Marine Air Compressors

Marine air compressors

Marine air compressors are used to compress air and store it in tanks for various uses. they are crucial equipment of the ship as they provide compress air for a day to day maintenance on board ship when an emergency and Especially to start ship’s main engine and auxiliary engines. Marine air compressors are designed to function within the ship environment.

These machines need to withstand wet conditions, as well as the presence of salt in the water and air. Not all metals handle the conditions as well, so using an air compressor that isn’t designed for these specific conditions can result in device failure or even safety hazards.

Air Compressor produces pressurized air by decreasing the volume of air and in turn increasing its pressure. Different types of air compressors are used according to their usage.

Types of Air Compressors

There are mainly four types of compressors:

1. Centrifugal compressor
2. Rotary vane compressor
3. Rotary screw compressor
4. Reciprocating air compressor.

However, onboard ship reciprocating air compressor is widely used.

Uses of Air Compressor on Ship

Based on application, different air compressors are kept for a particular usage. Normally, air compressors onboard ships are:

• Main air compressor,
• Topping up compressor
• Deck air compressor
• Emergency air compressor

Main air compressor

The main air compressor is used for supplying high pressurized air for starting of main and auxiliary engines. The pressurized air generated by the air compressor is stored in an air storage bottle. These are high capacity compressors and the air pressure that is required from these compressors to start the main engine is 30 bars. Control air is also supplied from the air bottle through a pressure-reducing valve and a control air filter. Normally they are twice in number and can be more than that for redundancy.

Topping up compressor

Topping up the compressor takes up the lead to cover up for the leakage in the system. This means that as soon as the air pressure in the system goes below a particular level, the topping up compressor replenished the system with pressurized air.

Deck air compressor

Deck air compressor is used for deck use and as a service air compressor and might have a separate service air bottle for the same. These are lower capacity pressure compressors as the pressure required for service air is between 6 to 8 bar.

Emergency air compressor

The emergency air compressor is used for starting the auxiliary engine at the time of an emergency. This type of compressor can be motor-driven or engine-driven. If the motor is driven, it should be supplied from an emergency source of power.

Uses of Compressed Air on Ship

Compressed air is used for the following purposes on a ship.

• For starting of main engine, auxiliary engine, emergency generator and emergency fire pump.
• For automation and control air for main and auxiliary engine.
• For different application on the deck side and in engine room such as chipping, drilling, buffing, pressurized water jet cleaning etc. by use of pneumatic tools and machinery.
• For overhauling machinery by use of pneumatic tools and hydraulic jack.
• For pressure testing of different machinery parts, pipeline etc.
• Compressed air is also used for ships whistle and fog horn.
• It is used in life boat for heaving up the later, if air motor is attached as a heaving provision.
• For supplying water to accommodation and various parts of the ship through hydrophore by keeping the later pressurized with air.
• For conducting aerobic breakdowns of the on board sewage in sewage plant.
• For pressurized spray painting.
• Used in soot blowing of boiler and economizer.
• Used in portable pneumatic pumps like Weldon pumps for oil, water and bilge transfer.
• For general cleaning and services.

Purpose of Air Bottle

The high pressure is used for the initial starting of the marine I.C engines present onboard the vessel. It also supplies control air to the marine engines. Service air is supplied from the air bottle. If the quick closing valves are air operated, safety air is supplied through an air bottle. Spring air for the exhaust valve is supplied through the air bottle. Apart from the above-mentioned ones, there are several other uses as well.

Volumetric Efficiency

It is an efficiency measure of a compressor. Volumetric efficiency is the relationship between the volume quantity of air discharge when brought to standard atmospheric condition and the swept volume of air by L.P piston.

Volumetric Efficiency = Swept volume of air by L. P piston/The volume of air discharged as free air

Free air is assumed as air in the atmosphere at 15° C temperature.

Causes of reduced volumetric efficiency

1. Excessive ‘Bumping’ clearance.
2. Defective (leaky) valves.
3. Restrictions in the discharge lines.
4. Restrictions in Intercooler/ Aftercooler or reduced heat transfer.
5. Choked intake filter.
6. Worn piston rings.
7. Inlet air temperature (ambient temperature) too high.
8. Cooling water temperature too high.
9. Insufficient cooling water to the intercooler

How to check compressor efficiency during running?

