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MAINTENANCE. The plant will need a minimum of care and maintenance if it is kept: free of moisture; free of impurities; free of freon leaks; free of frost.
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The plant will need a minimum of care and maintenance if it is kept: • free of moisture; • free of impurities; • free of freon leaks; • free of frost.
Experience has shown that most problems with marine refrigerating plant involve refrigerant shortage caused by leakage. In rooms where frozen or chilled cargo below 5°C is carried, it is necessary to keep evaporators free from frost. Cleaning of filters is also important.
In cases where equipment is opened up, neither air nor moisture must enter the refrigeration system, as either will cause trouble, e.g. in the form of increased condensing pressure. To avoid moisture, filter driers are installed in HCFC/HFC plants.
The drying agent should be changed every time any part of the system is opened. This also applies when charging with oil or refrigerant. Oil should not be filled from vessels that have not been tightly closed.
Drop in oil level • A leakage-free refrigerating plant does not consume any oil. The oil which has disappeared from the crankcase or oil separator is always somewhere in the system.
Oil level drops quickly at the start • This may be due to refrigerant being dissolved in the oil. At evaporation the oil is drawn with the refrigerant into the system. Fill the system with a small quantity of oil, as the ejected oil will gradually come back.
Oil level drops slowly • the plant is operated at lower evaporating temperature than usual or the refrigerant charge is too small; • refrigerant leakage in the system, by which the level in the evaporator has become too low; • condensing temperature is too low, the minimum condensing temperature should be maintained; • the cooling demand is too low, so the gas velocity becomes too low in the evaporator and the oil remains in the system.
In those plants with piston compressors which have oil separators, the shut-off valve in the oil return line should always be kept closed for about 1/2 hour after compressor start in order to avoid the carriage of condensate from the oil separator to the crankcase.
Brine specific gravity • In those cargo refrigerating plants where brine serves as the heat transfer medium, it is of great importance that the correct brine specific gravity for the required cargo temperature is maintained. If this specific gravity is not checked, functional problems may occur.
Daily maintenance • The daily maintenance for a R22 installation should be completed as follows: • Check that condensing pressure and evaporating pressure are correct. • Inspect the compressor unit and check that there are no abnormal noises or vibrations. • Check the oil level. • Check the tightness of the shaft seal. Oil leakage can be tolerated whereas gas leakage can not. • If an oil separator is installed, check that oil is returned to the crankcase, and that the oil return line is warmer than the crankcase.
Leak detection • Refrigerating plants must be gas-tight to prevent refrigerant leakage and air entering the low-pressure side when under a vacuum. Systems which have been opened to the atmosphere during repairs, must be pressure tested for mechanical strength and leaks before charging with refrigerant.
Pressure tests • Pressure tests should be done with nitrogen. Water or other fluids must not be used as a test medium. The plant’s compressors must not be used to pressurise the plant. The pressure is gradually increased by pressurising with nitrogen gas the pressure is equal to 1.5 times the maximum working pressure of the system. This pressure should be maintained for about 10 minutes.
Each joint must be examined thoroughly for signs of gas bubbles which indicate a leak. After sealing any leaks, pressurise the system again with the test medium and some refrigerant as a trace gas, and repeat the leak test using an electronic leak detector, etc. It is imperative that all leaks are found and sealed before the system is charged with refrigerant, as even the tiniest of leaks can result in the loss of the whole refrigerant charge.
Leak detection equipment • Electronic leak detectors • Electronic leak detectors are the most sensitive and accurate method of leak detection. The detector contains an internal pump that draws air into a probe, or tube. If refrigerant gas is present in the sample, the electrodes in the sensing element generate a current, and an output signal is obtained.
Halide lamps • Halide lamp is used to locate leaks of HCFC refrigerants. This method is based on the colour of a flame that surrounds a glowing copper element. The flame turns blue-green if the air being consumed contains the refrigerant. • These lamps should only be used in well ventilated spaces.
Routine inspections • The high pressure side of the system may easily be tested for leaks with the plant running, but it may be necessary to stop the compressor, and allow the pressure in the low pressure side to rise sufficiently for leak test in the low pressure side. The amount of refrigerant in the system should also be strictly monitored as a drop in level may indicate the presence of a leak. In the event of a major leak, the initial leak test should be made using the soap bubble method, specially in areas where there are signs of an oil leak.
Drying by evacuation • This method of removing moisture is based on the fact that the boiling point of water decreases with falling pressure. In the course of evacuation, any water or ice in the plant will evaporate, and is carried away by the vacuum pump. • Connect a vacuum pump to the system using a short length of large bore pipe, and open all valves in the system (expansion valves, solenoid valves, etc., may have to be jacked open). • Evacuate the system to a pressure of 6mm Hg or less. If possible, carry out the evacuation at ambient temperatures above 10°C. • Close the line between the system and the vacuum pump. The pressure in the system may not rise more than 2mm Hg within five minutes. A rise of more than 2mm Hg indicates the presence of water, and/or a leak. Where water is present, the system will be colder than its surroundings. • Check for water and/or leaks, carry out any repairs, and repeat the evacuation procedure until the pressure rise is less than 2mm Hg. When this is achieved the system is free of moisture and non-condensable gases and ready for refrigerant charging.
Charging procedures • Refrigerating plants should not be overcharged with refrigerant, as this may overload or damage the compressor. • To ensure that the correct amount is added, the refrigerant should be weighed during charging. • The liquid refrigerant is decanted from the refrigerant bottle into the system via a charging valve just after, the liquid receiver.
