How Defrost Works on Aluminum Evaporators in Supermarket Refrigerated Cabinets

1. Why defrost is necessary

When cabinet air passes over the evaporator:

• Moisture condenses and freezes on fins and tubes

• Frost buildup increases air pressure drop

• Heat transfer efficiency drops

• Product temperature becomes unstable

• Compressor energy consumption rises

Defrost periodically melts this ice and removes water from the coil and drain system.

2. Key property of aluminum during defrost

Aluminum has:

• High thermal conductivity

• Low thermal mass

This means:

• It heats up quickly during defrost

• Defrost cycles can be shorter and more energy-efficient

• Temperature recovery after defrost is faster

3. Main defrost methods used

A. Electric Defrost (Most Common)

How it works:

• Electric resistance heaters are installed:

• Inside the coil

• Under the evaporator

• In the drain pan

• During defrost:

• Refrigeration stops

• Heaters warm the aluminum coil

• Ice melts and drains away

Why it suits aluminum evaporators:

• Aluminum distributes heat evenly

• Prevents localized overheating

• Reliable and simple control

Typical application:

• Low-temperature freezer cabinets (−18 °C and below)

• Glass-door freezers

• Island freezers

B. Hot Gas Defrost

How it works:

• Hot compressor discharge gas is redirected into the evaporator

• The hot gas condenses inside the aluminum tubes

• Latent heat melts frost from inside out

Advantages with aluminum coils:

• Rapid heat transfer

• No electric heater energy consumption

• Short defrost duration

Typical application:

• Large supermarket systems

• CO₂ (R744) refrigeration systems

• Centralized rack installations

Design considerations:

• Proper pressure control is critical

• Drain pan heaters still required

C. Off-Cycle (Air) Defrost

How it works:

• Compressor stops

• Fans may continue running

• Ambient cabinet air melts light frost naturally

Why aluminum helps:

• Fast response to small temperature rise

• Effective for minimal frost loads

Typical application:

• Medium-temperature cabinets (2–8 °C)

• Open display cases

• Beverage coolers

4. Defrost control and termination

Modern supermarket systems use intelligent defrost control:

Initiation methods:

• Time-based (fixed schedule)

• Temperature-based (coil sensor)

• Demand defrost (pressure or frost detection)

Termination methods:

• Coil temperature sensor (typically +8 to +12 °C)

• Time limit safety backup

Aluminum coils respond quickly, enabling precise termination and reduced energy waste.

5. Drainage and water management

During defrost:

• Meltwater flows through aluminum fins and tubes

• Water collects in drain pan

• Heated drain prevents refreezing

Design essentials:

• Proper fin spacing to avoid ice bridging

• Corrosion-resistant aluminum coatings

• Adequate pan heater capacity

6. Impact of defrost on food safety

Correct defrost design ensures:

• Minimal product temperature rise

• Short recovery time

• No water dripping onto food

• Compliance with food safety regulations

Aluminum’s fast thermal response is critical in maintaining tight temperature control.

7. Common defrost problems and solutions

8. Summary

Defrost in aluminum evaporators works by temporarily reversing or interrupting refrigeration and applying controlled heat to melt accumulated frost. Aluminum’s excellent heat transfer properties allow faster, more uniform defrost with lower energy consumption—making it ideal for supermarket refrigerated cabinets.