Air Compressors
Graph of Pressure against volume in a reciprocating compressor Volumetric efficiency Vh = Actual suction volume Vx/ Theoretical suction volume Vs
For greater efficiency air compression should be isothermal as this require requires s the minimu minimum m work input. input. In practi practice ce Isothe Isotherma rmall compres compressio sion n is not possib possible, le, an ideal ideal Isothe Isotherma rmall cycle cycle require requires s suffic sufficien ientt time time to allow allow all the required heat to be transferred out of the cylinder, practicality dictates that the piston must have a relatively high speed to give a reasonable output, Cylinder cooling on a single stage compressor gives better efficiency but there is a limitation in the surface area to cylinder volume that can be used for cooling effect, but multistage compressors with an efficient extended surface interstage cooler gives cycle improved compression efficiency better approaching that of the isothermal. In theory the greater the number of stages the closer the curve will approach the ideal isothermal compression curve, however there is an increase in cost, complexity, and the law of diminishing returns limit the number. Compression in stages has the following advantages; 1. The compr compress ession ion ratio ratio at each stage stage is lower lower and so the final final temper temperatu ature re is lower. This reduces problems with lubrication 2. . The The machine machine is smalle smallerr and and better better balance balanced d 3. water water can can be be drain drained ed off off at each each stage stage 4. Compres Compression sion bette betterr approach approaches es the ideal ideal isotherm isothermal al
It is important that the compressor clearance volume is kept small as possible in order to improve overall volumetric efficiency as the air trapped in this space must expand to below suction pressure before new air can enter, this is an effective loss of stroke. clearance is required in order to prevent the piston striking the cylinder cover when starting or stopping off load. !he clearance volume is sometimes referred to as the "#ump Clearance".
Crankcase lubrication $ubrication of the crankcase in a compressor does not pose any specific problems and normally consist of splash lubrication with pressurised oil being fed to shell bearings. %here drip cylinder lubrication is used, this should be kept to a minimum conducive with liner wear. standard mineral oil similar to that used in the main engine may be used, although due to carbon deposits, higher quality oils are generally used with the most effective being specifically designed synthetics which have allow a considerable reduction in maintenance but are costly. &ineral oils contain a blend of lighter e lements such as paraffin"s, and heavier elements such as asphaltenes. 'uring compression the lighter elements are vaporised leaving the heavy ends, these coat the piston rings and discharge valves in combination with oxidised oil depo sits. !hese deposits also coat passage ways and coolers resulting in higher interstage air temperatures. 'eposits on discharge valves cause them to become sticky and leak resulting in hot air being drawn back into the cylinder for recompression. !his increases the temperature and hence causes greater oxidation and deposits, and so the condition deteriorates with increasing rapidity. !emperature can become very high, this may result in oily deposits at the discharge valves carbonising. (ventually this carbon could glow red and cause detonation. It is more likely, however, that oily deposits will be carried over to the air receiver and air start manifold to be ignited by blowpast at the cylinder air start valve. 'eposits 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. )egular maintenance will minimise oily deposits build up and hence the risk of explosion
Materials and design of a reciprocating compressor !he compressor casing, cylinder covers and piston rings are generally of cast iron. *istons may be of cast iron, steel of aluminium. luminium being the preferred material for use on the $* piston due to its larger diameter. +alves are usually made so that parts can be interchanged between the suction and discharge valves. eats are of mild steel with small diameter air passages to prevent the fragments of broken valve plate from entering the cylinder. +alve plates are of vanadium steel heat treated and ground to provided the required hardness and surface finish. prings should be arranged such that they lift and seat squarely. -neven spring force or deposits on the seat cause valves to bend resulting in fatigue cracking.
For compressors designed for starting air re quirements a water acket relief valve is fitted.
