COZBY ENTERPRISES, INC.

Superheat and Reheat and Pressure
John Cozby       June 2012

     Because superheated steam and reheated steam are so important to high fuel efficiency in advanced Rankine cycle steam engines a simple brief discussion of the subject is in order.  Reference to the Flow Diagram will be helpful.

     Water is found in three states: solid (ice), liquid (water), and vapor (steam).  When enough heat energy is added to the solid state it becomes liquid.  When enough heat energy is added to the liquid state it becomes vapor.  Steam engines are sometimes referred to as vapor engines.  Conversely, when enough heat energy is removed from steam it becomes water, and when enough heat energy is removed from water it becomes ice.  Liquid water at 212 °F at atmospheric pressure at sea level is only hot water.  If enough heat energy is then added to the hot water it becomes steam.  The additional heat energy which must be added to produce steam is called the latent heat of evaporation.  A boiling kettle on a stove contains both hot water and steam.  The steam coming out of the kettle spout is clear vapor.  A short distance from the spout a cloud is visible.  This cloud is made up of liquid and vapor.  What is seen are tiny water droplets that are cooling and condensing back into liquid.  As the pressure of the steam increases so does its temperature.  If the kettle were strong enough with a tight lid, and the spout were strong enough with a closed valve, high pressure steam could be produced in the kettle.  If this high pressure steam in the kettle is connected to a steam engine, the engine can then do useful work.  (This type of “kettle” is then usually referred to as a boiler or a steam generator.)  As the steam enters the engine cylinder the steam pushes on the piston  forcing the crankshaft to turn.  The steam expands producing work.  As the steam expands it cools.  There is an exact relationship between the amount of work produced and the cooling of the steam by expansion.  If the steam expands enough in the engine, the cooling due to expansion will cause some of the steam to begin to condense back to liquid.  A steam engine is sometimes referred to as an expander.  If the steam is then simply exhausted from the engine the steam cools and condenses back into liquid water.

     The hotter the steam is, the more it can expand and the more useful work it can do before it begins to condense.  In the illustration of the kettle spout something interesting can be done.  If a long metal tube is connected to the spout and the tubing is in a hot fire the steam in the tubing can be heated a lot more.  This hotter steam is then called superheated steam.  The temperature of the superheated steam can be raised several hundred degrees Fahrenheit.  That much additional heat is referred to as high superheat.  High initial pressure and high super heat with great expansion in the cylinders are necessary for the best efficiency in a steam engine.

     Steam that is superheated is called dry steam because there are no tiny liquid water droplets in the steam.  Steam that has some tiny liquid water droplets in the steam is called wet steam.  The point at which steam goes from dry steam to wet steam is called the saturation point.  The heat of dry steam is above the saturation point, while wet steam is below the saturation point.
 
      The term supercritical steam applies to the steam pressure.  Basically, steam that is above 3,208.2 pounds per square inch pressure is supercritical.  The term ultra-critical is sometimes applied to pressures above 5,000 pounds per square inch.  A three stage advanced Rankine cycle steam engine is well suited to subcritical and supercritical pressures.  A four stage advanced Rankine cycle steam engine is better suited to high supercritical  and ultra-critical steam pressures.  For best efficiency, the pressure of the exhaust steam from the final stage cylinder is well below atmospheric pressure.  The final exhaust is then condensed in the condenser back to liquid water and retained in the system for reuse.  The condenser is similar to an automobile radiator and uses air to cool the steam back to liquid water. The steam condenser pressure is maintained as a vacuum. Initial steam pressures to 5,437 pounds per square inch are in use.  Steam temperatures to 1,500 degrees Fahrenheit are in use.  With high pressure and high temperature, efficiencies of 55% are presently being realized, compared to around 20% for most automotive gasoline engines.

     Even very hot, high superheat, steam can only expand so far before it begins to have condensate, that is, before it is below it’s saturation point.  This is where the next stage comes into play.  The hot, high pressure steam is allowed to expand in the first stage cylinder to almost its saturation point.  The slightly superheated steam is then exhausted into a long metal tube which connects to a larger second stage cylinder.  The long steel tubing connecting the two stages is called a receiver / reheater because it receives the steam from the first stage cylinder and delivers the steam to the second stage cylinder.  The receiver / reheater takes the exhaust steam from the first stage cylinder through the hot furnace where the steam is highly reheated.  The reheated steam can then expand to a much greater degree, doing more useful work, before it reaches its saturation point.  Depending upon the initial pressure, the heat and reheats, and the degree of expansion –  three or four stages can be utilized.  That is why advanced Rankine cycle steam engines can be very efficient.  It is a practical and proven approach that is demonstrated in industry and has been in use for many years.

     The real issue in the ERDA Report, “An Assessment of the Technology of Rankine Engines For Automobiles”, was between simple single stage uniflow steam engines which could not achieve good efficiency and the re-heat advanced Rankine cycle engine which can achieve very good efficiency — even with only one re-heat.  The Cozby designs call for two or three re-heats.  There is NO LOSS in the receiver(s) of re-heat engines.  There IS an EFFICIENCY GAIN from the additional heat added to the steam in the receiver/re-heater(s) of re-heat engines.  This is because of two reasons: (1) the range of the expansion ratio is greatly extended, and (2) because the vapor which is re-heated does not go through the loss of latent heat of vaporization from liquid to vapor.

     The terms, counter-flow and multi-stage, can be misleading and confusing.  A steam engine can be counter-flow and multi-stage without having re-heating.  A re-heat steam engine must be a counter-flow and multi-stage engine as well as re-heating .  Lacking greater definition and clarity leads to confusion.  When speaking of “advanced” Rankine steam engines I am referring to re-heat engines.  A better way to describe such steam engines could be according to re-heat.  A single stage engine has no re-heat, such as the uniflow.  A single re-heat engine has two stages and is referred to as a compound.  A two re-heat engine has three stages and is referred to as a triple.  A three re-heat engine has four stages and is referred to as a quadruple.  We should refer to our designs as the two re-heat engine and the three re-heat engine for more precise definition. 

BOILER
by John Cozby, Oct. 13, 2012 

     Steam is made in the furnace/boiler section.  The type of boiler that is used in high pressure, high temperature applications requiring fast start-up time goes by many names.  This can be confusing, but all of the following names are describing basically the same type of boiler. 

     A monotube boiler: The boiler is a single length of tubing from beginning to end that is heated by the furnace gasses.  The exiting steam is super-heated. 

     A once through boiler: The water, becoming super-heated steam, makes a single pass through the boiler. 

     A safety boiler: There is only a small quantity of water in the boiler and the boiler is not subject to “blowing-up” and releasing large quantities of steam.  The monotube boiler is the safest type of boiler and it is especially applicable to high pressure and highly super-heated steam. 

     A flash boiler: The water quickly flashes to steam and is super-heated in the monotube.  Some have referred to this type of boiler as “flash power”. 

     A steam generator: Steam is made, or generated, and super-heated in the tubing. 

     A single tube boiler: The same meaning as monotube.

     A water tube boiler: The water is only in the tubing and not in a drum.  Some boilers are called fire tube boilers.  In fire tube boilers the furnace gasses pass through tubes.  These tube bundles are inside a drum containing large quantities of water.  Low pressure locomotive boilers were of this type.  Water tube boilers are uniquely conducive to super-heating high pressure steam. 

     Sometimes these terms will be used in combination such as, “a monotube steam generator”.

     The “White” and “Doble” steam cars used flash boilers and were some of the best steam automobiles.