Case Analysis | 6 Reasons For High Air Compressor System Energy Consumption

At present, the air compressor system energy consumption accounts for about 6% ~ 9% of country’s total power generation. It ranks third in high power consumption equipment after fans and water pumps. Now, energy is increasingly tight. Energy saving is an inevitable subject in the air compressor industry. Therefore, it is very import to control the air compressor system energy consumption.  

Compressed air system case

Currently, the compressed air system contain the following equipment: 5 sets of oil injected screw air compressors and 5 sets of heat regeneration combination desiccant air dryers. The system design pressure is 1.0 MPa. The maximum air flow is about 7300 Nm3/h. And design dew point of compressed air dryer≤-40℃. The air compressor pressure setting range is 0.86 ~ 0.93 MPa. The average dosage is about 3500 Nm3/h. The dew point of desiccant air dryer for compressor is about -60°C. Transport them to various air sites through the pipe network. Max air flow of 1#~5# air compressors are as follows: 1152 Nm3/h, 1074 Nm3/h, 1074 Nm3/h, 1326 Nm3/h and 2760 Nm3/h. In addition, the rated power is as follows: 110 kW, 112 kW, 112 kW, 132 kW and 250 kW. Among them, 1# is vsd air compressor. And 2#~5# are power frequency compressors. The rated air capacity of 1#~4# combination adsorption dryer is 1380 Nm3/h. They are rated at 11.5 kW. In addition, the air capacity of the 5# combination absorption dryer is 3000 Nm3/h. Its power rating is 26 kW. At present, operation mode of the air compressor system is blow: 5# air compressor + 2# or 3# air compressor. Three combination regenerative air dryers are running. Another 2 dryers are on standby. The energy consumption of air compressor system is about 288100 (kW·h)/month. It is equivalent to about 400 (kW·h)/h. So the energy consumption per unit air supply is 0.114 (kW·h) /Nm3. Refer to JB/T 50159 “Energy Classification of Compressed Air Stations” to judge. The power consumption index is only third. Operational efficiency is very low. So there is a lot of room for improvement in system performance.  

Why is the energy consumption of the air compressor system high?

The high energy consumption of compressed air is mainly due to the following reasons:

(1) Improper combination of air compressors

At present, the air compressor combination is not reasonable. Use large flow power frequency compressor (5#) with small air volume power frequency compressor (2# or 3#). In addition, the air-use sites are scattered. They are more than 20. Besides, the air use is irregular. Therefore, the total air consumption fluctuates greatly. The amplitude exceeds 20%. Moreover, air compressor loading and unloading frequently. And the motor idling often occurs. The energy consumption during unloading is high. It accounts for about 20% ~ 35% of the full load energy consumption of air compressor system. Therefore, the operation efficiency is low.

(2) Air compressor set pressure is too high

At present, there is only one air-use site that requires the pressure of ≥0.8 MPa. Its usage accounts for about 10% of the total. Other air sites require pressure ≥ 0.70 MPa. But the terminal centralized supply. Therefore, set outlet pressure range at 0.86~0.93 MPa. Overcome filter resistance and pipe resistance. Finally, the air terminal pressure is ≥ 0.81 MPa. We need to meet the high pressure needs of a small volume site. Increase the air compressor set pressure by 0.1 MPa. This is clearly unreasonable.

(3) Energy consumption of air compressor dryer is high

We need to make sure the dew point meets requirements. Therefore, the system uses 5 micro-heat combination dryers for drying. The dryer operation consumes a lot of regeneration air. Air consumption is about 7%. This occurs during desiccant heating and cold purge. In fact, this is the energy consumption of air compressor system. Plus the electricity consumed by the electric heater. Compared with other dryers, the combination dryer consumes higher energy. Assume that the dew point of the compressed air is -60°C. The energy consumption comparison of different dryers is shown in below Table 1. Take the combination dryer with air capacity of 1380Nm3/h as an example. The unit energy consumption is calculated as 0.1(kW·h)/Nm3. It can be seen from Table 1: The energy consumption of the micro-heat combination dryer (second type) is high. It is about 5 times that of the traditional waste heat regeneration combined dryer (third type). In addtion, it is about 10 times that of the new type of waste heat regeneration combined dryer (fifth type). It is only lower than the heatless combination dryer (first type). In the air compressor system, the energy consumption is more than 9%. A single unit can exceed 12 kW of the third type dryer. So 3 units consume more than 36 kW. At present, the overall energy consumption of air compressor system is about 400 kW.

(4) Large pressure loss in compressed air system

The compressed air system was completed in batches. The piping layout is complex. Pipe resistance is great. Besides in order to ensure the air quality, installed many precision filters. There are 20 pcs in total. There are also filters inside the dryers. Therefore, the pressure loss of this air compressor system reaches 0.05 MPa.

(5) There is a air leakage in the air compressor system

There are more than ten system equipments. Moreover, the pipeline network is complicated. The main pipeline is over 600 m. There are more than 20 air-use sites. There are many locations where leaks can occur. About 2% of air leakage are found based on operating conditions and flow comparisons. This also affects the air compressor system energy consumption.