Rotary Compressor
!he rotary compressor may be of the impeller type similar to that used in the turbocharger , scroll, twin rotating lobes or of the sliding vane type similar to the one shown above. In practice there would be several more vanes than shown. )otary compressors are capable of handling large quantities of low pressure air much more efficiently than a reciprocating compressor. In order to produce increased pressures it is possible to stage rotary compressors but leakage problems increase at higher pre ssures as well as stress on the vanes. !he sliding vane compressor consists of a slotted rotor with its axis offset from that of the cylindrical casing. +anes fit in the slots and have contact with the casing /n the suction side the space contained between the casing, the rotor and an adacent pair of vanes is gradually increasing allowing air to be drawn in. /n the compression side this same space is gradually reduced causing the pressure increase. %hen the leading vane uncovers the discharge port air will flow to outlet. $arger compressors of this type are water0cooled, smaller compressors tend to be air cooled. !he main problems related with sliding vane compressors concern wear at the vane tips and sealing of the ends
Rotary/reciprocating Compressor )otary compressors in general do not require internal lubrication but they are not suitable alone for providing air at a pressure for starting duties. !hey can, however, be linked to reciprocating stages to produce a hybrid compressor. !he compressor is lighter, more compact and better balanced than an equivalent all reciprocating unit. In basic terms the rotary first stage supplies air
to the reciprocating second and subsequent stages. ll stages being driven by the same shaft
Safety Valve
Materials Cast iron0Casing, $iners, *istons1 the $* piston is sometimes made from an aluminium alloy, Cylinder covers Steel0 Crankshaft, Conrods, *istons, +alve seats Vanadium Steel0 +alve plates
Starting air compressor circuit tarting and stopping sequence is adustable, the magnetic valves are open when the compressor is stopped so any residual pressure is blown off. /n starting the magnetic valve are sometimes delayed to close so as to allow the compressor motor to reach full speed before the compressor is loaded up. !he non0return valves prevent 2* air leaking back from the receiver on which the filling is also of the non return type.
Calculation of reuired cylinder compression for a multistage reciprocating compressor r ) for r 3 )456
3 3
stage compression
pressure pressure
a
for
two
a
three
ratio ratio stage
stage
r 3 )457 for example, a 7 stage compressor requiring a final pressure of 89bar would have the following interstage pressures 4st stage 4bar compressed to 9bar 6nd stage 9bar compressed to 48bar 7rd stage 48bar compressed to 89bar It would appear that most of the work is being carried out in the final stage, this is untrue as with the increase in pressure is a complimentary reduction in volume, if the temperature conditions remain the same then work will be equally divided between the stages. #y reducing the suction pressure, the cylinder is required to do more work on the air before the discharge valve opens. !his means that the air will be delivered at a higher pressure. !he higher temperature can lead to problems with the cylinder lubrication as well as a drop in efficiency as well as carbonising of the oil and increased deposits on the valves and piston rings and interstage passages. In the extreme it can lead to sei:ure and possible diesel detonation of the oil laden air. !he reduction in pressure at the suction can be due to a partially blocked suction filter or partially choked suction valve. !he lower pressure conditions in the cylinder at the start of compression can cause oil laden air to be drawn from the crankcase up the liner. !his oil can lead to increased deposits in the compressor as well as further downstream in the distribution system 1*4.+45 !4 3 1*6. +65!6 and 1*45*6.1!65!4 3 1+65+4
*4.+4g 3 *6.+6g and *45*6 3
3
+6g 5+4g !4. 3 take
From 1*65*4
these
we
get;
1g
045g !6 g 4.9 and if we for and example *4 3 <.9 bar *6 3 4 bar *f 3 = bar !initial 3 7<< > we end with final temperatures for the two compression"s of !43 84? > and !6 3 9?= > from the graph it can be clearly seen that losses due to the bump clearance has increased and the period of constant pressure delivery has been reduced.
Coolers •
Plain Tube-easy to clean -very effective due to large surface area of large number small diameter tubes -plugging of failed tube allos cooler to continue in service with little loss in efficiency -must allow for thermal expansion by having one tube plate floating 'U' tube-suitable for higher pressures than plain tube -self compensating for thermal expansion -efficent due to large nomber small diameter tubes -failed tubes may be plugged -more expensive than plain tube and diificult to clean Coil tube -self compensating for expansion -suitable for high pressures -difficult to clean -inefficicent due to large tube diameter - not easy to plug -expensive • •
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•
•
• • • • •
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Air Start Systems Regulations
1. There must be at least two starting air receivers, the total capacity of which will give 12 starts for a reversing engine or 6 starts fo a nonreversing engine with C!!. 2. There must be at least two compressors 3. "n addition to these there must be a compressor which can be started by hand i.e. with a dead ship. #ote$ this is not necessary if one of the compressors is run off the emergency switchboard 4. i. % relief valve must be fitted to the &! discharge and be sufficient si'e to ensure that the pressure rise does not e(ceed 1)* of the w.p. when the compressor is running and the outlet valves on the bottle are shut. ii. % relief valve or bursting disc on the hp cooler casing in order to protect the casing from overpressure in the event of cooler tube failure #ote$ +ursting discs are generally preferred because they fail and stay failed giving complete protection. % relief valve will reseat when the compressor is stopped allowing water to enter the air side. iii. % drain must be fitted at each stage
!iesel start air system !he components of the air start system are taken to include compressors and storage bottles in addition to the engine air start arrangement. !he minimum of tow compressors should be matched to the starting air requirements of the engine. !he compressor after coolers should be protected by a bursting disc. ll high pressure lines in the system to be of solid drawn pipe.
