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• Basics of Steam
• Heating with Steam
• Steam Heat Transfer
• Steam Partial Pressure #1: Why the Temperature Doesn’t Rise
• Steam Partial Pressure #2: Air Removal (Part 1)
• Steam Partial Pressure #3: Air Removal (Part 2)
• Steam Trap Losses - what it costs you
• Types of Steam and Their Applications
• Vapor and Flash Steam
• Various States of Steam
• Steam Trap
• Insulating Traps
• Steam Locking and Air Binding
• Temperature Control Trap Precautions
• The History of Steam Traps #1
• The History of Steam Traps #2
• Trap Back Pressure
• Trap Installation Orientation
• Traps and Orifices Part 1
• Traps and Orifices Part 2
• What is a Steam Trap?
• Condensate Recovery
• Stall Phenomenon #1: Causes & Resulting Problems
• Stall Phenomenon #2: Methods for Preventing Stall
• Piping / Other
• Bypass Valves
• Check Valve Installation and Benefits
• Pressure Reducing Valves for Steam
• Types of Valves and Their Applications
• Air Trap
• Preventing Clogging of Air Traps

Steam Partial Pressure #2: Air Removal (Part 1)

Which is heavier, air or steam?

In ‘Steam Partial Pressure # 1’ we discussed the necessity of removing the air from steam-using equipment in order to be able to utilize the steam most efficiently. In this article, we’re going to touch briefly on the question ‘Which is heavier, air or steam?’ as we work up to the next article’s discussion of ‘From where in the equipment is the excess air removed?’

• The average molecular weight of air is 29, which means that 1 mole weighs 29 grams.
• The molecular weight of steam (water) is 18, which means that 1 mole weighs 18 grams.

Gas under standard conditions is 1 mole, so, compared with air, steam can be said to be the lighter of the two.

Reference note: Standard conditions for gas can be defined in the following two ways:

• STP definition: 1 atm, 0°C (273.15 K), 1 mol = 22.4 L
• SATP definition: 1 bar, 25°C (298.15 K), 1 mol = 24.8 L

However, under actual conditions…

• The specific weights vary widely depending on temperature and pressure.
• The steam space of a heat exchanger is filled with a mixture of steam and air.
• Partial pressures change depending on the proportions of steam and air.

As we can see, actual conditions are far from standard conditions, so the results gained under standard conditions do not necessarily apply.

If we examine the weights of steam and air under actual conditions such as those of the steam space inside a heat exchanger, quite interestingly we find that they vary greatly depending on the conditions.

As an example, let’s say that the pressure gauge indicates the pressure of the steam and air mixture at the time steam first enters the steam space is 1.0 [MPaG]. In this case, if the temperature of the mixture is lower than 162°C, then steam is lighter, but if the temperature is higher than 162°C, then air is lighter. In this way, whether air is lighter than steam or steam is lighter than air changes, depending on the temperature and partial pressures of the mixture of the two. Let’s take a look at the graph below to see the point at which the relative specific weights of air and steam become reversed.

In answer to the question, ‘from where should the air be removed?’ it is not enough just to say ‘from the top’ or ‘from the bottom.’ The state of air removal varies greatly depending on the figuration of the steam space, so it is necessary to consider the issue from various angles.