Why does the low pressure compressor comes before the high pressure compressor

The low pressure compressor (LPC) comes before the high pressure compressor (HPC) in a gas turbine engine for several operational and thermodynamic reasons:

**Staged Compression**: Gas turbine engines typically utilize two stages of compression to achieve high pressure ratios. The LPC first compresses the incoming air to a moderate pressure, which is then fed into the HPC for further compression. This staged approach allows for more efficient compression and reduces the overall work required from the compressors.

**Efficiency**: By initially compressing the air with a low pressure compressor, the high pressure compressor can operate more efficiently. Compressing air in stages allows each compressor stage to work within its optimal range of pressure ratios, leading to better performance and lower energy consumption.

**Temperature Management**: Compressing air generates heat, which can lead to increased temperatures in the compressor stages. By using the LPC first, the air is compressed at a lower pressure and temperature, which helps manage the thermal load on the HPC. This can help prevent issues like compressor stall or surge by allowing the HPC to operate under more favorable conditions.

**Flow Characteristics**: The LPC helps to maintain a steady airflow into the HPC. By the time the air reaches the HPC, its pressure is increased, leading to better flow characteristics and improved compressor stability.

**Weight and Size Considerations**: Designing the compressors in stages allows for a more compact and lighter engine design. It’s easier to manage the size and weight of individual compressor sections rather than having a single, large compressor that handles all the compression.

Overall, the arrangement of low pressure and high pressure compressors enhances the efficiency, thermal performance, and reliability of gas turbine engines.