Triple Expansion Steam Engine

Posted by PITHOCRATES - November 6th, 2013

Technology 101

Pressure and Temperature have a Direct Relationship while Pressure and Volume have an Inverse Relationship

For much of human existence we used our own muscles to push things.  Which limited the work we could do.  Early river transport were barges of low capacity that we pushed along with a pole.  We’d stand on the barge and place the pole into the water and into the river bed.  Then push the pole away from us.  To get the boat to move in the other direction.

In more developed areas we may have cleared a pathway alongside the river.  And pulled our boats with animal power.  Of course, none of this helped us cross an ocean.  Only sail did that.  Where we captured the wind in sails.  And the wind pushed our ships across the oceans.  Then we started to understand our environment more.  And noticed relationships between physical properties.  Such as the ideal gas law equation:

Pressure = (n X R X Temperature)/Volume

In a gas pressure is determined my multiplying together ‘n’ and ‘R’ and temperature then dividing this number by volume.  Where ‘n’ is the amount of moles of the gas.  And ‘R’ is the constant 8.3145 m3·Pa/(mol·K).  For our purposes you can ignore ‘n’ and ‘R’.  It’s the relationship between pressure, temperature and volume that we want to focus on.  Which we can see in the ideal gas law equation.  Pressure and temperature have a direct relationship.  That is, if one rises so does the other.  If one falls so does the other.  While pressure and volume have an inverse relationship.  If volume decreases pressure increases.  If volume increases pressure decreases.  These properties prove to be very useful.  Especially if we want to push things.

Once the Piston traveled its Full Stroke on a Locomotive the Spent Steam vented into the Atmosphere 

So what gas can produce a high pressure that we can make relatively easy?  Steam.  Which we can make simply by boiling water.  And if we can harness this steam in a fixed vessel the pressure will rise to become strong enough to push things for us.  Operating a boiler was a risky profession, though.  As a lot of boiler operators died when the steam they were producing rose beyond safe levels.  Causing the boiler to explode like a bomb.

Early locomotives would burn coal or wood to boil water into steam.  The steam pressure was so great that it would push a piston while at the same time moving a connecting rod connected to the locomotive’s wheel.  Once the piston traveled its full stroke the spent steam vented into the atmosphere.  Allowing the pressure of that steam to dissipate safely into the air.  Of course doing this required the locomotive to stop at water towers along the way to keep taking on fresh water to boil into steam. 

Not all steam engines vented their used steam (after it expanded and gave up its energy) into the atmosphere.  Most condensed the low-pressure, low-temperature steam back into water.  Piping it (i.e., the condensate) back to the boiler to boil again into steam.  By recycling the used steam back into water eliminated the need to have water available to feed into the boiler.  Reducing non-revenue weight in steam ships.  And making more room available for fuel to travel greater distances.  Or to carry more revenue-producing cargo.

The Triple Expansion Steam Engine reduced the Expansion and Temperature Drop in each Cylinder

Pressure pushes the pistons in the steam engine.  And by the ideal gas law equation we see that the higher the temperature the higher the pressure.  As well as the corollary.  The lower the temperature the lower the pressure.  And one other thing.  As the volume increases the temperature falls.  So as the pressure in the steam pushes the piston the volume inside the cylinder increases.  Which lowers the temperature of the steam.  And the temperature of the piston and cylinder walls.  So when fresh steam from the boiler flows into this cylinder the cooler temperature of the piston and cylinder walls will cool the temperature of the steam.  Condensing some of it.  Reducing the pressure of the steam.  Which will push the piston with less force.  Reducing the efficiency of the engine.

There was a way to improve the efficiency of the steam engine.  By reducing the temperature drop during expansion (i.e., when it moves the piston).  They did this by raising the temperature of the steam.  And breaking down the expansion phase into multiple parts.  Such as in the triple expansion steam engine.  Where steam from the boiler entered the first cylinder.  Which is the smallest cylinder.  After it pushed the piston the spent steam still had a lot of energy in it looking to expand further.  Which it did in the second cylinder.  As the exhaust port of the first cylinder is piped into the intake port of the second cylinder.

The second cylinder is bigger than the first cylinder.  For the steam entering this cylinder is a lower-pressure and lower-temperature steam than that entering the first cylinder.  And needs a larger area to push against to match the down-stroke force on the first piston.  After it pushes this piston there is still energy left in that steam looking to expand.  Which it did in the third and largest cylinder.  After it pushed the third piston this low-pressure and low-temperature steam flowed into the condenser.  Where cooling removed what energy (i.e., temperature above the boiling point of water) was left in it.  Turning it back into water again.  Which was then pumped back to the boiler.  To be boiled into steam again.

By restricting the amount of expansion in each cylinder the triple expansion steam engine reduced the amount the temperature fell in each cylinder.  Allowing more of the heat go into pushing the piston.  And less of it go into raising the temperature of the piston and cylinder walls.  Greatly increasing the efficiency of the engine.  Making it the dominant maritime engine during the era of steam.

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Phase Transition, Expansion Valve, Evaporator, Compressor, Condenser and Air Conditioning

Posted by PITHOCRATES - April 3rd, 2013

Technology 101

We can use Volume, Pressure and Temperature to change Water from a Liquid to a Gas and back Again

Liquids and gasses can do a lot of work for us.  If we can control three variables.  Volume.  Pressure.  And temperature.  For example, internal combustion engines work best when hot.  But excessive heat levels can damage the engine.  So we use a special anti-freeze/anti-boil liquid in the cooling system.  A pump circulates this liquid through the engine where it absorbs some of the excess heat of combustion that isn’t used in pushing the piston.  After leaving the engine it flows through a radiator.  Air blows across tubes in the radiator cooling this liquid.  Ejecting some of the heat of combustion into the atmosphere.  Lowering the temperature of the cooling liquid so it can flow through the engine again and absorb more heat.

Our first cars used alcohol in the winter for a lower freezing point.  So this liquid didn’t freeze in the engine and crack the block.  Letting the coolant flow out.  And with no cooling available the excessive heat levels would damage the engine.  In the summer time we used plain water in the cooling system.  And kept the cooling system sealed and under high pressure to prevent the water from boiling into steam.  But the high pressure often caused a hose or a radiator cap to fail.  Releasing the pressure.  And letting the cooling water boil out leaving the engine unsafe to operate.

If this happened on a hot summer’s day and you got a tow to a gas station you may have sat there waiting for them to complete the repairs.  Sipping on a cool bottle of soda from a refrigerated soda machine.  Soon drops of water would condense onto your cold bottle.  The cold bottle cooled the water in gas form (the humidity in the air) and turned it back into a liquid.  So in these examples we see how we were able to use pressure to keep water a liquid.  And how removing heat from water as a gas changed it back into a liquid.  This phase transition of a material has some very useful applications.

The High-Pressure Refrigerant Liquid from the Condenser loses Pressure going through the Expansion Valve

The phase transition between a liquid and a gas are particularly useful.  Because we can move liquids and gases in pipes and tubing.  Which allows us to take advantage of evaporation (going from a liquid to a gas) in one area.  While taking advantage of condensation (going from a gas to a liquid) in another area.  By changing pressure and volume we can absorb heat during evaporation.  And release heat during condensation.  Allowing us to absorb heat inside a building with evaporation.  And release that heat outdoors with condensation.  All we need are a few additional components and we have air conditioning.  An expansion valve.  An evaporator.  A compressor.  A condenser.  A couple of fans.  And some miscellaneous control components.

We install the expansion valve and the evaporator inside our house.  Often installed inside the furnace.  And the compressor and the condenser outside of the house.  We interconnect the indoor and the outdoor units with tubing.  Inside this tubing is a refrigerant.  Which is a substance that transitions from liquid to a gas and back again at relatively low temperatures.  As the refrigerant moves from the evaporator to the condenser it is a gas.  As it moves from the condenser to the evaporator it is a liquid.  The transition between these stages occurs at the evaporator and the condenser.

The refrigerant leaves the condenser as a liquid under high pressure.  As it passes through the expansion valve the pressure drops.  By restricting the flow of the liquid refrigerant.  Think of a faucet at a kitchen sink.  If you open it all the way the water flowing in and the water flowing out are almost equal.  But if we just open the faucet a little we get only a small trickle of water out of the faucet.  And a pressure drop across the valve.  With the full force of city water pressure pushing to get out of the faucet.  And a low pressure trickle coming out of the faucet.

As the Warm Air blows across the Evaporator Coil any Humidity in the Air will condense on the Coil

As the liquid leaves the expansion valve at a lower pressure it enters the evaporator coil.  A fan blows the warm air inside of the house through the evaporator coil.  The heat in this air raises the temperature of the refrigerant.  And because of the lower pressure this heat readily boils the liquid into a gas.  That is, it evaporates.  Absorbing heat from the warm air as it does.  Cooling the air.  Which the fan blows throughout the ductwork of the house.

As the gas leaves the evaporator it travels through a tube to the condenser unit outside.  And enters a compressor.  Where an electric motor spins a crankshaft.  Attached to the crankshaft are two pistons.  As a piston moves down it pulls low pressure gas into the cylinder.  As the piston moves up it compresses this gas into a higher pressure.  As the pressure rises it applies more pressure on the spring holding the discharge valve closed.  When the pressure is great enough it forces open the valve.  And sends the high-pressure gas to the condenser coil.  Where a fan blows air through it lowering the temperature of the high pressure gas enough to return it to a liquid.  As it does it releases heat from the refrigerant into the atmosphere.  Cooling the refrigerant.  As the liquid leaves the condenser it flows to the expansion valve to repeat the cycle.  Over and over again until the temperature inside the house falls below the setting on the thermostat.  Shutting the system down.  Until the temperature rises high enough to turn it back on.  A window air conditioner works the same way.  Only they package all of the components together into one unit.

There is one other liquid in an air conditioning system.  Water.  As the warm air blows across the evaporator coil any humidity in the air will condense on the coil.  Like on a cold bottle of soda on a hot summer day.  As this water condenses on the evaporator coil is eventually drips off into a pan with a drain line.  If the evaporator is in the furnace this line will likely run to a sewer.  If the evaporator is in the attic this line will run to the exterior of the house.  Perhaps draining into a gutter.  If it’s a window unit this line runs to the exterior side of the unit.  These simple components working together give us a cool and dehumidified house to live in.  No matter how hot and humid it gets outside.

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Thrust, Drag, Lift, Weight, Concorde, Center of Pressure, Center of Gravity, Boeing 747, Slats and Flaps

Posted by PITHOCRATES - January 16th, 2013

Technology 101

The Drawback to increasing Thrust and Lift with more Powerful Engines is the Weight of Greater Fuel Loads

To get an airplane off of the ground requires two things.  To produce thrust that is greater than drag.  And to produce lift that is greater than weight.  You do this and you’ll get any airplane off of the ground.  Of course, getting these two things is not the easiest thing to do.  Primarily because of the purpose of airplanes.  To move people and freight.  People and freight add weight.  Which increases the amount of lift needed.  And they make the plane bigger.  A bigger object displaces more air increasing drag.  And thus requiring more thrust.

Engines provide thrust.  And wings provide lift.  So the obvious solution to overcome greater drag is to produce greater thrust.  And the solution to overcome greater weight is to produce greater lift.  And we do both with fuel.  Greater amounts of fuel can power bigger engines that can produce more thrust.  And larger wings can produce greater lift.  But larger wings also produce more drag.  Requiring additional thrust.  And fuel.  Or, we can produce greater lift by moving air over the wings faster.  Also requiring additional thrust.  And fuel.

Of course, the obvious drawback to increasing both thrust and lift is the added weight of the fuel.  The more fuel carried the more weight lift has to overcome.  Requiring more powerful engines.  Or bigger wings.  Both of which require more fuel.  This is why our first planes were small by today’s standards.  The thrust of a propeller engine could not produce enough thrust to travel at high speeds.  Or operate at high altitudes.  And the first wings were relatively fixed.  Having the same surface area to produce lift at takeoffs and landings.  As well as at cruising altitudes.  Big wings that allowed the lifting of heavier weights produced a lot of drag.  Requiring more fuel to overcome that drag.  And the added weight of that fuel limited the number of people and freight they could carry.  Or they could trade off that fuel for more revenue weight.  The smaller fuel load, of course, reduced flying times.  Requiring an additional takeoff and landing or two to refuel.

