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Air
Air as Insulation
As we learned before, air is an excellent insulator. It just doesn't want to accept or give up heat readily. Think of thermal glass windows. They're based on the idea that trapped air between the panes will insulate your home from the weather outside. Fiberglass insulation is another good example. It's not the fuzzy stuff keeping out the heat and the cold. It's the air caught in the loft of the fuzzy stuff.
Are you getting the point here? Air is a good insulator. In fact, a film of air only .01 inches (.025 cm) thick has the same resistance to heat transfer as .02 inches (.05 cm) of water...or 15.5 inches (39 cm) of steel...or 11 FEET (3.4 meters) of copper.
Air in Steam Systems
In a steam system, the normal flow of steam toward the heat exchanger carries air and other gases along with it. Since these gases do not condense and drain by the natural pull of gravity, they create a barrier between the steam and the heat exchanger surface. Air makes a terrific insulator, preventing the transfer of heat.
Temperature Reduction Due to Air
Dalton's Law States:
The pressure of a mixture of gases is equal to the sum of all the partial pressures. Also, the partial pressure of each component is proportional to the volume fraction of that component.
In a steam space occupied by a mixture of steam and air, temperature and heat are less than if the space were occupied by steam alone.
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In a space containing 100 psia (6.8 bar)of saturated steam, we would expect a temperature of 328° F (164° C). In that same space containing a mixture of 30% air and 70% steam, the temperature will be 303° F (151° C). The pressure of the steam alone affects the temperature. Therefore, we know that we'll only realize maximum heat and temperature in a space occupied by 100% steam. |
Air Binding
When non-condensable gases (air) are allowed to accumulate, they may fill the heat exchanger with gases and stop the steam altogether. Many devices that drain condensate open to drain water and close when they sense gas. Unfortunately, these devices can't tell the difference between air and steam. So, when they sense a significant accumulation of gas, they close, lock, bind until the air is removed.
To see a synopsis for the video "The Air in the Steam," click here.
