Air Handling Unit, Outside Air, Exhaust Air, Return Air and Energy Recovery Unit

Posted by PITHOCRATES - March 27th, 2013

Technology 101

Things that Absorb Energy can Cool Things Down and Things that Radiate Energy can Warm Things Up

When two different temperatures come into contact with each other they try to reach equilibrium.  The warmer temperature cools down.  And the cooler temperature warms up.  If you drop some ice cubes into a glass of soda at room temperature the warm soda cools down.  The ice cubes warm up.  And melt.  When there is no more ice to melt the temperature of the soda rises again.  Until it reaches the ambient room temperature.  The normal unheated or un-cooled temperature in the surrounding space.  As the soda and the air in the room reach equilibrium.

When two temperatures come into contact with each other what happens depends on the available energy.  Higher temperatures have more energy.  Lower temperatures have less energy.  For heat is energy.  Things that absorb energy can cool things down.  Things that radiate energy can warm things up.  And this is the basis of our heating and cooling systems in our buildings and homes.

Boilers burn fuel to heat water.  A furnace burns fuel to heat air.  The heated water temperature and heated air temperature is warmer than the temperature you set on your thermostat.  When this very hot water/air circulates through a house or building it comes into contact with the cooler air.  As they come into contact with each other they bring the air in the space up to a comfortable room temperature.  Above the unheated ambient temperature.  But below the very hot temperature of the heating hot water or heated air temperature.

Heating and Cooling Buildings consume up to Half of all Energy on the Planet

Large buildings have air handling units (AHU) that ventilate, heat and cool the building’s air.  They’re big boxes (some big enough for grown men to walk in) with filter sections to clean the air.  Coil sections that heat or cool the air as it blows through these coils.  A supply and a return fan to blow air into the building via a network of air ducts.  And to suck air out of the building through another network of air ducts.  And a series of dampers (outside air, exhaust air and return air).

To keep the air quality suitable for humans we have to exhaust the breath we exhale from the building.  And replace it with fresh air from outside of the building.  This is what the dampers are for.  The amount they open and close adjusts the amount of outside air the AHU pulls into the building.  The amount of the air it exhausts from the building.  And the amount of air it recirculates within the building.  Elaborate computer control systems carefully adjust these damper positions.  For the amount of moving air has to balance.  If you exhaust less you have to recirculate more.  Otherwise you may have dangerous high pressures build up that can damage the system.

It takes a lot of energy to do this.  Buildings consume up to half of all energy on the planet.  And heating and cooling buildings is a big reason why.  Because it take a lot of energy to raise or lower a building’s air temperature.  And keeping the air safe for humans to breathe adds to that large energy consumption.  If you stand outside next to an exhaust air damper you can understand why.  If it’s winter time the exhausted air is toasty warm.  If it’s summer time the exhausted air is refreshingly cool.

An Energy Recovery Wheel is a Circular Honeycomb Matrix that Rotates through both the Outside & Exhaust Air Ducts

In the winter large volumes of gas fire boilers to heat water.  Electric water pumps send this water throughout the building.  Into baseboard convection heaters under exterior windows to wash this cold glass with warm air.  And into the heating coils on AHUs.  Powerful electric supply and return fans blow air through those heating coils and throughout the building.  After traveling through the supply air ductwork, out of the supply air ductwork and into the open air, back into the return air ductwork and back to the AHU much of this air exhausts out of the building.  That returning air is not as warm as the supply air coming off of the heating coil.  But it is still warm.  And exhausting it out of the building dumps a lot of energy out of the building that requires new energy to heat very cold outside air to replace it.  The more air you recirculate the less money it costs to heat the building.  But you can only recirculate air so long before you compromise the quality of indoor air.  So you eventually have to exhaust heated air and pull in more unheated outside air.

Enter the heat recovery unit.  Or energy recovery unit.  There are different names.  And different technologies.  But they do pretty much the same thing.  They recover the energy in the exhaust air BEFORE it leaves the building.  And transfers it to the outside air coming into the building.  To understand how this works think of the outside air duct and the exhaust air duct running side by side.  With the air moving in opposite directions.  Like a two-lane highway.  These sections of duct run between the AHU and the outside air and exhaust air dampers.  It is in this section of ductwork where we put an energy recovery unit.  Like an energy recovery wheel.  A circular honeycomb matrix that slowly rotates through both ducts.  Half of the wheel is in the outside air duct.  Half of the wheel is in the exhaust air duct.  As exhaust air blows through the honeycomb matrix it absorbs heat (i.e., energy) from the exhaust air stream.  As that section of the wheel rotates into the outside air duct the unheated outside air blows through the now warm honeycomb matrix.  Where the unheated air absorbs the energy from the wheel.  Warming it slightly so the AHU doesn’t have use as much energy to heat outside air.  It works similarly with air conditioned air.

Many of us no doubt heard our mother yell, “Shut the door.  You’re letting all of the heat out.”  For whenever you open a door heated air will vent out and cold air will migrate in.  Making it cooler for awhile until the furnace can bring the temperature back up.  It’s similar with commercial buildings.  Which is why a lot of them have revolving doors.  So there is always an airlock between the heated/cooled air inside and the air outside.  But engineers do something else to keep the cold/hot/humid air outside when people open doors.  They design the AHU control system to maintain a higher pressure inside the building than there is outside of the building.  So when people open doors air blows out.  Not in.  This keeps cold air from leaking into the building.  Allowing people to work comfortably near these doors without getting a cold blast of air whenever they open.  It allows people to work along exterior windows and walls without feeling any cold drafts.  And it also helps to keep any bad smells from outside getting into the building.


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