A Study on the Causes and Treatment of the Decline in Operating Efficiency of Screw Air Compressors

Against the backdrop of increasing energy scarcity, countries have reached a consensus in areas such as energy saving and carbon reduction, and have put forward higher requirements for industrial energy saving. Screw air compressors, as a kind of common industrial equipment, generate high energy consumption on a daily basis and need to tap into the potential of energy saving by improving the operational efficiency of the equipment. Therefore, it should focus on how to improve the operating efficiency of the equipment and systematically analyze the reasons for the decline in the operating efficiency of the equipment, so as to realize the upgrading and transformation through the operating efficiency and scientific evaluation mechanism.

1. Overview of the project

Zhaotong Cigarette Factory adopts screw air compressor for air compression and delivery, the air compressor uses four-stage compression, three-stage cooling design, the intermediate cooler are arranged on both sides of the air compressor. The equipment models are ZR5R-52 (4740Nm³/h), ZR500VSD (3600Nm³/h), ZR400 (3600Nm³/h), and ZR4-51 (1800Nm³/h), with an overall gas production of 13740Nm³/h. Since its commissioning, the equipment has been maintained in good operating condition for a long time, and the parameters such as exhaust pressure and flow rate basically meet the requirements. However, recently, the outlet flow rate of the air compressor has continued to drop and could not reach the design flow rate (see Table 1), and there is a problem of reduced efficiency in the operation of the equipment.

2. Reasons for the decline in operating efficiency of screw compressors

2.1 Analysis of operating problems

The air compressor system consists of several parts such as the air compressor, refrigerated air dryers, filter W, piping system, and drain. Even if the main engine reaches a high specific power, in the case of other equipment with large resistance losses, the motor operating efficiency will decline, thus triggering a reduction in the efficiency of the whole machine. Evaluation of the operating efficiency of the air compressor can be seen

The above formula(1): ω is for the host power (kW); N is for the host shaft power (kW); Q is for the host volume flow (m³/min). An analysis of the relationship between the inputs and outputs of the air compressor shows that, the specific power input of the air compressor under the specified operating conditions satisfies

In the above formula: ω’ is for the input specific power [kW/(m³*min)]; N’ is for the whole machine input power (kW); Q is for the actual volumetric flow rate (m³/min). Analysis of the actual situation of the air compressor shows that since the operation of the air compressor, the failure rate is low, and the input specific power can reach 4.58[kW/(m³*min)], and the overall performance is good. It can be seen that the air compressor unit appears to be operating at a reduced efficiency, mainly affected by other factors.

2.2 Analysis of system leakage problems

During the operation of an air compressor system, pressure stabilization needs to be maintained, if pressure fluctuations occur, they can also adversely affect the operation of the mainframe, leading to problems with frequent starting and stopping, loading and unloading of the system, and subsequently a reduction in operational efficiency. Based on the data in Table 1, it can be seen that the outlet pressure of the unit decreases significantly in 2022, which occurs in relation to a decrease in the airtightness of the air line connections, This leads to “bubbles” and “leaks” in the system and pressure drop problems, which result in significant energy consumption. As can be seen from Table 2, when there are different sizes of air release holes, as the diameter of the holes increases, a larger leakage will be triggered, leading to a consequent decrease in the exhaust pressure, which in turn triggers a decrease in the operating efficiency. Then we check the pipe connecting flange, shaft end seal and other places, and found no obvious leakage. Therefore, it was determined that a leak had occurred within the system, causing the unit to deviate from its original design operating conditions.

2.3 Analysis of system blocking problems

Problems with the system experiencing a drop in exhaust volume may be related to a blockage in the system. In the case of high filter resistance, this will result in a decrease in the suction pressure of the compressor, triggering an increase in the pressure ratio and ultimately a decrease in the exhaust volume. A check of the pressure in the upper tower of the air separation plant shows that there is no significant pressure loss, therefore, system filter blockage is ruled out. In addition, problems such as excessive ash buildup on the guide vane structure can also trigger a decrease in exhaust volume. And as evidenced by the February 2020 overhaul records, the guide vanes are cleaner overall and there are no structural anomalies.

