The core idea
The expansion valve (Lesson 14) drops the liquid refrigerant to low pressure, and the compressor (L15) keeps pumping vapour away, so the whole evaporator sits at low pressure. Low pressure means the refrigerant boils at a low temperature — for R-134a at about 4 bar absolute, that is 10 °C; in a freezer the coil boils at −28 °C. From that moment the job is simple: any air warmer than the coil wall loses heat to it, and that heat is exactly what feeds the boiling.
Boiling is the key. While liquid turns to vapour at constant pressure, its temperature does not budge — all the incoming heat goes into breaking molecules free (latent heat), not into warming anything (compare L7). So the coil wall sits pinned at the saturation temperature (L12's P–T rulebook) while air flows past: warm air enters at 27 °C on a summer day, gives up sensible heat, leaves at perhaps 13 °C. And because the coil surface is below the room's dew point, moisture condenses out too — that is why every evaporator has a drain pan. The puddle under a cold-room cooler is air-conditioning happening.
Design rule of thumb: keep the temperature difference (TD) between room air and evaporating refrigerant around 8–12 K. Too small and the coil cannot swallow enough heat; too large and the system chokes itself (see the pitfall). Near the coil's end the last liquid is gone and the vapour superheats a few kelvin — the very superheat of L6, which the expansion valve's sensing bulb reads as "feed me exactly right."
Air-side duty: Q = ṁ · cp · ΔT — where Q is cooling delivered (kW), ṁ the air mass flow (kg/s), cp the specific heat of air ≈ 1.0 kJ/kg·K, and ΔT the air temperature drop across the coil (K).
Real-world example
A restaurant walk-in freezer holds −18 °C; its evaporator boils refrigerant at −28 °C, so TD = 10 K. Every gram of moisture in the infiltrating air frosts straight onto those fins. Frost is a double tax: it insulates (heat struggles through the ice layer to reach the coil) and it blocks the fin passages (air flow collapses). Capacity sags, the compressor runs longer, food temperature climbs. So the system melts it on schedule — electric or hot-gas defrost every six hours or so, with heated drain pans — then returns to cooling. A freezer that never defrosts is a freezer dying politely.
Common pitfall
"Colder coil = more cooling." Cranking the setpoint down drops the evaporating temperature — but thinner suction vapour means the compressor moves less mass per second (L15), so capacity falls while frost races up the widening TD and chokes the coil. Ice on the coil looks like hard work; it is the system suffocating.