China Centrifugal Air Compressor Manufacturer & Supplier: The Definitive Guide
Centrifugal Air Compressor Controls and Sizing Basics
Centrifugal air compressors are commonly used in manufacturing facilities, processing plants, and refineries. These are typically large compressors of above 500 horsepower. These compressors are not very different from rotary screw air compressors. They work by compressing air to plant pressure from the atmosphere and delivering it to the dryer. Centrifugal air compressors have very few internal wearing parts, apart from the bearings and seals. This aspect makes them highly reliable and efficient. In this article, you will learn more about centrifugal air compressor controls and sizing basics.
What Makes Centrifugal Compressors Reliable and Efficient?
Centrifugal air compressors deliver relatively constant airflow at any given pressure and inlet temperature. In rotary screw air compressors, this aspect is considered the base-load. This aspect means that the compressor should work at full capacity at any given time. In other words, centrifugal air compressors are like screw compressors with constant conditions. But do you know how this is controlled?
Centrifugal air compressors running on actual systems do not actually run on base-load. However, the inlet conditions vary according to external factors.
Sometimes, centrifugal air compressors are run in a centrifugal-only system with a backup and the main machine. Accordingly, they can operate as multiple units in different plants. Lastly, these units can operate as a mixed system with screw compressors running parallel for efficiency. All the applications come down to centrifugal compressor controls and maintenance.
Centrifugal Air Compressor Basics
A centrifugal air compressor works by developing air pressure drawn from the atmosphere. This situation happens when air is accelerated from the center of the impeller by centrifugal force. The velocity is then reduced once the air passes through the diffuser, resulting in the pressure rise. At this point, the kinetic energy is converted into pressure. Therefore, it is safe to say that the centrifugal air compressor working principle takes after Bernoulli’s principle.
Typically, centrifugal air compressors have three stages. They come with intercoolers between the stages and aftercoolers after the last stage. Modern centrifugal compressor configurations are highly reliable, efficient, and compact. These machines are mounted on a common base. They also come with integrated drivers, coolers, gears, piping, and controls. Accordingly, the controls are designed to help the compressor manage capacity and reliability at constant pressure. The basic components of a centrifugal air compressor are:
Centrifugal Air Compressor Control
Are you familiar with the older rotary screw air compressor controls? Then you can say centrifugal air compressor controls are a refined version of the upper-range unloading and modulation, with the only difference being on the minimum throttle limits.
Here is a summary of centrifugal air compressor controls:
Centrifugal air compressors have a primary control independent variable, and it is the pressure. The unit’s pressure controls the inlet valve to adjust its capacity to remain stable with the demands. PID or the proportional-integral-derivative is the mode of control. PID balances the pressure and may only deviate with a fraction of the psi. in some centrifugal compressor configurations, recirculation valves are designed to control the part load.
Sometimes, the pressure is raised to control the blow-off valve. In some configurations, the valves are stroked at the same time as the compressor unit approaches the surge line. At this point, the pressure is maintained constant.
Centrifugal air compressors have two control modes, the continuous run and the low demand mode. This is often termed dual control. While in the low-demand mode, the centrifugal compressor can unload after the blow-off has gone past a given level for some time. The inlet valve shuts while the blow-off valve remains open. The air compressor shuts when it is left unloaded for a given time.
In some cases, the blow-off valve, and not the inlet valve, may control the pressure. The only problem is that this results in constant power at all capacity levels, which is more wasteful. The situation is most rampant when the compressor is left in manual control or when it experiences surging. So, how can you control this problem?
How To Control and Protect Surging
Some centrifugal air compressor manufacturers design surge control and protection. These designs are available for constant pressure and auto-dual control systems. Accordingly, some technicians surge compressors manually to set up the control system. This can also happen in two ways.
Motor Current Control
Centrifugal air compressors can be correlated with the motor current. The motor current decreases as the airflow reduce. The situation can be correlated to the compressor’s surge point. With this control mode, the unloading valve opens when the motor reaches the minimum set current value to prevent surging. The technique may be simple, but it does not optimize the compressor’s actual turndown range.
Surge Anticipation Control Optimization
There is another way controllers use to optimize surge anticipation control. For instance, controllers can monitor the actual surge line position in relation to the existing condition of the ambient inlet. This prevents the compressor from surging. The control optimizes the turndown. Additionally, it enables the centrifugal compressor unit to operate at actual turndown based on the condition of the existing ambient inlet.
How Do You Control Capacity on A Centrifugal Air Compressor?
You can control the capacity of a centrifugal air compressor by enabling the centrifugal air compressor unit to move back on its characteristic curve. You can do this by throttling the discharge valve. Additionally, you can use speed control and common control techniques. For a reciprocating air compressor, speed control is the most logical control method. Nevertheless, large low-speed units are often driven by large synchronous motors at a fixed speed. External recycles and unloaders are, therefore, the primary control technique. This method can either hold the suction valve open or add clearance volume to the cylinder.
The Implications of Centrifugal Air Compressor Sizing
Here are the three implications of centrifugal air compressor sizing.
- You can get an accurate airflow profile of the compressor unit before sizing a new compressor.
- Additionally, you can choose a centrifugal air compressor with enough turndown and no blow-off to match your load variance. In case there is a 50% drop off the peak, you will be forced to allow blow-off. This is primarily true if you are sizing one compressor unit to the system. Accordingly, you might want to consider multiple compressor units with varying loads.
- Ensure you have a sufficiently large motor for the full capacity at low temperatures.
The Bottom Line
Most advanced control systems designed by centrifugal air compressor manufacturers result in trouble-free, high reliable, and efficient operation. With multiple centrifugal compressor control techniques, you can optimize your compressor performance to suit your applications. You can further improve reliability and efficiency by understanding the influence of environmental conditions on the centrifugal compressor performance.
