Ammonia Blast Evaporators: Principles and Applications Explained

Ammonia Blast Evaporators: Principles and Applications Explained

Ammonia Blast Evaporators utilize ammonia (R717, a natural refrigerant) as the working fluid. Designed with “direct expansion liquid supply + high-pressure ammonia liquid defrosting” technology, these heat exchange units deliver efficient heat transfer (cooling) and rapid defrosting in low-temperature refrigeration systems, preventing frost buildup that impairs heat exchange efficiency. They are widely applied in large-scale cold chains, food processing, and industrial cryogenic processes, particularly suited for systems requiring substantial refrigeration capacity and operating at low temperatures (typically -10°C to -40°C).

The operational mechanism of ammonia flash-freezing evaporators revolves around two phases: “normal refrigeration” and “frost removal.” It leverages ammonia's thermodynamic properties—low boiling point and high latent heat of vaporization—to achieve efficient heat exchange. Simultaneously, it addresses frost buildup in low-temperature environments through the “impact + melting” action of high-pressure liquid ammonia.

1. Normal Refrigeration Cycle (Heat Exchange Phase)

This phase constitutes the core cooling function of the evaporator, adhering to the “evaporative heat absorption” principle of ammonia refrigeration systems. The specific process is as follows:

Ammonia Liquid Supply: Low-temperature, low-pressure ammonia liquid (temperature: -15°C to -45°C; pressure: 0.15MPa to 0.6MPa, adjusted based on evaporation temperature) from the system's throttle valve is uniformly distributed via a manifold to the evaporator's heat exchange tube bundle (typically seamless steel or stainless steel tubes).

Evaporative Heat Absorption: Within the heat exchange tubes, the liquid ammonia absorbs heat from the external medium (air, brine, or process fluid), rapidly vaporizing into low-temperature, low-pressure ammonia vapor (this phase transition releases significant cooling capacity). Consequently, the temperature of the external medium decreases (e.g., in cold storage applications, air temperature can be lowered to -10°C to -30°C);

Vapor Recirculation: The vaporized ammonia vapor is drawn into the ammonia compressor via the suction line. After compression and condensation, it reverts to a liquid state and enters the next cycle, forming a continuous refrigeration process.

2. High-Pressure Defrost Cycle (Defrost Phase)

During low-temperature operation, frost easily forms on the evaporator surface due to condensation of airborne moisture (especially when air humidity exceeds 60%). This frost layer increases thermal resistance, reducing heat exchange efficiency (each 1mm increase in frost thickness decreases efficiency by 8%-12%). At this point, the “high-pressure liquid ammonia defrost” is initiated, following this procedure:

System Switchover: Close the evaporator liquid supply valve and suction valve; open the high-pressure liquid ammonia defrost valve (connected to the high-pressure ammonia liquid line from the condenser outlet, operating at 1.2MPa to 1.8MPa pressure and 30°C to 45°C temperature).

Defrosting: High-pressure, ambient-temperature ammonia liquid is directly injected into the evaporator heat exchange tubes. Through “heat transfer,” it melts residual ammonia liquid inside the tubes and the frost layer outside the tubes. The heated frost layer outside the tubes turns into water, which is drained via the drip pan and drain pipe;

Drainage and Resumption: After defrosting concludes, open the evaporator drain valve to discharge ammonia liquid (including melted frost water) into the receiver. Subsequently, close the defrost valve and drain valve, then reopen the liquid supply valve and suction valve to resume evaporator refrigeration operation.

Note: Defrosting typically takes 15-30 minutes depending on frost thickness. The entire process requires no system shutdown (some large systems employ “alternating dual evaporator operation” to ensure continuous refrigeration).

Ammonia defrost evaporators, valued for their “high refrigeration capacity, efficient defrosting, and environmentally friendly ammonia refrigerant (ODP=0, GWP=0),” are primarily used in large-scale industrial refrigeration and cold chain systems. Specific applications include:

1. Large-scale food cold storage (core application scenario)

Application Scenarios: Used for air cooling in freezing chambers (temperatures -30°C to -40°C) and refrigerated chambers (temperatures -10°C to -20°C), particularly suited for storing and processing meat, seafood, and frozen foods (e.g., large meat processing plants, seafood cold storage facilities);

Core Advantages:

High Refrigeration Capacity: Single evaporators deliver 100kW to 1000kW cooling capacity, meeting the cooling demands of warehouses spanning tens of thousands of cubic meters;

High-efficiency defrosting: In low-temperature, high-humidity environments (warehouse humidity 70%-85%), frost buildup is common. High-pressure ammonia liquid defrosting rapidly removes frost layers, eliminating the hassle of manual defrosting and associated downtime losses;

Environmentally friendly and safe: Ammonia is a natural refrigerant with no greenhouse effect, complying with environmental regulations (e.g., the Montreal Protocol).

2. Industrial Low-Temperature Process Cooling

Applications: Low-temperature process fluid cooling for chemical, pharmaceutical, and electronics industries, such as cooling chemical reactors (temperatures -5°C to -25°C) and creating low-temperature testing environments for electronic components (temperatures -20°C to -35°C).

Core Advantages:

High Corrosion Resistance: Heat exchange tubes made of stainless steel accommodate certain corrosive process fluids (e.g., weakly acidic solutions).

Precise Temperature Control: By regulating ammonia liquid supply, evaporation temperature fluctuations can be controlled within ±1℃, meeting process requirements for temperature stability.

3. Brine/Secondary Coolant Cooling System

Applications: Prepares low-temperature brine (e.g., calcium chloride solution, ethylene glycol solution) to supply cooling to distributed points (e.g., small cold storage units, food processing lines). Commonly used in large food processing complexes and logistics cold chain centers.

Core Advantages:

- Indirect Heat Exchange Safety: Indirect cooling via brine prevents direct ammonia contact with food, reducing leakage risks.

- High System Flexibility: A single ammonia defrost evaporator can drive multiple brine heat exchangers, accommodating dispersed cooling demands while minimizing equipment investment.