Notes on Energy Efficient Operation for
Compressed Air Systems
There are many different ways to set up and operate a compressed air system in a manufacturing facility. After examining dozens of compressed air systems throughout multiple industries, we have seen highly efficient systems and very inefficient systems. The following list of recommendations details the best practices to achieve energy efficiency when purchasing and running compressed air systems that we commonly see in industry.
1) When using air-cooled air compressors, install ducts to vent hot air into the plant during winter months. This will help offset heating costs and make use of useful heat that is otherwise wasted. The ductwork should be installed so that the hot air can easily be directed into the plant in the winter and outside in the summer. A simple schematic is shown in the figure below. When purchasing new air compressors, consider buying air cooled compressors if you can make use of the hot air as space heating.
2)
Compress outside
air. Thermodynamics show that it
takes less energy to compress cold air versus hot air. The average year round air temperature for
3) Stage pressure settings or install controller to automatically stage air compressors. When multiple air compressors are required to meet plant demand, the activation pressure settings should be staged so that lag compressors only operate when demand increases. Installing a controller is a more expensive option, but simplifies the staging process, especially when purchased concurrently with the rest of the compressed air system.
4) Locate all compressors in the same room to facilitate staging. Staging air compressors can be very complicated if the air compressors are spread throughout the plant. It is important to have the compressors adding compressed air at the same point so the staged compressors are all reading the same line pressure. This can also simplify other issues, such as the need for multiple air dryers, maintenance, and the ductwork mentioned above in #1 and #2 above.
5) Operate lag compressor in load/unload mode with auto shutoff, or install a VFD. There are numerous operation modes for air compressors. Increasingly, the most common operating mode is what is commonly known as “modulation.” However, this mode is very inefficient when demand is low. Analysis shows that even when compressing no air, modulating compressors draw up to 70% of their rated power. A more efficient operating mode is called “load/unload.” In this mode, the compressor “loads” when the storage tank pressure drops to a specified point, and continues to load until it reaches the “unload” set point. This significantly improves the part load efficiency with no capital cost, as most compressors can easily be switched between operation modes. Alternately, installing a variable frequency drive (VFD) allows the compressor to follow load with high accuracy, achieving the highest part load efficiency available. However, the VFD requires a higher initial cost or retrofit cost.
6) Install sufficient compressed air storage. Adding receivers to store compressed air will help dampen pressure swings when using load/unload compressors, as well as allow load/unload compressors with auto-shutoff to unload and shut off more frequently.
7) Use refrigerated dryers instead of desiccant dryers when possible. Desiccant air dryers are required to get the driest air possible. However, many times plants have desiccant dryers when compressed air humidity is not a top concern. In this case, refrigerant dryers are highly recommended. They use far less energy than desiccant dryers, because most desiccant dryers use about 15% of the total compressed air generated to blow down and dry the desiccant.
8) Lower compressed air pressure setting to lowest possible level. Many plants Compress air to higher pressures than necessary. Few compressed air tools require above 90 psig, and the pressure drop across most compressed air distribution systems is only a few psig. Compressing air to the lowest possible pressure is an excellent way to cut back on compressed air cost.