• Check by filling time with the previous record and also check the first stage discharge pressure.
• If compressor efficiency is lower, the compressor will run longer and compressor temperature will rise.
• The first stage and second stage pressure gauge must be correct and stable.
• No escape of air from the suction filter.
• Intercooler and aftercooler outlet air temperature should not be high.
• If open drain valve nothing can be found.
• Low L.O consumption.
• Oily air mixture must not blow out from breather pipe.
• Total no. of Air Compressors must be sufficient to fill the empty Air Bottle to maximum pressure within 1 hour

Advantages of Intercooler

1. Reduce air temperature and volume, and increase air density for the next stage.
2. Increase Compressor Capacity and Volumetric Efficiency,
3. Better lubrication for cylinder and rings.
4. Water and excess Oil can be drained out, preventing fouling of Intercooler and pipes, Air Bottle corrosion, and starting airline explosion.
5. Work done is saved.
6. Metal stresses reduced., due to control of temperature.

What is the difference between an aftercooler and an intercooler

The intercooler is fitted between intermediate stages

• saving power
• increase volumetric efficiency
• No, carbonize materiel from the discharge valve
• Moisture separation is easier through an intercooler drain

Aftercooler fitted on the final stage at the discharge side

• Reduce final discharge air temperature to room temperature
• Increase volumetric efficiency
• Air Bottle size is smaller
• Moisture separation is easier through aftercooler drain

Effect on intercooler starvation of water

If inter-stage cooler starves cooling water or cooler, efficiency is drop due to depositing. The slope of the curve will increase and be far away from the Isothermal compression curve. The area under the curve will increase and more work done to be applied. Work save will not exist due to temperature increase.

The following defects will occur,

• Reduce volumetric efficiency and rapid fall of discharge pressure. Carbonizing to the valves, Sluggish operation and rapid valve deterioration.
• Adversely effect to lubrication
• High temperature running of compressor and lead to explosion in extreme case.

Bumping Clearance

It is the clearance between the piston and the cylinder cover, at the top dead center. This is necessary to prevent mechanical contact between the moving piston and the valves and gear. The clearance volume should not be too less or too more.

Significance and Effects of Bumping Clearance In an air compressor, when the discharge valve closes at the end of the compression cycle, a small amount of high-pressure air is trapped in the clearance volume. Before again taking suction, the air trapped in the clearance volume must expand below the suction pressure i.e. below the atmospheric pressure. The expansion of this trapped air in the clearance volume causes effective loss of stroke due to which the volumetric efficiency of the compressor drops. Therefore, the clearance volume has a significant effect on the efficiency of the compressor.

Effects Due to Less Bumping Clearance

Small clearance volume may result in piston banging or colliding to the cylinder head. This is dangerous when the compressor is running in unloaded condition without any resistance to the movement of the piston.

Effects Due to Large Clearance Large bumping clearance retards the formation of vacuum on the suction stroke and thus less air is drawn inside for compression and accordingly the weight of the air delivered is reduced proportionally to the clearance volume.

Reasons for Change Clearance volume

During overhauls of the air compressor, if the gasket fitted between the cylinder head joints is of the wrong type, then the bumping clearance will increase, resulting in wear down of bottom bearings or wrong bearings are put in place.

Checking Bumping Clearance

Bumping clearance is checked by putting a lead ball or plastic gauges over the piston and then turning the compressor one revolution by hand. By doing this the lead ball will compress and the thickness obtained is the clearance volume. This thickness is measured with a vernier caliper or micrometer and is then compared with the manufacturer’s value. Adjustments are made in case there is an offset in the value.

The Bumping clearance is Effected

• due to the main bearing, crankpin bearing and gudgeon bush wear down.
• due to piston crown wear, or
• even by the incorrect thickness of the gasket of the cylinder cover when overhauling.

Adjustment of bumping clearance

Adjustment is usually by shim packs between the connecting rod and bottom-end bearing block or even between cylinder cover and block. With tandem type pistons, it is necessary to be able to adjust each stage separately (since the piston is common).

What should be the Bumping Clearance?

Generally bumping clearance depends on the manufacturer but as a thumb rule, it should be between 0.5% and 1% of the bore of the cylinder.

Advantages of Multistage Compressors

Multistage Intercooling compressors are widely used onboard since it has so many advantages compared to the single-stage compressor.

• Improve Compressor Efficiency
• Reduce in Air Compressor size
• Lower Work done to Compressing Air
• Easier to control the air temperature
• Reduce the thermal stresses
• More Stages are needed to achieve required final pressure

Starting and Stopping of Air Compressor

Starting procedure

Before starting the machine the drains must be opened to give an unloaded start and allow the machine to purge itself of any accumulated moisture as soon as it starts to turn after short interval drains are closed. And the machine put on load.

