I. What does energy saving in compressed air systems mean?
1. The Context of Carbon Emission Reduction and the “Carbon Zero” Goal
Today, the global environment is being seriously affected by climate change and increasingly frequent natural disasters. The rise in carbon emissions has led to major problems such as the greenhouse effect and urban heat island phenomena. At the same time, uneven energy consumption and shortages have made the transition to new energy structures—particularly away from coal—more urgent than ever.
To address this issue, China has set the goals of “carbon peaking” and “carbon neutrality” as part of its 14th Five-Year Energy Development Plan. The industrial sector accounts for approximately 65% of the country’s total energy consumption, making it a key focus for energy saving and carbon emission reduction. Among industrial energy sources, compressed air ranks just after electricity in importance. It plays a vital role in daily operations and is indispensable in industrial production.
2. Energy Saving in Compressed Air Systems
A compressed air system consists of key components such as the air compressor, air receiver tank, filters, and dryers. Energy-saving measures can be applied across various parts of the system—including the compressor, control systems, drying processes, and filtration units.
This article will focus on three main energy-saving technologies: (1) Inlet air treatment for compressors, (2) Cooling and heat recovery in air compressors, (3) Post-compression air treatment
II. Pre-treatment of Compressor Inlet Air
1. Core Concept of Compressor Inlet Air Pre-treatment
The pre-treatment of compressor inlet air primarily addresses challenges posed by high temperature and humidity during summer. When ambient air temperature rises, it leads to a series of issues such as increased energy consumption, reduced operational efficiency, and elevated discharge air temperature. Excessively high inlet air temperature can also cause the compressor to malfunction. Therefore, inlet air pre-treatment not only improves operational stability but also significantly contributes to energy savings.
Moreover, there is a growing contradiction between high power consumption and limited electricity supply, along with rising energy costs. Studies have shown that inlet air temperature has a direct impact on power consumption: for every 1°C reduction in inlet temperature, energy consumption can decrease by approximately 0.65%. Thus, developing effective solutions for inlet air pre-treatment—especially in regions with hot climates—is a meaningful approach to improving energy efficiency.
The main goals of inlet air pre-treatment are to reduce temperature and humidity. But is it truly necessary to reduce humidity? To what extent should the temperature be lowered? Let’s analyze this from a theoretical perspective. Assuming ambient air pressure is 100 kPa, temperature is 38°C, humidity is 21.75 g/kg, isentropic efficiency is 0.72, and the cooling equipment’s coefficient of performance (COP) is 3.4.
When the inlet air temperature drops from 38°C to 18°C and humidity decreases from 21.75 g/kg to 17.75 g/kg, the specific power consumption (i.e., power required per unit air flow) decreases from 6.98 kW/(m³·min) to 6.27 kW/(m³·min).
Considering the impact of inlet air temperature and humidity on compressor energy consumption, it is clear that temperature has a significantly greater effect than humidity. Therefore, the core objective of pre-treatment should be to reduce temperature, while humidity control is not particularly necessary due to the high cost involved.
This is because when the surface temperature of the heat exchanger is lower than the dew point, condensation may occur, releasing a large amount of latent heat and drastically increasing the cooling load. In other words, achieving the same cooling result would require much higher cooling capacity if condensation takes place.
As such, the key approach to inlet air pre-treatment is: focus on reducing temperature, not humidity. Reducing humidity only yields limited energy-saving benefits. This principle also highlights the difference from conventional air conditioning products—using an air conditioner alone to pre-treat inlet air is not entirely appropriate.
Air conditioners can lower the temperature slightly, requiring a cooling source at a relatively higher temperature. For example, studies have shown that using an air conditioner to reduce inlet temperature from 35°C to around 28–29°C is quite feasible, but it should not be lowered below 26°C. This solution is both simple and practical, offering real-world energy-saving benefits.
2. Drawbacks of Traditional Cooling Methods
Currently, traditional cooling methods exhibit several limitations:
High energy consumption and operating costs, with low control precision.
Significant variation in energy usage of inlet air treatment systems across different regions.
Lack of theoretical guidance for designing or selecting inlet air treatment equipment.
Incomplete evaluation methods and criteria; no standardized products currently available.
3. Comparison of Inlet Air Pre-treatment Solutions Using Different Cooling Sources
3.1. Analysis of Energy Saving Potential
The pre-treatment of compressor inlet air using vapor-compression cooling systems can directly utilize commercially available air conditioning units as pre-treatment equipment. However, due to their limited cooling capacity, these air conditioners are only suitable for applications with low cooling load requirements.
