THE HISTORY OF TURBO COMPRESSORS
What is a Turbo compressor and how does it work?
The turbocompressor is one from a family of dynamic compressors, these machines are widely used to compress air and gas. Compression is affected by spinning the air at high speed, imparting kinetic energy to the air. There is a small degree of compression across the impeller, this usually varies between 1:1 up to 2:1.
These machines use dynamics to produce compression. This means that unlike a positive displacement machine, there isn't any mechanical squashing as the air passes through it. Inside a turbo compressor is a rotating element called an impeller. There are no pistons or other types of compressing parts between the inlet eye and the discharge tip at the other end.
A turbo compressor sucks air into the inlet eye, spins it at an incredibly fast speed and throw it out of the tip. The forces exerted on to the air are centripetal and the force that the air exerts on its surroundings are centrifugal. This type of machine is also often called a centrifugal compressor, a dynamic compressor or even an aerodynamic compressor.
This type of compressor has the following performance characteristics, often called 'the three fan laws'.
- Flow is proportional to impeller speed.
- Differential pressure* across an impeller is proportional to the square of the impeller speed.
- Power absorbed by the impeller varies with the cube of the impeller speed.
*If you try and use absolute pressures the law fails. For example, if a machine produces 5 psi at 1000 rpm, it will produce 25 psi at 2000 rpm (assuming that it doesn't fall to bits due to the increase in stresses). If you did the same calculation (incorrectly) using absolute pressures, we have 14.7 + 5 = 19.7 psia. The square of this would be 388!!
Where did the Turbo Compressor come from?
The first turbo compressors were manufactured at the turn of the 1900's. They were originally developed by steam turbine manufacturers and were widely used for ventilation purposes in deep shaft mining, in particular the coal industry. At that time, the method of producing an impeller relied upon fabrication. It would be decades before technology allow highly efficient turbo compressors to be made.
Typically, some blades were riveted on to a hub and the resulting impeller would then be balanced as best as possible. It wasn't until the early 1950's that sufficient money was pumped into technology, to allow the production of efficient high speed compressors and then the market took off. As with most things, the American market dwarfed the European market, so most advances in technology have been imported.
The pressure ratio across an early riveted impeller would have only been in the order of 1.2 : 1. This would have meant that to reach a final working pressure of 7 barg, a turbo compressor needed as many as ten or eleven stages. Modern impellers can produce a single pressure ratio as high as 8:1. (Perhaps higher).
However, a modern single stage high efficiency turbo-compressor would still be fairly in-efficient because of absence of intercooling. Nevertheless, this gives an idea of the advances in design technology, materials of manufacture and machining accuracy.