Normal running (while running)

Periodical draining of the accumulated moisture is drained by opening the drains

The machine is checked to ensure it is getting adequate cooling water and stage pressure are correct and forced lubricated lube oil pressure is correct

Stopping the machine

A short time before stopping the machine drains should be opened and drain accumulated moisture for an idling period

The above functions have been mechanized to perform better by introducing modulator unloaders that vent the systems through the filters and that help keep the air intake filter

Starting air compressor circuit

Starting and stopping sequence is adjustable, the magnetic valves are open when the compressor is stopped so any residual pressure is blown off. On starting the magnetic valve is sometimes delayed to close to allow the compressor motor to reach full speed before the compressor is loaded up.

The non-return valves prevent HP air from leaking back from the receiver on which the filling is also of the non-return type.

Compressor Control

The operation of the compressor can usually be done by two methods

The first method is where solenoid valves at each stage outlets are held open when the compressors start and are held open for a short time afterward. This allows the compressor to run up to speed with a minimum load on the machine. When the required pressure is obtained the solenoids valves are again held open and then the compressor stops. The control of this is by the use of pressure switches and timers. The second method is to have the compressor running continuously, with the machine loading up and unloading at set pressures. By holding the suction valve in the open position the compressor can be run in unloaded condition.

CrankCase Lubrication

Lubrication normally consists of pressurized oil being fed to shell bearings while splash oil may lubricate the liner surfaces. Some air compressors have a drip cylinder lubrication system and this should be kept to a minimum conducive to liner wear. A standard mineral oil similar to that used in the main engine may be used, although due to carbon deposits, synthetics oils are generally used more effectively which reduces carbon deposits thus reducing maintenance. But oil is costly.

Mineral oils contain a blend of lighter elements such as paraffin’s, and heavier elements Such as asphaltenes. During compression the lighter elements are vaporized leaving the heavy ends, these coat the piston rings and discharge valves in combination with oxidized oil deposits. These deposits also coat passageways and coolers resulting in higher interstage air temperatures. Deposits on discharge valves cause them to become sticky and leak resulting in hot air being drawn back into the cylinder for recompression. This increases the temperature and hence causes greater OXidation and deposits, and so the condition deteriorates with increasing rapidity.

Temperature can become very high, this may result in oily deposits at the discharge valves carbonizing. Eventually, this carbon could glow red and cause detonation. However, it is more likely that oily deposits will be carried over to the air receiver and air start manifold to be ignited by blow past at the cylinder air start valve.

Deposits at piston rings cause leakage allowing oil to enter the cylinder from the crankcase thus increasing the danger it is essential that crankcase lubrication be kept to a minimum compatible with an acceptable wear rate. Regular maintenance will minimize oily deposits build up and hence the risk of explosion

Lubrication Selection

According to normal lubrication requirements and to minimize coke deposit

Plain mineral oil is not used as it Oxidises easily at a high temp. and pressure Lube oil must have anti-oxidant additives for good oxidation resistance

Low viscosity easy spreading, effective sealing,

Hydrodynamic lubrication with low friction

Flashpoint above 220°C

Oil with minimum heavy ends

If oil has too broad a distillation range, the more volatile portion may tend to evaporate leaving behind heavier ends in the hot zone.

Synthetic Oil

Product made of chemical synthesis

Raw materials or the base stock may be mineral oil derived

The chemical structure is planned and controllable

The molecular structure is variable so that product performance may be modified

Additive treated to produce a lubricant with superior properties to mineral oil

Synthetic Oil Advantages

• Superior to mineral oil & excellent protection against corrosion.
• Suitable for severe operation.
• Good thermal & oxidation stability
• Operate over wide temperature range’
• may be cost etfective
• Good viscosity vs temp. properties.
• Low pour point (-33°C to – 48°C).
• Good wear resistance with low viscosity.
• Reduced maintenance Cleaner compressor due to minimum deposits.
• Long service life.
• Enhanced safety due to high flash point (246 to 266 deg C)

Synthetic Oil Disadvantages

• Less compatible with seals & paint
• Initial cost is high, but overall it  may be cost effective
• Possible mineral oil incompatibility
• Potential toxicity
• normally consist of splash lubrication with pressurized oil being fed to shell bearings

Safety of the Air Compressor

• Oil dip
• L.O law pressure alarm
• Sight Glass
• Relief Valves are fitted to discharge side of every stage
• Bursting disks are fitted on the cooler shells (at water side)
• Drain valves (also work as unloaders) fitted at each stage

LP discharge pressure 4 bars: HP discharge pressure 30 bars

intercooler inlet air 130`C: Intercooler outlet air temperature 35`C