The cooling system runs on the following standard layout:
The central freshwater cooling system with single-stage scavenges aircooler and a separate HT circuit.
As freshwater is the standard cooling medium of the scavenge air cooler(s), this involves using a central freshwater cooling system.
The central freshwater cooling system comprises “low-temperature” (LT) and “high-temperature” (HT) circuits. Freshwater-cooling systems reduce the amount of seawater pipework and its attendant problems and provide for improved cooling control. Opttmizrng central freshwater- cooling results in lower overall running costs compared to the conventional seawater cooling system.
Cooling Water Treatment
The correct treatment of cooling fresh water- is essential for safe engine operation. Only totally demineralized water or condensate must be used. In the event of an emergency, tap water may be used for a limited period, but afterward, the entire cylinder cooling water system is to be drained off flushed, and recharged with demineralized water.
In addition, the water used must be treated with a suitable corrosion inhibitor to prevent a corrosive attack, sludge formation, and scale deposits. Monitoring the corrosion inhibitor level and water softness level is essential to prevent downtimes due to component failures resulting from corrosion or impaired heat transfer. No internally galvanized steel pipes should be used in connection with treated freshwater because most corrosion inhibitors have a nitrite base. Nitrites attack the zinc lining of galvanized piping and create sludge.
Pre-Heating
To prevent corrosive liner wear when not in service or during short stays m the port, it is important that the main engine is kept warm. Warming through can be provided by a dedicated heater, using boiler-raised steam or hot water from the diesel auxiliaries, or through direct circulation from the diesel auxiliaries.
If the requirement for warming up is from the cooling water systems of the diesel auxiliaries, it is essential that the amount of heat available at the normal load is sufficient to warm the main engine If the main and auxiliary engines have a cooling water system that can be cross-connected. it has to be ensured that when the cross-connection is made, any pressure drop across the mam engine will not affect the cooling water pressure required by the auxiliaries. If the cooling water systems are apart, then a dedicated heat exchanger is required to transfer the heat to the mam cylinder water system.
If the mam cylinder water pump is to be used to circulate water through the engine during pre-heating, the heater is to be arranged parallel with the cylinder water system, and on/off control is to be provided by a dedicated temperature sensor at the cylinder water outlet of the engine The flow through the heater is set by throttling discs but not by valves, to assure flow through the heater.
If the requirement is for a separate pre-heating pump, a small unit with 10% of tire mam pump capacity and an additional non-return valve between the cylinder cooling water pump and the heater are to be installed. In addition, the pimps are to be electrically interlocked to prevent two pumps from running at the same time.
The recommended temperature for stalling and operating the engine is 60 C at the cylinder cooling water outlet. If the engine has to be started below’ the recommended temperature, the engine power should not exceed 80% of CMCR until the water- temperature has reached 60 C.
The ambient engine room temperature and warm-up tune are the key parameters for estimating the heater-power capacity required to achieve the target temperature of 60 C.
The figure shows the warm-up tune needed in relation to the ambient engine room temperature to arrive at the heat amount required per cylinder. The graph covers tire warming up of the engine components per cylinder, also taking the radiation heat into account.
The readable figure is then multiplied by the number of cylinders to show the heater- capacity required for the engine. All the figures are related to the requirements of the engine and should be used only for the first rough layout of the heater capacity. During pre-heater- selection, however, the shipyard or ship designer must also consider other aspects, such as the heat losses in the external piping system, the water volume inside the system, the pipe lengths, and the volume of the ancillary equipment.
