Every ship has a provision cooling plant, which can range from a refrigeration-cooling combination to 3 to 4 cold storage rooms depending on the ship size and type. Cold storage rooms on passenger ships can include up to 30 – 50 rooms, including dedicated storage rooms for wine, flowers, cigars, and so on.
The purpose of cold storage rooms is to maintain optimal storage conditions in terms of temperature and air humidity. To avoid radiation losses, insulation layers are built on the ceilings, walls, and floors. Windows are not supplied, and special heat-insulated doors are fitted. Antechambers are installed to avoid cold loss during entry and leave.
The layout is determined by the vessel’s trading region and duration of voyage, as well as national and international legislation, classification society regulations, environmental regulations, and so on.
The refrigeration plant is fully automated, with two compressors (which can be scroll type or semi-hermetic reciprocating piston type), two condensers and each cooling and refrigerating room has an evaporator coil and an air circulation fan.
A direct expansion Freon gas (usually R134a) system provides cooling for the cooling and refrigerating rooms. The expansion valve regulates the amount of liquid flowing to the evaporator in accordance with the current room temperature; if the temperature has risen, more liquid passes to the evaporator. The liquid travels through the expansion valve, converting to gas and absorbing heat from the evaporator.
An electrically powered fan circulates air over the evaporator coil, cooling the provision room. The supply of refrigerant to the expansion valve is controlled by the provision room thermostat via a solenoid valve in the supply line. On some systems a suction line heat exchanger is fitted to protect the compressor from any refrigerant liquid droplets by vaporizing any droplets by the hot refrigerant liquid.
The refrigeration unit is controlled by an electronic controller, which performs the following functions:
- Setting of thermostat set points
- Selection of defrost times
- Selection of fan operating options during defrosting
- Monitoring plant’s functions, such as room temperatures
- System alarm indication and self-check functions
- Temperature probe recalibration
Electric defrosting elements are installed in the refrigerating room evaporator. The frequency of defrosting is controlled by a timer that is part of the electronic control system of the refrigerating unit, but defrosting can also be initiated manually.
Compressors function automatically and should not require any configuration changes. The cut-in, cut-out, and pressure differential settings on the controllers can be adjusted, although this should only be done if a new controller is installed. High pressure, low pressure, low lubricating oil pressure, and condenser cooling water failure cut off switches protect the compressors.
Under typical circumstances, one compressor/condenser unit is in use, with the other ready for manual start-up. All valves on the standby unit should be closed until it is required for service. The compressor collects R134a gas from the cooling and refrigerating room evaporator coils and discharges compressed/hot gas to the condenser. Water circulating from the central cooling FW system cools the condenser, which cools the heated gas, which condenses into a cool liquid. The liquid refrigerant is returned to the evaporators in the cooling and refrigerating room via a dryer unit and filter.
Some compressors (scroll type) have additional cooling provided by the economizer which allows a small amount of refrigerant into the middle of the compression process. The refrigerant is allowed into the middle of the compressing scrolls by means of a capillary tube. The cooling effect of this refrigerant improves efficiency.
A thermostat fitted in each room enables the liquid refrigerant solenoid valves to operate independently, this reduces the number of compressor starts and the running time of the compressors. It is important to note that the operation of the compressor plant is not designed for continuous parallel running. When one compressor is running, the standby compressor must be isolated from the system. If the valve in the liquid line is not shut, R134a in the system will accumulate in the standby condenser, which will be at the lowest pressure. This will result in the system to stop working due to the lack of refrigerant. Emptying the system of R134a will also transfer lubricating oil from the running compressor and damage the in-use compressor.
The expansion valves of the evaporators accept the refrigerant as a supercooled vapor and the temperature of the chamber being served by the evaporator determines the opening of the expansion valve. The vapours are then returned to the compressor after being heated by the evaporator.
The room thermostats open and close the solenoid valves on the air coolers (evaporator units), allowing refrigerant gas to flow to the evaporator while open. With the solenoid valves closed, there is no gas flow to the evaporators; thus, there is no gas flow to the compressor suction, and the low pressure switch will shut down the compressor.
The evaporator in the refrigeration room is equipped with electrical defrosting equipment, which comprises of electric heating components attached to the evaporator and drip tray.
Any refrigerant gas escapes from the system will cause the system to become undercharged. Low suction and discharge pressures will indicate an undercharged system, which will eventually become useless. There will be bubbles seen in the liquid gas flow sight glass.
When necessary, more refrigerant can be injected through the charging line after venting the connection between the refrigerant bottle and the charging connection to prevent air from entering the system through the connection pipe.
Before the extra refrigerant enters the system, it is dried. Any trace of moisture in the refrigerant system will result in the thermostatic expansion valve icing over and being obstructed.
Refrigerant R134a is a hydrofluorocarbon (HFC) refrigerant and if gas is lost from the system, only R134a should be used to top the system up. To comply with the Montreal Protocol, the maximum annual leakage of this gas into the atmosphere should be restricted to 10% of the total system charge. To verify this and to monitor the number of times the system has to be recharged, a record has to be made in the Refrigerant Recharge Log. A regular system of leak detection to minimise gas leaks is to be implemented to ensure leaks are detected at an early stage.
The start and stop procedure of the system is similar with the one from the air conditioning system which has been explained on an earlier post which can be found here.
