Abstract: This article summarizes the experience in the operation and maintenance of lithium chemical refrigerators, and proposes a full-process management plan and matters that need to be paid attention to in design and manufacturing.
Keywords: lithium; refrigeration machine; repair and maintenance; whole process management
The steam double-effect lithium absorption refrigeration unit is a device that uses steam as the driving heat source, lithium solution as the absorbent, and water as the refrigerant to produce low-temperature water. Since the 1990s, the coking industry has undergone large-scale transformation of the coke oven gas purification process. The new gas purification process requires low-temperature cooling water to ensure gas purification production. Lithium absorption refrigerators have begun to be widely used in the coking industry.
Our company’s six double-effect lithium absorption refrigerators operate very stably under the half-load production conditions of the gas purification device. After the gas purification device was fully loaded, the cooling capacity of the refrigerator showed a trend of decreasing year by year. In 1999, the cooling effect deteriorated, and the cooling capacity of the unit was only 50% to 60% of the rated cooling capacity. The number of breakdowns and shutdowns became increasingly frequent, which directly affected the Production of gas purification equipment. Starting from the end of 1999, the unit has been drilled, cleaned and inspected, and the circulating water system in the recycling workshop has been transformed and optimized. After more than three years of hard work, the operation of the unit is basically normal, and the cooling capacity has returned to 85% to 90% of the rated cooling capacity. The various indicators of the gas purification system have also increased significantly and achieved good results.
1There is a problem
⑴ When the unit is running, the pressure and temperature at various points inside the unit are relatively high. The pressure and temperature at various points inside the unit are higher than the design values. Taking the absorber as an example, the design temperature of its dilute solution (operating concentration is 53% ~ 55%) is 39 ~ 41°C, and the corresponding pressure is 1403 ~ 1117Pa. During operation, the actual temperature is 49°C, and the corresponding pressure is 2460~1988Pa, which is almost doubled.
⑵The properties of the lithium solution have deteriorated. The color of the solution is brown, and the pH value is greater than 10.5. The lithium chromate in the solution is consumed quickly, and a large amount of corrosion products precipitate after the sample is left to stand, causing frequent damage to the canned electric pump.
⑶ It is difficult to discharge non-condensable gas. A large number of bubbles can be observed at the sight glass of the unit’s automatic air extraction device, and they are never completely discharged. The vacuum pump must be started frequently to pump air. The vacuum pump oil is easy to emulsify and requires a large amount. The unit was severely corroded, and the solution and refrigerant water distribution devices in the machine were seriously blocked.
⑷The unit is severely corroded. The heat exchange tubes of the unit’s heat exchanger are corroded and perforated, causing leakage of dilute and concentrated solutions. The copper tubes of the high and low pressure generators ruptured, causing unit shutdown and refrigerant water contamination. The small hole blocking rate of the concentrated solution distribution plate of the absorber reaches 50% to 80%, which seriously affects the absorption effect. The clogging rate of the refrigerant water secondary spray nozzle is more than 80%, the evaporation effect is extremely poor, and the temperature difference between the refrigerant water and the refrigerant water is more than 5°C.
The above problems have greatly reduced the cooling capacity of the unit. The temperature of the refrigerant water leaving the unit has reached 22-26°C, which is far from the required 16-18°C. The gas purification device cannot operate normally.
2Problem analysis and improvement measures
The reason why the cooling capacity of the unit decreases significantly is that the operating state of the unit seriously deviates from the designed operating state (variable operating conditions). The main reasons are as follows.
2.1 Reasons for the unit
In the past, lithium chemical absorption refrigerators were mainly used in centralized air conditioning systems, and the refrigerant water inlet and outlet temperatures were designed at 12°C and 7°C. For the gas purification device, the inlet and outlet temperatures of the refrigerant water (that is, the low-temperature circulating water required for the gas purification device) are required to be 23°C and 16°C. When the vacuum degree in the evaporator of the chiller remains unchanged, the higher the inlet temperature of the refrigerant water, the greater the evaporation of the refrigerant water. In other words, the working pressure of the evaporator of the chiller used in the coke oven gas purification device is higher than that of the air-conditioning unit.
As the evaporation pressure increases, the mass transfer driving force for the concentrated lithium solution to absorb refrigerant vapor in the unit’s absorber increases, and the absorption capacity of the absorber increases accordingly. As the inlet temperature of the refrigerant water increases, the cooling capacity of the unit will increase accordingly. For every 1°C increase in the temperature of the refrigerant water entering the unit, the cooling capacity of the unit can increase by 3% to 5%. But this only applies within a specific range (within a rise in the refrigerant water inlet temperature of 3 to 5°C), because after the evaporation capacity of the evaporator increases, and the working capabilities of the absorber, generator and condenser have been determined, then the absorption The absorption capacity of the absorber will be insufficient, and the water vapor evaporated from the evaporator cannot be effectively absorbed, causing the operating temperature and pressure of the absorber to rise; the corresponding working capacity of the generator and condenser will be insufficient, resulting in poor generation of dilute solutions in the unit. , the condensation temperature increases. The unit has been operating under variable operating conditions for a long time, and the operating temperature and pressure of each part deviated from the design values. 2.2 Influence of external conditions
⑴The unit is operating at overload. The low-temperature water produced by the chiller is used as the second-stage circulating cooling water of the gas purification device, while the first-stage circulating cooling water is an open circulating cooling water. As a result, the cooling capacity of the first stage of the purification device is insufficient and the cooling effect is poor. Under a large heat load Moving to the second stage further increases the temperature of the low-temperature water entering the unit, which can reach 25-28°C in summer. The deviation of the working conditions further increases, and the chiller is in an overloaded state, making it difficult to maintain long-term operation.
