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3-stage air-cooled compressors
With a fll-up capacity of 460 m³/h, the SAUER Levante WP460L Marine is Sauer’s biggest 3-stage air-cooled compressor.
The WP100L was the frst 3-stage air-cooled Sauer compressor installed on a ferry in 1975.
CONTAINER SHIPS, TANKERS, CRUISERS – THE MAJORITY OF SEAGOING VESSELS WITH AN ENGINE POWER OF AT LEAST 20,000KW IS EQUIPPED WITH A 3-STAGE AIR-COOLED STARTINGAIR COMPRESSOR NOWADAYS. THE POWERFUL TECHNOLOGY REDUCES HEAT AND ALLOWS FOR ENORMOUS COST SAVINGS. SAUER COMPRESSORS DISCOVERED THE POTENTIAL OF 3-STAGE AIR-COOLED COMPRESSORS FOR THE SHIPPING INDUSTRY EARLY ON AND HAS THEREFORE BEEN A PIONEER IN DEVELOPING THESE COMPRESSORS FOR ALMOST 50 YEARS.
ALL IMAGES COURTESY OF SAUER COMPRESSORS. In 1975, when the Railship I frst entered service on the Travemünde (Germany) – Hanko (Finland) route, it has been the world’s largest railway ferry. While its length of about 180m was quite remarkable at that tme, it would almost look like a toy ship next to today’s ULVCs (Ultra Large Container Vessels). On board the ferry were Sauer’s 3-stage air-cooled compressors, which was a frst at that tme. Almost fve decades have passed since then, during which Sauer Compressors delivered over 30,000 of these startng-air compressors. Despite many developments, the basic operatng principle of the technology has not changed a lot. “Today, about 80% of large container ships are equipped with 3-stage air-cooled compressors, making them industry standard”, says Stephan Behrens, Global Head of Business Development Commercial Shipping at Sauer Compressors. >>
While the compression temperatures of 2-stage water-cooled compressors are above 250°C, the 3-stage air-cooled compressors divide the compression ratio by three, enabling the temperatures to be up to 30% lower. This reduces maintenance costs.
The SAUER Passat WP81L Marine is Sauer Compressors’ smallest 3-stage air-cooled machine.
For 3-stage air-cooled compressors, the service life of valves is up to 4,000 hours as opposed to 2,000 hours for water-cooled machines.
Stage valve of an air-cooled compressor afer 2,500 hours. Stage valve of a water-cooled compressor afer 800 hours. However, the comprehensive establishment of the 3-stage air-cooled startng-air compressors took some tme. Many shipping companies and shipyards stuck with the previously common 2-stage water-cooled compressors, instead of switching to generally advantageous air-cooled machines.
Large ships, large engines, large fll-up capacity
To start the ship’s engine, compressors fll 30 bar compressed air into air receivers whose volume is determined by the engine size. As required by the classifcaton societes, the amount of compressed air must be sufcient for a certain amount of engine starts. The increase of the internatonal trading volume startng in the 1950s led to bigger ships and engines and therefore a higher demand for compressed air. 2-stage water-cooled compressors were the most common soluton for startng the ever larger diesel engines. Due to their comparably low fll-up capacity, the 2-stage air-cooled compressors that were commonly used for smaller ships were not a viable alternatve. But Sauer Compressors was eager to enable the use of air-cooled compressors for large ships and engines as well. In the early
Stephan Behrens, Global Head of Business Development Commercial Shipping at Sauer Compressors.
1970s, the company came up with the idea of adaptng 3-stage air-cooled compressors as an established soluton from the industrial sector to commercial shipping. However, despite their successful applicaton on the Railship I and several other ships, the 3-stage air-cooled compressors could not establish themselves as industry standard. “The building of large cargo ships was relocated to Asia and the local shipyards simply took over the established design standards”, explains Stephan Behrens. Only with the new millennium, the mult-stage air-cooled compressors were fnally put on the standard makers list of the Asian shipyards. Sauer Compressors was able to convince ship owners and builders not only with references but also with specifc demonstratons. For this purpose, a 3-stage air-cooled compressor was installed in a TEU container and presented worldwide.
Reduced temperature
The 3-stage air-cooled compressors are bigger and therefore have a signifcantly higher fll-up capacity than the 2-stage air-cooled machines. The total heat to be dissipated is divided between three stages instead of two and the heat-dissipatng surface area increases while the heatgeneratng volume remains the same.
Air beats water
3-stage air-cooled compressors are superior to water-cooled machines for multple reasons. By dividing the total compression rato by three, the compression temperatures in the cylinders and valves of 3-stage air-cooled compressors are up to 30% lower than those of 2-stage water cooled compressors, which may well reach 250°C. Due to these comparably low temperatures, there is no thermal cracking of standard mineral oils. The maintenance intervals of valves therefore increase to 4,000 hours in contrast to 2,000 hours for water-cooled compressors. Instead of expensive synthetc oils, conventonal SAE-30 mineral oils such as those used in 2-stroke marine diesel engines for circulatng lubricaton of the crank drive may be used to lubricate the compressors. Thus, no expensive synthetc oil is required. With ongoing operaton, the long-term maintenance costs are up to 30% lower for
SAUER Passat WP271L Marine, installed on the Hapag-Lloyd Vancouver Express.
air-cooled than for water-cooled compressors.
Quick amortisation of air-cooled compressors
Cooling the startng-air compressor with room air makes a cooling water circuit obsolete. “This can save installaton costs of over Eur 6,400”, states Stephan Behrens. Moreover, it avoids leakages and troublesome condensate in the crank drive that occurs when travelling through diferent climate zones. Other benefts are reduced space requirements in the engine room, higher reliability, and minimum operatng efort of the self-regulatng air cooling. There is no need to modify the fan output of the engine room ventlaton, since the waste heat of the compressor is negligible compared to the heat of the ship’s engine.
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Photo courtesy of Stena RoRo.
