Adiabatic (isentropic) compression takes place when there is not heat added to or removed from the system. Compression follows the formula p1V1k=p2V2k, where exponent k is the ratio of the specific heat capacities. Although an adiabatic cycle is never totally obtained in practice, it is approached typically with most positive-displacement machines and is generally the base to which they are referred. Isothermal compression takes place when the temperature is kept constant as the pressure increases, requiring continuous removal of heat generated during compression. Compression follows the formula p1V1=p2V2. However, in practice it is never possible to remove the heat of compression as rapidly as it is generated. Polytropic compression is a compromise between the two basic processes, the adiabatic and the isothermal. It is primarily applicable to dynamic continuous-flow machines such as centrifugal or axial compressors. Compression follows the formula p1V1n=p2V2n, where exponent n is experimentally determined for a particular type of machine. It may be lower or higher than the exponent k used in adiabatic cycle calculations.