Analyzing the Importance of Enhancing Energy Efficiency in Air Compressors

Published on: 2024-09-24      Views: 224

Air compressors play a critical role in modern industrial production, serving as a key source of pneumatic power across various sectors such as machinery manufacturing, petrochemicals, food and pharmaceuticals, electronics, and transportation infrastructure. According to relevant data, air compressors account for approximately 8-10% of industrial electricity consumption. However, only 66% of this energy is used effectively, with 34% being wasted. Additionally, over the life cycle of an air compressor—typically 8 to 10 years for oil-injected screw compressors—purchase costs account for only 5% of the total expenses, maintenance costs for around 18%, while operational electricity costs make up a staggering 77%. Given this, implementing energy-saving measures for air compressors is of paramount importance.

China's current energy efficiency standards for screw compressors, as outlined in the GB19153-2019 regulation, were officially implemented on July 1, 2020. These standards stipulate that the lower the specific power consumption, the higher the energy efficiency. The standard classifies air compressors into three efficiency levels: Level 1, Level 2, and Level 3. For oil-injected screw compressors, there is an approximate 10% efficiency difference between levels, varying depending on discharge pressure and rated power.

Mainstream methods for improving the efficiency of compressed air systems include selecting appropriate compressor models, configuring post-processing equipment based on actual air usage, optimizing air pipeline layout, using intelligent centralized control, preventing leaks, recovering residual heat, and ensuring high-quality maintenance. Of course, if the budget allows, investing in compressors with higher energy efficiency ratings is the most effective way to save energy.

Over the past decade, oil-injected twin-screw compressors have experienced rapid development due to the accelerated localization of key technologies. These compressors, with power ranges from 7.5 to 355 kW, have largely replaced traditional piston compressors, becoming the mainstream models in the market. The domestic market sees annual sales of around 500,000 units, with a growth rate of about 10% per year. Against this backdrop, improving the energy efficiency of screw compressors holds significant practical importance.

While screw compressors consist of many components, the most critical factor affecting overall energy efficiency is the screw host, also known as the “Airend.” Currently, twin-screw air compressors on the market are mainly divided into single-stage and two-stage compression models. Single-stage compression is the earliest and most widely used type, but in large-scale operations requiring high output, two-stage compression models have become increasingly popular alongside permanent magnet variable frequency screw compressors. In recent years, many companies in the industry have sought to be included in the "National Industrial Energy-Saving Technology and Equipment Recommendation Catalog" and the "Energy Efficiency Star" program, with many of the selected high-efficiency compressors being two-stage models.

The working principle of a two-stage screw compressor involves combining the first-stage and second-stage compression rotors within a single housing, both driven directly by helical gears. Ambient air enters the first-stage compressor through an air filter, where it mixes with a small amount of lubricating oil in the compression chamber and is compressed to intermediate pressure. The compressed air is then cooled by contact with oil mist, significantly reducing its temperature. After cooling, the air enters the second-stage rotor for further compression until it reaches the final discharge pressure, completing the entire compression process. Compared to single-stage models, two-stage screw compressors offer unique advantages.

First, staged compression reduces compression work. The two-stage screw compressor divides the compression process into two stages, reducing the compression ratio in each stage. This effectively lowers the resistance encountered during the engagement and compression of the screw rotors, which in turn reduces the load on each bearing, thus lowering the power required for each stage of compression.

In theory, the total power required for single-stage compression equals the sum of the power required for multi-stage compression. However, in practice, power losses from couplings, bearing friction, and coolant viscosity all increase with greater force, resulting in inefficiencies. Therefore, by reducing the compression ratio in each stage, the useless work generated during the compression process can be minimized, allowing the total power required for multi-stage compression to be less than that for single-stage compression.

Secondly, intermediate oil cooling lowers the temperature of the air entering the next stage. All gas compression processes involve friction between the gas and moving parts, generating heat. This heat causes the gas to expand, increasing the internal pressure. By cooling the gas between stages, two-stage compressors prevent unnecessary expansion, enhancing overall energy efficiency.

The temperature increase in compressed gas naturally raises its pressure during the compression process, thus increasing the compression ratio. This inevitably requires additional power to drive the equipment to compress the air to the desired pressure. To address this, two-stage screw air compressors are equipped with a cooling injection system. After the first stage of compression, a mist of coolant is injected into the compressed air, lowering its temperature before it enters the second stage. This cooling mechanism effectively mimics the impact of reducing environmental or intake air temperatures for single-stage compressors. Not only does this significantly reduce the energy needed for secondary compression, but it also eliminates the need for an intermediate cooler.

Moreover, the cooling mist not only lowers the temperature of the compressed air but also reduces the overall temperature of the entire compression system. This leads to less coolant evaporation, helping the lubricating oil maintain optimal performance for extended periods and reducing both maintenance frequency and costs.

In recent years, data from the National Compressor Testing Center shows that two-stage compression air compressors perform significantly better in terms of energy efficiency than single-stage models, especially in machines over 110 kW. Single-stage compressors in this power range often struggle to achieve a Level 1 energy efficiency rating and sometimes even fall short of Level 2. However, with the implementation of two-stage compression technology, high-powered air compressors can more easily reach Level 1 efficiency. Industry data indicates that two-stage screw compressors, depending on their power output, are at least 10% more efficient than single-stage models, with an average improvement of about 15%. Some models even achieve up to a 20% efficiency boost.

This significant increase in efficiency with two-stage compressors stems not only from the aforementioned cooling injection system, which brings the compression process closer to an isothermal compression model, but also from another key factor: reduced internal leakage and improved volumetric efficiency.

Improving the energy efficiency of air compressors is a comprehensive process and cannot be achieved by focusing on one or two aspects alone. Beyond adopting two-stage compression, efficiency can also be enhanced by optimizing rotor profiles, improving rotor machining accuracy, and minimizing rotor engagement gaps and exhaust end clearances to reduce internal leakage. However, it’s important to note that not all two-stage screw compressors are the most energy-efficient. Some companies have achieved high energy efficiency using single-stage compressors. Systematic energy-saving efforts depend on meticulous attention to every step, from product development and testing to manufacturing and assembly, as well as the dedication of all participants to continuous improvement.