Evaporator Coil

Detailed Explanation

The evaporator coil is the heat exchanger in the refrigeration cycle where the refrigerant transitions from liquid to gas phase (evaporates). During this phase change, the refrigerant absorbs a large amount of heat; this heat is supplied by cooling the air passing across the evaporator. In a typical dehumidification application, the evaporator cools air down to 2–8°C.

Structurally, evaporators are usually copper-tube + aluminum-fin (fin-and-tube). As air passes between the fins, it transfers heat to the refrigerant evaporating in the copper tubes. When air is cooled below its dew point, moisture condenses on the fin surfaces and drains into the bottom collector pan.

Evaporator design parameters: • Surface area (m²) — larger surface = lower air velocity = lower pressure drop, higher efficiency • Fin pitch — typically 1.8–3.0 mm; lower pitch gives higher heat transfer but freezes more quickly • Tube diameter — 7.9 mm (5/16"), 9.5 mm (3/8"), 12.7 mm (1/2") are common • Refrigerant distribution — homogeneous distribution via distributor + capillaries is critical, otherwise local freezing • Drain pan slope — minimum 1/100 gradient, antibacterial coating

The evaporator outlet air temperature approaches the refrigerant evaporation temperature, so the "approach temperature" is 2–4°C; a lower approach means a larger coil.

Performance Calculation

Evaporator capacity (total cooling load): Qevap = ṁa × (h1 − h2)

Qevap: evaporator capacity (kW) ṁa: dry air mass flow rate (kg/s) h1: enthalpy of inlet air (kJ/kg) h2: enthalpy of outlet air (kJ/kg)

Log Mean Temperature Difference (LMTD): LMTD = (ΔT1 − ΔT2) / ln(ΔT1/ΔT2)

ΔT1 = Tair,in − Tref,evap ΔT2 = Tair,out − Tref,evap

Heat transfer equation: Qevap = U × A × LMTD

U: overall heat transfer coefficient (W/m²·K) A: surface area (m²)

Under humid conditions, U increases by a factor of 1.3–2.0 versus dry conditions (condensation on the outer film improves heat transfer).

Practical Example

Evaporator analysis for an NKT CD400 model dehumidifier:

Inputs: • Air flow: 4,000 m³/h ≈ 1.33 kg/s dry air • Inlet air: 30°C, 70% RH → h1 = 78.4 kJ/kg, W1 = 18.9 g/kg • Outlet air (evaporator exit): 12°C, 95% RH → h2 = 33.8 kJ/kg, W2 = 8.4 g/kg • Refrigerant (R454B) evaporation temperature: 5°C

Capacity: Qevap = 1.33 × (78.4 − 33.8) = 59.3 kW

Sensible portion: 1.33 × 1.012 × (30 − 12) = 24.2 kW Latent portion: 59.3 − 24.2 = 35.1 kW Moisture removed: 1.33 × (18.9 − 8.4) / 1000 = 0.0140 kg/s ≈ 50.4 kg/h

LMTD: ΔT1 = 30 − 5 = 25°C ΔT2 = 12 − 5 = 7°C LMTD = (25 − 7) / ln(25/7) = 14.1°C

U·A = Q / LMTD = 59,300 / 14.1 = 4,206 W/K For a wet evaporator with typical U ≈ 50 W/m²·K → A ≈ 84 m²

This surface area corresponds to a standard 4-fan industrial evaporator. Compact units use higher U values (microchannel design) to reduce surface area by 30–40%.

Engineering Note

Practical considerations for evaporator coils:

• Freezing risk — when the evaporator surface temperature drops below 0°C, ice begins to accumulate between fins. The smaller the fin pitch, the faster it clogs. In low-temperature applications, wide pitch (3–4 mm) + hot gas defrost are used. • Defrost strategies — electric resistance (simple, energy-hungry), hot gas (efficient, requires compressor off-cycle), bypass damper (continuous operation, high cost). Duration is typically 5–10 minutes, frequency up to once per hour depending on conditions. • Corrosion protection — in pool, coastal, and chemical industry applications, copper/aluminum surfaces are exposed to chloride/sulfur attack. Epoxy coating, pre-tinning, or aluminum/aluminum (Cu-free) coils are preferred. • Hygiene — wet surfaces carry Pseudomonas and Legionella risk. UV-C lamps, antibacterial epoxy, or silver-ion coatings are mandatory in pharmaceutical/healthcare applications. • Microchannel (MCHE) coils — a modern alternative: aluminum monolithic construction, 30% less refrigerant, 25% lighter. Ideal for integration with low-GWP refrigerants (R32, R454B). NKT next-generation condensation units are certified with MCHE.

Evaporator selection directly determines the unit's annual energy performance and service life; a cheap coil pays back as 10 years of COP loss.