Waste-Heat Recovery Technology and Engineering Practice

In the production systems of industrial and mining enterprises, oil-injected screw air compressors are core power equipment that operate continuously throughout the year. Industry data shows that only about 15% of the input electrical energy is converted into effective work for compressed air, while the remaining 85% of energy is lost as waste heat. Traditionally, most companies use air-cooling or direct water-cooling methods to dissipate heat, leading to significant waste of low-grade thermal energy, as well as issues such as heat accumulation in workshops, increased environmental temperatures, and higher energy consumption for cooling fans.

Compared to the simple single-stage heat exchange mode with direct contact between hot and cold media, the independent closed-loop system featuring a primary oil-to-water heat exchange followed by a secondary water-to-water heat exchange is currently more stable, durable, and suitable for complex water quality conditions in industrial settings. This process uses two completely independent circulating water circuits to isolate the air compressor’s internal circulation from the terminal heating circuits in the factory, ensuring a stable output of low-temperature hot water at 60°C. This effectively avoids issues such as water quality corrosion, scaling, and cross-contamination. The secondary heat exchange system used in this application has an independent circulation structure, with the two water circuits not connected and operating independently, solving the problem of water quality interference from a structural standpoint:

First Stage: Primary Oil-Water Heat Exchange (Closed-loop circulation within the air compressor) uses the air compressor’s lubricating oil as the heat source and dedicated pure circulating water as the medium, completing the initial heat exchange through an oil-water heat exchanger. This closed-loop circuit ensures stable water quality without external contamination, specifically responsible for absorbing waste heat from the air compressor and heating the circulating water to approximately 60°C, ensuring stable cooling conditions for the air compressor and protecting the main unit and oil circuit system.

Second Stage: Secondary Water-Water Heat Exchange (Closed-loop circulation at the factory heating end) uses the 60°C water generated in the first stage as an intermediate heat source, transferring heat to the factory’s terminal water use (boiler makeup water, drying cycle water) through a plate heat exchanger. The terminal water only circulates in the second loop and does not come into contact with the air compressor’s core heat exchange equipment. This dual-system isolation design is the core advantage of the secondary heat exchange: it completely avoids corrosion, blockage, and wear of the precision heat exchanger plates due to industrial wastewater, significantly reducing equipment failure rates and maintenance costs. Additionally, it ensures that the heating and usage ends do not interfere with each other, allowing the air compressor to operate without being affected by fluctuations in terminal heating, resulting in more stable heating temperatures. The entire system’s long-term operational stability is far superior to single-stage heat exchange schemes, making it very suitable for 24/7 continuous production in factories.

Case 1: Air Compressor Waste Heat Used for Preheating Boiler Makeup Water (1) Project Site Conditions A large manufacturing company’s compressed air station is equipped with three 250kW oil-injected screw air compressors (SIND SILVERLUBE S250A), operating in a 2-on-1-off mode throughout the year, with an annual operating time of about 8000 hours. The factory’s production steam relies on a gas boiler system, which requires continuous replenishment of softened makeup water. In the original production process, the initial boiler makeup water temperature was only around 10°C, and cold water was directly fed into the boiler, requiring a large amount of natural gas to heat and pressurize it. If a single-stage heat exchange were used, even though the boiler makeup water is softened, it still contains trace impurities, which can cause scaling and reduced heat exchange efficiency over time if directly connected. Therefore, the company chose an independent secondary heat exchange solution, using the stable 60°C waste heat source to preheat the boiler makeup water, with a controllable preheating range of 10°C to 50°C. (2) System Transformation Process The project strictly follows the design of a dual-loop structure with “primary oil-water heat exchange and secondary water-water heat exchange.” The first stage closed-loop continuously absorbs the waste heat from the air compressor’s lubricating oil, producing 60°C hot water; the second stage uses an independent water-water heat exchange unit to indirectly preheat the boiler makeup water using the intermediate heat source. The entire system is completely physically isolated, with the boiler makeup water not entering the air compressor’s heat exchange circuit, eliminating the risk of corrosion and scaling. During operation, the system first uses the waste heat recovery oil circuit to heat the first stage to 60°C; then, through secondary heat exchange, it stabilizes the 10°C cold makeup water to 45-50°C before sending it to the boiler. After the transformation, the boiler no longer needs to significantly increase the temperature from zero, only requiring a small amount of additional heating to reach the production process temperature, reducing fuel consumption at the source. Additionally, the dual-loop structure ensures that the air compressor maintains good cooling conditions throughout the year, eliminating equipment failures caused by seasonal temperature differences and water quality variations. (3) Energy Saving and Operational Benefits Analysis From the actual operational results, the waste heat preheating reduces the boiler makeup water temperature increase by nearly 40°C, effectively lowering the boiler combustion load and significantly reducing natural gas consumption. The long-term fuel savings are considerable. More importantly, the secondary heat exchange structure brings hidden operational value. Previously, single-stage heat exchange required annual shutdowns for cleaning the heat exchanger and clearing the pipes, with scaling and corrosion leading to a gradual decline in heat exchange efficiency. After the transformation, the air compressor’s closed-loop circuit has clean water without impurities, with the heat exchanger showing minimal scaling and corrosion, resulting in extremely slow equipment degradation and significantly reduced annual maintenance costs and downtime losses. Additionally, the preheated water reduces the cold-heat shock to the boiler, minimising thermal stress on the boiler body, slowing down the ageing process, and significantly improving the overall system’s safety, stability, and lifespan.

