An ideal reciprocating air compressor demands less work to run if the compression follows

Explanation:

For reciprocating air compressor, the law of compression desired is isothermal and that may be possible at very low speeds.

• Due to minimum work input required in isothermal compression, it is the ideal process for compression.
• Constant pressure line 4-1 represents the suction stroke.
• Area 1234 represents the adiabatic work.

Isothermal compression:

• If the compression is carried out isothermally, then it follows the curve 1-2 which has less slope than both isentropic and polytropic processes.
• This work done that is area 1234 in isothermal process is considerably less than that due to adiabatic compression.
• Thus, the compressor will have higher efficiency if compression follows the isothermal process.
• It is not possible in practice to achieve an isothermal process, as the compressor must run very slowly.
• In practice, compressors run at high speeds which results in a polytropic process.
• The cold-water spray and multi-stage compression are used for approximating to isothermal compression while still running the compressor at high speeds.