1.Introduction
Marine air compressors are used for a variety of purposes, ranging from main engine starting to general cleaning. Air compressors used on board ships are the auxiliary machinery for important use related to the propulsion of ships and the safety of lives and ships, so they are subject to the plan approval of the Korean Register of Shipping(KR). In KR's Rules and Regulations, the number and total capacity of air compressors are regulated. It is stated that where the main engines are designed for starting by compressed air, at least two starting air compressors are to be provided and arranged so as to be able to charge each reservoir.
It is a requirement that where the air compressors are arranged for main engine starting, an emergency air compressor powered by an independent power source shall be provided. The reservoirs must store sufficient air for the starts without the need for top-up from the compressors. The reservoirs shall have capacity that allows the main engines to be started twelve times if they are reversible (i.e., running in reverse as well as forward directions) and six times if non-reversible(www.krs.co.kr, 2015).
Where compressor air is used for starting of auxiliary engines, there shall be at least two independent auxiliary(Lee et al., 1998) reservoirs whose combined capacity allows all the auxiliary engines to be started three times, while the compressors are idle. If none or only one auxiliary reservoir is available, the compressed air that is sufficient for three auxiliary engine starts shall be supplied from the main air reservoir via independent pipelines.
Marine air compressors are essential auxiliary machinery and so great care is taken in the operation and maintenance of air compressors. However, it is not uncommon to see costly damages to pneumatic control systems related to condensation (a mixture of moisture and contaminants) in the compressed air. A substantial amount of research has been carried out to minimize condensate generated by the compressing of air and to remove it from compressed air systems.
To reduce installation space requirements and manufacturing costs, reciprocating air compressors are changing cooling methods, from water-cooled to air-cooled. Lee (Lee et al., 2011) studied the principal noise sources of reciprocating air compressors and developed 8 countermeasures against noise. It was reported that these countermeasures decreases compressor noise by up to 10 dB(A).
As air compressors are highly needed, the performance of different types of compressors becomes significant. Sung(Seong et al., 2007) estimated the performance of marine air compressors and confirmed that there is an efficiency advantage to multi-stage compression when intercooling is properly used.
Moisture or oily water mixtures contained in the compressed air would condense in the compressor, causing secondary damage to the compressed air systems. To address this problem, this work examines a way to reduce condensate produced by air-cooled reciprocating marine air compressors through experiments.
2.Theoretical background
2.1.Moisture in the air
Atmospheric air normally contains some water vapor (moisture). It can be referred to as "dry air" or "moist air" according to the proportion of moisture in the air. Removing this moisture from the compressed air is essential to prevent pneumatic machines and tools from experiencing premature wear or damage(Kim et al., 2014).
Relative humidity (RH), denoted by Rh , indicates the amount of water vapor in the air. RH depends on the air temperature. At higher temperatures, the air can hold more water vapor. RH is expressed as a ratio of the actual (partial) amount of water vapor in the air to the amount of saturated water vapor at a given temperature(M. G. Lawrence, 2005).
Temperature and humidity determine the amount of moisture the air can hold. This is also influenced by the pressure. The amount (or mass) of water vapor per unit volume increases when the air is compressed by the air compressor.
When the compressed air is cooled, the amount of saturated water vapor decreases as the air temperature drops. Moisture condensation occurs when the amount of water vapor in the air exceeds the amount of saturated water vapor.
The compressed air discharged from an air compressor has a very high temperature and pressure. Even if its RH is below 100 %, compressed air at high temperatures contains large quantities of water in vapor form, which will condense into a liquid form as it cools down.
where,
Dr: Amount of condensate produced [g/m3]
rs: Amount of saturated water vapor at the beginning [g/m3]
r's: Amount of saturated water vapor after compression-cooling [g/m3]
P: Absolute pressure at the beginning [kgf/cm2]
P': Absolute pressure after compression-cooling [kgf/cm2]
T: Absolute temperature at the beginning [K]
T': Absolute temperature after compression-cooling [K]
φ: Relative humidity at the beginning [%]
2.2.Performance and power consumption of the air compressor
Where the humidity is considered, the actual power (W) of the air compressor is defined as the ratio of the theoretical power (Wt ) of the compressor to the mechanical efficiency (ηm ) and thermal efficiency (ηt ). It is calculated using the following equation.
The energy consumption index (ECI) represents the relationship between the actual power consumption (W) and the discharge flow rate (Q outlet) of the compressor. ECI is expressed as follows.