Charging Procedure • Weigh refrigerant bottle • Connect refrigerant bottle to charging valve with flexible charging line. • Crack bottle liquid valve before tightening line to blow out air. • Close main liquid line valve and pump down system. • Open charging valve and carefully open liquid valve on refrigerant bottle. • Liquid refrigerant will flow into the system. • Start compressor. • Continue charging until required amount of refrigerant has been charged. • Check weighing scale reading and observe liquid level in receiver. • Close charging valve and open main liquid valve and observe liquid flow through sight glass. Bubbles indicate the need for further charging. • If charge is complete, close bottle valve, and disconnect charging line. • Store empty refrigerant bottle for reuse.
Condensers • Once a year, the tubes should be cleaned with atube brush in order to remove deposits which would cause a high condensing pressure. The gaskets must be glued to the condenser end plate with good contact. If the partition wall gasket is not properly installed, there is a risk that it will ‘blow’, leading to an excessively high water velocity and to damage to the tube plate. • If a tube has become defective, it does not have to be replaced immediately. Up to 10% in each pass (flow direction) can be plugged. • Corrosion plugs should be renewed if required.
Defrosting • Another task which maybe regarded as maintenance is to keep frost deposits on provision and cargo refrigerating plants under control. Frequent defrosting assures that the plant will cause few problems.
Investigating trouble • In investigating trouble, there are certain things to which attention should always be paid in the first instance: • the temperature of the refrigerated space; • evaporating pressure; • condensing pressure; • suction pipe temperature; • discharge temperature; • liquid line temperature; • compressor running time; • noise from compressor, motor, expansion valve, etc.
TROUBLE SHOOTING • When problems are incurred in a refrigerating plant, these can be attributed in most cases to a shortage of refrigerant. Bearing this in mind, always commence trouble-shooting by checking the refrigerant charge. • For example, in the case of HCFC plants, too little refrigerant prevents the oil, which always circulates in the system, from being returned as the gas velocity is low, and this leads to various functional troubles.
CONTAMINANTS • If the moisture present in a refrigerating system exceeds the amount that the refrigerant can hold in solution it will exist as free water. At temperatures of 0°C or lower, the free water will freeze into ice in the expansion valve or evaporator, restricting the flow of refrigerant. To avoid freeze-ups, the moisture content in low temperature HCFC refrigerant systems must be maintained at a very low level.
Oil • In refrigerating systems some oil is always carried over from the compressor into the condenser by the refrigerant gas, from where it is carried by the liquid into the evaporator. The presence of oil in the circulating refrigerant reduces the heat transfer capacity of the various heat exchangers, the problem being greatest in the evaporator, since oil becomes more viscous and tends to congeal at low temperature. To prevent oil related problems, the operation of the oil separator should be checked regularly to ensure oil is being returned to the compressor lubrication system. The amount of oil added to the lubrication system should also be strictly monitored; an excessive amount indicates that oil is being trapped in the evaporator or suction line.
Air and non condensable gases • The presence of air and other non-condensable gases is detrimental to the efficient operation of a refrigerating plant, as these gases collect in the condenser, and so increase the condensing pressure. Abnormally high condensing pressures cause overheating of the compressor, excessive discharge temperatures, losses in compressor capacity and efficiency, excessive power consumption and possible overloading of the drive motor.
Testing for Non-condensable Gases • Air and non-condensable gases, if present in the system, are pumped through the system and discharged by the compressor into the condenser. • These gases are trapped in the condenser and cause excessive condensing pressures. In order to check the condenser for the presence of air or non-condensable gases, it is essential that gauges and thermometers be accurate and that the system has sufficient charge so that the liquid refrigerant present in the receiver will seal the liquid line connection.
Check for non-condensable gases • Close liquid line valve and allow system to pump down. • Shut off compressor and close suction line valves. • The thermometer in the sea water outlet of the condenser will indicate the actual condensing temperature, when there is no further drop in temperature. • Record the condensing pressure. • On a refrigerant pressure gauge, look up the saturation temperature that corresponds to the condensing pressure. • If the condensing temperature is less than the corresponding saturation temperature of the condensing pressure, it is necessary to purge.
Purging Non-condensable Gases • Pump down the refrigerant by shutting the liquid valve at the outlet of liquid receiver. • Continue cooling the condenser for 10 to 15 minutes. • Open purge valve on top of condenser, and slowly release gases. • Since it is difficult to tell if excessive refrigerant is being purged with the non-condensables, purge slowly.
A type of pneumonia which may be fatal to older people, has been blamed on the presence of a bacteria associated with the air conditioning plant of large buildings. Because the outbreak which heralded the disease, occurred at a convention for American ex-servicemen (The American Legion), the identified cause of the problem, was labelled legionella bacteria and the sickness is referred to as legionnaires disease.
There is a risk that the bacteria could flourish in the air conditioning systems of ships. The organisms breed in stagnant water or in wet deposits of slime or sludge. Possible locations for bacteria colonies, are mentioned as being at the air inlet area and below the cooler (stagnant water), in the filter, in humidifiers of the water spray type and in damaged insulation. • Provision of adequate drainage is recommended to remove stagnant water.
Regular inspections and cleaning as necessary of filters and other parts, using a 50ppm super-chlorinated solution as the sterilizing agent is required. The solution is to be used also on the cooler drain area at not more than three month intervals. Regular sterilization is necessary for water spray type humidifiers (steam humidifiers being preferred).
