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1. Hot Gas Defrost (Most Common Method)How it works:
High-pressure ammonia vapor discharged from the compressor is routed into the evaporator coil. As the hot gas condenses inside the tubes, it releases latent heat, melting the accumulated frost and ice on the fin surface.
Key characteristics:
Fast and effective defrosting
No external heat source required
Well suited for large industrial NH₃ systems
Allows centralized control of multiple evaporators
Design considerations:
Proper condensate drainage to avoid refreezing
Pressure relief and check valves to prevent liquid hammer
Accurate control sequencing to avoid coil overpressure
Hot gas defrost is the preferred solution for blast freezers and IQF tunnels due to its speed and efficiency.
How it works:
Warm liquid ammonia from a high-pressure receiver is circulated through the evaporator during defrost. The sensible heat of the liquid melts frost on the coil surface.
Key characteristics:
Gentler temperature rise compared to hot gas defrost
Lower pressure stress on coils
Suitable for pumped-liquid ammonia systems
Limitations:
Slower defrost cycle
Requires careful liquid management and control
This method is often used where system design prioritizes pressure stability over defrost speed.
How it works:
Electric heating elements are installed near or within the coil to melt frost.
Key characteristics:
Simple to control
Independent of refrigeration circuit
Limitations:
High energy consumption
Generally impractical for large blast evaporators
Rarely used in industrial ammonia systems
Electric defrost is typically reserved for small auxiliary evaporators rather than primary blast freezers.
Regardless of the defrost method, proper control is essential:
Evaporators are defrosted in sequence to avoid excessive load on the refrigeration system
Fan operation is stopped during defrost to prevent warm air circulation
Defrost termination is controlled by time, temperature, or pressure sensors
Post-defrost drip time is included to ensure complete condensate removal
All defrost components must be rated for ammonia service
Pressure relief protection is mandatory
Defrost piping must allow complete drainage
Systems must comply with IIAR, EN 378, or ISO standards
In ammonia blast evaporators, hot gas defrost is the most widely used and effective method, offering rapid ice removal and high operational efficiency. Pumped liquid defrost is an alternative in specific system configurations, while electric defrost is rarely applied in large industrial NH₃ installations. Proper defrost design ensures stable airflow, consistent freezing performance, and long equipment life.
