Some manufacturers utilize certain of the advantages of the aero-derivative machines, i.e. high powerto- weight ratio and easy maintenance. The high power-to-weight ratios are achieved by running the machines with high combustion and exhaust temperatures and by operating the primary air compressors at reasonably high compression ratios i.e. above 7. A minimum of metal is used and so a more frequent maintenance programme is needed. Easier maintenance is achieved by designing the combustion chambers, the gas generator and compressor turbine section to be easily removable as a single modular type of unit. The ratings of machines in this category are limited to about 10 MW.