Study on the leakage behavior and concentration reduction of ammonia leakage using mechanical ventilation in the ammonia refrigeration machine room for tube ice factory

Abstract

Frequent leakage is observed in industrial refrigeration systems that use ammonia refrigerant. Respiratory toxicity and adverse health effects among large populations are caused by leakage accidents. Ventilation in emergency situations is prescribed by the Ministry of Industry’s regulations. Differences were identified when the requirements were compared with TIS and IIAR standards. The suitability of emergency ventilation for machine rooms in ammonia refrigeration systems was evaluated using a mathematical model. Leakage rates under transient conditions can be analyzed by the model in accordance with ACGIH ventilation principles. From the transient analysis, it was shown that the buildup of ammonia vapor concentration follows a characteristic time-dependent function. When the interplay between system pressure and leak size was examined, increasing system pressure was found to amplify the effect of a given increase in orifice size on the leak rate. The response speed of ammonia detectors is significantly affected by the buildup stage. During an emergency ammonia release, the purging stage is important so that hazardous concentrations are reduced more rapidly to allow a longer evacuation period and safety is improved. Purging time can be estimated and changes in ammonia concentration can be monitored through transient-state analysis and parameter adjustments. The purging time is affected by the initial concentration and by air-mixing conditions in the machine room. A longer purging time is required if the initial concentration is high or if air mixing in the machine room is poor. The purging time is affected by the air-change rate required for the emergency situation. If the air-circulation volume is high, the purging time can be significantly reduced. The dynamic simulation technique developed in this research is expected to serve as an efficient tool for analyzing time-varying ammonia concentrations and can be applied to machine rooms of different sizes. A machine room with a volume of 2,475 m³ was considered as the case study, under normal operating conditions and typical outdoor weather in Thailand. The time evolution of concentration from 0 to 50 ppm was primarily examined. (1) Buildup: When the leak-orifice diameter was increased from 1 mm to 4 mm, the leak rate was increased by a factor of 16. When the refrigeration-system pressure was increased from 1,400 kPa to 1,600 kPa at a fixed orifice size, the leak rate was increased by 35%. For the rise from 0 to 50 ppm, required times of 15.25, 6.77, and 3.81 s were obtained for orifice diameters of 2, 3, and 4 mm, respectively. (2) Purging: For initial concentrations of 35, 40, 45, and 50 ppm, the time required to reach 25 ppm was 1.68, 2.35, 2.94, and 3.47 min, respectively, when K = 2.5. When emergency ventilation of 30 ACH was applied to reduce the concentration from 35 to 25 ppm, times of 0.67, 1.01, and 1.68 min were obtained for K = 1, 1.5, and 2.5, respectively. For reductions from 35 to 25 ppm with emergency-ventilation rates of 3 (ministerial regulation), 10.8 (TIS), 20, 25, 30, 35, and 40 ACH, the required times were 16.82, 4.67, 2.52, 2.02, 1.68, 1.44, and 1.26 min, respectively.