(6) Electronic timing drainer consumes a lot of air

There are 27 pcs of drainer in the compressed air system. They can automatically drain according to the season. However, compressed air is always discharged during the draining process. In spring and summer, the drainage interval is 1 min. 5 s each time. In addition, in autumn and winter, the drainage interval is 3 minutes. 15 s each time. So average annual drainage interval is 2 min. 10 s each time. The diameter of the drain core is about 4 mm. The discharge of a single drain is about 0.25 Nm3/h. Most of them are air. Calculated at 90%. Air loss is about 0.23 Nm3/h. A total of about 6 Nm3/h. So about 50,000 Nm3 of compressed air is wasted throughout the year.

Plans to change the energy consumption of air compressor system

(1) Adjust the air compressor combination

Give several different combinations of air compressors to supply air. And count the operating energy consumption. The energy consumption per unit of air production is shown in Table 2. In fact, the combination includes the desiccant dryer and refrigerated air dryer.
Dryer Type Air Consumption (/Rated Air Flow) Power Loss (KW) Power Consumption (KW) Total Power (KW)
Heatless Regeneration Combination Dryer about 14% 19.3 0.5 19.8
Micro Heat Regeneration Combination Dryer about 7% 9.7 5.8 15.5
Heated Regeneration Combination Dryer about 4% 5.5 8 13.5
Traditional Waste Heat Regeneration Combination Dryer about 2% 2.8 0.5 3.3
New Type Waste Heat Regeneration Combination Dryer 0 0 1.5 1.5
Table 1 “Energy Consumption Comparison of Different Types of Dryers“”
Combination Type Air Compressor System Energy Consumption (KW*h)/h Unit Energy Consumption (KW*h)/Nm³
5#+1# 345.6 0.0987
5# + 2# or 3# (The combination that has been always used) 358.3 0.1024
5# + 4# 360.5 0.103
1#+2#+3#+4# 362.9 0.1037
Table 2 “Air Compressor Energy Consumption Under Different Combinations”
It can be seen from Table 2: the energy consumption of the combination 5#+1# is the lowest. 1#+2#+3#+4# has the highest energy consumption. The former reduces the energy consumption by about 3.5% compared with the second type.

Energy consumption analysis of air compressor combination

The air compressors load and unload according to the pipe pressure. Sometimes, the actual air consumption is greater than the air supply. At this time, the pipe pressure will gradually decrease. Once below the loading set pressure, air compressor starts loading at full capacity. In addition, sometimes the actual air consumption is less than the air supply. At this time, the pipe pressure will gradually increase. Once exceed unloading set pressure, the compressor begins to unload. At this time, the intake valve closes. And the vent valve opens. Vent the high pressure air from the exhaust end to the inlet end. Reduce the inlet and outlet pressure ratio at no-load. At this time, the energy consumption accounts for about 20% to 35% of the full load state. The difference is that the motor speed of power frequency compressor is constant. So it can’t adjust air volume. Changes in the pipe pressure will cause the valves to operate frequently. Finally, it will reduce the operation efficiency of air compressor. Besides, it will affect the stability of the air supply pressure. However, the motor speed of VSD compressor can adjust independently. The adjustment range is generally 60%~100%. The intake valve and vent valve operate very little. It can effectively reduce the idling of air compressor system and energy consumption. Air supply pressure is also very stable. Therefore, we plan to use the combination of 5# + 1# air compressor. Between them, the 1# air compressor is variable speed.

(2) Different regional pressure supply

Add a new compressed air system. Supply air separately for the high-pressure air use site. The compressed air terminal operates at reduced pressure. Reduce the air compressor pressure from 0.86~0.93 MPa to 0.76~0.83 MPa. Energy consumption formula: E=∫ Edt=∫PaQaln(p/pa)dt Pa: atmospheric pressure absolute pressure Qa: volume flow in atmospheric state p: absolute pressure of air compressor outlet t: time Either variable can reduce air compressor system energy consumption. Assume the flow and time remain unchanged. For every 0.1 MPa decrease in pressure, energy consumption decreases by about 7%~8%. The new adding compressed air system is as follows: include 2 sets oil-free screw VSD compressors and 2 sets combination dryers. The design working pressure is 1.0 MPa. Each max air capacity is 1068 Nm3/h. Use permanent magnet variable frequency motor. It can start and stop arbitrarily. And the times is unlimited.

(3) Use GM zero air consumption smart drainer

Don’t use the electronic timing drain again. Change it to GM smart zero air consumption drainer. Its working principle is as follows: condensate flows into the drain. The fluid level in the chamber keeps rising. When reaching high level, the level sensor signals. Activate the solenoid valve to start draining. The condensate level drops to a low level. At this time, the sensor sends a signal. Close the solenoid valve to stop draining. The liquid level is still slightly above the drain port. It will form a liquid seal. So prevent the compressed air from escaping. Then it achieves the effect of zero air consumption. Judging from the actual situation on site, there is no air emission during GM drainer working. It really eliminates the unnecessary energy consumption of the air compressor system.

Energy consumption performance of the air compressor system after change

System change completes. The monthly operation energy consumption of the compressed air station is about 239,906 (kW·h). The new adding energy consumption is about 25205 (kW·h). Thus, total is 265111 (kW·h)/month. Before change, the monthly energy consumption was 288100 (kW·h)/month. Therefore, it saves energy by 22989 (kW·h). It reduce air compressor ystem energy consumption by about 8%. The energy saving effect is very obvious. In conclusion, we should monitor the compressed air system frequently. And need maintain the air compressor, dryer, filters and pipeline well. In addition, the system also needs to be optimized and upgraded. So it can reduce the energy consumption of the air compressor system.