Grease-Fouled Window Unit in a Canteen
A 1.5 HP window-type air conditioner cools the staff canteen of a small factory, running 06:00–22:00 daily, seven days a week. The kitchen hood exhaust discharges 3 m from the unit's outdoor grille. Airflow from the supply grille has dropped noticeably and the evaporator ices up after about 2 hours of running. The caretaker has cleaned the filter every month. The technician finds the evaporator fins coated with a sticky brown film, the blower wheel caked with the same deposit, and the condensate drain partly clogged. He proposes to spray the evaporator with muriatic acid, rinse it with a high-pressure washer with the unit in place and the power on so he can watch the fan, then "add a little R-22 because the suction pressure is low". Before cleaning, suction pressure was 58 psig and amp draw 7.9 A (nameplate RLA 8.4 A); ambient 33 °C. The customer wants a one-visit fix.
1. Critique the proposed cleaning method and state the correct approach.
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Three parts of the plan will damage the unit. Muriatic (hydrochloric) acid attacks aluminium fins; residue left in the coil causes pinholes in the tubing months later. A high-pressure washer at close range flattens fins and drives water into the blower motor, control box and insulation. And running the fan while spraying water is a shock hazard and a reliable way to short the PCB.
Correct approach: isolate and lock off first. Apply a neutral or mildly alkaline cleaner labelled for aluminium evaporators with a low-pressure sprayer (a hand pump is enough), pre-wet the fins, respect the dwell time, then rinse thoroughly with the nozzle at 45° to the fins and 150–250 mm away, shielding the electrics. Remove the blower wheel and clean it separately in a parts bath - it cannot be cleaned in place. Flush the drain pan and prove the drain with water. Dry, megger the fan motor, then re-energise.
2. Explain the iced evaporator and the 58 psig suction reading. Should refrigerant be added?
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The icing and the low suction are symptoms of airflow, not of charge. Grease-coated fins and a caked blower wheel cut the air volume across the coil; less heat is absorbed, so the evaporator runs colder than design and the saturated suction temperature falls - 58 psig on R-22 is roughly 0 °C at the coil - and in a humid canteen that surface ices.
The amp draw confirms it: 7.9 A against 8.4 A RLA is a lightly loaded compressor, exactly what a starved evaporator produces. Adding R-22 now would push the charge above design while the real fault remains; once the coil is finally cleaned the unit would be overcharged - high head pressure, possible floodback, and compressor damage on a unit that runs 16 hours a day.
Clean first. Run 20–30 minutes at stable indoor and outdoor conditions, then judge the charge by the manufacturer's method (weigh-in, or subcooling on a capillary system where superheat is fixed by the cap tube). If the readings then sit inside spec, no refrigerant is added at all.
3. Set out a maintenance plan and the customer communication that prevents a repeat.
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Schedule: filter cleaning every two weeks rather than monthly, because the unit sits downstream of a kitchen hood; evaporator and blower degreasing quarterly, or sooner if the air temperature split falls; drain check monthly in the wet season. Record at every visit - suction pressure, ambient, supply/return temperature split (target roughly 8–11 K for a window unit) and amp draw - so the trend drives the next service rather than a single reading.
Root cause: the grease comes from the hood exhaust 3 m away. Recommend extending or redirecting the duct, or relocating the condenser intake, otherwise the coil re-fouls at the same rate and the quarterly clean becomes permanent.
Communication: show the before/after photographs and the amp reading, explain plainly that the charge was not touched and why, and price the quarterly degrease against a compressor replacement. Write the interval and the readings on the service record so the next technician inherits evidence, not opinions.