Air Receivers !here must be a means of access to allow cleaning and inspection of internals. !he internal surface should be protected by a coating which is flexible enough to move when the metal distorts. Copal varnish is generally used because it has these properties and willnot easily oxidise. -sually precautions are taken the same as for an enclosed space when entering. +entilation is required to the solvent fumes in the varnish 'rains must be fitted in the lowest part of the receiver )eceivers must be protected by means of a relief valve, if the relief valve can be isolated from the reciever than a fusible plug or plugs must be fited. !hese are usually fitted because in the event of a fire near to the bottle they will fail and release the entire contents of the bottle rapidly. relief valve however will only release air down to its closing pressure which is set point less blowdown. If the structure of the bottle becomes weakened by the heat then its ability to withstand even the reduced pressure is weakened an possible rupture could occur. !he inlet and outlet valves are to be arranged to prevent direct flow through the bottle with insufficient residence time for moisture to rpecipitate. +alves to be of the slow opening type to prevent excessive pressure rises. ll
attachments
should
be
via
a
support
plate
Safety devices !he automatic valve 1&ain air start block valve prevents connection between the air receiver and air start manifold unless actually in the process of starting. !his minimises the risk of an explosion in the air manifold actually propagating back to the air receiver where a much more severe explosion is possible. afety devices are encorporated in the air start manifold in order to dissapate the energy of an explosion thus keeping its effects local. uch devices include flame traps, relief valves and bursting discs
$oss of air can be kept to a minimum by rotating moveable outer hood to blank off relief ports. !he failed cap should be replaced as soon as possible.
Starting air e"plosions Causes0continuous leaking of start valve followed by it sticking open on start. n oil film may build up on the start air pipe due to oil dscharge from the compressor. !his oil may come from general lubrication or sticky scraper ring or from the engine room air, %ith a continuous leaking valve hot gasses with unburnt fuel will enter the pipe and turn the oil film into a hot incandescent carbon. %hen high pressure air is put on the pipe line an explosion can occur with resultant high speed high pressure shock wave.
lternately, air discharged into cylinder during starting may have an oil mist which can ignite in a hot cylinder. !he hot gasses can return through the start valve. !o prevent this the non return valve should be properly maintained, oil discharge from the compresors should be kept to a minmum and pipelines inspected nad cleaned when necessary. !o minimise effects a flame gau:e should be fitted to the start valve and ample relief valves, bursting discs or caps fitted. n isolated valve on the discharge side of the manouervring control valve.
Starting air valve#
Starting Air Regulations $irst start reuirements (quipment for starting the main and auxiliary engines is to be provided so that the necessary initial charge of starting air or initial electric power can be developed on board ship without external aid. If for this purpose an emergency air compressor or electric generator is required, these units are to be power driven by hand starting oil engine or steam engine, except in the case of small installations where a hand operated compressor of approved capacity may be accepted. lternatively, other devices of approved type may be accepted as a means of providing the initial start
Air Compressor reuirements
Air Compressor number and capacities !wo or more air compressors are to be fitted having a total capacity, together with a topping0up compressor where fitted, capable of charging the air receivers within 4 hour from atmospheric pressure, to the pressure sufficient for the number of starts require t least one of the air compressors is to be independent of the main propulsion unit and the capacity of the main air compressors is to be approximately equally divided between them. !he capacity of an emergency compressor which may be installed to satisfy the requirements of first start is to be ignored.
Ma"imum discharge air temperature !he compressors are to be so designed that the temperature of the air discharged to the starting air receivers will not substantially exceed @7"C in service. small fusible plug or an alarm device operating at 464AC is to be provided on each compressor to give warning of excessive air temperature. !he emergency air compressor is excepted from these requirements.
Safety Valves (ach compressor is to be fitted with a safety valve so proportioned and adusted that the accumulation with the outlet valve closed will not exceed 4< per cent of the maximum working pressure. !he casings of the cooling water spaces are to be fitted with a safety valve or bursting disc so that ample relief will be provided in the event of the bursting of an air cooler tube. It is recommended that compressors be cooled by fresh water.
Air Receiver reuirements Air Receiver capacity %here the main engine is arranged for air starting the total air receiver capacity is to be sufficient to provide without replenishment, not less than 46 consecutive starts of the main engine, alternating between ahead and astern if of the reversible type and not less than six consecutive starts if of the non0reversible type. t least two air receivers of approximately equal capacity are to be provided. For scantlings and fittings of air receivers For multi0engine installations, the number of starts required for each engine will be specially considered.
