in the hope of contributing positively to the promotion of air-temperature gasifiers in China.

The general structure of a finned tube consists of a central tube surrounded by radial fins. Liquid flows inside the tube, while the fins absorb heat from the surrounding environment and transfer it to the liquid inside the tube, enabling vaporization. After multiple experiments and selections, a new type of finned tube was developed that differs from the conventional ones, with a heat transfer coefficient that is more than 50% higher. The working principle of this new finned tube is illustrated using liquefied petroleum gas as an example: When liquefied petroleum gas flows inside the tube (vertically from bottom to top), the liquid near the inner wall of the finned tube first exchanges heat with the surroundings and vaporizes into tiny bubbles. As these bubbles accumulate and form a gas, they escape, leading to boiling. However, due to the short contact time between the liquid petroleum gas and the tube wall, convection occurs only under the influence of gravity, resulting in uneven wall temperatures throughout the finned tube. This prevents a significant increase in the heat transfer coefficient and results in suboptimal overall heat exchange performance. These are precisely the shortcomings present in most current air-temperature gasifiers in China. The ATC series developed by our company improves the heat transfer coefficient of finned tubes through the use of an internal liquid-absorbing core and spiral guide strips, increasing it by 1.5–2.0 times compared to finned tubes without this technology. The principles behind the use of the internal liquid-absorbing core and spiral guide strips are as follows:

(1) The internal liquid-absorbing core is a porous stainless steel strip that is wound into a cylindrical shape and attached tightly to the inner wall of the finned tube. When the core is immersed in liquefied petroleum gas or natural gas liquid, the capillary action caused by surface tension causes liquid from the liquid film to be absorbed through the narrow gap between the core and the inner wall of the tube. This liquid then absorbs heat and vaporizes, forming tiny bubbles. As these bubbles rise and grow into larger bubbles, they escape through small holes on the upper surface of the core, eventually separating from the liquid surface and reaching boiling. After the tiny bubbles leave the core, liquid is replenished into the gap through the small holes, facilitating micro-convection heat transfer. Additionally, the separation of liquid between the internal core and the spiral guide strips prevents it from being washed away and carried away too early, thus promoting the formation of tiny bubbles.

(2) The internal spiral guide strips change the direction of liquid flow. Without these strips, the liquid flows vertically, but with them, the flow becomes spiral upward (similar to an Archimedes spiral). This increases the flow rate of the liquid, prolongs its contact time with the finned tube, and enhances the separation of liquid between the core and the guide strips, creating an airflow that carries a small amount of liquid droplets. As this airflow rises, it continuously moistens the fins above the liquid surface. The gas then absorbs more liquid droplets with the same temperature as the rising airflow, which flow downward along the inner wall. After mixing with the liquid at a slightly lower temperature at the surface, the temperature at the surface increases, enhancing boiling inside the finned tube and further improving its overall heat exchange performance.

Thanks to these two technologies, the uniformity of the temperature on the inner wall of the finned tube is significantly improved, the heat density is increased, and the heat transfer coefficient is markedly enhanced. This leads to better heat exchange efficiency in air-temperature gasifiers, ensuring more effective vaporization of liquefied gases such as LPG and LNG. An air-temperature gasifier composed of high-efficiency heat exchange tubes with an internal liquid-absorbing core and spiral guide strips for the gasification section, finned tubes with spiral guide strips for the superheating section (to raise the temperature of the gas after vaporization), along with a main pressure regulating device, a control system for monitoring the liquid-phase pressure regulator and gas-phase pressure regulation system, an anti-overflow device, and a safety vent pipe, can maximize vaporization capacity within a given volume.

3. Defects of Air-Temperature Gasifiers and Solutions
Air-temperature gasifiers rely on their own sensible heat and heat absorption from the surrounding atmospheric environment to achieve vaporization—a feature that is an advantage. However, their vaporization capacity is greatly affected by temperature. In colder climates (such as in the north), the vaporization capacity may fall short of the rated value. According to our company’s calculations, at 0 degrees Celsius, the vaporization capacity is only 65% of the rated flow rate. Nevertheless, this issue can be resolved. On one hand, a backup system can be activated during cold seasons; on the other hand, an additional backup system can be installed. Considering costs, this approach remains more economical than using only electric heating gasifiers.

Well-designed and manufactured air-temperature gasifiers are not only environmentally friendly and energy-efficient but also yield significant economic benefits. In an era of growing global energy shortages, the use of air-temperature gasifiers should be vigorously promoted and supported.