Schemes for the use and safety of ammonia refrigerants

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Schemes for the Use and Safety of Ammonia Refrigerants I. About Ammonia Refrigerants Ammonia is an excellent refrigerant and belongs to natural refrigerants with an ODP of 0； With a GWP of 0, it boasts the best thermodynamic properties at present, offering the highest cooling efficiency, low cost, and low operational and maintenance expenses. It has been in use for over a hundred years; currently, more than 85% of large-scale industrial and commercial cooling systems in China use ammonia. 　　Vapor is colorless and has a strong, pungent odor; even small leaks of ammonia (3–5 ppm) can be detected, and its explosive limit in air is 15–28%. 　　II. Sources of hazard for ammonia Operation room: manual labor, high density of personnel, enclosed environment; in the event of an accident, emergency evacuation is difficult. 　　Refrigerated storage: Food is piled up, the environment is enclosed, and leaks are difficult to detect. 　　Machine room: It stores a large amount of ammonia liquid, making it a key location in the event of another explosion. 　　Surrounding environment: It should be kept away from areas with high population density. 　　III. An objective assessment of ammonia Some companies, when promoting ammonia as a natural refrigerant, overstate its advantages and downplay its disadvantages. Ammonia’s flammability, toxicity, and corrosiveness mean that it is necessary to strengthen the safety measures for ammonia-based systems. Compared to industries such as chemicals, coal mining, petroleum, and petrochemicals, ammonia poses a relatively lower level of danger; yet in daily life, hazards related to electricity, water, gas, buildings, and so on are always present. All accidents arise from negligence, carelessness, and management flaws. Man-made disasters are worse than natural disasters! 　　IV. Causes of disasters
1. Fires trigger secondary disasters; ignition and high temperatures lead to increased pressure, resulting in explosions ； 　　2. Improper manual defrosting procedures can result in liquid hammer or closed-loop defrosting, leading to pipe bursts ； 　　3. Defrosting accidents in single-freezer units have caused numerous casualties. Aging equipment and pipelines, or poor management ； 　　4. Aging of system pipelines, valves and other facilities, leakage, and non-standard design and installation ； 　　5. Use of materials that do not meet fire safety standards; open-flame operations such as electric welding performed in the presence of ammonia. Images V. Issues That Cannot Be Avoided 1. Large charging volume for refrigeration systems where ammonia evaporates directly ； 　　2. Ammonia directly enters densely populated operating rooms and food freezing/chilling rooms ； 　　3. In the event of force majeure such as fires or earthquakes, secondary disasters caused by ammonia are inevitable. 　　VI. Plans for ammonia safety 1. Minimize the amount of ammonia loaded, in accordance with GB18218-2009 \"Identification of Major Hazard Sources for Hazardous Chemicals\", the critical amount for ammonia is specified as 10 tons. 　　2. Confine ammonia to specific areas within the machine room, keeping it away from areas such as control rooms and cold storage areas that could pose risks to human health and food safety. 3. Implement proactive defense measures for the ammonia system, including concentration monitoring and alarm systems for early detection. Active defense: alarm, high-altitude discharge, spraying, and interlocked emergency shutdown of equipment. 　　VII. Ammonia Safety Measures
In the event of an ammonia leak in a certain area, the active safety defense system responds according to the amount of ammonia released:
1. Audible and visual alarms alert operators to the occurrence of an ammonia leak ； 　　2. The accident exhaust fan is activated to discharge the leaked ammonia outside ； 　　3. The water curtain spray uses water to separate the dangerous area from the ammonia release point. 　　If more ammonia leaks, the IEMC interlock function will shut down the compressors in the affected areas to reduce their load. The key aspects of the water curtain spraying system are preventing the formation of an explosive mixture of ammonia, preventing water from entering electrical switches and other components. Shutdown controls must be carried out in accordance with the procedures of the refrigeration system, to avoid simultaneous shutdowns and to ensure the integrity of the equipment after an accident. It is also important to ensure the reliability and timeliness of the spraying water treatment system. 　　VIII. Defrosting method: Automatic instead of manual
Liquid inlet piping: Shut-off valve → Filter → Solenoid valve → Check valve → Shut-off valve ； 　　Return air circuit: Shut-off valve → Two-stage solenoid valve → Shut-off valve ； 　　Hot ammonia piping: Globe valve → Filter → Solenoid valve → Check valve → Globe valve ； 　　Drain piping: Bypass valve. 　　IX. Liquid explosion and water hammer
Liquid explosion: The incompressibility of liquids and their expansion when heated. 　　Prevention: Operate correctly to avoid liquid hammer caused by overfilling. 　　Causes of liquid explosion:
1. During the defrosting process, the heat exchange tube bundle becomes filled with liquid, or there is an excess amount of liquid. 　　2. The valves at both ends of the cryogenic liquid pipeline are closed; upon heating, the pressure increases (for example, the valves from the ammonia pump to the regulation station). 　　Liquid hammer: Impact of high-speed fluid. 　　Precautions: Operate correctly, increasing and decreasing pressure slowly. 　　Liquid hammer is primarily caused by the following two situations: 1. The supply valve is closed before defrosting, resulting in incomplete recovery of the ammonia liquid from the heat exchange tube bank; liquid remains in the return pipeline. When defrosting begins and the hot ammonia inlet valve opens too quickly, the high-pressure gas pushes the liquid in the return pipeline, accelerating its flow (due to a pressure difference of over 8 bar), which creates a high-speed liquid flow in the return pipeline. Liquid hammer occurs when this flow encounters an obstacle. Be sure to open the hot ammonia inlet valve slowly. 　　2. When defrosting is complete, the pressure of the liquid in the heat exchange tube bundle does not decrease; the return air valve is opened too quickly. 　　X. Add emergency handling facilities in the warehouse 1. Ammonia concentration detection and alarm devices, as well as a water supply system. 　　2. Add a shielding barrier to the reservoir. 　　3. High liquid level alarm in the reservoir. 　　4. A dedicated high-altitude emergency ammonia release system is provided to discharge ammonia into a water container. 　　5. During ammonia safety retrofitting for thermal ammonia defrosting, the pressure before entering the evaporator must not exceed 0.8 MPa. 　　6. It is prohibited to use the method of reducing or closing the condenser inlet valve to speed up the defrosting process. 　　7. Active defense measures using ammonia systems can be applied in both computer rooms and warehouses. 　　8. The hazards can only be reduced by minimizing the amount of filling.