2.4 Analysis of system cooling problems: Screw Air Compressors

In the case of insufficient cooling water in the system, the same will lead to a reduction in exhaust. According to the parameters of the equipment, the design flow rate of the cooler is 1200m³/h, and the circulating water pressure is 0.35MPa. The actual water intake reaches 1400 m³/h, and the water pressure of the mother pipe reaches 0.45MPa. The cooling water entering the air compressor is designed for a maximum temperature of 33°C. The actual maximum summer temperature can reach 41°C, requiring more cooling water. Inadequate cooling water supply is related to cooling water leaks and suboptimal heat transfer from the cooler.

The air compressor unit contains three stages of coolers. Leaks in the first two stages will cause problems such as clogging and scaling, and leaks in the latter stage will cause a reduction in exhaust gas flow. Upon inspection, there is no leakage problem. As can be seen from the pressure changes in the air compressor unit and at various levels in recent years (see Table 3), there was a vapor-side fouling problem, and inspection revealed that the fins were peeling off due to corrosion, which triggered a pipe blockage. When the gas passes through the fins and tubes to exchange heat with the circulating water, the blockage of the tubes results in poor heat exchange, which ultimately leads to a decrease in the exhaust volume.

3. screw air compressor operating efficiency degradation problem processing

3.1 Doing a good job on internal leaks

The problem of internal leakage in air compressors is related to the condensate produced by the equipment during the compression of air. Condensate contains heavy metals, sulfides, and other highly corrosive substances, which will remain inside the equipment for a long period of time if they cannot be discharged smoothly, resulting in corrosion of the body and system piping, which can lead to serious downtime problems. In order to determine the location of internal leakage, we can analyze the internal structure of the air compressor. The intercooler uses rectangular fins, which are expanded with the heat exchanger tube to form a unit heat exchanger tube. The unit heat exchanger tube constitutes a tube bundle, with water flowing inside the tube and exhausting outside the tube, and the tube wall is thinned after a long period of time, which triggers the internal pipeline leakage problem.

Internal leakage causes the air path to carry water, which cannot be effectively separated through the separator when the water content is high, and is suspended on the separator when it enters the next stage of compression, resulting in a drop in airflow. After disassembling the unit, the piping will be inspected along the air flow, and cracks and holes will be patched and welded if found. For the thinned pipe wall, considering the poor effect of patch welding, both sides will be blocked and the pipeline will be replaced to avoid recurrence of internal leakage within a short period of time afterwards. To avoid the recurrence of similar problems, it is necessary to ensure that the condensate is discharged in a timely manner.

The condensate from the air compressor system before the retrofit was mainly discharged manually, which would have triggered prolonged venting losses if the manual valve had been left in the open position at all times. It was confirmed on site that the manual valve discharge caliber reached 4mm and gas loss reached 0.745m³/min. In order to reduce the waste of resources, we remodel the condensate discharge pipeline, and add an electronic level control drainer to the pipeline, which automatically opens the valve and discharges according to the level of condensate in a timely manner. By adopting fully automated means, it is possible to reduce venting losses while avoiding internal leakage problems. In addition, in order to avoid unsuccessful discharge of condensate due to device failure, a bypass pipe is added to the pipeline. And add a drain check item in the inspection content, check every 4h and open the bypass valve in time to discharge the condensate when a fault is found, which effectively eliminates the occurrence of internal leakage and ensures that the compressor system can operate efficiently.