3.Marine compressed air systems and damage incidents
3.1.Marine compressed air systems
A marine compressed air system consists of the compressor main body, power supply unit, and pipelines that deliver the compressed air to the air reservoirs.
Marine air compressors are the auxiliary machinery for important use that requires a plan approval. Shipboard air compressors are roughly classified into turbo and reciprocating types. Despite the structural drawbacks of reciprocating motions, multi-stage reciprocating air compressors are most widely used in favor of rotation speeds and flow volumes. The performance of reciprocating air compressors is largely influenced by cylinder's compression ratios, valve weights, and flow resistance in the air lines.
The moisture or oily condensate in the compressed air might give rise to mechanical failures. It must be removed from the compressed air prior to contaminating downstream equipment. One way to achieve this is supplying dry air to the air intaker of the compressor, thus preventing moisture from coming inside the compression chamber. Another method is utilizing cooling fans to remove heat from air between compressor stages in the two-staged or three-staged compression process.
This work exploits a cooling fan to cool the discharge air in each compression stage. In the experiments, an extra cooling fan was installed in the compressor and changes in the amount of condensate from the compressor were examined in connection with the use (or non-use) of the cooling fan.
Fig. 1 shows an overview of the marine compressed air system. The air is compressed in the Compressor and then goes through the Cooler. The Separator automatically or manually removes condensation from the compressed air. In the Main Air Reservoir, condensation is removed again. Finally, the compressed air is supplied for starting of the engines, or it is delivered to pneumatic control systems after being decompressed.
Fig. 2 is a cross section of the air compressor. The names of the various parts of the compressor are listed in Table 1. The engine room air is drawn into the compressor via the Air-intake, and moves through the compressor circuit until it reaches the cooling and storage systems.
3.2.Secondary damages caused by compressed air
Atmospheric air contains moisture or water vapor. When it is compressed by the air compressor, the moisture contained in it is retained at high temperatures and then condenses into liquid water as the compressed air is cooled to a usable temperature. The produced water needs to be regularly removed by a drain as it may cause a variety of operational problems and failures in the pneumatic control systems. Note that the term "drain" is sometimes used to refer to a slurry of crud that is a mixture of condensed water and contaminants (e.g., lubricant, pipe dope, etc.) from the compressed air system.
Where pneumatic control devices are used, a compressed air dryer is used along with the air compressor for condensation removal. The two most common air dryers are refrigerated and desiccant dryers. Desiccant air dryers utilize the adsorption method of moisture removal. That is, water in the compressed air is chemically bound to an adsorption material (e.g., activated alumina gel, molecular sieve) and dissolves. Shipboard air compressors usually employ refrigerated air dryers. When a refrigerated dryer cools the compressed air as low as 11°C, a proportion of water vapor in the air is decreased by 50%. Compressed air dryers have to operate properly; otherwise, the pneumatic control and actuator equipment are exposed to the water and contaminants in the compressed air.
Condensation is caused by either compression or cooling. As the air is compressed by the compressor, the amount of water vapor per unit volume increases. In terms of cooling, the amount of saturated water vapor decreases as the air temperature decreases. When the amount of water vapor contained in the air exceeds the amount of saturated water vapor, condensation (drops of water) begins to form.
The compressed air in the receiver right after the compressor runs has a significantly increased temperature and pressure. It becomes super saturated, thus holding more moisture than 100% of the moisture it can normally carry. When this air cools as low as the ambient temperature, a large amount of water condenses in the compressor.
Every compressor brings in some moisture and contaminants (e.g., dust, pipe rust, water, water vapor, oily distillates) with the intake air. The volume of the air is decreased after compression but the amount of contaminants remains constant. The air is heated when it is compressed and it is able to hold more water in vapor form. That is, the saturation temperature goes high, which allows the water to remain a vapor state, but only temporarily. The high temperature and high pressure air discharged from the air compressor is getting cooled while moving through the pneumatic control systems. When the temperature drops below the pressure dew point, the water vapor condenses into liquid water. This is why condensate is often found in the air lines. Timely and effective condensate removal is important to protect pneumatic equipment and control systems from damage.Table 2
Fig. 3 shows a damaged air cylinder used for the large-scale marine diesel engine. The root cause of this incident is reported to be the contaminants in the compressed air rather than product defects.
