When the gas is used for external work through the expansion of the expander, the brake must consume this part of work. Otherwise, the rotating speed of the expander will lose control, which will cause the flying accident. The fan or generator driven by the expander is the function of absorbing and consuming the power generated by the expander.
The advantage of fan braking is that the fan impeller can be directly installed at one end of the expansion machine shaft, the equipment is simple, the unit is compact, the volume is small, the manufacture is easy, the cost is low, the maintenance work is minimum, and the operation is convenient. Its disadvantage is that all the power generated by the expander is wasted, and at the same time, the draught fan will produce a lot of noise, which requires a special isolation chamber, and the exhaust port also needs to be equipped with a silencer.
The advantage of generator braking is that the power generated by the expansion machine can be recycled, and it can be converted into electric energy to send power to the power grid. So as to reduce the power consumption of the air separation plant and reduce the operation cost. In addition, the noise of motor braking is also small. Its disadvantage is that it needs a set of generator
Equipment. Meanwhile, because the rotating speed of the expander greatly exceeds the rotating speed of the generator (the maximum rotating speed of the generator is 3000r/min), there must be a set of reducer with high precision, so the cost is relatively high. In addition, when the power grid breaks down, the expansion machine is suddenly removed from the braking load, which is easy to cause damage to the machine parts due to overspeed.
Considering comprehensively from the two aspects of investment and operation economy, generally speaking, in order to improve the economical efficiency of equipment operation, the way of power recovery should be adopted for the turbine expansion machine with larger power. Some countries start to use generators to recover power when the power is above 15kw; some start to recover when the power is above 100kw.
With the development of air separation design and manufacturing technology, it is more and more common to use the technological process of using booster (booster turbine compressor) to absorb the power of expansion machine. The impeller of the booster is installed at the other end of the axis of the expansion machine, and the expansion air works on the impeller of the expansion machine to make it rotate, and the impeller of the booster also rotates at the same speed, the expansion gas before entering the expansion machine is pressurized and then introduced into the working wheel of the expansion machine. In this way, the power of the expansion machine is recycled to the expansion working medium itself, which increases the inlet pressure of the expansion working medium into the expansion machine, thus increasing the refrigerating capacity per unit working medium of the expansion machine, reducing the inflated air volume into the upper tower is beneficial to improve the extraction rate of oxygen. This kind of braking mode is a relatively advanced braking mode at present and is being widely promoted and used.
At present, our country basically adopts the braking mode of energy recovery on the air separation plants with the capacity above 6000 m 3/h.
