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Sunday, August 18, 2013

The steam cycle

I have been meaning to do a post on the steam cycle, otherwise known as the Rankine Cycle. It's pretty simple to describe the four-step process. The complicated part is designing, maintaining, and operating the machinery that makes it happen continuously, year after year.

The water/steam cycle is the most practical and commonly used means of converting heat energy into electrical energy.  The process works like this:  Fuel is burned for heat.  The heat vaporizes a liquid (typically very pure water), which is then expanded through a steam turbine.  Lastly it is condensed at very low pressure to extract all possible thermal energy and also to re-use the pure water. 

Not all power plants use water as an operating fluid.  In one case, a power plant was built using a mercury vapor turbine!  Currently there are a few power plants that use organic liquids such as HCFCs and ordinary flammable hydrocarbons (Iso-Butane) as a working fluid.  Organic fluids are used where the heat source is below the boiling point of water and/or corrosive, as in geothermal power.

Back to the Steam Rankine Cycle though...  Below is a diagram showing the water and steam flow through a simple thermal power plant.  The four steps of the steam cycle occur between the large numbers.

Beginning at number 1, the physical location in the power plant is the main condenser hotwell, where very pure water has collected at the bottom of the main condenser.

The first step of the process occurs between 1 and 2.  This is where pressure in the water is increased.  The W with the little dot above it indicates that power is being added to the system.   This power is required by electric motors which drive high pressure pumps.

Physically in the power plant, this is a two-stage process.  Water is pumped out of the hotwell with a condensate pump.  The discharge pressure on the condensate pump is about 300 psi.  The water is pumped up in elevation to a de-aerator, which uses steam to scrub oxygen (a corrosive contaminant) from the liquid.  A boiler feedwater pump takes suction from the de-aerator and pressurizes the water further, to around 2500 psi, so that it has enough pressure to enter the boiler.

A Small Boiler Feedwater Pump

The second step of the process occurs between 2 and 3.  This step is where heat is added, and the water is turned into steam.  The Qin with the little dot indicates that heat is flowing into the system.   This heat can come from any number of sources - Nuclear, Coal, Trash, Distillate Oil, Direct fired Natural Gas, or waste heat from a Gas Turbine, to name the major ones.

Physically in the power plant, this step occurs insde a boiler.  Water is preheated before entering the boiling section.  This is done by using exhaust gas to heat water inside an economizer, usually toward the back of the boiler, where most of the heat in the exhaust gas has already been spent boiling water.  This helps to recover as much heat as possible from the exhaust gas, improving fuel efficiency.

After this preheating, the water pressure is reduced, and water is allowed to boil inside tubes.  Within the tubes, the water boils, and rises as a blend of both steam and water.  This steam/water mixture enters a steam drum, which separates the water and steam - and (importantly) allows us to control the water level inside the boiler.  The separated water is returned to the boiler tubes to be boiled again.  The steam is dried out and sent to a superheater, and eventually to the steam turbine.

Below, a small coal-fired boiler.  The steam drum is the round silvery object near the top.  The silver pipes attached to the bottom of the steam drum are where the separated water is returned to the bottom of the boiler.

The third step of the steam cycle occurs between 3 and 4.  This is where energy is extracted from the steam, and its pressure and temperature are converted to rotational energy, shown as "Wturbine", or power out.  Physically this process takes place within the steam turbine.  The generator is attached by a coupling to the steam turbine, and the generator converts the rotational energy to electrical energy.

Modern steam turbines are usually of the reaction type, where the rotating sections are shaped like nozzles.  As steam passes through the nozzles of the rotating sections, the reaction pushes the blades in the opposite direction - like when you squirt the garden hose and it pushes back against you.

In this post post I discuss Steam Turbine Designs in slightly better detail.

In the animation below, the non-moving sections are not causing the steam to push down on the moving sections.  Instead they align the steam to enter the moving sections at the correct angle of attack.  Motion is achieved because the moving nozzles squirt the steam out at higher velocity than when it enters them. 

Below is a simple reaction turbine.  The rotor is turned by the equal and opposite reaction produced by the steam jets. 

Below, a large nuclear plant steam turbine.

The final step in the steam cycle is condensation, which occurs between steps 4 and 1.  After passing through the turbine, the steam enters a condenser, which is a large vessel with cooling water tubes running through it.  The steam side of the condenser is under a vacuum to reduce backpressure on the turbine, which allows us to extract all the useful energy from the steam.  Steam exits the turbine at about 100 degrees, but it is still steam, due to the very low pressure. 

When the low pressure, low temperature steam comes in contact with the cooling tubes, it condenses into water.  This serves to maintain vacuum in the condenser, because water has 1/1000 the volume of an equal mass of steam.   Tiny amounts of air in-leakage are continuously removed by steam jet air ejectors or vacuum pumps. 

The heat extracted from the steam by the cooling tubes in the condenser is shown as Qout at the right side of the original diagram.  Physically this heat is removed to the cooling tower by low-purity Circulating Water, which is then partially evaporated to remove the heat from it.
Below, a diagram of a condenser.  Steam enters from the top, comes in contact with tubes carrying cool water, condenses outside the tubes and falls into the hotwell.  Low-purity Circulating Water enters on the lower left, exits on the lower right, and is sent to the cooling tower to remove the added heat.
Below, a small condenser.  Steam enters at top.  Cooling water inlet and outlet are at the near side.  The hotwell is the section between the support legs, where the condensate pump will take suction.

