Air-conditioning and ventilation-related conditions on ships have a considerable impact on the comfort of passengers, the well-being of the crew, and the efficient operation of equipment, systems, and installations, regardless of the type of ship.
Both temperature and humidity must be controlled in order to provide a comfortable environment. Higher temperatures are more tolerable if the air is dryer. The cooling impact on the air as it travels over the evaporator coil removes moisture, but comfort requires a certain level of humidity, thus it is required to humidify the air again by sprinkling water into the circulating air flow.
The air conditioning plant is designed in principle to be self-contained within the fire zones or water tight compartments and to perform the following functions:
- Supply cool air to the accommodation and wheelhouse.
- Provide heating to the accommodation and wheelhouse air when necessary.
- Remove excess moisture from the air or humidify it to a comfortable level if necessary.
- Filter the air before it passes to the accommodation and wheelhouse.
The air is supplied to the accommodation by an air handling unit (AHU), which is usually located inside accommodation block, in the air conditioning unit room. The unit consists of an electrically driven fan drawing air through the following sections starting from inlet to the outlet:
- One air filter
- One steam preheating unit
- One enthalpy exchanger of the rotating composite type (Econovent)
- One steam reheating unit
- Two air cooler evaporator coils
The exhaust section of the air handling unit comprises, from inlet to the outlet:
- One return air filter
- One exhaust ventilator
Automatic control for the humidification of the air is installed in the outlet portion of the AHU and the humidistat controller is positioned in the room housing the air handling unit. The humidifier nozzles are supplied with steam from the ship’s steam system. The air is driven through the distribution trunking that supplies the living quarters and wheelhouse. Air may be drawn into the system either from outside or from the accommodation via the recirculation trunking. With heating or cooling coils in use, the unit are generally designed to operate on 70% fresh air supply. The ratio of circulation air may be varied manually using the damper in the inlet trunking. The inlet filters are of the washable mat type and preheating of the air is provided by coils supplied with steam from the ship’s steam system.
This rotary heat exchanger (Econovent) is installed in the accommodation ventilation system for heat recovery purposes, where it exchanges thermal energy from one counter-flow air stream to another over a rotary wheel. The rotary wheel consists of corrugated aluminum/stainless steel foil, which is driven slowly by an electric motor and V-belt/pulley arrangement.
The Freon gas (usually R134a) system with direct expansion provides cooling. The plant is automatic and is comprised of two compressor/condenser units that supply the evaporators housed in the AHU. The expansion valves for the coils are supplied with liquid refrigerant from the air conditioning compressor, which have been compressed in the compressor and condensed in the condenser. The liquid is then delivered to the evaporator coils via dryer units, where it expands under the supervision of expansion valves, before being returned to the compressor as a gas. It removes heat from the air that passes over the evaporator coils.
The compressor is usually fitted with an internal oil pressure activated unloading mechanism which affords automatic starting and variable capacity control of 100%, 67% and 33% of full capacity by unloading groups of cylinders. This variable capacity control allows the compressor to remain running even when the load is relatively light and thus avoids the need for frequent stopping and starting. The compressor is protected by high and low pressure cut-out switches, a low lubricating oil pressure trip, a cooling water pressure trip and a high pressure and oil supply pressure differential trip and crankcase heater and cooler are also fitted.
Any refrigerant gas loss from the system will cause the system to become undercharged. Low suction and discharge pressure will indicate an undercharged system, and the system will eventually become useless. Low refrigerant gas charge is accompanied by an obviously low oil level in the sump, as the low charge level will result in the entrapment of extra oil in the circulating refrigerant gas, causing the sump level to decrease. This extra oil will be segregated and returned to the sump when the system is charged to its maximum capacity. The level indicated by the condenser level indicator will fall during operation.
If the system does become undercharged, the whole system pipework should be checked for leakage as the only reason for an undercharge condition after operating previously with a full charge is that refrigerant is leaking from the system. When required, additional gas can be added through the charging line, after first venting the connection between the gas bottle and the charging connection. The added refrigerant is dried before entering the system. Any trace of moisture in the refrigerant will lead to problems with the thermostatic expansion valve icing up and subsequent blockage.
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 minimize gas leaks is to be implemented to ensure leaks are detected at an early stage.
As refrigerant R134a is mostly used nowadays and as is a gas mixture, if is lost, it may be one component of the mixture, and a top-up with new refrigerant can result in a slight change in the composition. If a substantial leak has occurred, the system should be evacuated and refilled with a fresh charge.
