Steam Properties and Qualities: A Primer

When the laws and characteristics of steam are respected, fewer operating problems occur and unnecessary costs are minimized.

Four Types of Steam

Steam is an invisible gas created by adding heat energy to water. It is liquid water changed to its gaseous state.

  • Saturated steam – steam in immediate contact with the water from which it is being generated. If the pressure remains constant, any loss of heat will result in condensation.
  • Dry saturated steam – if more heat is added to saturated steam at a constant pressure, stopping only when all of the water is converted to steam, dry saturated steam is produced.
  • Superheated steam – If more heat is added to dry saturated steam at a constant pressure, increasing its temperature and specific volume, superheated steam is produced. Heat must be lost and temperature reduced before condensation occurs.
  • Flash steam – when condensate, at saturation temperature and pressure, is discharged into a region of lower pressure, it automatically adjusts to the saturated conditions at the lower pressure. In effect, some of the condensate is “re-evaporated” into steam.

Properties of SteamSteam

The physical phenomenon of converting water from a liquid to a vapor (steam) is the same, whether it occurs on a stove with a tea kettle, in a small package boiler, or in a the most sophisticated high-pressure boiler in a power plant. Steam obeys a precise, well defined, documented set of physical laws.

Two atoms of hydrogen and one atom of oxygen make up one molecule of water (H2O). The energy that the molecules of any substance possess is of two forms: Kinetic energy and potential energy.

When energy is added to a substance under conditions which do not change its state, the energy takes the form of motion (kinetic energy); and an increase in motion of molecules results in an increase in temperature. When energy is added to a substance under conditions which do change its state, the energy takes the form of change of position of the molecules (potential energy), and results in a change of state without any change in temperature.

In the case of steam, sensible heat has kinetic energy and latent heat has potential energy.

Qualities of Steam

  • Steam is clean, odorless, and tasteless
  • Steam is a highly useable heat content
  • Steam gives up heat at a constant temperature
  • Steam heat can be used over in the form of flash steam
  • Steam is easily distributed and controlled
  • Steam’s constant characteristics mean performance is repeatable under similar conditions

Calculating Flash Loss

The following example illustrates the dramatic savings in energy and dollars possible when the flash losses are eliminated from a system vented to atmosphere:

Operating Conditions

A             50 PSI flashing to 0 PSI

B            10,000 lbs/hr

F            Cost of steam, $5.00 per 1000 lbs.

G            Specific volume of make-up water 0.120 gal/lb (40° – 60° F)

From the “Properties of Saturated Steam” Table

C            Heat of liquid at initial pressure          267 BTU/lb

D            Heat of liquid at final pressure            180 BTU/lb

E            Latent heat at final pressure                 970 BTU/lb

 

Estimated Flash Loss Calculation

1.       (C-D)/E x 100 = % flash loss

(267-180)/970 x 100 = 9% flash loss

2.       B x % flash loss = lbs/hr loss

10,000 x 9/100 = 900 lbs/hr loss

3.       Lbs/hr x 24 = lbs/day loss

900 x 24 = 21,600 lbs/day loss

4.       Lbs/day x 365 = lbs/yr loss

21,600 x 365 = 7,884,000 lbs/yr loss

Estimated Dollar Loss

5.       Lbs/yr x (F) = $loss/yr

7,884,000 x $5.00 /1000 = $39,425.00 loss/yr

Estimated Energy Loss

6.       Lbs/yr loss x (D+E) = BTU loss/yr

7,884,000 x (180 + 970) = 9,066,600,000 BTU loss/yr

Estimated Water Loss

7.       Lbs/yr loss x G = gal/yr make-up

7,884,000 x 0.120 = 946,080 gal/yr

                                                           

Limiting energy loss by providing a system for recovering flash steam can result in considerable savings.

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