# How an AHU Works See how an air handling unit mixes, filters, cools, dries, reheats and moves air, in constant and variable air volume systems By Omkar Marathe Topic: Machines [Open the interactive Jig](https://ask.jig.com/j/Sh4ngKyHHQrx) ## Transcript ### 1. How an AHU Works An air handling unit, or AHU, is a big metal box that breathes for a building\. It pulls air in, cleans it, cools it, dries it and sends it to the rooms\. Each little dot is a puff of air, coloured by how warm it is\. Drag to look around\. Orange dots are warm air, blue dots are cool air\. ### 2. Mixing fresh and return air Air enters the mixing box from two sides\. Fresh air \(FA\) comes from outdoors, and return air \(RA\) comes back from the rooms\. Two dampers, like window blinds, set the mix: when one opens, the other closes\. Slide to let in more fresh air\. Some fresh air always comes in, so people have enough fresh oxygen\. ### 3. Two filters catch the dust First the air passes a G4 filter, a coarse screen that catches big bits like fluff and pollen\. Then comes an F7 bag filter, with deep pockets of fine fabric that trap tiny particles\. A gauge measures the pressure across the filters, so you can tell when they are clogged\. G4 and F7 are filter grades\. The higher the number, the finer the filter\. ### 4. Cold coils cool and dry the air Next the air flows through two cooling coils in series, one after the other\. Each coil is a block of thin metal fins on copper tubes, chilled inside by a refrigerant called R410A\. Water vapour in the warm air turns into droplets on the cold fins, like on a cold glass, and drips into the drain pan\. Air usually leaves the coils at about 13 °C\. ### 5. Outdoor condensing units The heat taken from the air must go somewhere\. Refrigerant carries it outside to an air\-cooled condensing unit, where a compressor squeezes it and a fan blows the heat away\. On the way back, an expansion valve drops its pressure so it turns icy cold again\. Each coil has its own 5\.5 TR unit: one working and one on standby\. 1 TR \(ton of refrigeration\) is about 3\.5 kW, so each unit moves about 19 kW of heat\. Units can swap duty to share the wear\. ### 6. Reheating to control humidity To dry the air, the coils must make it colder than the rooms need\. So a reheater, a row of electric heating elements, warms it back up a little\. The coils control the humidity, and the reheater controls the temperature\. Switch it off to see what would happen\. RH means relative humidity: how full of water vapour the air is\. About 50% feels comfortable\. ### 7. The plug fan and its VFD The plug fan is the heart of the AHU\. Its wheel spins inside the box, without a snail\-shaped housing, and pulls air through everything in front of it\. A VFD \(variable frequency drive\) changes the motor's speed\. Slide to change the speed and watch the airflow and the power\. At half speed the fan needs only about one eighth of the power\. ### 8. Dampers that stop a fire A supply air \(SA\) damper at the outlet can shut off the air\. Where the SA and RA ducts pass through a fire wall, they also have fire dampers\. A small fusible link, which melts at about 74 °C, holds their blades open\. If a fire heats it, the link melts and springs snap the blades shut, with no electricity needed\. Fire dampers react to heat\. A new link is fitted after they close\. ### 9. Sensors: the AHU's senses Sensors tell the controller what is happening\. They measure the temperature \(T\) and relative humidity \(RH\) of the fresh, return and supply air, the filter pressure, the air leaving the coils and the duct pressure\. The controller compares them with what the rooms need and adjusts everything\. A comfortable room is often set to about 24 °C and 50% RH\. ### 10. Configuration 1: constant air volume In a constant air volume \(CAV\) system, the fan always moves the same amount of air\. To keep the room comfortable, the controller changes how cold or warm that air is\. On a hot day the coils work hard\. On a muggy day the coils dry the air and the reheater warms it back up\. CAV suits one space, like a hall, with one room sensor for T and RH\. ### 11. Configuration 2: variable air volume In a variable air volume \(VAV\) system, the supply air stays cool and each zone has a VAV box that opens or closes\. When rooms need less cooling, the boxes close, the duct pressure rises and the VFD slows the fan to save energy\. The coils still dry the air, and the reheater stops rooms getting chilly\. A pressure sensor about two\-thirds along the duct tells the fan how fast to run\.