Fluid Process Control: Steam Knowledge

1. Steam & Steam Trap Relationship

Condensate is generated after steam is used in steam transmission and utilization equipment.

A steam trap is a valve that automatically discharges this condensate, making it closely related to steam equipment and steam itself.

2. Steam Definition & Characteristics

Steam is a transparent gas produced by heating water in a boiler.

At a specific pressure, sufficient energy is required to raise the water temperature to its boiling point. Any additional energy afterward only converts water into steam without increasing the temperature further.

Steam is an efficient and easily controllable heating medium.

It is commonly used to transfer energy from a centralized point (boiler-generated steam) to various locations in a factory for heating air, water, or process media.

When steam is delivered to steam-using devices, its latent heat is utilized for heating (steam turns into condensate after releasing latent heat) and is discharged outside the devices.

To further utilize the sensible heat of condensate and the condensate itself effectively, condensate recovery is performed—recycling it to boilers or other equipment (see Figure 1).

3. Water State Changes & Heat Types

Matter generally exists in three states: solid, liquid, and gas.

• Water is typically in a liquid state.

• Heating water converts it into gaseous steam; removing heat converts it into solid ice.

Key Heat Definitions

• Sensible Heat: The heat absorbed when heating water from its melting point (freezing point) to its boiling point.It can be measured with a thermometer, and no state change occurs in the heated substance.

• Latent Heat: The heat required to convert boiling water into steam.The substance changes state after absorbing this heat.

Figure 2 illustrates the temperature and heat changes when 1kg of ice at -40℃ is heated to steam at 130℃ under standard atmospheric pressure.

4. Impact of Pressure on Steam Properties

The aforementioned heat and state changes of water-to-steam conversion occur under standard atmospheric pressure.

However, saturation temperature of water changes with pressure—the total heat of saturated water (including sensible heat) and the latent heat of saturated steam both vary accordingly.

As shown in Figure 3:

• At 0.7MPa pressure, more heat and a higher temperature are required to boil water compared to atmospheric pressure.

• Conversely, less heat is needed to convert water into steam at higher pressures.

In summary:

With increasing pressure, sensible heat increases continuously while latent heat decreases gradually.

5. Key Term Explanations