In the production chain of the alumina industry, as a core equipment, the operating efficiency of the evaporator is directly related to production benefits and cost control. However, from an alumina plant in Hebei in 2023 to another in Shanxi in March 2024, both successively invited us to address the issue of insufficient water evaporation capacity of their seven-effect evaporators. This situation reveals numerous unprofessional problems in the design of evaporators within the industry. An in-depth exploration of these cases not only helps to clarify the root causes of the problems but also provides a practical guide for the industry to avoid potential pitfalls.

I: In-depth Analysis of Typical Cases

01: The 370-ton evaporator of an alumina plant in Shanxi: The dilemma of inaccurate area design

Since its commissioning, the 370-ton evaporator of an alumina plant in Shanxi has been plagued by production capacity issues, with the actual water evaporation volume hovering around 220 tons for a long time. Upon investigation, it was found that when designing the evaporator, the manufacturer heavily relied on empirical values of evaporation intensity to determine the heat transfer area of the equipment, ignoring the unique thermodynamic characteristics of the seven-effect evaporator. Compared with the six-effect evaporator, the temperature difference at each effect point of the seven-effect evaporator is significantly reduced. This requires meticulous heat balance calculations during the design process to significantly increase the heat transfer area and ensure the heat transfer efficiency. However, the manufacturer simply applied empirical formulas, resulting in an insufficient heat transfer area of the equipment. As a result, the equipment fails to meet the heat requirements during the evaporation process, ultimately causing the water evaporation volume to fall far short of the designed production capacity.

02: Two 500-ton evaporators of an alumina plant in Hebei: Seasonal crises triggered by deviations in heat balance calculations

The two 500-ton evaporators at an alumina plant in Hebei exhibit seasonal operational disparities: they can barely sustain production in winter but suffer a sharp decline in evaporation capacity during summer. The crux lies in the manufacturer's miscalculation of the evaporation volume at the final effect. In their heat balance calculation sheet, the evaporation volume at the final effect was set at only 81 tons, far lower than the actual requirement. This value directly dictates the scientific allocation of cooling water, and the erroneous estimate led to insufficient cooling capacity. In winter, the lower ambient temperature allows the system to maintain a vacuum of around -0.088 MPa. However, in summer's high temperatures, the inadequate cooling capacity causes the vacuum to plummet, impeding the evaporation process and resulting in subpar evaporation volumes.

II: Common "Pitfalls" in Unprofessional Evaporator Design

01: Over-reliance on experience with a lack of scientific calculations

During the design process of evaporators, there is an over-reliance on past experience, simply applying empirical parameters such as evaporation intensity without conducting thermodynamic calculations tailored to specific operating conditions. In the alumina production process, factors such as raw material composition, processing volume, and process conditions are complex and variable. Designing solely based on experience can easily lead to a disconnection between equipment parameters and actual requirements.

02: Inadequate understanding of the characteristics of multi-effect evaporators

There are complex heat transfer and material balance relationships among the various effects of multi-effect evaporators. Parameters such as inter-effect temperature differences and pressure distribution directly affect the equipment's performance. Unprofessional designs often overlook these characteristics. For example, in the design of seven-effect evaporators, the increased requirement for heat transfer area caused by the reduction in inter-effect temperature differences is not fully considered, resulting in inherent deficiencies in the equipment.

03: Miscalculation of key parameters

Key parameters such as the evaporation volume at the final effect directly determine the rational configuration of cooling and vacuum systems. A miscalculation in these parameters can trigger a chain reaction: insufficient cooling water fails to maintain the required vacuum, reducing evaporation efficiency, while excessive cooling water leads to energy waste and increased operational costs.

04: Lack of systematic design thinking

Evaporator design is a systematic engineering project that involves multidisciplinary knowledge such as heat transfer, fluid mechanics, and thermodynamics. Unprofessional design teams often view individual components in isolation, ignoring the coordinated operation of the entire equipment. This oversight leads to various "incompatibility" issues during the actual operation of the equipment.

III: Pitfall Avoidance Strategies for the Alumina Industry

01: Strictly screen design teams

Prioritize teams with extensive experience in designing evaporators for the alumina industry and professional thermodynamic calculation capabilities. Review their past project portfolios, focusing on successful designs for similar scales and operating conditions to ensure they can handle complex  operating conditions.

02: Strengthen Control over the Design Process

1. Require detailed calculation reports: During the design phase, demand that the design party provide a complete and detailed heat balance calculation report, covering the calculation processes of parameters such as temperature, pressure, flow rate, and heat transfer area for each effect.

2. Enterprises can organize internal technical teams or hire external experts to review the reports, ensuring the scientific validity and accuracy of the calculations.

3. Establish a communication mechanism for design changes: During the design process, if any issues are identified or process conditions change, promptly communicate with the design party, request them to adjust the design plan, and conduct new heat balance calculations and performance evaluations to ensure that the design always aligns with actual requirements.

03: Attach Importance to Equipment Performance Testing and Verification

After the equipment is manufactured, conduct performance tests strictly in accordance with the design parameters. Simulate actual production conditions and comprehensively inspect key indicators of the evaporator, such as evaporation volume, energy consumption, and vacuum degree. If the performance fails to meet the standards, promptly communicate with the design and manufacturing parties, requiring them to analyze the causes and make rectifications to ensure that the equipment can meet production requirements when delivered for use.

In conclusion, to avoid the many problems caused by unprofessional evaporator design in the alumina industry, it is essential to address the issue from the source and strictly control the design process. Only by scientifically screening design teams, strengthening process management, emphasizing performance verification, and enhancing the enterprise's own technical capabilities can efficient and stable evaporator equipment be developed, providing a solid guarantee for the efficient operation of alumina production.