What is Vacuum Cooling?
The Evaporative Cooling Principle
Evaporative cooling is the process of cooling by vaporizing water to remove heat. Water evaporates when it receives its latent heat of evaporation. Water absorbs heat until it receives its latent heat of vaporization but the hot medium also loses the same amount of heat.
The evaporative cooling system makes active use of this. The principle of evaporative cooling is also used in a surprisingly familiar place.
For example, some high-performance automobiles are equipped with water injection systems for evaporative cooling, to improve the cooling performance of air-cooled intercoolers combined with turbochargers.
On hot summer days, sprinkling water outside is also a form of evaporative cooling that uses evaporative heat to lower the temperature of road surfaces and other surfaces.
Vacuum Evaporative Cooling
Water can even evaporate at room temperature under atmospheric pressure. In this case, the water mass is only evaporating and not boiling. For example, if water is boiled and vaporised at 40°C [104°F], this will result in the transfer of large amounts of heat equal to the latent heat of evaporation.
At atmospheric pressure, water boils at 100°C [212°F]. Then, if the pressure is lowered below atmospheric pressure, the boiling point also falls below 100°C [212°F]. This is because the saturation temperature is below 100°C [212°F]. The details are explained in Vacuum Steam Heating Systems.
In vacuum steam heating, saturated steam at 100°C [212°F] or lower is used as a heating source, whereas in vacuum steam cooling, the boiling phenomenon that occurs at 100°C [212°F] or lower is used.
Vacuum Evaporative Cooling in Production Equipment
While technically under vacuum conditions, the saturation pressure of steam in the temperature range used in vacuum evaporative cooling is in the range of several kPa, and it is not so difficult to lower the pressure to that level. The trouble is that in the evaporative cooling process, the more it cools, the greater the quantities of steam that are generated on the heat transfer surface, which needs to be handled somehow.
In this pressure range, the volume of steam is a thousand or several hundred times larger than that of water, so the pressure will quickly rise if the generated steam is not discharged. If the pressure increases, evaporative cooling will no longer be possible at the desired temperature.
For this reason, the evaporative cooling system uses a high-capacity vacuum pump that can not only create a vacuum by air suction, but also discharge the steam generated during evaporative cooling.
Vacuum Evaporative Cooling Applications
As described above, evaporative cooling promotes the evaporation of water in the vacuum pressure range and achieves heat transfer performance on the cooling side comparable to that of steam heating. In industry, evaporative cooling is used for the following purposes.
- Fast and even cooling to reduce the formation of impurities
- Increasing production yield through high-performance cooling that suppresses exothermic reactions.
- Reducing process batch times by faster cooling
In addition, by combining evaporative cooling technology with vacuum steam heating, quick switching between heating and cooling is possible, and high-precision temperature control can be achieved. Because of its high reproducibility and control, vacuum evaporative cooling systems are commonly used in the following situations.
- When quick temperature regulation is crucial, such as raw material drop-in, exothermic or endothermic reactions, heat transfer in agitated vessels, etc.
- When there are many utilities used and it is desirable to eliminate the temperature variation between production batches.
- When quick determination of process conditions is needed for piloting new products or for actual production
- When testing multiple production conditions for new product development
We hope you enjoyed this series on steam control and vacuum heating and cooling. Effective control of steam can allow improved productivity and reduced batch times, while reducing energy expenditure and equipment footprint.
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