Centrifugal compressor – for making base load, or for adjusting?

For this, as for many matters, there are two schools of thought. The solution is influenced by many factors.

First of all, it should be remembered that the adjustment range of the centrifugal compressor depends on how it was originally dimensioned. Its impellers are selected to produce a certain pressure at a certain air flow.
The target end pressure is limited by the stall limit and choke limit. It is not desired to go below the stall limit, because then the flow inside the machine reverses, which causes large loads on the bearings, and in the worst case, damages the whole machine. To avoid this danger, stall guards have been developed. After the choke limit, on the other hand, the flow rate can no longer increase because it is already the same as the speed of sound. No matter how much power is increased, the flow rate will not increase, the temperature will only rise. It should be avoided to go beyond that limit.
These limits define the operation curves that also move as circumstances change. The lower the pressure at which the compressor is operated, the wider the adjustment range between the surge and choke curves. The higher the pressure at which the compressor is dimensioned, the wider the regulation range.
A compromise is worthwhile when sizing a centrifugal compressor
In practice, a small compromise often has to be made: sacrificing little for efficiency, but achieving a wider adjustment range, thus flexibility in air production.
A compressor buyer should ask himself the question: do I want to size and optimize the compressor to a single operating point – for a certain output at a certain pressure – or do I think a little flexibility is better for operation: a wider adjustment range, even if a percentage or a couple of the efficiency is lost.
Not all centrifugal compressors are the same
When the parties sit at a table to plan an investment, they are very happy to start comparing the specific power demand figure directly between competitors. At worst, just look at the price tag. It is a fatal simplification. Not all centrifugal compressors are really the same.
The life of a centrifugal compressor can be easily 20 years. If the device is dimensioned at some operation point today, who knows what the need will be in five years? Of course, many factories have been running for four decades, but nowadays the situations change quickly, and it is worth investing in flexibility.
How are centrifugal compressors regulated?
Screw compressors and centrifugal compressors should not be compared directly. Sizing a centrifugal compressor can go worse into the woods than sizing a screw. Regulating a centrifugal compressor with a control system is not quite as easy as with a screw compressor. After all, a centrifugal compressor adjusts itself, but only within its operating range.
The question arises: if there are several centrifugal compressors in the plant and some screws in addition to them, how is the system controlled as efficiently as possible?
The answer depends on the situation. It is more the rule than the exception that both types of compressors are in the same system. Of course, there are systems that have only centrifugal compressors. Then one should definitely emphasize the wide regulation range of the compressor.
If the need for regulation, i.e. the variable part of the compressed air consumption, is controllable by the centrifugal compressor, it regulates the output in its own control range almost without loss of efficiency. In this case, it is definitely a viable alternative to load-unload control, which consumes electricity even when it does not produce air.
In mixed systems, one model is that the centrifugal compressor produces the base load and screws are used for adjusting. In the worst case, even more screws unload at the same time. After all, there are also variable speed screws, but in the case of a large system, it must be remembered that the operating range of the screw ends at the level of 350 kW, at which the operating range of the turbo begins.
In the second model, the adjustment is handled by several screws or a single centrifugal compressor with sufficient adjustment range. When the centrifugal compressor’s control range is maximized, up to 45 percent of its capacity is reached. In this case, the control range of a 400 kW compressor is almost 200 kW. This cannot be achieved with a normal screw.
Sarlin Balance compressed air management system optimizes the use of various types of compressors so that regulation of the air production is as lossless as possible.
The control philosophy is often on a shaky basis. When a new compressor is needed, it is only dimensioned for a specific operating situation and the specific power requirement is considered at only one point.
Going through the easiest way, the hardware investment focuses on comparing the features of just one of its new devices and forget to explore how the device fits into the system as a whole. It’s so much easier to purchase one new device than to think about its suitability for the whole and appreciate flexibility, adjustability and change management.
There is no average air consumption
Sometimes we think of the average: “on average” so much compressed air is consumed. In reality, the plant may be working two shifts. On weekends, only a few devices require a little compressed air, while on weekdays, the consumption profile can be completely different. The average is an imaginary value. The average need never exists. The sizing criteria are then incorrect.
Honestly, if screw compressors are replaced with centrifugal compressors, the efficiency of the system will be improved thanks to the better features of the equipment. However, if the adjustment is then done with a screw that runs loaded/unload, which is the easy way, improving the efficiency will be halfway.
What about buying compressed air as a service instead of air production device?
The situation is exacerbated when not just one device is purchased, but the entire compressed air as a service. On paper, one service can look exactly the same as another one. It can be a really troublesome situation for a buyer when you should know so many things. Do you trust the seller then? The level of expertise of sellers also varies. You receive a different message depending on who you ask.

Checklist for the service buyer

  • Feel free to ask any questions, including the regulation of air production
  • Refine the content of the service; know what you are getting
  • Don’t focus on just one factor
  • Remember, comparing price alone is easy – too easy