3 spool vs 2 spool

In the Trent 1000, there is 3 spool arrangement of compressors and turbines. This means that there are three sets of compressors and turbines which are connected by three separate shafts. They are the low pressure compressor (LPC) which is the fan and turbine (LPT), Intermediate pressure compressor (IPC) and turbine (IPT) and high pressure compressor (HPC) and turbine (HPT). This is on contrast to the two spool arrangement currently used in many aircrafts where a LPC/LPT and HPC/HPT are used only. All the three shafts are located within each other.
The fan positioned at the front of the engine is run by the LPT via a gearbox so that it can operate at optimum speeds. Since the large diameter fan requires a large torque to start rotating, it limits the high speed attained by the LPT due to which its efficiency decreases. Additions of an additional set of compressor and turbine eliminates the influence of this problem but does not eliminate the problem itself!



The problem is eliminated by using a planetary gearbox between the IPC and the fan so that the LPT and fan can operate at their maximum speed and hence max. efficiency. This is called a geared turbofan. In 2 spool arrangement, the gearbox is situated in between the fan and the LPC.
 Employing and additional set of compressor ensures the air gets compressed to a much higher pressure due to which the efficiency increases.
 Although the overall weight of the engine and its complexity is increased, it is compensated with a considrable increase in thrust. The thrust produced by the Trent 1000 varies in the range of 60,000-80,000 lbf. Due to these reasons, three spool is more preferred over 2 spool arrangement.




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