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What is Steam?

Many issues of Info-Tec have discussed items used in steam systems such as valves, regulators, traps, controls, etc.  This Info-Tec will deal with steam itself.  Understanding steam, why and how it works, will help one to understand the devices used to control steam.

Just what is steam?

Steam is water in its gaseous state.  Enough heat has to be added to the water to raise the temperature of liquid water to its boiling point, and then more heat is added to cause a change of state to steam without an increase in temperature.

The amount of heat required to raise the water to boiling temperature is called sensible heat.  The amount of heat required to change the water to steam is called latent heat of vaporization.  The latent heat of vaporization is exactly the same as the “latent heat of condensation”.  This is the principal that steam systems utilize.  As we will see, this latent heat is the primary reason steam is used as a heat energy transfer medium.

In order to illustrate sensible and latent heat, we must recall the definition of a BTU (British Thermal Unit), a measure of an amount of heat.  A BTU is defined as the amount of heat needed to raise one pound of water one degree Fahrenheit.

Sensible heat is heat that can be readily sensed.  It can be felt, even “seen” by using a thermometer.  Latent heat is heat that is “there” but not readily perceptible. 

A simple experiment demonstrates sensible and latent heat.

Figure 1 shows a glass beaker containing one pound of water.  A thermometer can be put into the water.  The thermometer shows the water is at a room temperature of 70°F.  The beaker of water is placed over a burner and the burner is turned on.  The burner raises the temperature of the pound of water to 212°F.  This required 142 BTU’s.  212 - 70 = 142.  (Remember the definition of a BTU.)

Figure 1.

This 142 BTU’s is sensible heat.  We can “see” the heat added to the water by the burner as evidenced by the thermometer.  We could put our hand in the water and “feel” the heat that has been added; “sense” it.  (Not advisable.)

The continued addition of heat will cause the water to boil, but the thermometer will go no higher!  At atmospheric pressure, it will stay at 212°F!  How can this be?  The burner is still on.  We can see that heat is still being added to the water.  Where is all this additional heat going?

It is going into causing a change of state.  The water is being changed into steam.  This change of state requires a large amount of heat, far more heat than that needed to raise the temperature of the water from 70°F to 212°F.  An additional 970 BTU’s is needed to change a pound of water into a pound of steam at atmospheric pressure!

We cannot “see” this heat.  We cannot “feel” this heat, but it is there.  It is “latent” heat, hidden heat.  The exact term is “Latent Heat of Vaporization”.

Latent heat of vaporization is exactly the same as latent heat of condensation.  That is; if we condense the pound of steam at 212°F back into a pound of water at 212°F, we must extract 970 BTU’s from the steam.  This is why steam is so widely used.  The pound of steam containing a large amount of heat energy can be quickly and easily transported by a distribution system to remote locations where the energy can be recovered and put to useful work.

The boiling temperature of water is not constant.  Varying the pressure of the water can change its boiling point.  This requires a closed system so the pressure can be controlled.  Water can then be boiled at 50°F, to say, 500°F as easily as at 212°F.  The only thing necessary is to change the pressure above the water to one corresponding to the desired boiling point.

As an example, if the pressure in a boiler is raised to 52 psig. (67 psia.), the water will boil at 300°F.  Conversely, if the pressure were lowered to a vacuum of 29.6 inches of mercury, the water would boil at 40°F.

Changing the boiling point of water by varying the pressure results in other physical property changes.  Under atmospheric pressure, the latent heat of vaporization was 970 BTU’s per pound, but at 100 psig, it is 889 BTU’s per pound.

Steam tables showing the properties of steam are attached.  Table 1 and Table 2 are essentially the same, the difference being that Table 1 is a temperature table in Column 1, Table 2 is a pressure table in Column 1.  They work well together, since horizontal entries on table one fills the gaps on the other table.

Table 1.

Table 2.

If the latent heat of vaporization for steam at 240°F needed to be known, referring to Table 1, shows no 240°F line.  The entries are 212°F or 250°F.  Using Table 2, Column 2, the entry 240.07°F appears.  (This is showing water at 25 psia. boils at 240.07°F.)  The latent heat appears as 952.1 BTU’s per pound, Column 6.

Enthalpy

No discussion of steam is complete without mentioning enthalpy.  Enthalpy is total heat.  Enthalpy is a property of substances that is a measure of their heat content.  It is convenient for finding the amount of heat needed for certain processes.  From Table 1, total heat of steam at atmospheric pressure (0 psig. or 14.696 psia) is given as 1150.4 BTU per Lb.  This total is made up of two parts, sensible and latent heat.  The sensible heat raises the temperature of water from 32°F to 212°F, 180.07 BTU per Lb. (Column 6).  The latent heat of vaporization of water is at 212°F, 970.3 BTU per Lb. (Column 7).  The sum is 1150.4 BTU per Lb. (Column 8).  This information can be used to determine how much heat would be required to change water to steam at any temperature and pressure.  For instance, what amount of heat is required to change water at 70°F to steam at 250°F?  From Table 1, line 250°F, Column 8, the enthalpy of steam is 1164 BTU per Lb.  From Column 6, line 70°F, the enthalpy of water is 38.04 BTU per Lb.  1164 represents the total heat content of the steam, and 38.04 the heat content of the water at 70°F.  The difference, 1164 - 38.04, or 1125.96 BTU per Lb. is the amount of heat that must be added to the water at 70°F to change it into steam at 250°F.

Superheated Steam

Some mention should be made about superheated steam.

It is impossible to superheat steam in the presence of water because all the heat supplied will only evaporate the water.  As we saw in Figure 1, the temperature of the water will stay constant until all the water has boiled off.  Steam at the same temperature as the boiling water is “saturated” steam.  Superheated steam is steam at a higher temperature than boiling water under the same pressure.  Superheated steam is used mainly in the generation of power.  Turbines are more efficient, require less maintenance, and last longer running on superheated steam.  Usually, in commercial industrial heating and process work, we will be dealing with saturated steam.

(An interesting aside concerning air-conditioning is the fact that all moisture in atmospheric air exists as superheated steam at very low pressure.  The latent heat load of de-superheating this steam can comprise over 50% of an air-conditioner’s load.  In cooling a mixture of air and superheated steam, the steam is de-superheated until it reaches a point at which it condenses to water.  This point is called the “dew point”.  Actually, it is the condensing temperature of low-pressure steam.)

Steam is widely used.  Almost every plant will have one or more steam units in operation.  Figure 2 illustrates some of the uses in a typical plant.

Figure 2.

The steam generated in a boiler can be transferred to remote locations through piping systems to accomplish many useful tasks.  The higher pressure in the boiler pushes the steam to where it is needed, and while some losses occur in any distribution system, a carefully designed and insulated system will minimize this waste and deliver steam where it is intended to heat.  Here the same latent heat of vaporization now becomes the latent heat of condensation used to heat air, water, food cooking vessels, etc.

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