Posted by PITHOCRATES - January 22nd, 2014

Technology 101

A Bridge is a Fixed Structure that requires no Active Systems to Function

Bridges are dumb.  While tunnels are smart.  You can build a bridge and walk away from it.  And it will still work.  That is, you can still cross the bridge without anyone at the bridge doing anything.  It can even work in a power outage.  Even at night.  It may be dark.  But a car’s headlights will let a person cross safely.  Because a bridge doesn’t have to do much for people to use it.  All it has to do is stand there.  A tunnel, on the other hand, needs smart systems to make the tunnel passable and safe.

Bridges are high in the air.  Where there is plenty of fresh air to breathe.  If there is a car fire on the bridge all of that fresh air will allow other drivers to breathe as they drive around it.  And for first responders to breathe as they put that fire out.  They can use all the water they bring onto the bridge, too.  Even in a driving downpour.  For that water will just run off of that bridge without causing a drowning hazard.  Visibility doesn’t change driving onto or off of the bridge.  Unlike with tunnels.  Where you can go from bright daylight into a dark hole.  And from a dark hole into bright daylight.

A bridge is a fixed structure that requires no active systems to function.  Just some maintenance.  Painting and roadway lighting.  Maybe some traffic control signals.  But that’s about it.  Tunnels, on the other hand, need machinery.  Equipment.  Systems.  And people.  Because tunneling below grade causes a whole host of problems.  Problems that have to be addressed with machinery, equipment and systems.  And if they don’t work people can die in a tunnel.

Powerful fans at each end of the tunnel pull in fresh air and blow it through the duct under the roadway

Cars have internal combustion engines.  They exhaust carbon monoxide after combustion.  Which is poisonous if we breathe it.  A big problem in tunnels filled with cars with internal combustion engines.  Which is why if you look at a cross-sectional view of a tunnel you will see that the biggest section of these underground structures are used for moving air.

If you have driven through a tunnel you probably remember driving through a rectangular tube.  Little bigger than the vehicles driving through it.  What you don’t see is the air duct beneath the roadway.  And the air duct above the roadway.  Powerful fans at each end of the tunnel pull in fresh air from the atmosphere and blow it through the duct under the roadway.  It exits the duct at about exhaust pipe level.  This fresh air blows into the rectangular tube where cars are pumping in carbon monoxide.

Other powerful fans are also located at each end of the tunnel that pull air out of the tunnel.  Via the duct over the roadway.  Fresh air comes in from below.  Mixes with the poisonous carbon monoxide.  This gets sucked into openings overhead.  Into the duct over the roadway.  And vents to the atmosphere at either end of the tunnel.  Allowing these poison-making machines to travel underground in an enclosed space without killing people.

A Tunnel is a Complex Machine that requires Intelligent Programming not to put People in Danger

Tunnels through mountains go through porous rock that drip water into the tunnel.  Tunnels under bodies of water are low in the middle and high at the ends.  Making each tunnel portal a massive storm drain when it rains.  And water in a tunnel is a dangerous thing.  It can freeze.  It can get deep.  It can cause an accident.  It can drown people.  So when it enters the tunnel you need to pump it out.  Tunnels have storm drains that drain any water entering the tunnel to a sump at a low point.  And pumps move this water from the sump out of the tunnel.

Ever spend an hour or so shoveling snow on a bright day?  And then go inside only to temporarily lose your vision?  This is snow blindness.  Your pupils shrink down to a tiny dot outside to block much of the bright sun and the light reflecting from the snow and ice. And when you walk inside that tiny dot of a pupil won’t let enough light into your eye so you can see in the reduced lighting level.  After awhile your pupils begin to dilate.  And you can see.  Same thing happens when driving into a tunnel.  Of course, temporarily losing your vision while driving a car can be dangerous.  So they add a lot of lights at the entrance of a tunnel.  To replicate sunlight.  And as you drive through the tunnel the lighting levels fall as your eyes adjust.  At night they reduce the lighting levels to prevent blinding drives as they enter.  And prevent snow blindness when exiting the tunnel.

A bridge doesn’t need any of these systems.  But a tunnel won’t work without them.  As people could die in these tunnels.  Because it’s dangerous when people go below grade with machines that create poison.  So tunnels need computers and control systems.  To monitor existing conditions such as exterior lighting levels, carbon monoxide levels, smoke and fire detection, water levels and high water alarms, etc.  Based on these inputs a control system (or a person) turns lights on or off, increase or decrease supply and exhaust fan speeds, pump down the sump when it reaches a high water level, etc.  Only when these systems are on line and operating properly is driving through a tunnel as safe as driving over a bridge.  Because bridges are dumb things.  They only need to stand there to work.  While a tunnel is a complex machine.  That requires intelligent programming not to put people in danger.



