Steam

Steam is an interesting subject. Even more so after seeing the pictures of a demolished boiler during the October ISA presentation. Thanks Bob. I learned something.

Steam is a versatile and efficient way to transport energy. It is used to heat buildings, processes, and run turbines to generate electricity. There are two parts to the heat energy of steam. The first is the sensible heat. It is the heat used to bring the water up to the boiling point. In the steam tables that is the enthalpy. The second is the latent heat or heat of vaporization. It is the energy used to change hot water to steam at the same temperature. This energy is released as the steam condenses back into water or condensate; preferably at the other end of the pipe where the heat is needed.

At first glance it would seem that hot water would do the same thing. Saturated steam is the same temperature as hot water condensate but the amount of energy to vaporize the water is a lot more than the amount of energy to heat water to the boiling point. It takes 142 BTUs to heat one pound of water from 70 degF to 212 degF. It takes 970 BTUs to vaporize that water. At the destination the steam will release 970 BTUs of heat to the air or process while the hot water will only release 142 BTUs of heat.

But the above numbers are at atmospheric pressure. In order to transport something it must have pressure drop to push it to the location you need it. It is interesting that as the pressure goes up the sensible heat goes up but the latent heat goes down. That means it takes a lot more energy to heat up 200 psig water before it boils but it takes less heat to turn it from water to steam. This also means the 200 psig steam condensing to 200 psig condensate releases less heat than atmospheric steam. To use this to your advantage, transport steam at high pressure and reduce the pressure at the destination to release the most BTUs from the steam.

The temperature that water vaporizes to steam is set by the laws of physics and charted in steam tables and liquid-vapor phase diagrams. It seems counterintuitive that when you add more heat to a boiling liquid that the temperature stays the same. If you want to change the temperature of a boiling liquid you must change the pressure not the heat. This also goes for all boiling liquids like Freon in a refrigeration cycle or chemicals that change phase at ambient temperatures like ammonia.

Once all the water is converted to steam then the temperature can rise again. This is super heated steam. The advantages of superheated steam for certain applications is a discussion for another time. The disadvantage of letting all the water turn to steam is that the boiler tubes can heat to flame temperature and crack. When a tube fails then the pressure falls rapidly. Any high pressure hot water somewhere else in the boiler will be too hot for the liquid-vapor curve and will instantly turn to steam. Water expands 1600 times when it turns to steam - instantly. This will blow parts off the boiler with the force to knock down walls and send projectiles hundreds of feet.

The pictures were very impressive.

Until next month,
Grace MacMillen
Self proclaimed Automation Expert, CAP, and PE