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When I was a lad at sea many years ago, I sailed on motor and steamships as an Engineering Officer. In those days we had evaporators which used steam from the boilers or the main diesel cooling water as a heating medium to evaporate the seawater. As I gained experience and promotion, one of my duties as 4th Engineer was looking after the vaps, as we called them (among other things).
Nowadays, there are several very efficient types of evaporators still using the same heat sources, and of course we now use osmosis as well.
In the following sections we will examine the current evaporators in use, fresh water and condensate storage tanks, and condensate feed water testing. In this article we shall examine two categories of water evaporators, tube and flash, and have a look at how osmosis equipment operates to produce fresh water from seawater.
We begin with an examination of the types of evaporators used aboard ships.
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Types of Fresh Water Evaporators
There are numerous types of evaporators and osmosis equipment used to produce fresh water from seawater on our ships today. Here we shall examine the following types:
- Multi-stage Flash Evaporator
- Tube or Coil Evaporator
Multi-stage Flash Evaporators
This type of evaporator uses a multi-stage process which has two components, the seawater heater and the flash drum, with these being two separate units.
The seawater can be heated using steam or the main engine cooling water, depending on the main propulsion unit.
The heated seawater is pumped into the flash drum, which has numerous sections all at a lower pressure than that of the water heater. Some of the hot seawater flashes of to steam in the first section, before going on through remaining sections, flashing as it moves through them. The steam rises up the flash drum through a demister, and upon contacting the condenser tubesis condensed and pumped via a salinometer to the fresh water or boiler water feed tanks. Should the salt content in the distillate rise to an unacceptable level, the salinometer alarm will be activated and the distillate diverted to bilges.
A sketch of a typical multi-stage evaporator is shown below:
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Coil or Tube Seawater Evaporator
This is a modern version of the type used when I was at sea in the 1960s. They used heating coils in those days as opposed to the pipe nest heaters of today. The coils used to become scaled in salt, with the attendant loss in output of distillate.
I was in charge of the vaps and I remember the old Chief coming down to the engine room on my watch and balling me out for the downturn in distillate. We were having problems with the boiler feed water purity (another article will cover the testing and treatment of boiler feed water), so I was blowing down the boiler regularly, which with the associated make-up requirement meant we needed more water pronto.
Anyway I took him up to the vaps and showed him the scaling on the heating coils, reminding him that I was pumping Foss chemicals into the beast to try and break this away.
He pushed me aside and shut off the seawater supply opening up the steam supply which rapidly dried the salt layer on the coils. He then opened the seawater inlet and hey presto – the salt scale cracked and fell of the coils. I used this system several times until I was up for Seconds ticket and examiner wasn’t too pleased to hear of this method, and called the old Chief several unprintable names!
Today we don’t have to resort to these measures as there is an innovative device which uses a material that emits oscillations counteracting the natural seawater oscillations, thereby altering its properties and preventing calcium carbonate scale. (See references section.)
A tube and coil evaporator consists of a steel vessel which has a nest of heating pipes near the bottom of the vessel being fed by steam or hot water from the main engine.
There is a tube condenser cooled by seawater installed near the top of the vessel. A vacuum is drawn in the vessel by air ejectors operated by steam or pressurised seawater.
Seawater is fed into the evaporator just covering the heating pipes. Heat is supplied to the pipes and, this combined with the vacuum conditions begins to boil the seawater producing steam. The steam rises up through a demister into the tube condenser where it is evaporated to distilled water. This is collected and pumped via the salinometer to the storage tanks.
A typical tube condenser is shown below.
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Reverse Osmosis Process
Osmosis is a natural process which occurs due to osmotic pressure between two substances divided by a semi-permeable membrane. When the membrane divides two substances of different concentrations of solids, the solvent from the less concentrated solution will flow into the higher concentrated solution, with the membrane blocking the solids.
In an engine room, reverse osmosis takes place in a pressure vessel which contains a tank holding a quantity of seawater and freshwater separated by a semi-permeable membrane. In natural osmosis the freshwater would flow into the seawater, however when pressure is applied to the seawater side the process is reversed. This causes the seawater to flow into the freshwater side, the solids being stopped by the membrane.
A sketch of osmosis in action on ships blackwater is shown below. This can be applied to freshwater osmosis water-makers.