Fig. 4 presents an air compressor used for starting of the generator engines. The turbines and guide vanes have damages caused by cavitation even though they are in an enclosed space where only compressed air is allowed to enter. This shows that water and other contaminants in the compressed air can cause critical mechanical damage.
4.Experiments and observations
4.1.Experimental method
Ship SAENURI, a training ship owned by the Mokpo National Maritime University, is equipped with two water-cooled air compressors and one air-cooled air compressor. In order to run the water-cooled air compressors, one sea water pump and one fresh water pump need to be operated as well. The fresh water that is cooled by the Central Fresh Water Cooler is supplied to the water-cooled air compressors. Whenever the ship lies at anchor or whenever necessary, the required compressed air is produced by running only the air-cooled air compressor, so as to reduce operational cost.
To find a way to minimize condensation that can be harmful to the compressed air system, the amount of condensation in the compressed air discharged from the air-cooled air compressor was measured in the experiments. The technical specifications of the air compressor used in the experiments are given in the table below.
4.2.Experiment results and observations
The experiments were conducted from June 24, 2014 through July 23, 2014. Five cases with different operation conditions were experimented. During 46.6 running hours, the amounts of condensation produced in the five different cases were measured by turning on and off the installed cooling fan. The executions of the five cases with different experiment conditions were regularly shifted in order to prevent the results from being susceptible to the weather and humidity conditions.
In Case 1, 1,825 ml (about 446.2 ml per hour) of condensation was produced while running the air compressor for 4.1 hours without using the cooling fan. When the cooling fan was used and the compressor ran for 1.4 hours, 600 ml (about 428.6 ml per hour) of condensation was generated. This indicates that the use of cooling fan contributes to decreasing the amount of condensation by 3.9 % (17.6 ml per hour).
In Case 2, 503.1 ml per hour was produced. When the cooling fan was used, 55.1 ml per hour was produced, thus reducing the amount of condensation by 10.9 %.
In Case 3, 9,535 (496.6 per hour) was produced while running for 19.2 hours without using the cooling fan. Where the cooling fan was used, condensate production decreased by 9.8 %.
In Case 4, the running duration of the cooling fan was extended as long as that of the air compressor. While running the compressor and cooling fan for 3.3 hours, 1,780 ml (539.4 ml per hour) was produced. Where the cooling fan was turned off, the produced condensation was 579.0 ml per hour. That is, the running of cooling fan decreased the amount of condensation by 6.85 % (39.6 ml per hour).
In Case 5, the air compressor and cooling fan ran for 3.3 hours. In this condition, 1,780 ml of condensate was generated. Where the cooling fan was not used, 591.7 ml per hour was produced. The amount of condensation generated was decreased by 8.8 % (52.3 ml per hour) with the use of cooling fan
The running hours, temperatures, and humidity levels might be slightly different in the five cases, but they were executed in turn in a similar condition in order to increase experiment accuracy. Fig. 6 presents the amounts of condensation and the compressed air temperatures at the outlet of the compressor in the five cases.
Fig. 7 presents all the condensation measures collected over the entire measurement period. The cases where the cooling fan was used were executed on date 6.27, 6.30, 7.12, 7.13, and 7.14. One can see that the amounts of condensation produced per hour are relatively small in those data points.
Fig. 8 shows the amount of condensate and the humidity levels that were measured when the cooling fan was used and when not used. When the cooling fan was used, the amount of condensate was decreased by 7 % and the humidity levels were slightly lower. Fig. 8 and 9 present the production of condensate in accordance with the humidity. When the temperature and humidity are higher, more condensate was produced. When the air temperature at the compressor outlet is lower, less condensate was generated.
5.Conclusion
Compressed air has many uses on board ship, ranging from diesel engine starting to the cleaning of machinery during maintenance. In an effort to enhance the performance of the marine compressed air system, this work studied a way to reduce condensation from the air compressor via experiments. The following findings were obtained in the study.
-
More condensation is produced when the temperature at compressor outlets and the humidity of the air are higher.
-
The amount of condensation is reduced by up to 10.9 % when the cooling fan is used along with the air compressor.
-
In the experiments, cooling fan's wind directions are not taken into account and changes in the temperature and humidity are not sufficiently varying. The limitations of experimental research need to be considered.
-
Condensed water and oily containments built up inside the compressed air control system can give rise to mechanical or system failures that might endanger the safety of lives and ships. In the future, the amount of condensate from the compressor in connection with compression load variations and the type and amount of silica gel at the air intaker will be studied to minimize condensate production.
