Generally speaking, vacuum cooling technology is based on the following 2 points:
1. Liquid water needs to absorb amount of heat to change to water vapor. Since vaporous water molecules have higher energy than liquid water molecules, water must absorb heat when vaporized.
2. The boiling point of water decreases as the decrease of air pressure. The lowering of the boiling point makes the liquid water easier to vaporize. For example, at a standard atmospheric pressure (101 325 Pa), water boils at 100 °C; the boiling temperature of water is 76 °C at 40 196.6 Pa, 29 °C at 4 kPa, and 14°C at 1.6 kPa.
In an air-tight vacuum room, the vegetables to be treated are placed and then start the vacuum cooling system. The boiling point of the water decreases as the decrease of air pressure in the vacuum chamber. To some certain degree, the liquid water in the vegetables begins to vaporize, while at the same time taking a lot of heat from the vegetables.
Through this physical approach, rapid and even cooling of vegetables can be achieved!
Then, you may have these questions:
Question A: The vacuum pump can take out the water vapor from vacuum chamber to the environment, if so, why refrigerating system is needed?
Answer A: the volume is 1120 times bigger when 1L liquid water become vapor in standard ambient environment. This dramatically change in volume brings another problem.
In the limited vacuum chamber will be full of vapor soon and reaches saturation condition. And in saturation condition, liquid water cannot be change to vapor continuously anymore. It means: The vacuum cooling is stopped!
What’s the solution?
You may say: We need powerful vacuum pump! But there are 3 new problem:
1. It’s not a good idea when considering the investment;
2. The service life of vacuum pump will be decreased when the vapor mixed with vacuum pump oil. (Note 1: check the following related links);
3. The result is no good if only using vacuum pump. (Note 2: check the following related links)
The solution is the cooling system: The evaporator of the cooling system will catch the vapor (hence, we also call evaporator “water catcher”), vapor will become liquid water and be collected, and then drain off after one complete vacuum cooling cycle is finished.
Now, we guess you have a basic understanding about vacuum cooling technology!
Testing result of vacuum cooling test without cooling system)