An air-conditioned vest that can cool workers in the heat has become popular and sold out during the current heat wave in southern China.

Wang Yan, a 40-year-old businessman in Nantong, Jiangsu province, invented the vest. He recently revised the device so that it can lower the temperature around the wearer by up to 20 degrees.

"It means when the temperature is 40 C, someone wearing the vest will feel as if it were 20 C," he said.

Wang opened a factory to make the vests he invented, and has sold about 4,000 of them in the last three months — 40 percent more than last year.

Each vest sells for about 200 yuan ($33).

Wang was an electric welder in the 1990s, and temperatures in his workplace often reached 50 C in the summer.

"Normally, manufacturing workshops are not equipped with air conditioning because the area is too spacious and cannot be tightly closed, so I think it's best to wear a personal air conditioner," he said.

The vest is made of impermeable fabric to lock in the cool air, he said. Cool air is distributed through small tubes with tiny holes that are inside the vest.

The key to the device, Wang said, is a vortex tube that produces cool air.

"Compressed air is delivered into the vortex tube, swirls quickly and forms a funneled airflow, like a tornado, causing hot air to sink and cool air to float," he said. "The cool airstream is distributed through the vest through a tube, while the small amount of hot air is emitted."

Wang originally wrapped the cool air tubes around him when he worked. He later managed to integrate the tubes into the vest. He then continued renovating the vest to make it cooler.

The vest requires compressed air that can be produced from an air compressor pump.

A pump can sustain several vests, he said, adding that many factories are equipped with compressed-air pipelines that make using the vest more convenient.

Two valves are on each side of the vortex tube. One controls the inflow of compressed air, and the other controls the cool-air output, with which the user can regulate the temperature.

"Normally when the temperature is around 35 C, the vest can lower it to about 25 C in 30 seconds," Wang said.

Wang Hongxin, an electric welder in Nantong, feels comfortable while using the vest.

"I wear the vest under my work clothes and feel my entire upper body cool down very fast," he said, adding that the vest is popular among his workmates.

Wang Yan said the vest does not use harmful or flammable gas, making it safe to wear.

Agreeing with that assurance is Li Tianbao, who has sold labor protection appliances in Nantong for years and tried the vest himself.

"The vest is widely welcomed by workers in shipyards, container producers and other manufacturing companies," he said.

Because of the lingering heatwave, the vests have been out of stock in Li's shop.

"I sold about 400 vests this summer, 50 percent more than last year. I want to order more from Wang's factory, but he can't supply them," Li said.