Case 2: Air Compressor Waste Heat Used for Drying Room Temperature Control (1) Project Site Conditions A deep processing company’s workshop is equipped with three 110kW oil-injected screw air compressors (SIND SILVERLUBE S110A), operating in a 2-on-1-off mode continuously. The workshop has a material drying room with a constant temperature requirement of 35°C. Before the transformation, the drying room relied entirely on electric heaters to maintain a constant temperature, with electric heaters running continuously to ensure uniform material drying and meet moisture content requirements. This led to high electricity costs in the workshop. If a traditional single-stage water-cooling direct heating mode were used, the return air from the drying room would contain dust and moisture, easily causing corrosion of the air compressor’s heat exchange equipment, posing significant equipment risks. The company ultimately chose a secondary heat exchange system, using the stable 60°C waste heat source to meet the low-temperature drying requirements. (2) System Transformation Process This project also uses a closed-loop circulation structure. The first stage oil-water closed-loop heat exchange stably collects waste heat from the air compressor, generating a constant 60°C clean heat source water; the second stage transfers heat to a dedicated circulation water line for the drying room through water-water heat exchange, then uses a heat exchanger for convective heat exchange to provide constant temperature heat to the drying room. The two systems are completely independent, with the drying room’s dusty and humid conditions never coming into contact with the air compressor’s core heat exchange components, completely avoiding corrosion and blockage risks. The system relies on the 60°C heat source’s excess temperature difference, combined with intelligent flow rate regulation, to precisely balance the heat supply, maintaining the drying room temperature at a constant 35°C. When the air compressor load fluctuates and the waste heat production changes, the first loop automatically stabilizes the temperature; once the drying room temperature reaches the target, the second loop automatically reduces the flow to maintain the temperature, ensuring stable drying processes while avoiding overheating risks for the air compressor, suitable for continuous production throughout the year. (3) Energy Saving and Production Benefits Analysis After the transformation, the original high-power electric heaters in the drying room were completely shut down, and the production heating was entirely achieved using the air compressor’s waste heat. The system only consumes a small amount of electricity for pumps and auxiliary equipment, significantly reducing the electricity costs for the drying process. In terms of product quality, the waste heat heating is gentle and constant, with a 35°C constant temperature environment free from localised high temperatures and sudden temperature changes, ensuring uniform and stable material drying, effectively avoiding issues such as cracking, discoloration, and uneven moisture content. This indirectly improves product quality. Leveraging the secondary heat exchange and the independent heat exchange structure’s advantages, the equipment can operate throughout the year without cleaning or maintenance, with extremely low operational failure rates, truly achieving multiple benefits of energy savings, stable production, and low maintenance.