Tags: , , , , , , , , , , , , , , , , , , , , ,

Engineering Tradeoffs, Security System, Fire Alarm System and HVAC System

Posted by PITHOCRATES - March 13th, 2013

Technology 101

A Security System basically locks Doors while a Fire Alarm System unlocks Doors

Engineering is basically a study of compromise.  Of tradeoffs.  For solving one problem often creates another problem.  For example, boiling water creates steam.  And the pressure of steam is so strong that it can do useful work for us.  However, the pressure of steam is so strong that it can also blow up boilers.  Which was common in the early days of steam.  So we install pressure relief valves on boilers.  To safely dump excessive steam pressure.  So they don’t explode violently.

We want steam pressure to do work for us.  And the higher pressure the steam is the more work it can do for us.  But the higher the pressure the greater the chance for a catastrophic explosion.  So the engineering of steam systems is a tradeoff.  We design them to produce the maximum steam pressure that won’t blow up any part of the system.  Trading additional useful work for safety.

Then there are systems that come together with opposing design criteria.  Such as security and fire alarm systems.  A security system basically locks doors in a building.  Preventing the free passage of unauthorized people.  While a fire alarm system basically unlocks doors.  To allow the free passage of everyone.  Authorized and unauthorized.  For example, few people can get into the maternity area of a hospital.  Even the elevator won’t stop on that floor if you don’t have a security card to swipe in the elevator.  But if there is a fire in the building, all the secured doors will release to allow everyone to get out of the building.

If the Duct Smoke Detector detects Smoke it will Break the Safety Circuit and Shut Down the HVAC Unit

Interfacing the fire alarm system to HVAC systems require additional compromises.  The primary design criteria of a heating, ventilating and air conditioning unit (basically a big box with a supply fan and a return fan with filters, heating/cooling coils and air dampers to blend in a varying amount of outside air) is to move air.  To prevent the dangerous buildup of carbon dioxide from our exhaled breath.  They also cool buildings in the cooling season.  And help to heat the building in the heating season.  In addition to the floor-mounted perimeter hot-water heating system.  Located under most exterior windows.

Keeping the air moving helps to keep the air safe to breathe.  Which allows us to work safely within enclosed buildings.  But this moving air can be a problem if there is a fire in the building.  For in a fire it’s smoke inhalation that kills most people.  So if there is a fire someplace in a building you don’t want the HVAC system to blow that smoke throughout the building.  Especially in areas where there is no fire.  Which is why we interface the HVAC system to the fire alarm system.  When there is no fire alarm condition the HVAC system is free to operate to meet the HVAC design criteria.  Keeping dangerous levels of carbon dioxide from building up.  If there is a fire alarm condition the fire alarm system takes control of the HVAC system to meet the fire alarm system design criteria.  Preventing smoke from spreading throughout the building.  In exchange for a less dangerous buildup of carbon dioxide.  For in a fire alarm condition people will be leaving the building.  So they will be out of the building before any buildup of carbon dioxide can harm them.

Air moves through ductwork.  There is a supply-air duct system.  And a return-air duct system (or a ceiling plenum where all the airspace above the ceiling is the return-air pathway back to an HVAC unit).  They both terminate to an HVAC unit.  The return-air fan pulls air from the building and the supply-air fan blows air back into the building.  Located shortly downstream of an HVAC unit in the supply-air duct is a duct smoke detector.  We wire this into the safety circuit of the HVAC unit.  Which is basically a lot of switches wired in series.  They all have to close for the HVAC unit to start.  Such as the freeze-stat on the heating coil.  Which prevents the unit from blowing freezing air onto a cold heating coil to prevent the water from freezing and breaking the coil.  Also in the safety circuit are end-switches installed on the air dampers.  Which close when the unit isn’t running to prevent heated air from venting out and cold air from migrating in.  Before the fans start these damper have to open.  And once they fully open switches close in the safety circuit clearing these safeties.  Also in this safety circuit is the duct smoke detector.  When the duct smoke detector is powered it closes a set of contacts.  The duct smoke detector safety runs through these contacts.  When closed it clears this safety.  If there is smoke in this duct (or if the duct smoke detector loses power) this set of contacts opens.  Breaking the safety circuit.  And shuts down the HVAC unit.

