Basic Steam Cycle – Generation

basic steam cycle steam generation

Typical boiler doing work.

To make it easier to understand steam production or generation, you should know what happens to the steam both inside, and after it, leaves the boiler. There are basically four phases or sections of steam production. These four phases of a main steam system are generation, expansion, condensation, and feed. Over time, we will discuss them all.

This post will cover the first in that list of four – steam generation.

As you go through, do your best to try and tie in the other concepts and ideas we’ve talked about – heat and energy transfers, the laws of the conservation of energy, the laws of gases, etc. It’s all related.

Probably the best way to learn how the steam plant or system in your building, facility, or home operates is to trace the path of steam and water throughout its entire cycle of operation. In each phase of a basic steam cycle, the water and steam flow through the entire system without any exposure to the atmosphere.


Generation of Steam

combustion in a boiler

We need the fire triangle for combustion inside the boiler right?

The first energy transformation happens inside the boiler when the fuel burns. Through combustion, which you can read about here, the chemical energy stored in the molecules and molecular bonds of the fuel is transformed into thermal energy.

Thermal energy flows from the burning fuel to the water and creates our precious steam. The thermal energy is now stored as internal energy in the steam.

How do we know this? From the increase we see in our steam pressure and temperature.

When a liquid boils, it generates a vapor. Some or all of the liquid changes its physical state from liquid to gas (or a vapor.)

This is an important concept in steam production, and fluid theory:

  • As long as the vapor is in contact with the liquid from which it is being generated, it remains at the same temperature as the boiling liquid.

In this condition, the liquid and its vapors are in equilibrium contact with each other.

The temperature at which a boiling liquid and its vapors may exist in equilibrium contact depends on the pressure under which the process takes place. As the pressure increases, the boiling temperature increases. As the pressure decreases, the boiling temperature decreases. Figuring out or determining the boiling point depends on the pressure at which this is all happening or operating.

As a liquid is boiling and generating vapor, the liquid is a saturated liquid and the vapor is a saturated vapor. The temperature at which a liquid boils under a given pressure is the saturation temperature and the corresponding pressure is the saturation pressure.

Each pressure has a corresponding saturation temperature, and each temperature has a corresponding saturation pressure. You should be able to convert or calculate between the two. Here are a couple examples of the saturation pressures and temperatures for water:

Pressure
(psi absolute)
Temperature
(degrees F)
11
198
14.7
212
110
335
630
567
3206.2
705.4

From our earlier fundamentals module on pressure, we know that atmospheric pressure is 14.7 psia at sea level and decreases as we get higher (greater altitude.)

Have you ever read cooking directions on the back of a package before? I’m certain then that you have seen the alternate directions for cooking the food at higher altitudes. This post should be helping to explain why the differences.

Boiling water on top of Mt. Everest takes a longer than at sea level (where we are at 14.7 psia.) Why?

Remember, temperature and pressure are basically indications of internal energy that we can measure and read. Since we cannot raise the temperature of boiling water above the saturation temperature for that pressure, the internal energy available for boiling water is less at higher altitudes than at sea level. For your knowledge, the atmospheric pressure at the top of Mt. Everest is 1/3 that of sea level, about 5 psia then.

Now, using this thinking in reverse, why does a pressure cooker cook faster than an open pan?



Hold Your Horses

It’s not all easy and straightforward though. Something happens to water and its steam when it hits an absolute pressure of around 3200 psia and its corresponding saturation temperature at 705F, which is why it’s in the saturation pressures example above.

This is called the critical point. The vapor and liquid are indistinguishable. No change of state occurs when pressure increases above this point or when heat is added.

At the critical point, we no longer refer to the products as water or steam. At this pressure and temperature you cannot tell the water and steam apart. Instead, the substance or fluid is called a working substance.

Side note – I have no idea how you’d even begin to tell. What, am I going to melt off my entire face at 3200 psia and 700F. I’ll just take the scientists word for it. They’re smart.

Supercritical

steam superheat

View from the top of a reactor compartment.

Nuclear themed movies back in the 80’s would sometimes have reactors that went supercritical. Boilers that have been designed to operate at pressures and temperatures above the critical point are called supercritical boilers. Supercritical boilers are used in stationary steam power plants and I have never seen one.

Plus if your belt doesn’t match your socks, these boilers become ultra, hyper critical. Ha ha.

If we generate steam by boiling water in an open pan at atmospheric pressure, 14.7 psia, the water and steam that is in direct contact with the water will remain at 212°F until all of the water has evaporated.

Pressure-Temperature Relationship

If we fit a perfectly sized cover to the pan so that no steam can escape while we keep adding heat, both the pressure and temperature inside the vessel (pan) will rise. The steam and water will both increase in temperature and pressure, and each fluid will be at the same temperature and pressure as the other.

A boiler is neither an open vessel nor a closed vessel while operating. It is a pressure vessel designed with specific openings that allow steam to escape at a uniform rate while feed water is brought in at a uniform rate. Steam generation takes place in the boiler at constant pressure and constant temperature, minus the fluctuations. Fluctuations in constant pressure and constant temperature are caused by changes in steam demands. Like a hot water heater calling for more steam to raise hot water supply temperature to a pre-determined set point.

We cannot raise the temperature of the steam in the steam drum above the temperature of the water from which it is being generated until the steam is removed from contact with the water inside the steam drum and then heated.

Superheat
heat transfer equation

A heat transfer equation.

Steam that has been heated above its saturation temperature at a given pressure is called superheated steam.

The vessel in which the saturated steam is superheated is a superheater. The amount by which the temperature of superheated steam exceeds the temperature of saturated steam at the same pressure is the degree of superheat.

The primary advantage is that superheating steam provides a greater temperature differential between the boiler and the condenser. This allows more heat to be converted to work.

Another advantage is that superheated steam is dry and therefore causes relatively little corrosion of machinery and piping. Also, superheated steam does not conduct or lose heat as rapidly as saturated steam. The increased efficiency we see from using superheated steam reduces the amount of fuel needed to generate each pound mass of steam. This also reduces the space and weight requirements of the boilers. Incredible isn’t it?

Conclusion

This massive, overwhelming post has been brought to you by steam generation. It is the first phase of the four in the basic steam cycle that we are going to cover.

Important things that you take away from this are concepts like saturation temperature and pressure and the relationship between the two. Also keep thinking about the internal energy of a substance like water and how we use pressure and temperature as indicators of this energy. I wonder how the internal energy of gasoline differs from water. What do you think?

Take a break and then come back ready to push ahead into the next stage of the basic steam cycle – expansion. Is that where steam does work? Hmmm.


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