Compared to conventional air conditioners, chiller systems (water-cooled chillers) offer higher cooling capacities and efficiency, along with more stable and reliable performance. However, they require the installation of additional cooling water circulation pipelines, which increases the system’s complexity in terms of management and maintenance.
This solution can control the inlet air temperature by adjusting the flow rate and temperature of the cooling water entering the air–water heat exchanger. The circulating water is pre-cooled by a closed-loop cooling tower, which helps reduce the auxiliary energy consumption of the inlet air treatment system. However, the cooling effectiveness of the tower can be affected by ambient conditions such as air temperature and humidity. Adding a closed-loop cooling tower and a water circulation system also increases equipment investment costs and system complexity.
For comparison, two regions with distinct climates—Nanjing and Urumqi—were selected. After applying the inlet air pre-treatment solution, the specific power consumption of the compressed air was significantly reduced. In Nanjing, the maximum reduction reached 2.32%, while in Urumqi it was 2.58%. This indicates that although the difference between solutions is not substantial, in dry climates, evaporative cooling solutions demonstrate a clear advantage.
3.2. Economic Feasibility Analysis
Under the climate conditions of Nanjing and Urumqi, Solution 3 had the shortest payback periods—23.1 months and 15.3 months, respectively. The use of a cooling tower for pre-cooling significantly improves the system’s economic efficiency.
This shows that if the compressor system operates continuously year-round (365 days), the investment payback period will be very short. However, if the system is only used during the summer, investing in a dedicated cooling system may not be economically viable due to its low utilization rate, resulting in a prolonged payback period.
Solution 3—a hybrid pre-treatment system—is not suitable for applications with low air flow, especially those under 60–80 m³/min. In such cases, Solution 1—vapor-compression cooling—is more feasible.
Conversely, the hybrid solution is particularly suitable for large-scale compressor stations, where high air volume and performance are required. In general, with this solution, the investment payback period is usually no less than 10 months.
4. Using a Refrigerated Air Dryer (Cold Dryer) for Inlet Air Pre-treatment of Compressors
Based on the above analyses, a new idea has emerged: there is no need to install a separate dedicated cooling source. In many factories, air conditioning systems are already available, so the cooling source from these systems can be redirected and used to cool the inlet air. However, this solution has limited applicability and may not always be feasible.
This led to another new idea: adding an evaporator to the refrigerated air dryer, effectively equipping the compressor with a “mini air conditioner.” The air is pre-cooled by the evaporator before entering the compressor, which helps reduce the inlet air temperature efficiently.
The first evaporator uses a finned tube structure. When the surface temperature of the evaporator drops below the dew point of the air, water vapor condenses and forms a thin water film on the evaporator surface, which reduces heat exchange efficiency.
Based on the condensation coefficient, the amount of condensed water vapor can be calculated after the inlet air passes through the pre-treatment step. From there, the optimal inlet air temperature can be determined.
4.1. Evaluation Method
The evaluation conditions are set as follows: when the ambient temperature exceeds 30℃, the inlet air pre-treatment system is activated; the compressor’s inlet air flow rate ranges from 45–55 m³/min, and the exhaust pressure is 0.8 MPa. The air is cooled to 26℃ before entering the compressor. The refrigerant evaporation temperature varies with the inlet air temperature, and the temperature difference between the air and the refrigerant is set at 10℃.
When the ambient temperature exceeds 30℃ and the inlet air temperature is controlled at 26℃, the system can save 5747.85 kWh of electricity annually, equivalent to 4598.28 RMB in electricity cost. Thus, the payback period is very short.
The next step is to apply the research results to practical products by designing the structure and proceeding with specific implementation.
III. Compressor Cooling/Heat Recovery
1. Liquid Injection During Compression
One of the key solutions for improving energy efficiency in the main compressor unit is the method of liquid injection during air compression. This injection serves three main purposes: cooling, sealing, and noise reduction. Among them, sealing enhances energy efficiency, while cooling improves thermal control effectiveness. In oil-lubricated compressors, oil is injected continuously throughout the operation. For oil-free compressors, water injection can be considered as an alternative.
Currently, manufacturers on the market offer various liquid injection methods. This article lists three common techniques:
First method: injecting liquid from the suction chamber.
Second method: drilling multiple injection holes along the twin-screw chamber.
