Aircraft De-Icing Systems

Posted by PITHOCRATES - October 23rd, 2013

Technology 101

A build-up of Ice on Airfoils causes a Reduction of Lift and a Loss of Stability

In the classic movie Airport (1970) after the guy pulled the trigger on his briefcase bomb the plane suffered a massive decompression.  When Dean Martin got back to the cockpit he told the flight engineer to give them all the heat they had.  Because it’s very cold flying above 10,000 feet without pressurization.  That’s why World War II flight crews wore a lot of heavy clothing and thick mittens in their bombers.  As well as oxygen masks as the air was too thin to breathe.  The B-17 even had open windows for the waste gunners.  Making it very cold inside the plane.  Because the air is very, very cold at altitude.

There is another problem at altitude.  Because of these very frigid temperatures.  Water droplets in the air will freeze to any surface they come into contact with.  They can reduce engine power for both propeller and jet engines.  They can freeze on ports used for instrumentation and give inaccurate readings of vital aircraft data (such as engine pressure ratio, aircraft speed, etc.).  And they can freeze on airfoils (wings, rudder, tail fin, etc.).  Disturbing the airflow on these surfaces.  Causing a reduction of lift and a loss of stability.

Ice and airplanes are two things that don’t go together.  As ice forms on a wing it disturbs the airflow over the surface of the wing.  Increasing drag.  And reducing lift.  Causing the plane to lose speed.  And altitude.  If the ice continues to form on the wing eventually it will stall the wing.  And if the wing stalls (i.e., produces no lift) the plane will simply fall out of the sky.  In the early days of aviation pilots were highly skilled in flying their planes where there were no icing conditions.  Flying over, under or around masses of air containing water droplets in subfreezing temperatures.  Today we have anti-icing systems.

The most common Anti-Icing System on Commercial Jets is a Bleed Air System

One of the most common anti-icing systems on turboprop aircraft is the use of inflatable boots over the leading edge of the wing.  Basically a rubber surface that they can pump air into.  When there is no ice on the wing the boot lies flat on the leading edge without interrupting the airflow.  When ice forms on the leading edge of the wing the boot inflates and expands.  Cracking the ice that formed over it.  Which falls away from the wing.

Commercial jets have larger airfoils.  And require a larger anti-icing system.  The most common being a pneumatic manifold system that ducts hot air to areas subject to icing.  Which works thanks to a property of gas.  If you compress a gas you increase its temperature.  That’s how a diesel engine can work without sparkplugs.  The compressed air-fuel mixture gets so hot it ignites.  This property comes in handy on a jet plane as there is a readily available source of compressed air.  The jet engines.

As the air enters the jet it goes through a series of fast-spinning rotors.  As the air moves through the engine these rotors push this air into smaller and smaller spaces.  Compressing it.  Through a low-pressure compressor.  And then through a high-pressure compressor.  At which time the air temperature can be in excess of 500 degrees Fahrenheit.  It is in the high-pressure compressor that we ‘bleed’ off some of this hot and pressurized air.  We call this a bleed air system.  The air then enters a manifold which ducts it to at-risk icing areas.  From the engine cowling to the wings to the instrumentation ports.  Using the hot air to raise temperatures in these areas above the freezing temperature of water.  Thus preventing the formation of ice.

The Drawback of a Bleed Air System is Reduced Engine Efficiency

The bleed air system does more than just anti-icing.  It also pressurizes the cabin.  As well as keeps it warm.  Which is why we don’t have to dress like a crewmember on a World War II bomber when we fly.  It also powers the air conditioning system.  And the hydraulic system.  It provides the pressure for the water system.  And it even starts the jet engines.  With the source of pressurized bleed air coming from the auxiliary power unit mounted in the tail.  Or from an external ground unit.  Once the jets are running they disconnect from the auxiliary source and run on the bleed air from the engines.

There is one drawback of a bleed air system.  It bleeds air from the jet engine.  Thus reducing the efficiency of the engine.  And a less efficient engine burns more fuel.  Raising the cost of flying.  With high fuel costs and low margins airlines do everything within their power to reduce the consumption of fuel.  Which is why pilots don’t top off their fuel tanks.  They’d like to.  But extra fuel is extra weight which increases fuel consumption.  So they only take on enough fuel to get to their destination with enough reserve to go to an alternate airport.  Even though it seems risky few planes run out of fuel in flight.  Allowing the airlines to stay in business without having to raise ticket prices beyond what most people can afford.

To help airlines squeeze out more costs Boeing designed their 787 Dreamliner to be as light as possible by using more composite material and less metal.  Making it lighter.  They are also using a more efficient engine.  Engines without a bleed air system.  In fact, they eliminated the pneumatic system on the 787.  Converting the pneumatic components to electric.  Such as using electric heating elements for anti-icing.  Thus eliminating the weight of the bleed air manifold and duct system.  As well as increasing engine efficiency.  Because all engine energy goes to making thrust.  Which reduces fuel consumption.  The key to profitability and survival in the airline industry.

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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.

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