Home page||Cooling ||
De-aerators for motor ships
Mention has been made of the need for clean, neutral boiler feed, free from dissolved gases and of the consequent use of efficient de-aerators. Figure 1 shows one of several which liberate the dissolved gases from the feed and provide a measure of feed heating simultaneously.
This type of de-aerator has a great range of capacity and given a temperature rise of at least 20 deg C, an oxygen content of 0.2 cc/litre can be reduced to 0.005 cc/litre, when working between one-half full load and full load in a closed feed system.
Normally, the de-aerator is mounted directly on a storage tank, into which the de-aerated water falls, to be withdrawn through a bottom connection by a pump or by gravity. The tank usually has a capacity sufficient for 10 minutes' running supply of water but this is not necessarily the case.
Figure 1 : Typical de-aerator (Weir Pumps Ltd)
The feed water enters the de-aerator head and so that its surface area may be increased to the maximum possible, it is divided into sprays of minute droplets by being forced through the spray nozzles into the shell; here it meets the heating steam and is brought rapidly to its saturation temperature. Most of the dissolved gases are released and with some vapour rise to the vapour release opening.
The header may be divided and provided with two feed inlet connections, so that the efficiency of de-aeration may be maintained at low rates of flow, by reducing the number of nozzles in use.
Cascade trays
Three cascade trays are set one above the other in the lower part of the shell. The upper and lower of these trays have a raised lip on the outer periphery, have the central opening blanked and have a series of perforations arranged in rings towards the raised lip. The middle tray has a central opening with a raised lip and is perforated similarly.
The falling spray collects on the upper tray and is again broken up as it passes through the perforations to the middle tray where the process is repeated, to be repeated again as it passes through the lower tray to the tank below. The combination of spray, heating and cascade ensures the liberation of all but a minute fraction of the gases in solution or suspension. The final water temperature depends upon the pressure of the controlled steam supply.
It will be apparent that de-aerators of this type must be installed at such a height in the engine room that the pressure head at the extraction or feed pump suction is greater than that corresponding to the water temperature. The maintenance required is the control of corrosion, the cleaning of the nozzles, the renewal of those showing signs of erosion (which will seriously impair the efficiency), the overhaul of fittings and the maintenance of the safety valve,
Summarized below various circulating systems for motorships, some of the basic procedure of heat exchangers & control of temperatures:
- Sea water circulation-systems
The usual arrangement for motorships has been to have sea-water circulation of coolers for lubricating oil, piston cooling, jacket water, charge air, turbo-charger oil (if there are sleeve type bearings) and fuel valve cooling, plus direct sea-water cooling for air compressors and evaporators....
-
Shell and tube heat exchangers for engine cooling water and lubricating oil cooling
Shell and tube heat exchangers for engine cooling water and lubricating oil cooling have traditionally been circulated with sea water. The sea water is in contact with the inside of the tubes, tube plates and water boxes....
-
Plate type heat exchanger
The obvious feature of plate type heat exchangers, is that they are easily opened for cleaning. The major advantage over tube type coolers, is that their higher efficiency is reflected in a smaller size for the same cooling capacity....
-
Details of charged air cooler
The charge air coolers fitted to reduce the temperature of air after the turbo-charger and before entry to the diesel engine cylinder, are provided with fins on the heat transfer surfaces to compensate for the relatively poor heat transfer properties of air....
-
Maintenance of heat exchangers
The only attention that marine heat exchangers should require is to ensure that the heat transfer surfaces should remain substantially clean and flow passage generally clear of obstructions. Indcation that fouling has occured is given by a progressive increase in the temperature difference between the two fluids, and change of pressure....
-
Central cooling system & Scoop arrangement for motorships
The corrosion and other problems associated with salt water circulation systems can be minimized by using it for cooling central coolers through which fresh water from a closed general cooling circuit is passed. The salt water passes through only one set of pumps, valves and filters and a short length of piping.....
-
Circulating systems for steamships
The main sea-water circulating system for a ship with main propulsion by steam turbine is similar to that of a motorship with a central cooling system. The difference is that the sea water passes through a ....
- Closed feed system and feed heating for motor ships
To ensure trouble-free operation of water-tube boilers the feed water must be of high quality with a minimal solid content and an absence of dissolved gases. Solids are deposited on the inside surfaces of steam generating tubes,....
- Marine condenser assembly
A condenser is a vessel in which a vapour is deprived of its latent heat of vaporization and so is changed to its liquid state, usually by cooling at constant pressure. In surface condensers, steam enters at an upper level, passes over tubes in which cold sea water circulates, falls as water to the bottom and is removed by a pump (or flows to a feed tank)....
-
Three stage air ejector with internal diffusers
A steam-jet ejector may be used to withdraw air and dissolved gases from the condenser. In each stage of the steam-jet ejector, high pressure steam is expanded in a convergent/divergent nozzle. ...
- Pressure governor for motor ships
The main feature of the governor is that if the pump loses suction the steam ports are opened wide, allowing the pump to accelerate rapidly to the speed at which the emergency trip acts....
- Liquid ring pump- Nash rotary liquid ring pumps
Nash rotary liquid ring pumps, in association with atmospheric air ejectors, may be used instead of diffuser-type steam ejectors and are arranged as shown...
- The Weir electro-feeder - a multi-stage centrifugal pump
A multi-stage centrifugal pump mounted on a common baseplate with its electric motor. The number of stages may vary from two to fourteen depending upon the capacity of the pump and the required discharge pressure....
- Feed water heaters for motor ships
Surface or direct contact feed heaters, play an important part in the recovery of latent heat from exhaust steam. Direct contact feed heaters are also known as de-aerators....
- Devaporizer & turbo-feed pump
If the de-aerator cannot be vented to atmosphere or to a gland condenser satisfactorily, a devaporizer is connected to the vapour outlet condensing the vapour vented with the non-condensable gases and cooling these gases before they are discharged. ...
- Typical de-aerator & Cascade trays
Normally, the de-aerator is mounted directly on a storage tank, into which the de-aerated water falls, to be withdrawn through a bottom connection by a pump or by gravity. The tank usually has a capacity....
Home page||Cooling ||Machinery||Services ||Valves ||Pumps ||Auxiliary Power ||Propeller shaft ||Steering gears ||Ship stabilizers||Refrigeration||Air conditioning ||Deck machinery||Fire protection||Ship design
||Home ||
General Cargo Ship.com provide information on cargo ships various machinery systems -handling procedures, on board safety measures and some basic knowledge of cargo ships that might be useful for people working on board and those who ashore. For any remarks please
Contact us
Copyright © 2010-2016 General Cargo Ship.com All rights reserved.
Terms and conditions of use
Read our privacy policy|| Home page||