Air is circulated through ducting to outlets in the cabins and public rooms and the air flow through the outlets can be controlled at the individual outlets. On some vessels the system allows reheating of the air at the outlets from circulating hot water. A valve at the outlet allows hot water to circulate around a heating coil over which the air flows before discharge through the cabin outlet. The reheat water comes from a separate accommodation heating water system. Two water circulation pumps are fitted, one working as the duty pump and the other set as the standby pump. These pumps circulate water through a heater which is heated by steam and the water is then circulated through the accommodation spaces with branches to individual air outlets. A valve at each air outlet vent allows for individual control of reheat.
Air is extracted from various parts of the accommodation by the exhaust fan in the air handling unit. Areas from which air is extracted include toilets, bathrooms and public rooms. The exhaust air passes through the enthalpy heat exchanger unit before discharge to atmosphere. Heat energy in the exhaust air is recovered in the exchange unit and used to warm incoming air.
The air conditioning system is designed to run with the one compressor at a time meeting the full air conditioning load of the accommodation. Capacity control of the compressor is automatic and controlled by the suction pressure, but for borderline temperatures capacity can be controlled manually.
When starting the ventilation system, the following procedure generally apply:
- Check that the air filters are clean.
- Set the air dampers to the outside position.
- Start the AHU supply fan, exhaust fan and enthalpy exchanger.
- Check that air is flowing to all parts of the accommodation.
The starting procedure of air conditioning compressor is generally as follow:
- All stop valves, except the compressor suction, in the refrigerant line should be opened and fully back seated to prevent the pressure in the valve reaching the valve gland.
- The crankcase heater on the compressor to be used should be switched on a least 3 hours prior to starting the compressor.
- Check that the crankcase oil level is correct.
- Check the quantity of refrigerant charge.
- Start the cooling water supply for the condenser cooling. The cooling water comes from the engine room low temperature central cooling FW system and will probably already be running.
- Start the compressor.
- Slowly open the suction stop valve until it is fully open. If the compressor starts making a knocking noise, close the valve immediately as this indicates that liquid refrigerant has been drawn into the machine. When the noise has stopped open the suction valve again very slowly. Repeat this operation if necessary, until the compressor runs smoothly with the suction valve fully open.
It is important to note that a fully closed suction valve with the compressor running might cause
foaming of the lubricating oil in the crankcase.
If the compressor need to be stopped for short period of time, the following procedure will generally apply:
- Close the condenser liquid outlet valve and the outlet from the filter.
- Allow the compressor to pump down the system to the condenser so that the low level pressure cut-out operates.
- Isolate the compressor motor.
- Close the compressor suction valve.
- Close the compressor discharge valve.
- Close the inlet and outlet valves on the cooling water supply to the condenser.
- Switch on the crankcase heater
If the cooling system is to be shut down for a prolonged period, it is essential to pump down the system and isolate the refrigerant gas charge in the condenser. Leaving the system with full refrigerant pressure in the lines increases the tendency to lose charge through the shaft seal.
- Shut the liquid outlet valve on the condenser and the outlet from the filter.
- Run the compressor until the low pressure cut-out operates. The refrigerant gas will be condensed and will remain in the condenser as the condenser outlet valve is closed.
- After a period of time the suction pressure may rise, in which case the compressor should be allowed to pump down again. This procedure should be repeated until the suction pressure remains low and the compressor does not start again automatically.
- Shut the compressor suction and discharge valves.
- Close the cooling water inlet and outlet valves and drain the condenser of water.
- The compressor discharge valve should be marked closed and the compressor motor isolated, in order to prevent possible damage.
In conclusion, the air conditioning system will cool the air if required, will provide heating to the air if needed, will remove excess moisture from the air if necessary, and will humidify the air to the correct level for comfort. A comfortable atmosphere is a combination of temperature and humidity, and both must be controlled. Higher temperatures are more tolerable if the air is drier. The cooling effect on the air as it passes over the evaporator coil removes moisture, and a level of humidity is important for comfort, so it is necessary to humidify the air again by spraying steam into the circulating air flow.
It is also important to note that the moisture removed from the air is collected into a drain pan inside AHU and therefore it is essential that no water should be lying in the air conditioning system as this can become a breeding ground for legionella bacteria, which can have serious, or even fatal, consequences. Drains should be kept clear and areas where water can lie should be sterilized at frequent intervals.
It is most important that the AHUs are kept clean as follows:
- AHUs should be regularly cleaned internally.
- Air filter material should be replaced at suitable intervals depending on dust concentration in the air.
- Damper control mechanism must be lubricated at regular intervals.
- Steam heating coil steam traps are to be regularly checked for correct operation.
- In the cooling section, ensure that drip pans are kept clean and that drains are clear.
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