Third method: injecting liquid at the intersection of the two rotors.
Although there are multiple injection approaches, there is still a lack of detailed experimental studies comparing and evaluating how the injection location affects overall performance.
From a theoretical standpoint, as the screw rotor speed increases, the contact time between the liquid and the compressed air becomes shorter, requiring higher atomization efficiency (fine mist spray) of the liquid. However, this reduces sealing effectiveness. Therefore, for multi-hole injection designs, functional zoning is necessary. For example:
Large holes primarily serve the sealing function.
Small holes focus on fine mist atomization to aid cooling.
Such zoning and dedicated functional control can result in higher overall system efficiency.
2. Heat Recovery from Air Compressors
Heat recovery from air compressors primarily involves two main heat sources: the heat from the lubricating oil and the heat from the compressed air. We once conducted experiments using a closed-loop system aimed at directly recovering heat through evaporation, similar to the process of boiling water to produce steam. However, our core design utilizes a heat exchanger to achieve direct evaporative heat recovery.
In addition, the author has designed a semi-closed system that uses water as the heat transfer medium. This system features direct-contact heat transfer, enabling heat recovery without discharging emissions into the environment. In this configuration, the compressed air after the first compression stage, along with a portion of the heat, continues into the second-stage compressor for further compression and energy supply. Simultaneously, this system allows for the adjustment of steam generation and the direct production of hot water.
From a green industrial production perspective, one of the key challenges faced by our industry is the issue of waste oil emissions and wastewater treatment. To address these concerns, part of the recovered heat can be used in the humidification and dehumidification process, known as HDH (Humidification-Dehumidification Heating). Compared to traditional radiators, HDH systems have lower anti-corrosion requirements because the method mainly uses hot air to evaporate the water contained in wastewater.
During this process, water from the wastewater evaporates into the air, increasing the air’s humidity. The moist air is then cooled, allowing the water vapor to condense at a lower temperature, thereby achieving the goal of wastewater treatment. This method not only reduces waste oil emissions into the environment but also improves air quality, making it a green and highly efficient solution.
IV. Post-Compression Air Treatment
Post-compression air treatment essentially includes two aspects: filtration and drying. Filters are used to remove oil, while dryers eliminate water vapor. Today’s market offers a wide range of dryers, including adsorption, refrigeration, and hybrid types. The core objective of these devices is to meet pressure dew point requirements while ensuring energy-saving performance, reducing electricity consumption and pressure losses, and lowering both equipment investment and maintenance costs.
Nowadays, air compressors have shifted toward inverter (VSD) technology, and many types of dryers have also adopted inverter-based systems. As a result, the entire compressed air system can operate with variable speed control. In practice, even though our products are energy-efficient, improper use by end users can still lead to energy waste. Therefore, scientific and systematic control is crucial. Additionally, reducing pressure loss at the heating component in refrigerated dryers is also a key issue.
For adsorption dryers, the choice of desiccant material is critical. In HVAC systems, desiccant wheels are commonly used for dehumidification. However, desiccants typically exist in gel form, and their performance and quality can vary depending on the production batch and process technology. To accurately evaluate desiccant performance, scientific methods and technical assessments are required.
With advancing technology, it’s essential to select and configure new technologies appropriately based on specific pressure resolution requirements. There remains significant work to be done in researching regeneration methods for adsorption dryers and improving structural design to enhance efficiency and energy savings.
Moreover, hybrid dryers have become a technology worth noting. These systems combine the advantages of both refrigeration and adsorption methods. The key issue lies in the theoretical foundation and practical application of heat recovery. The author recently participated in a hybrid dryer project at a company in Zhejiang Province. The dryer was used to dehumidify and provide anti-corrosion protection for the compressed air system of a bridge engineering project.
Currently, most compressed air systems use static methods. Compared to that, rotary wheels—a dynamic method—have unique advantages but also face challenges, particularly in sealing effectiveness. Therefore, we are experimenting with the application of rotary wheel technology in low-pressure blower systems, aiming to integrate cooling, heating, and rotary wheels to achieve energy-saving goals.
At present, our team is working on structural design, production preparation, and planning for testing.
At the same time, the author is considering how to integrate energy-saving technologies and experiences from other industries into the air compressor field. Since rotary wheels are modular in design and allow for individual wheel replacement, they significantly enhance user convenience. It is believed that through continuous exploration and innovation, we can bring more technical breakthroughs and energy-saving solutions to the air compressor industry.
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