What is a steam table?
A steam table is a collection of experimental data on temperature, pressure, volume and the energy contained in water and steam. The data is organized in columns for readability. It can be based on a scale of either absolute or relative pressure. Relative (gauge) pressures are used in our steam table.
Utility of steam tables
Steam tables help us make the calculations required to make good use of steam in the various situations encountered in industries. Steam tables can also help diagnose problems.
Steam tables provide the temperature of steam if we know its pressure and inversely, they provide the pressure if we know the temperature. Consequently, one of the most common way to use a steam table is to infer the approximate steam pressure in a pipe after measuring its outer temperature with an infrared thermometer (infrared heat gun). This is extremely useful in a situation where there is no gauge on the pipe.
Steam tables also give the volume of water and steam at various temperatures and pressures, which is useful for correctly sizing equipments.
More importantly, steam tables give us the amount of energy used by water when it changes from liquid to steam (vaporization) and vice versa (condensation). Steam tables also provide information about energy changes in steam as it goes from one pressure to another or from one temperature to another. When we need steam to do work requiring a certain quantity of energy, we can find how many pounds of steam are required by referring to steam tables.
In short, steam tables contain information about the amount of energy exchanged, the relationship between temperature and pressure and the respective volumes of water and steam.
Origin of steam tables
The experimental data in steam tables have been measured in laboratories with precision instruments. They have been verified many times throughout the years and are therefore very reliable.
A very simple experience is used to build a steam table. In a closed device, in which a specific flow of water circulates, we introduce a known electric current. With this current, we know the exact amount of heat added and we simply record the pressure and temperature at which the system stabilizes. Figure 1 shows the experimental setup used to collect data for a steam table.
Units of steam tables
The units in our steam table are part of the English system. However, the metric system units for temperature and pressure are also given. Other steam tables will supply units from the metric system or a mix of units. No matter the units used, the same experimental values are used to build all steam tables and it is relatively easy to switch from one system to another by using the right formula.
In our steam table, the relative pressure is shown in both lb/in2 (psig) and kPa and they occupy the first two columns. The temperatures in degrees Fahrenheit (oF) and Celsius (oC) occupy the following two columns. The 5th column shows the latent heat of vaporization (the energy stored in steam after liquid water has reached saturation temperature) in Btu/lb. The 6th column shows the sensitive heat (the energy stored in water at saturation temperature) in Btu/lb. The 7th column shows the total energy (the total energy stored in the steam, including the sensitive heat and the latent heat of vaporization) in Btu/lb. The next to last column shows the volume of steam in ft3/lb while the last column shows the volume of water in ft3/lb.
Observations on steam tables
It is important to note that one and only one saturation temperature corresponds to each pressure. Conversely, one and only one pressure of saturation corresponds to each temperature. This data is valid for pure water steam. The presence of impurities will alter the values. Remember that the more the pressure rises, the more the boiling temperature increases.
As the pressure rises, the latent heat of vaporization of steam decreases while the sensible heat of water rises. At a pressure of approximately 1 300 psig, these two values are practically the same. At a pressure of 3 193 psig, the critical point is reached and the latent heat of vaporization is nil. Beyond the critical point, the steam is known as “supercritical“.
As we can see in figure 4, the total energy in one pound of water increases with pressure up to 300 psig and then starts to decrease. Total energy is the amount of energy required to make the water temperature rise until the water boils and vaporizes completely.
The volume of steam decreases as the pressure increases. This can be explained by the fact that pressure forces steam to occupy a smaller volume. At first, at low pressures, the decrease in volume is very important: the volume of steam goes from 26.8 pi3/lb at 0 psig to 4.87 pi3/lb at 75 psig.
There are important consequences to this variation in volume. For example, operating a boiler at low pressure induces carry-over in the pipes because the bubbles of steam in the boiler water are much bigger than at high pressure. When these big bubbles burst on the surface, they throw droplets of water that get carried over in steam pipes. Also, the greater volume of steam at low pressures implies that much bigger pipes are required for steam to circulate at the desired flow rate.
A similar problem happens when steam demand is very high, which causes the pressure inside the boiler to decrease. The sudden increase in boiling movement inflates the mass of water in the boiler, causing carry-over.
The volume of liquid water is very stable and hardly changes as pressure increases. This is why we say that water is incompressible, a characteristic used in hydraulic systems for heavy machinery. When we reach a pressure of 3 193 psig, the volumes of liquid and steam are equal. It is the critical point.
Note that the first values in the steam table are below the atmospheric pressure. In other words, it is a vacuum situation. The greater the vacuum, the lower the liquid boiling temperature. In a vacuum of 25 inches of water, water boils at the ambient temperature: 21oC. Obviously, boiling water at a temperature below freezing is impossible, since water is then solid.
The volume of steam increases very rapidly as the vacuum gets greater. Using steam under these conditions would require huge pipes. For example, vacuum steam heating used in the past offered good temperature control, but required enormous pipes.