What is Vacuum Steam?

Vacuum steam is the general term used for saturated steam at temperatures below 100°C.

As briefly discussed in Principal Applications for Steam, both positive pressure steam and vacuum steam can be used in heating processes. The following article will discuss the properties of vacuum steam and its advantages over heating with hot water.

The higher the pressure, the higher the temperature of saturated steam. At regular atmospheric pressure, saturated steam is roughly 100°C. Saturated steam generated from boilers, however, is generally much higher in temperature because it is generated at higher pressures. This steam (positive pressure steam) is therefore frequently used in industry for heating processes requiring temperatures above 100°C.

Alternatively, producing saturated steam for heating processes below 100°C is also possible. Such steam is often referred to as vacuum steam because it requires pressures below regular atmospheric pressure. Vacuum steam is generally generated at higher pressures after which pressure is reduced by using equipment such as an inlet control valve. A vacuum pump is also usually used to help achieve lower pressures at start-up and enable the smooth release of condensate.

Use of vacuum steam requires careful temperature and pressure reading. To determine steam temperature, referring to a steam table such as the one above is recommended. For example, through this steam table, we can see that if a process requires saturated steam at temperatures of 60°C or 90°C, saturated steam pressures should be set to 19.946kPa and 70.182kPa, respectively.

Vacuum Steam vs. Hot Water

Heating with vacuum steam offers the same advantages as heating with steam at temperatures of 100°C or higher:

Property	Advantage
Rapid even heating through latent heat transfer	Improved product quality and productivity
Pressure can control temperature	Temperature can be quickly and precisely established
High heat transfer coefficient	Smaller required heat transfer surface area, enabling reduced initial equipment outlay

Overall Heat Transfer Coefficient

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