The procedure for defrosting the refrigerating room evaporator is usually as follow:
The evaporator in the refrigeration room is fitted with electrical defrosting equipment, i.e. the evaporator and drip tray are provided with electric heating elements. The frequency of defrosting is chosen by means of a defrosting relay built into the starter panel. The defrosting sequence is automatic and as follows:
- The compressor stops and all the solenoid valves in the system are closed.
- The fan in the refrigerating room stops working. The fan in the cooling room continues the circulation of the warm air over the coolers, in this way keeping the cooling surfaces free from ice.
- The electric heating elements in the refrigerating room are switch on.
- As long as the coolers are covered with ice, the melting takes nearly all of the heat supplied and the temperature of the cooler and the refrigerant is constantly kept near zero. When the ice has melted, the refrigerant temperature rises in the refrigerating room. When the temperature reaches the set point (approximately +10°C) of the defrosting thermostat, the heating elements are switched off.
- The compressor will now restart and the solenoid valves will be opened according to operating conditions..
- When the coil surface temperature has gone below the freezing point, the fan in the refrigerating room starts.
The system is now back on the refrigerating cycle again. If the defrosting is not completed at the expiration of the defrosting period, a new defrosting cycle will commence.
If the plant is to be shut down for maintenance or repair and it involves opening up the compressors or breaking into the refrigerant lines, the refrigerant must first be pumped down to the condenser and isolated in a similar way to that described in my previous post regarding air conditioning system which can be found in here. However, pumping down until the LP cut-out trips the machine will usually not capture all the refrigerant, which may be entrained in the lubrication oil in the compressor sump and around the system. To ensure that the entire refrigerant charge is pumped into the condenser, the system is run until the LP cut-out trips the compressor and the condenser is isolated. The low pressure in the system will allow any refrigerant to evaporate and the process of pumping down until the LP cut-out trips the compressor again. This process is repeated at hourly intervals until there is no rise in system pressure following LP cut-out.
When complete, the inlet and outlet valves must be kept closed until all maintenance work has been completed and the system returned to normal operation.
If more substantial repairs are to be undertaken, it may be necessary to remove all of the refrigerant gas/liquid from the system. Because this operation involves evacuating the condenser and pressurising recovery cylinders, it should only be undertaken by a member of ship’s staff trained in this operation, or by a qualified service engineer. Additionally, for safety reasons, reference should be made to the manufacturer’s operating manuals before undertaking this task.
The first stage in this process is to shut down the refrigeration plant in accordance with the previously described procedure. For full refrigerant evacuation, a designated gas recovery unit must be used together with dedicated gas recovery cylinders. Normally the cylinders used to charge the system cannot be used, as they will typically only have a single non-return valve on the top of the cylinder as opposed to a normal recovery cylinder that has two isolating valves, one for gas and one for liquid, and an internal dropper pipe from the liquid valve.
The liquid valve from the recovery cylinder must first be connected via a flexible hose and isolating valve to the bottom of the condenser, and the gas valve of the cylinder connected in a similar manner to the suction side of the recovery pump. The discharge side of the recovery pump must then be connected to the top/gas side of the condenser. With the valves open to the recovery cylinder and recovery pump, the pump is to be run until all of the liquid refrigerant has been evacuated. The purpose of the pump is to compress gas evaporating from the top of the recovery cylinder, and use it to put a positive pressure inside the condenser on top of the liquid refrigerant.
When all of the liquid has been expelled, the connections to the recovery pump need to be changed. The suction side of the pump now has to be connected to the condenser. The pump’s discharge should be connected to the liquid connection on the recovery cylinder, and the gas valve on the recovery cylinder either left closed or also connected to the inlet side of the recovery pump.
Using this method, all of the gas is then removed from the condenser. The gaseous refrigerant passes through the recovery pump where it is condensed in its own air-cooled condenser and pumped into the liquid connection on the recovery cylinder.
When the unit has been run sufficiently and allowed to pull a small vacuum on the main condenser, all of the refrigerant gas will have been removed.
Switching off the pump and closing all of the valves will allow any necessary maintenance work to be undertaken. During this operation, a set of weighing scales must be used to ensure the recovery cylinders are not overfilled. It is important to ensure that any cylinder used is only filled to 80% capacity. The scales will also allow a record of the amount of gas recovered to be logged.
Before reintroducing any refrigerant into the system, all repair works must have been completed, and the pipelines and compressors visually checked for integrity. A vacuum pump, not the recovery pump, must then be used to create a vacuum in the system. A vacuum of 10 torr will be sufficient. This will allow leakage checks to be undertaken and also ensure any atmospheric moisture has been removed before refrigerant is introduced.
It is important to note that If there is water in liquid form in the system, rapid evacuation may cause the water to freeze. If this happens, there will be a rise in pressure (loss of vacuum) which could be confused with a pipework leak.
The vacuum is broken by flooding the system with dry nitrogen and again a pressure test, this time with positive pressure carried out. The nitrogen pressure is released to atmosphere until there is a slight positive pressure in the system and the vacuum pump then used to re-evacuate the system. The vacuum needs to be held for sufficient time to confirm that there is no leakage. Obviously the ship may not be carrying sufficient nitrogen for this process to be carried out, but it is the recommended practice.
The system is now ready to introduce refrigerant via a flexible hose into the condenser. The exact charge will be noted in the manufacturer’s handbook, and checked by suspending the refrigerant bottle from a set of scales. With modern refrigerants being gas mixtures, charging is always done via the liquid line as this ensures correct composition.
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