Providing Smoke-Free Routes out of a Building gives People the best Chance of Surviving a Fire

HVAC units may feed more than one zone in a building.  And if the ductwork serving these units pass through a wall (i.e., a fire/smoke barrier) there will be a fire damper in the ductwork at this location.  Either one with a fusible link that melts in a fire.  And when it melts energy stored in a spring releases and closes the damper.  Preventing smoke from crossing this barrier.  Often times they will install a combination fire/smoke damper.  That will have both a fusible link that will melt in a fire.  And a duct smoke detector and a motor.  When powered up the motor winds up a spring and holds open the damper.  These will also have end-switches on them.  And we will also wire these into an HVAC unit’s safety circuit.  Either hard-wired.  Or by computer programming.  If the detector detects smoke or loses power the contacts open the holding circuit and the energy in the spring will close the damper.  As well as shutting down the HVAC unit connected to that duct.

The reason why we tie these into the safety circuit is that if the HVAC units start up without opening these dampers first dangerous pressures will build up in the ductwork.  And blow them apart.  Which is why there are end switches on the air dampers at the unit.  For if the unit starts with those closed they will blow the dampers apart.  All of a building’s HVAC units and dampers are controlled by a building management system (BMS).  Which makes all the components in the building work harmoniously together.  Varying the speeds of the fans, the positions of the dampers, the position of the valves on the piping serving the heating/cooling coils, etc.  Unless there is a fire alarm condition.  Then the fire alarm system takes control.  And sends a fire alarm signal to the BMS system.  Which, upon receiving this, executes an orderly shutdown of all systems.  So when the fire alarm condition clears it can begin an orderly and safe startup.  Often staggering the starting of the HVAC units to prevent dimming the lights from the power surge if they all started at the same time.

These systems can be even more complex in large buildings.  Stairwells may have a stairwell pressurization system.  If there is a fire alarm condition a dedicated fan will start up and blow air into the stairwell.  And shut down any HVAC units serving areas outside these stairwells.  So there will be a higher pressure inside the stairwell than outside the stairwell.  So air, and smoke, blow out of and not into the stairwell.  Making them safe for people to use to leave a building during a fire.  An even more complex fire alarm system will take over control of the fans and dampers of the HVAC system to ventilate smoke out of building.  Smoke evacuation systems are very complex.  And costly.  But they can save a lot of lives.  As most people die from smoke inhalation in a fire.  So having the ability to provide smoke-free routes out of a building or venting it out of a building gives people the best chance of surviving a fire.  Which we can do when we make some engineering compromises.  And make some tradeoffs between the security, HVAC and the fire alarm designs.



Tags: , , , , , , , , , , , , , , , , , , , , , , , , , , , ,

Kerosene, Jet Fuel, Lockheed Constellation, Boeing 707, Boeing 747-400, Newton’s Third Law of Motion, Turbojet and Fan Jet

Posted by PITHOCRATES - October 3rd, 2012

Technology 101

The only way to make Flying Available to the General Public is to put as many People as Possible on an Airplane

Refined petroleum products have made our lives better.  We have gasoline to drive wherever we want.  We have diesel fuel to transport things on ships and trains like petroleum oil, iron ore, coal, food, medicine, smartphones, coffee, tea, wine, scotch whisky, bourbon whiskey, beer, fresh fish, sushi, etc.  Pretty much everything we buy at a store or a restaurant got there on something powered by diesel fuel.  And sometimes kerosene.  If it must travel fast.  And if it does then it finds itself on a jet aircraft.

Today aviation has shrunk the world.  We can order a new smartphone sitting on a shelf in California and have it the next day in New York.  We can even travel to distant countries.  Some in the time of a typical working day.  Some a half a day or longer.  When but a 100 years earlier it took a couple of weeks to cross the Atlantic Ocean.  While 200 years ago it took a couple of months.  We can travel anywhere.  And get there quickly.  Thanks to the jumbo jet.  And not just the super-rich.  Pretty much anyone today can afford to buy a plane ticket to travel anywhere in the world.  And one thing makes this possible.  The jet engine.

Airplanes are expensive.  So are airports, air traffic control and jet fuel.  Airlines pay for all of these costs one passenger at a time.  Their largest cost is their fuel cost.  The longer the flight the greater the cost.  So the only way to make flying available to the general public is to put as many people as possible on an airplane.  Dividing the total flying cost by the number of passengers on the airplane.  This is why we fly on jumbo jets for these longer flights.  Because there are more people to split the total costs.  Lowering the cost per ticket.  Before the jet engine, though, it was a different story.