A Wing that produces sufficient Lift at 600 MPH does not produce sufficient Lift at Takeoff and Landing Speeds

The supersonic Concorde was basically a flying gas can.  It was more missile than plane.  To travel at those great speeds required a very small cross section to reduce drag.  Limiting the Concorde to about 100 revenue paying passengers.  Its delta wing performed well at supersonic flight but required a drooping nose so the pilot could see over it to land and takeoff due to the extreme nose pitched up attitude.  As Concorde approached supersonic speeds the center of pressure moved aft.  Placing the center of gravity forward of the center of pressure.  Causing the nose to pitch down.  You correct this with trim controls on slower flying aircraft.  But using this on Concorde would create additional drag.  So they trimmed Concorde by pumping the remaining fuel to other fuel tanks to move the center of gravity to the center of pressure.

They designed Concorde to fly fast.  Which came at a cost.  They can only carry 100 revenue paying passengers.  So they can only divide the fuel cost between those 100 passengers.  Whereas a Boeing 747 could seat anywhere around 500 passengers.  Which meant you could charge less per passenger ticket while still earning more revenue than on Concorde.  Which is why the Boeing 747 ruled the skies for decades.  While Concorde flies no more.  And the only serious competition for the Boeing 747 is the Airbus A380.  Which can carry even more revenue paying passengers.  How do they do this?  To fly greater amount of people and freight than both piston-engine and supersonic aircraft?  While being more profitable than both?  By making compromises between thrust and drag.  And lift and weight.

Jet engines can produce more thrust than piston engines.  And can operate at higher altitudes.  Allowing aircraft to take advantage of thinner air to produce less drag.  Achieving speeds approaching 600 mph.  Not Concorde speeds.  But faster than every other mode of travel.  To travel at those speeds, though, requires a cleaner wing.  Something closer to Concorde than, say, a DC-3.  Something thinner and flatter than earlier wings.  But a wing that produces lift at 600 mph does not produce enough lift at takeoff and landing speeds.

Planes need more Runway on Hot and Humid Days than they do on Cool and Dry Days

The other big development in air travel (the first being the jet engine) are wings that can change shape.  Wings you can configure to have more surface area and a greater curve for low-speed flying (greater lift but greater drag).  And configure to have less surface area and a lesser curve for high-speed flying (less lift but less drag).  We do this with leading-edge slats (wing extensions at the leading edge of the wing).  And trailing-edge flaps (wing extensions at the trailing edge of the wing).  When fully extended they increase the surface area of the wing.  And add curvature at the leading and trailing edge of the wing.  Creating the maximum amount of lift.  As well as the greatest amount of drag.  Allowing a wing to produce sufficient lift at takeoff speeds (about 200 mph).  Once airborne the plane continues to increase its speed.  As it does they retract the slats and flaps.  As the wing can produce sufficient lift at higher speeds without the slats and flaps extended.

But there are limits to what powerful jet engines and slats/flaps can do.  A wing produces lift by having a high pressure under the wing pushing up.  And a low pressure on top of the wing pulling it up.  The amount of air passing over/under the wing determines the amount of lift.  As does the density of that air.  The more dense the air the more lift.  The thinner the air the less lift.  Which is why planes need less runway on a cold winter’s day than on a hot and humid summer’s day.  If you watch a weather report you’ll notice that clear days are associated with a high pressure.  And storms are associated with a low pressure.  When a storm approaches meteorologists will note the barometer is falling.  Meaning the air is getting thinner.  When the air is thinner there are fewer air molecules to pass over the wing surface.  Which is why planes need more runway on hot and humid days.  To travel faster to produce the same amount of lift they can get at slower speeds on days cooler and dryer.

For the same reason planes taking off at higher elevations need more runway than they do at lower elevations.  Either that or they will have to reduce takeoff weight.  They don’t throw people or their baggage off of the airplane.  They just reduce the fuel load.  Of course, by reducing the fuel load a plane will not be able to reach its destination without landing and refueling.  Increasing costs (airport and fuel expenses for an additional takeoff and landing).  And increasing flying time.  Which hurts the economics of flying a plane like a Boeing 747.  A plane that can transport a lot of people over great distances at a low per-person cost.  Adding an additional takeoff and landing for refueling adds a lot of cost.  Reducing the profitability of that flight.  Not as bad as a normal Concorde flight.  But not as good as a normal Boeing 747 flight.

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