Air Conditioning: Charging and Diagnosing the Cooling Cycle
Charging and diagnosing the cooling cycle
You must move fluidly between pressures and temperatures. Convert suction pressure to evaporating saturation temperature and liquid-line pressure to condensing temperature, then compute superheat (suction line temperature minus evaporating temperature) and subcooling (condensing temperature minus liquid line temperature). Fixed-orifice/piston systems are charged by superheat using the indoor wet-bulb and outdoor dry-bulb target chart; TXV systems are charged by subcooling, commonly 8-12°F.
Benchmark numbers: 400 cfm per ton nominal, an 18-22°F indoor split (return dry-bulb minus supply dry-bulb), and a condenser split of roughly 15-25°F above outdoor ambient.
Traps: charging a TXV system by superheat, charging a piston system by subcooling alone, and blaming charge for an airflow problem. Low airflow and low charge both produce low suction pressure and high superheat; airflow problems show a small temperature split, while low charge usually shows a larger split. Overcharge shows high subcooling, high head pressure and high compressor amps. A restriction (kinked line, plugged filter-drier) shows low suction, high superheat, low head pressure and little or no subcooling. Non-condensables raise head pressure with abnormally high subcooling and no other explanation.
Worked example: return air at 75°F and supply air at 58°F gives a 17°F split, which is acceptable. If the split is only 8°F with low suction pressure, inspect the blower, filter and evaporator coil before adding refrigerant.