Principles of naval engineering - Historic Naval Ships Association

Chapter 17. -COMPRESSED AIR PLANTS(^ ^TO OIL SPRAY FORTURBINE PINIONOIL TO TURBINE"PINION BEARING47.156Figure 17-8. — Lubrication system for turbine-driven high pressure air compressor.Automatic temperature shutdown devices arefitted to all recent designs of high pressure aircompressors. Such a device stops the compressorautomatically (and does not allow it torestart automatically) when the cooling watertemperature rises above a safe limit. Some compressorsare fitted with a device that shuts downthe compressor if the temperature of the airleaving any stage exceeds a preset value.Control or regulating systems for naval aircompressors are mainly of the start- stop type.With this type of control, the compressor startsand stops automatically as the accumulator pressurerises or falls to predetermined limits. Onelectrically driven compressors, the system isvery simple: the accumulator pressure operatesagainst a pressure switch that opens when thepressure upon it reaches a given limit andcloses when the pressure drops a predeterminedamount. On steam-driven compressors, theaccumulator pressure is piped to a controlvalve (also called a pilot, trigger, or aux-leakage or bleeding to atsteamis thereby permittedthe governor valve and re-iliary valve) which, when the designed cutoffpressure is reached, admits air to a plungerconnected with the turbine governor valve.This causes steam to be shut off and thecompressor to stop, When the pressure fallsto a predetermined level, the control valvecloses and the air acting upon the plungeris released bymosphere. Theto flow throughstart the turbine.Constant- speed control is a method of controllingthe pressure in the air accumulator bycontrolling the output of the compressor withoutstopping or changing the speed of the unit. Thistype of control is used on compressors that havea fairly constant demand for air, where frequentstopping and starting is undesirable.445