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Vacuum Steam Basics

What causes steam to blow out so strongly?

Steam is commonly thought of as blowing out vigorously when a valve is opened and released to the atmosphere. It is also blown out from valve packing and pinhole leaks. What does this phenomenon mean? The answer is simple: steam has a higher pressure than atmospheric pressure.


So, is the pressure of steam always higher than atmospheric pressure? Actually no, it is possible to generate and use steam below atmospheric pressure: vacuum steam.

It is common knowledge that water has a lower boiling point at higher altitudes. In other words, at lower pressures, water boils at a lower temperature. Similarly, steam generated at pressures below atmospheric pressure will have a temperature lower than 212°F.

Low-Temperature Saturated Steam (Vacuum Steam)

Referring to a saturated steam table, we can see that the pressure of saturated steam at 212°F is 14.7 psia, which is near atmospheric pressure. The steam table also shows data for steam at temperatures below 212°F.

Temperature Based Saturated Steam Table

Temp. Press.
Specific Volume Specific Enthalpy
°F psig ft3/lb Btu/lb
T P Vf Vg Hf Hfg Hg
130 -12.473 0.016247 157.33 97.96 1019.8 1117.8
150 -10.978 0.016343 97.07 117.95 1008.2 1126.1
170 -8.7034 0.016451 62.06 137.97 996.2 1134.2
190 -5.356 0.016572 40.957 158.04 984.1 1142.1
212 0.093 0.016719 26.799 1803.17 970.3 1150.5
220 2.490 0.016775 23.148 188.23 965.2 1153.4
230 6.083 0.016849 19.381 198.33 958.7 1157.1
240 10.272 0.016926 16.321 208.45 952.1 1160.6
250 15.129 0.017006 13.819 218.59 945.4 1164.0
260 20.731 0.017089 11.762 228.76 938.6 1167.4

Steam below 212°F

Saturated steam at temperatures below 212°F has a pressure below atmospheric pressure, approximately 14.7 psia. While not widely known or utilized, vacuum steam does exist and excels in certain heating applications, as we will discuss later.

How to Create Vacuum Steam

Since saturated steam with a temperature below 212°F can only exist in a vacuum state below atmospheric pressure, it is necessary to start by depressurizing the steam system itself, from piping, heat exchanger, steam trap and condensate recovery system, to below atmospheric pressure.

There are several ways to generate a vacuum, but the most common is to use an electric vacuum pump in a vacuum steam system.

There are several ways to generate vacuum, but electric vacuum pumps are commonly used in vacuum steam systems.

Characteristics of Vacuum Steam

Vacuum steam requires some additional work and equipment to use, but when it is saturated, it offers the same advantages as saturated steam above atmospheric pressure. In other words:

  • 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

Steam is an excellent heat medium even at lower temperatures.

Need help in achieving fast and uniform heating below 212°F?