FUNCTION AND CONSTRUCTION
Service air compressor onboard is used for low pressure air compression (around 7bar) which is used as general purpose air( cleaning, filter cleaning, operating pneumatic hand tools and pumps, ship air horns) and also for control air for different pneumatic control systems (blower dampers, E/R control air, air operated quick closing valve system, air for boiler burner etc.). It is the responsibility of fourth engineer to maintain this to best of its effeciency. Screw compressors are very reliable and only timely maintanance routines such as filter and seperator change are enough to have smooth operation.
In this series we will discuss every aspect of service air compressor starting from general introduction to different case studies regarding malfunctioning of service air compressors on board ships. This series also includes FAQ regarding maintainance and construction of service air compressor.
Air is taken in via suction air filter. The clean air, filtered of dirt and dust, is sucked into screw-end. This air is compressed at screw-end and transferred to oil separator, together with a lot of cooling oil, which has also been injected during the operation. Air and oil are separated within oil tank and oil separator. The separated air goes through minimum pressure valve and is transferred to the after cooler, which cools down the air (Please note that there is no separate oil pump in service air compressor, min pr. v/v holds the air till air pressure rises to a certain value inside the tank, after that it allows air to go through aftercooler. The compressed air inside oil tank provides required pressure to oil.)
The cooled air is then transferred to air reservoir. On the other hand, oil in separator element is transferred to oil filter and then to cooler, pushed by air pressure inside separator and finally reinjected to screw end. Here the oil is cooled by the cut in and cut out of oil/after cooler fan according to the temperature (some models also have thermostatic valve after oil tank which transfer the oil directly to air end if the oil temp is below certain temp e.g. 75°c. if temp is more thermostatic valve sent fraction of oil to oil/after cooler accordingly and finally oil is reinjected to screw end for lubrication and cooling.)
A branch is bled from oil/air tank as control air which is supplied to unloader or air control valve through separate piping.
When unloading inner pressure of oil tank, is exhaust to the atmosphere side of unloader.
DIFFERENT PARTS AND THEIR FUNCTION:
Suction filter– Its purpose is to remove dust from inlet air. It contains dry-type element to remove dust of more than about 10 µ. It is provided with maintenance indicator/ differential pressure indicator for suction filter.
Screw-end (Compressor body)- Screw-end is composed of male rotor with n lobes and female rotor with n+1 sockets. Each rotor is supported by a roller bearing and 2 pieces of tapered roller bearing. As the rotor turns and air is sucked from upper inlet port and is compressed in the space between casing and rotors. Compression continues until the space comes to the outlet port at bottom side and the air is exhausted.
Screw-end (Compressor body)- Screw-end is composed of male rotor with n lobes and female rotor with n+1 sockets. Each rotor is supported bya roller bearing and 2 pieces of tapered roller bearing. As the rotor turns and air is sucked from upper inlet port and is compressed in the space between casing and rotors. Compression continues until the space comes to the outlet port at bottom side and the air is exhausted.
Oil separator element– The oil separator separates oil in the air discharged by screw-end. This machine has a cartridge style separator element. Air flows from the outside to the inside of separator element, and oil is separated. The oil which has been removed collects inside and is eventually reinjected into the screw-end.
Minimum pressure valve– it is mounted at separator outlet. This compressor has no oil pump.
Lubricating oil pressure is so designed as to be held by air pressure inside oil tank.
Accordingly, lubrication must be performed by raising lubricating oil pressure immediately after starting. On that account, minimum pressure valve installed so that air will not be discharged until separator inner pressure is approximately 0.5MPa(5kg/cm2). It also functions as check valve since it is closed at unloading and stop.
Oil Filter- it filters particle larger than 5-10 µ in oil. It is of cartridge type.
Air and oil cooler/ After cooler- oil and after cooler are integrated into a single piece cooler made of Aluminum. One side of which acts as a oil cooler and other half as air/after cooler. Cooling air is blown with a fan towards the cooler.
Thermostatic valve– In order to prevent condensation of water in oil, oil flow in to oil
cooler is automatically regulated so that lubricant temperature will be kept constant.
In the case temperature is low, as bypass circuit of thermostatic valve is open, oil enter oil filter not going through oil cooler and transferred to screw end. When lubricating oil temperature rises and became 70°C, bypass circuit of thermostatic valve will close down and transferred to screw-end.
1. Valve position when lubricant temperature is low. 2. Valve position when lubricant temperature is high
Pressure Switch-Pressure switch is used turn capacity control solenoid valve on and off.