⑵ Vacuum management. The refrigeration machine is a refrigeration equipment that operates under a high vacuum state. Once there is a problem with the vacuum degree, first the cooling capacity will decrease, and secondly it will cause operational failure, which in serious cases will endanger the service life of the unit. The purpose of vacuum plating management is to ensure that the cold machine is always in an optimal vacuum state. Its core is to control the non-condensable gas content in the machine within the allowable range. The existence of non-condensable gas increases the resistance of the absorption process and weakens the mass and heat transfer process. Even a small amount of non-condensable gas will cause a substantial attenuation of cooling capacity. The presence of oxygen in non-condensable gases is the main cause of internal corrosion of the unit, so the oxygen content in the unit must be strictly controlled.
⑶Water quality management of circulating cooling water. The problem with circulating cooling water is that the side filtration and bacterium and algae sterilization are not effective, resulting in too much sediment and dirt in the water, which affects the cooling effect, causing the temperature of the absorber and condenser to be too high, and the deviation of the unit’s operating conditions to increase.
⑷Daily management of lithium solution. In the early stages of construction, the pH value and lithium chromate content of the lithium solution were not strictly controlled. Coupled with the insufficient capacity of the steam temperature and pressure reduction device, the temperature of the steam entering the unit is generally controlled at 180 to 190°C, which increases the amount of clay-like complexes and low-priced chromium products in the lithium solution, and deteriorates the properties of the solution.
Due to various reasons, the unit has been operating under variable working conditions for a long time, and the degree of deviation has become larger and larger, resulting in serious corrosion inside the machine, blockage of the lithium solution and refrigerant water distribution device, significant reduction in cooling capacity, and an increasingly passive production situation. .
In response to the above problems, the unit was drilled and cleaned to remove most of the corrosion products, the solution was replaced and regenerated, and the leaking heat exchanger and cracked copper tubes were replaced. At the same time, the temperature and pressure reduction device was modified to control the steam temperature between 155 and 165°C. The circulating cooling water system and refrigeration circulating water system in the first section of the workshop were transformed and the water volume was balanced and optimized, effectively reducing the downward shift of the heat load and reducing the burden on the chiller. Establish and improve various management systems for refrigeration units and implement them in place. At present, the working conditions of the unit have been greatly improved. Taking the absorber as an example, the working temperature of the dilute solution has been controlled below 43°C. The refrigerant water outlet temperature has also dropped below 20°C, and the corrosion in the machine has been controlled and will no longer worsen.
Several years of unit operation and Keji maintenance practice have shown that the whole process management of the unit must be carried out from the following aspects.
⑴ Careful calculation and special design must be carried out according to the changes in refrigerant water temperature. The absorber, condenser and high and low pressure generators need to be enlarged to a certain extent, and the solution and refrigerant water circulation should also be adjusted accordingly.
⑵ Since the refrigerant water inlet temperature is required to be 23°C, the problem of rising working pressure of the evaporator and absorber cannot be avoided. Therefore, the cooling capacity of the unit should be larger than the required cooling capacity, that is, there is a certain margin, which can effectively alleviate this problem. contradiction.
⑶ The manufacturing level and testing methods of the unit body must be improved to ensure good air tightness and minimize the impact of external leakage on the unit.
⑷The vacuum management of the unit focuses on controlling the content of non-condensable gases. Two points must be emphasized: First, the absolute pressure of the unit is the sum of the partial pressure of water vapor and the partial pressure of non-condensable gas. Therefore, it is impossible to determine the amount of non-condensable gas inside the unit by simply measuring the absolute pressure to manage the vacuum of the unit. The absolute pressure and the water vapor saturation pressure of the solution should be measured separately, and then the difference between the two should be used as a reference to determine The content of non-condensable gases in the machine. Second, the unit uses a rotary vane vacuum pump, and its ultimate vacuum degree must be checked regularly, and the actual measured value must not be greater than 1.33Pa.
⑸The most important thing in the daily management of the solution is to detect and control the pH value and corrosion inhibitor content of the solution.
⑹ Perfect cooling water management should include controlling water temperature, controlling circulation volume, controlling system water balance, and stabilizing water quality. The core is water quality stabilization, which must ensure that the fouling coefficient is less than 0.086m2•℃/kW.
⑺The unit itself and external instruments should be accurate. Only by accurately understanding the parameters of each part can the operating effect and quality of the unit be correctly grasped and judged.
Link to this article：Experience in using steam double-effect lithium bromide absorption refrigerator
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