Compressed air is a very high cost component in the production of goods and services at a plant. As such, improving the efficiency of an existing system offers a large savings opportunity. To realize the potential, the system dynamics must be understood and the supply from the compressors must always match the real system demands.
Production processes get their energy from the air stored at higher pressure in the piping distribution system. The air compressors simply replenish the air that is consumed.
The energy input in compressing the air is supplied to the connecting pipes for delivery to the various demands throughout the facility. The energy extracted from the system to perform the required tasks actually comes from air already stored in the pipes.
The inefficiencies of a plant air system are affected as much by how the air escapes the system as by how it is generated in the compressor room. Matching the supply with the demand at an optimal level requires that both generation and storage issues be addressed.
Every air system reaches a balance between the air compressor’s supply into the system and the downstream demands that use the air. The energy input from compressing the air equals the energy used plus the system’s inefficiencies. Any more or less energy goes into or is released from storage. Every time there is a change to either side of the equation the system rebalances at a new point.
- Air stored at an elevated pressure in a fixed volume vessel
- Reserve rotating energy of off loaded operating air compressor motors
Air Storage: Volume alone does not equal storage. In order to replenish or release the energy of the stored air, the fixed volume must realize a change in pressure.
Pressure/Flow Control: The resultant pressure fluctuations from bringing compressors on and off line and the impact of short duration surge demands throughout the plant air system forces the system to continuously seek a rebalance point. The addition of the properly sized air storage receiver mitigates the magnitude and rate of change in system pressure but does not by itself eliminate it. System pressure must still be raised high enough to compensate for the cyclical profile. To stabilize delivered air pressure, the air release out of the receiver must be controlled.
A Pressure/Flow Controller installed downstream of the properly-sized air storage receiver(s) and upstream of the main piping header leaving the compressor room is designed for this task. It senses the pressure at its outlet and modulates the flow control valve(s) accordingly to control the air flow from the receiver to hold the pressure constant.
Reserve Rotating Horsepower: Significant reserve energy is available from air compressor motors that are running but not fully loaded. In combination with the Pressure/Flow Controller and air storage receiver, this reserve energy can be applied in a proactive manner to maintain an optimal balance point. As the receiver pressure changes, the trim compressor loads and unloads accordingly.
Running a partially loaded fixed speed compressor is inefficient and can be costly. Storage, therefore, is typically sized to allow unneeded compressors to time out and shut down. Ideally, all operating compressors run at full load with only one compressor trimming at any given time. Substantial air storage must be applied to cover any peaks so a shut down compressor doesn’t have to restart.
With both the supply and demand profile under control, any steps taken to reduce air consumption will positively translate back to the compressors and reduce the input energy. Leak repairs, regulating use points, and the application of high efficiency blow off devices are some additional cost effective measures to take.