The Boeing 747-400 can take up to 660 Passengers some 7,260 Miles at a Speed of 567 MPH

One of the last intercontinental propeller-driven airplanes was the Lockheed Constellation.  A plane with four (4) Wright R-3350-DA3 Turbo Compound 18-cylinder supercharged radial engines putting out 3,250 horsepower each.  Which is a lot considering today’s typical 6-cyclinder automobile engine is lucky to get 300 horsepower.  No, the horsepower of one of these engines is about what one modern diesel-electric locomotive produces.  So these are big engines.  With a total power equal to about four locomotives lashed up.  Which is a lot of power.  And what does that power allow the Constellation do?  Not much by today’s standards.

In its day the Lockheed Constellation was a technological wonder.  It could take up to 109 passengers some 5,500 miles at a speed of 340 mph.  No bus or train could match this.  Not to mention it could fly over the water.  Then came the age of the jet.  The Boeing 707 being the first largely successful commercial jetliner.  Which could take up to 189 passengers some 6,160 miles at a speed of 607 mph.  That’s 73.4% more passengers, a 78.5% faster speed and a 14.1% longer range.  Which is an incredible improvement over the Constellation.  But nothing compared to the Boeing 747-400.  Which can take up to 660 passengers (506% more than the Constellation and 249% more than the 707) some 7,260 miles at a speed of 567 mph.

Now remember, fuel is the greatest cost of aviation.  So let’s assume that a intercontinental flight costs a total of $75,000 for each plane flying the same route.  Dividing that cost by the number of passengers you get a ticket price of approximately $688, $397 and $114 for the Constellation, the 707 and the 747-400, respectively.  So you can see the advantage of packing in as many passengers as possible into an airplane to lower the cost of flying.  Which is why the jumbo jets fly the longest routes that consume the most fuel.  And why we no longer fly propeller-driven aircraft except on short routes to airports with short runways.  These engines just don’t have the power to get a plane off the ground with enough people to reduce the cost of flying to a price most people could afford.  Only the jet engine has that kind of power.

The Fan Jet is basically a Turbojet with a Large Fan in front of the Compressor

Newton’s Third Law of Motion states that for every action there is an equal and opposite reaction.  Think of a balloon you just blew up and are holding closed.  If you release your hold air will exit the balloon in one direction.  And the balloon will move in the opposite direction.  This is how a jet engine moves an aircraft.  Hot exhaust gases exit the engine in one direction.  Pushing the jet engine in the opposite direction.  And because the jet engines move the plane moves.  With the force of the jet engines transferred via their connection points to the aircraft.  The greater the speed of the gas exiting the jet the faster it will push a plane forward.

The jet engine gets that power from the continuous cycle of the jet engine.  Air enters one end, gets compressed, enters a combustion chamber, mixes with fuel (kerosene), ignites, expands rapidly and exits the other end.  The hot (3,632 degree Fahrenheit) and expanding gases pass through and spin a turbine.  Then exit the engine.  The turbine is connected to the compressor at the front of the engine.  So the exhaust gases spin the compressor that sucks air into the engine.  As the air passes through the compressor it compresses and heats up.  Then it enters the combustion chamber and joins fuel that is injected and burned continuously.  Sort of like pouring gas on a burning fire.  Only enormous amounts of compressed air and kerosene are poured onto a burning fire.  As this air-fuel mixture burns it rapidly expands.  And exits the combustion chamber faster than the air entered it.  And shoots a hot stream of jet gas out the tail pipe.  Which produces the loud noise of these turbojets.  This fast jet of air cuts through the surrounding air.  Resulting in a shear effect.  Which the next generation of jet engines, the fan jet, greatly reduces.

The fan jet is basically a turbojet with one additional feature.  A large fan in front of the compressor.  These are the big engines you see on the jumbo jets.  They add another turbine inside the jet that spins the fan at the front of the engine.  Which feeds some air into the compressor of what is basically a turbojet.  But a lot of the air this fan sucks in bypasses the turbojet core.  And blows directly out the back of the fan at high speed.  In fact, this bypass air provides about 75% of the total thrust of the fan jet.  Acting more like a propeller than a jet.  And as an added benefit this bypass air surrounds the faster exhaust of the jet thereby lessening the shear effect.  Making these larger engines pretty quiet.  In fact a DC-9, an MD-80, a 707 or a 727 with standard turbojets are much louder than a 747 with 4 fan jets at full power.  They’re quieter.  And they can push a lot more people through the air at incredible speeds over great distances at a reasonable price per passenger than any other aircraft engine.



Tags: , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ,