Best Tutorial On Marine Air Compressor

Introduction

In this tutorial of Best Tutorial on Marine Air Compressor, we will discuss the — What is a compressor, Uses of main Air Compressor on Board a ship, Compressor working principles, General problem in compressors, How does a compressor work.

What is Air Compressor?

Air compressor is a machine used to increase the pressure by compressing from low pressure to higher pressure by utilizing some external source of energy.

The source of energy for driving the compressor will be either an electric motor or a diesel engine.

Compressed air is classified into the following 2 categories

1. Main Air
2. Instrument Air

Main Air: — (11 to 30 bar)

• Main air is used for starting the main and auxiliary engines onboard a ship.
• For supercharging purpose the engines
• For the operation of Lambda controller to boos the air at the change of loads in the engine.

Instrument Air; — (7 bars)

Instrument air is used for the operation of the following instruments

• Control system for several controls and automation.
• Service air and control air application
• Paint and spray instruments
• For the operation of pneumatic hand tools
• To clean machine
• For Hydrophore ( A system to maintain the water pressure)

Important terminology used for compressor

Compression Ratio; — Ratio of volume pre-compression to the volume of air post- compression is termed as compression ratio.

Compressor Capacity; — Absolute volume of air delivered by a compressor in M3 / minute is termed as compressor capacity.

Free Air Delivery; — Quantity of air in M3 / minute produced by compressor existing temperature and pressure at the suction of compressor is termed as free air delivered.

Swept Volume; — The volume of air displaced /swept during TDC and BDC movement of the piston.

Classes of Compressors

Positive Displacement; — In positive displacement type of compressors specific volume of air is compressed and forced to a smaller volume.

• Rotary compressor
• Reciprocating

Centrifugal;- in this type of compressor air is accelerated and the energy is converted into pressure energy.

• Centrifugal
• Axial

Reciprocating Compressor

In reciprocating type compressor air is sucked in the cylinder, it is compressed by piston and expelled in to discharge line. The valves fitted in the cylinder regulate the flow of air from the cylinder.

Reciprocating compressors are of the following types

1. Single-acting
2. Double acting

Single-acting

In the single-acting compressor, there is one discharge stroke in each revolution of the crankshaft.

Double acting

In double-acting compressor, there are two discharge strokes every revolution of the crankshaft.

• Screw Compressor
• Reciprocating compressor
• Centrifugal compressor
• Rotary compressor
• Diaphragm compressor

Generally, a Reciprocating compressor is used for the marine and industrial Diesel power plant starting and control systems.

Air Compressor working principle

In the compression stroke for a theoretical mono cylinder compressor, pressure increases marginally more than the delivery pressure. A spring-loaded Non- return delivery valve operates, and the compressed air flows through at relatively constant pressure. After completing the stroke, the differential pressure across the valve, supported by the valve spring, shuts the discharge valve, trapping a small quantity of high-pressure air in the clearance area between the piston and the cylinder head. On the induction stroke, the air in the clearance space expands; its pressure keeps dropping until spring loaded suction valve re-seats and another compression stroke starts.

How does a Marine compressor work?

In a single crank single-acting compressor, the low-pressure and high-pressure Pistons are connected in tandem to a single crankpin low-pressure stage.

The crankshaft is rotated by external power, which may be either an electric motor or a Diesel engine. Crankshaft gives drive to the piston.

During the downstroke of the piston, the atmospheric air is inducted from the first stage through the intercooler. Next, pressurized air is inducted into the second stage. In the upward stroke of the piston, compression takes place in both stages. Finally, the high-pressure air from the second stage is delivered through the aftercooler.

In single-acting crank compressors, the low-pressure and high-pressure pistons are connected to two separate cranks. As the low-pressure piston moves down, the air is inducted through the air filter. Compressed air is delivered to the second stage through the intercooler. The air is further compressed in the second stage, and high-pressure air is delivered through the aftercooler.

In a double-acting compressor, compression occurs during the upward and downward strokes of the piston. As per the Arrangement, the low-pressure first-stage piston is at the top, and the high-pressure piston is at the bottom. Thus, as the piston moves up in the first stage, the air is compressed, and the compressed air is inducted into the second stage.

In the downward movement of the piston, the pressurized air is further compressed in the second stage. In the meantime, the low-pressure stage inducts fresh air from the atmosphere through an air filter. In this way, the process is cyclical.

In three-stage compression, the air is inducted at the first stage compressed pool by the intercooler and delivered to the second stage .the the second stage, the air is compressed further and delivered to the third stage staged. Finally, the air is compressed to the required pressure and delivered through an aftercooler. In the upward stroke of the piston, the air is compressed in the first and third stages. There as air from the first stage is inducted into the second stage.

Advantage of Multistage compressors

• Power reduction in driving the compressor
• Enhanced mechanical balancing of the complete unit with uniform torque.
• Improved volumetric Efficiency
• Minimized losses due to leakages.
• Low working temperatures enable easy lubrication.
• Reduction in-cylinder weights.
• Due to low working temperature, could use low-cost material.

Compressor Efficiencies

1. Volumetric Efficiency;- It is the ratio of the absolute volume of the free air discharged at the standard atmospheric condition in one discharge stroke to the displaced volume by the piston during the stroke.
2. Isothermal Efficiency;- Ratio of the isothermal power to the indicated power is termed as isothermal Efficiency.
3. Mechanical Efficiency; — ratio of indicated power to the brake power is termed mechanical Efficiency.

Components of Air Compressor

For an easy understanding of the subject, we will discuss the popularly used reciprocating Air compressors. We will discuss the major working parts of reciprocating air compressors.

Piston

Cylinder Liner

Connecting rod

Main Bearings

Crankshaft

Filter

Valves

Gudgeon pin Bush

Gudgeon Pin

Piston Ring

Shaft seal

Procedure for starting the compressor (Best Tutorial on Marine Air Compressor)

1. Keep the compressor in unloading condition either manually or by keeping the supply of electro-pneumatic control in on position.
2. Turn the supply of cooling water on and check the flow and inlet pressure of cooling water.
3. Rotate the wheel drive belt manually by few turns for the freeness. This action is mandatory for the compressor to be started after a prolonged period.
4. Check the oil level and operate the force-feed lubricator for around 8 to 10 turns to ensure the availability of lube oil to cylinders.
5. Check the oil level in the sump and top up if needed.
6. Position of the safety shut down switches in normal working condition.
7. Drain the condensate from the intercooler and aftercooler.
8. Ensure the V belt is tight as per norms.
9. Open the stop valve for the air to flow to the air bottle/receiver.
10. Press on the button from the control panel to start the compressor.
11. Monitor the oil pressure with the pressure gauge, stop the compressor immediately if the pressure is not indicated on the indicator.
12. Check the lubricating oil flow from the lubrication pump.
13. Put the compressor load after 5 minutes of satisfactory operation.
14. Monitor the water flow to the cylinder and coolers.
15. The inter-stage pressure should be within range.
16. Check and rectify leakages in the system.
17. The voltage at each terminal must be as per the specified levels.
18. Check the discharge air temperature. The maximum permitted air outlet temperature is 75Deg C.
19. Check and tighten the fasteners at regular intervals of operation.
20. Listen to the abnormal noises and take immediate action if any obnoxious noise is present.

Stopping procedure of compressor

1. Unload the compressor with the help of the installed control/ device
2. Run the compressor at idle for 5minutes.
3. Stop the compressor
4. Close the cooling water supply
5. Drain the condensate from the points.
6. Isolate the power supply to the starter.

How to make a compressor more efficient? (Best Tutorial on Marine Air Compressor)

1. The air intake supply to the compressor must be cool. Every 40C decrease in air inlet temperature will save 1% energy consumption.
2. Regular cleaning of air inlet filters can save energy by 2 % at every 250 mm WC pressure drop across the filters.
3. Always maintain the compressor valves in excellent condition. Worn-out valves will reduce the compressor efficiency by 50%.
4. The manometer across the filters should be in excellent condition to better monitor pressure drop and indicate element replacement.
5. Always attempt to minimize the low load compressor operation.
6. Always follow cleaning the intercoolers regularly at the specified intervals or more temperature differences across the cooler.
7. Carry out the free air delivery test periodically to check the present capacity of the compressor against the design capacity. If the deviation is observed, then take the corrective action.
8. Always opt for a multistage compressor to get energy efficiency.
9. Curtail compressor delivery pressure wherever possible to save energy.
10. Set the minimum possible range between loads and unload pressure settings.
11. Minimize the frequency of drain.
12. Check the air compressor logs regularly for abnormal readings and corrections of abnormalities.
13. 40 to 50% of air leakage of compressed air is not uncommon. Carry out periodic leakage tests.
14. The machine should be with interlocked solenoid cut-off valve in the air system so that the air supply to the equipment can be switched off when not in use.
15. All pneumatic operated equipment should be well serviced and lubricated to reduce the friction power, prevent wear down of seals, and other rubber parts, thus preventing energy wastage due to excessive air utility or leakage.
16. Wrong use of compressed air, such as cleaning the body, agitation, floor cleaning, and other similar appliances, must be discouraged to conserve the compressed air and energy.
17. The operating pressure of pneumatic equipment should be monitored as overpressure will be a waste of energy and will lead to excessive equipment wear.
18. Pneumatic tools like grinders and drills consume about 20 times more energy than motor-driven tools. Must be replaced with electrically operated.
19. Welded connections are more good practice than threaded connections.
20. Always try to use ball or plug or gate valves than the globe valves in compressed airlines.

General Problem of Air Compressor on ship (Best Tutorial on Marine Air Compressor)