It is at this point that I want to mention the different types of heat.  First off, heat is not the same as temperature.  Heat is the movement of thermal energy from one place to another.  Temperature is a measure of the level of thermal energy in a substance.

Back to the power plant though...  There are two types of heat we are concerned with.  The first is "Sensible Heat".  We put a pan with 1 lb of 32 degree water on the stove and turn on the burner.  The temperature eventually rises to 212 degrees F.  This is called "sensible heat" - you heat the water and you can sense it getting warmer.

However, at 212 degrees, something changes.  The temperature stops rising.  At this point, the temperature cannot increase, because *much* more energy must be added to boil the water.  Even so, when the water eventually begins to boil, the temperature will not increase above 212 degrees.  This is odd characteristic is called "Latent Heat", and it is the energy that must be added to change water at 212 degrees to steam at 212 degrees.

The sensible heat required to bring one pound of water from 32 degrees to 212 degrees is only 180 BTU.  The *latent* heat required to convert a pound of water at 212 degrees to steam at 212 degrees is a whopping 970 BTU.  This is why steam has so much energy! 

This is also why you frequently see a steam plume coming from cooling towers wherever large quantities of steam are being condensed.  Let's talk for a moment about waste heat from the steam cycle:

To condense one pound of steam at 100 degrees F in a condenser to water at 100 degrees, (so that you can re-use that expensive pure water in your steam cycle again) you must remove that 970 BTU of latent heat.  Most power plants condense hundreds of thousands of  pounds of steam per hour, so a lot of heat has to be rejected into the environment, continuously. 

The most common technique is to evaporate a portion of the Circulating Water in a cooling tower.  By using evaporation, the water returning to the condenser can actually be cooler than ambient air temperature, increasing the efficiency of the steam cycle. 

Nothing here but steam coming from evaporating warm water...

Above is a natural draft cooling tower, which makes use of a chimney effect.  The 90+ degree water sprays out near the bottom of the cooling tower, warms the surrounding air, and the warm air/steam rises out the top, drawing in fresh cool air at the bottom, in a continuous operation.  This is a nice arrangement, because no fans are required.  Such a tall structure can easily support a continuous natural draft.

Smaller power plants use forced draft cooling towers, mainly because they are far less expensive to build than natural draft cooling towers.  The trade-off is that forced draft cooling towers require more maintenance, and the large fans require a substantial amount of power that would otherwise go to the electrical grid.
Below, a forced draft cooling tower.  Inside each shroud is a large fan to pull air upward.

Below, an inside view of a cooling tower fan.  Yes the man in the photo above is surrounded by 15 foot long blades whipping around behind 1/4 inch of fiberglass shroud.  Not a comfortable feeling :)

In water-challenged areas, such as the US SouthWest, wet cooling towers are no longer allowed to be used for power plant cooling.  A medium sized power plant (~ 500 MW) can burn upwards of 3000 gallons/minute of water on a hot, dry day.  That's a lot of useful drinking water to blow into the wind. 

When water use is an issue, an Air-Cooled Condenser is used, and so the power plant only uses a minute fraction of the water it would have needed.  An Air-Cooled Condenser is similar to a large car radiator, only it's condensing steam instead of circulating coolant to cool a motor.  The spent steam from the steam turbine is routed up on top of a large structure and split into several large headers.  Each header has multiple down-tubes which the steam can enter and condense in.  The water then drains back to a collection tank, and then returns to our steam system diagram as condensate.

Below, a side-profile of an Air-Cooled Condenser.

Air Cooled Condensers, while they save water, are not that desirable from an efficiency or cost standpoint.  They are expensive to purchase and to build, compared to a wet cooling tower.  

Air Cooled Condensers are enormous structures.  They have to be, because air cannot remove as much heat as evaporating water.  Air lacks mass and high thermal conductivity compared to water, so the cooling surface area needs to be much larger.  Furthermore, many more fans are required to produce the needed cooling air flow than would be needed to evaporate water.  This parasitic power loss reduces electrical efficiency, and therefore overall efficiency.

Lastly, because the steam turbine exhaust has to be routed into multiple paths, and the air-cooled condenser cannot reduce temperature below ambient, as the cooling tower can, the backpressure on the steam turbine is higher, further reducing steam cycle efficiency.  Thus for a given power output, more fuel must be burned.  

Below, an air cooled condenser.  30 fans hang underneath to force air up into the condensing section.

A final note about waste heat:  Nuclear plants (and geothermal) generate the most waste heat per Megawatt, and the reason is this:  Fossil-fired plants can superheat steam, and therefore a given pound of steam has more energy than a nuclear plant.  Nuclear plants cannot create superheated steam - they must use saturated steam.  So for the same output, a nuclear plant must boil (and condense) much more water than a conventional (superheated steam) power plant. 

Because more steam is being condensed in the nuclear (geothermal) plant than a fossil plant (for a given output), more heat must be removed.  This is why nuclear plants are frequently built near large bodies of water, so that they can use a "once through" cooling system in the main condenser, without having to vaporize massive amounts of drinkable water.

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