3.2 Improvement of heat transfer efficiency: Screw Air Compressors

Affected by the ambient temperature, air-trapped water and other issues, Screw Air Compressors are prone to a decline in the heat transfer efficiency of the cooler, which in turn leads to a decline in the unit’s exhaust volume and operating efficiency. In order to improve the heat transfer efficiency, the primary cooler needs to be replaced to improve the heat exchanger heat transfer efficiency. The heat exchanger temperature efficiency fulfills

In the above formula(3): η is the heat transfer efficiency; t1 and t2 are the temperatures of the heated mass in and out (℃); T1 and T2 are the temperatures of the heated mass in and out of the heat exchanger (℃); Through measurements and calculations, it can be seen that the original primary cooler heat transfer efficiency is only 80.33%, after replacing the high-efficiency cooler, the heat transfer efficiency can reach 98.72%, which effectively reduces the primary exhaust temperature. After measurement, it can be seen that the water inlet temperature of the secondary cooler is increased from 28℃ to 29.7℃ and the discharge temperature is reduced from 38.3℃ to 37.8℃ after the modification, the heat transfer efficiency was increased from 78.0% to 90.5%. And the drain temperature of the tertiary cooler was increased from 38.5°C to 39.8°C, and the heat transfer efficiency was increased from 71.6% to 75.1%. By increasing the heat transfer efficiency of the system, the problem of insufficient cooling water supply can be avoided in the case of high summer temperatures. In addition, considering that the cooler fins are susceptible to corrosion and cannot be simply repaired with fins, and appropriately increase the gas through the area, to achieve the goal of strengthening the cooler heat transfer.

In short, it is the choice of aluminum + anticorrosive coating fins, heat exchanger tubes are made of iron white copper, the heat exchange area increased from 1420m2 to 1510m2, and the number of heat exchanger tubes increased from 460 to 570. After completing the air compressor overhaul, it is necessary to carry out a comprehensive cleaning of the cooling water pipes, air runners, impellers, etc., in order to ensure that the internal piping is smooth and to avoid clogging.

3.3 Analysis of operational efficiency evaluation: Screw Air Compressors

After a series of modifications, a commissioning test is required to determine the optimization of the performance of the air compressor system, it can be found that the outlet pressure of the unit reached 608kPa and the gas production reached 13740Nm3/h, which meets the production requirements. The operating efficiency of the air compressor was evaluated before and after the improvement (see Table 4), and the exhaust volume increased significantly as the exhaust pressure increased.

Under the condition that the input power of the whole Screw Air Compressors remains unchanged, the input specific power can be further reduced by increasing the actual volumetric flow rate, so as to achieve the purpose of improving the operating efficiency of the compressor. Improve the system operation efficiency by 10%. In the case of ignoring the problem of emptying, the air compressor current is reduced by 6A under the same load, which can save 94kW-h of electricity, and according to the electricity price of 0.65 yuan/(kW-h), it can save about 500,000 yuan per year.

In order to ensure that the air compressor is maintained in good operating condition on a daily basis, it is necessary to establish inspection standards and analyze the economic data of equipment operation. The operating efficiency is evaluated by analyzing the mainframe power and input power, and by looking at parameters such as the inlet resistance and the temperature of the interstage gas after cooling we can confirm that there are no problems such as blockages or leaks.

The establishment of the nudge diagram can visualize the changes in equipment operation data, and can detect equipment abnormalities in a timely manner, and ensure that the equipment always maintains a high operating efficiency through timely intervention. In addition, in response to the decline in the operating efficiency of the Screw Air Compressors, it is also necessary to optimize the performance of the unit, taking into account that the impeller efficiency will continue to decline with the extension of the service life.

In fact, in the process of designing the air compressor, there is usually a margin in terms of flow rate and outlet pressure, etc. In the case of repeated superposition of the margin there will be a problem of deviation of the actual operating point from the design value, which will lead to a decrease in the operating efficiency of the unit. In conjunction with this characteristic, the unit outlet pressure selection can be subsequently redone, the impeller and other components are then fabricated based on the new outlet pressure to improve the overall efficiency of the unit’s operation without replacing other system components.

4. Conclusion

In response to the problem of decreasing operating efficiency of screw air compressors, we can find the root cause through scientific evaluation, and use internal leakage treatment and improve heat exchange efficiency to effectively solve the problem, and realize the goal of maintaining the efficient operation of air compressors. Combined with the data for systematic analysis, we can put forward the future direction of technical research from the perspective of improving the operating efficiency of air compressors, and accumulate rich experience for realizing the upgrading and transformation of the same type of equipment. Read here.