# Air\-Cooled Condensing Unit & AHU How an air\-cooled condensing unit and a DX air handling unit work together as one split refrigeration circuit By Omkar Marathe Topic: Machines [Open the interactive Jig](https://ask.jig.com/j/QyXq3S2T6JQL) ## Transcript ### 1. Split DX system A nominal 5\-ton \(17\.6 kW\) R\-410A air\-cooled condensing unit on grade serves a DX coil in a draw\-through AHU\. Two field lines close the circuit: a 3/8 in liquid line and an insulated 1\-1/8 in suction line\. Marker colour is refrigerant or air temperature, blue cold to red hot\. Drag empty space to turn the assembly\. The ACCU holds the compressor, condenser coil and fan; the metering device and evaporator sit in the AHU\. ### 2. Scroll compressor The hermetic scroll draws superheated suction vapour through the accumulator and discharges hot gas at condensing pressure\. At a typical 7 °C SST and 46 °C SCT that is roughly 9\.9 bar\(a\) to 28 bar\(a\) for R\-410A\. Suction gas also cools the motor windings\. Typical design values, not one unit's rating\. The accumulator protects the scroll set from liquid slugging\. ### 3. Condenser coil and fan Discharge gas enters the top header and passes down the wrapped coil\. It desuperheats, condenses about 10–15 K above ambient and leaves subcooled by roughly 5–8 K\. The top\-mounted propeller fan draws outdoor air in through three coil faces and discharges it vertically, rejecting the evaporator load plus compressor work\. Head pressure is held up in low ambient by fan cycling or ECM speed control\. ### 4. Liquid line Subcooled liquid leaves the liquid service valve and crosses the wall to the AHU\. Inline it passes the filter\-drier, the sight glass and moisture indicator, and the liquid\-line solenoid valve\. Size the line by equivalent length, including the drier and fittings, to keep the liquid subcooled at the TXV\. A clear sight glass with stable subcooling indicates a full liquid column\. Source: Daikin AG 31\-011 \(snippet level\)\. ### 5. TXV and distributor The thermostatic expansion valve throttles the liquid to a low\-pressure two\-phase mixture\. Its bulb, strapped to the horizontal suction outlet, and the external equalizer set the opening to hold about 5–8 K of superheat\. The distributor feeds the coil circuits in parallel through equal\-length feeder tubes\. Mount the bulb on a horizontal run, never in a trap\. Multi\-circuit coils need one TXV and distributor per circuit\. ### 6. Evaporator and air side Refrigerant evaporates through the coil rows and the last rows superheat the vapour\. The supply fan draws mixed air through the filter and coil, cooling and dehumidifying it; condensate falls to the sloped pan and leaves through a trap\. Open the doors to inspect the coil section and fan\. The draw\-through fan puts the drain pan under negative pressure, so the trap must provide a water seal deeper than the casing static\. ### 7. Suction return Superheated vapour leaves the coil header and returns through the insulated suction line\. Slope horizontal runs toward the condensing unit and avoid traps between the coil riser and the ACCU so oil returns to the compressor\. The insulation stops condensation and limits superheat gain\. The suction line is the large, cold line; it sweats if bare\. The liquid line runs near ambient\. ### 8. Controls and pumpdown A thermostat or BMS call closes the contactor and runs the compressor and condenser fan with the AHU fan\. On satisfaction the LLSV closes first; the compressor pumps the low side down to the low\-pressure cutout and stops with the charge stored on the high side\. Run the cycle to watch it\. High\- and low\-pressure switches protect the circuit; motor protection is internal to the compressor\.