Defrost

Detailed Explanation

Defrost is the periodic melting of ice that accumulates on evaporator surfaces in refrigeration systems. All cold storage rooms, freezers, spiral freezers, and low-temperature dehumidifiers operating below freezing require periodic defrost. Otherwise, ice clogging between fins blocks airflow, and both heat transfer and cooling capacity gradually decline.

The cause of ice accumulation: moisture in the air condenses on the evaporator surface, and because the same surface is below 0°C, this condensation freezes into ice. Once the ice layer reaches 1–2 mm: • Airflow decreases by 20–40% • Evaporator efficiency drops • Risk of compressor pulling vacuum (LP cut) • Temperature fluctuation in the cold room

Defrost methods: 1. Electric resistance defrost — heater rods placed in the fins; simple but energy-intensive (0.3–1.0 kWh/m² of evaporator surface per defrost) 2. Hot gas defrost — hot gas from the compressor discharge line is sent backward through the evaporator; efficient, requires compressor off-cycle 3. Water defrost — spraying warm water; common in agricultural sectors, presents hygiene and drainage issues 4. Natural (off-cycle) defrost — passive melting if ambient temperature is above 0°C; only in cold-room environments around 0°C

Defrost Energy Calculation

Energy load of one defrost cycle:

Qdef = mi × hsf + mi × cp,i × ΔTi + me × cp,e × ΔTe

Qdef: defrost heat load (kJ) mi: mass of ice to be melted (kg) hsf: latent heat of fusion of ice = 333 kJ/kg cp,i: specific heat of ice = 2.09 kJ/kg·K ΔTi: ice temperature rise (e.g. from −20°C to 0°C → 20K) cp,e: equipment/air specific heat ΔTe: evaporator material temperature rise

Actual defrost energy is typically multiplied by a factor of 1.5–2.5 over the theoretical requirement (losses, ambient heating, residual equipment heating).

Annual defrost energy ratio: Def rate = (Qdef × N) / Qcooling,annual

N: annual number of defrost cycles (typical 4–24 cycles/day × 365)

Good design: Def rate < 10% Poor design: Def rate 20–35% (uncontrolled ice in moisture-loaded environment)

Practical Example

Defrost analysis for a spiral freezer (meat processing facility):

Current state: • Evaporators: 4 units, each 60 m² surface • Operating temperature: −32°C • Defrost method: electric resistance • Defrost frequency: 4 times per day × 25 minutes • Ice accumulation rate: ~3 kg/hour (high moisture load → uncontrolled outlet air)

Daily defrost energy: • Ice to melt: 3 kg/h × 6 h operation = 18 kg/cycle × 4 = 72 kg • Melting heat: 72 × 333 = 23,976 kJ • Equipment heating (estimated 50%): + 12,000 kJ • Total defrost energy: ~36,000 kJ/day = 10 kWh/day • Annually: 3,650 kWh defrost + cooling losses (cold room re-cooling after defrost-related warming): approximately 8,000 kWh/year additional load

NKT improvement proposal: integrate a −45°Cdp silica gel rotor at the spiral freezer inlet. • Effect: moisture in air is removed before entry → ice accumulation reduced by 70% • New defrost frequency: 1 time per day × 20 minutes • Annual defrost energy savings: 9,500 kWh • Capacity gain: with fewer defrost interruptions, the production line operates 8–12% longer

Payback period: rotor system investment / annual savings ≈ 18–24 months.

Engineering Note

Considerations in defrost optimization:

• Defrost initiation method: – Time-based (timer): simple, but inadequate at high load/excessive at low load – Temperature-based (evaporator superheat): moderate efficiency – Differential pressure (DP): detects airflow blockage in real time — best choice – Demand defrost (smart algorithm + machine learning): next generation, additional 30–40% savings • Defrost duration setting — short duration = incomplete melting, long duration = excess heating. Automatic termination (drip pan thermostat reaching 18°C) should be standard. • Damper closing — during defrost, the room door/damper should be closed; otherwise, ambient temperature rises and later creates re-cooling load. • Drain heating — without drip pan and drain line heaters, melted water refreezes; pan heater + drain line heater are mandatory. • Dehumidifier integration — the only method to physically reduce defrost frequency is drying the inlet air. Effective at moderate level (30–50% reduction) for condensation-type, high level (60–80% reduction) for silica gel rotor. • Product temperature fluctuation — during defrost, cold room temperature rises 2–5°C, affecting product quality. In applications with narrow target ranges (ice cream, fisheries), defrost scheduling should be aligned with production breaks.

NKT periodic maintenance services include defrost performance audits and control algorithm tuning.