Utilities

Fresh Water Generators


Evaporators

Waste heat recovery


Even with a very efficient engine, only about 50% of the heat in the fuel is converted into useful work at the crankshaft. The remainder I potentially wasted. Heat energy is lost in the cooling systems and exhaust gas but some can be recovered whether it is worth recovering depends upon a number of factors including the amount of energy available, the time for which it is available, capitol costs of recovery plant. Modern highly pressure charged engines have a large amount of energy in scavenge air cooling water and this can provide a primary heating source for bunker fuel tanks.


Jacket cooling water also contains a considerable quantity of heat this may be recovered in fresh water evaporators provided operates at a pressure giving a corresponding saturation temperature for water lower than the jacket water entering the heating element. By heating the water to its saturation temperature gasses dissolved in it are liberated. Thus there is the requirement for air ejection to maintain a low pressure.


Any low temperature evaporator will operate over prolonged periods because scale does not form to any great extent. Joints and seals do not deteriorate. Types of plant There are two methods for generating fresh water, Reverse Osmosis (RO) and distillation. Reverse osmosis is generally used were large quantities of relatively low quality water is required. Typical examples of water produced are;


Treatment

Total Hardness

Calcium Hardness

Silica

NaCl

TDS

Sea Water

250

200

14

15000

15000

Evapourator

<0.2

<0.2

<0.2

<20

<20

Reverse Osmosis

20

5

<1

<750

<750

Demineraliser

0

<1

Trace

<2

<3

Distillation

The most commonly used form of shipboard freshwater generation is evaporative distillation, which uses engine jacket cooling water or steam heat from exhaust or gas fired boilers to evaporate sea water, which is then condensed into fresh water.

Evaporation distillers comes in two main forms, multistage flash (MSF) and multi effect (ME) evaporators.


Single (and Multi-effect) submerged tube distillation was one of the early types of fresh water generation. It uses heat passing through submerged coils or tube bundles immersed in sea water to produce the distillate, which when condensed becomes the fresh water.


The system above shows an evaporator typically heated by Main Engine Jacket water with means to supply steam when the engine is shut down.

To start this evaporator

Single Stage Flash Evaporator


An alternative arrangement to the shell evaporator is the flash evaporator were heating takes place externally, the hot brine enters the low pressure chamber into a weir where some of the water flashes off. Water overflowing the weir is either educted out or passed on to a second stage.

Multi stage units with each stage maintained at a lower pressure allow improved efficiency and high outputs.



Multi Stage Flash Evaporator


A typical multi stage flash system is based upon preheating of a pressurised sea water stream, or more typically a recycle brine stream to which the feed sea water is added; the stream is heated in the heat input section brine heater. From here the recycle stream is passed into the first stage of a series of flash chambers. Here the pressure is released, permitting a portion of the brine stream to flash to form salt-free vapour which is condensed to give the fresh water. In condensing the vapour gives off its latent heat to the recycle brine stream. From the first stage the flashing brine stream is passed to the second stage which is kept at a slightly lower pressure; more vapour flashes off. In the same way the flashing brine stream passes to the next stage and so on through the plant with a portion of the vapour flashing off at each stage. A heat balance shows that the heat supplied in the brine heater has to be rejected. This is done in the last two stages of the plant which are cooled by a sea water stream which subsequently passes to waste.

Modern Developments

Since the introduction of MSF, more efficient types of ME evaporators have been introduced.




Large Multi-effect Alfa laval evaporator


In 1990 Alfa-Laval Desalt introduced its D-TU concept-a ME desalination system based on tube type distillers, using evaporation under vacuum with the rising film principle. This means that the inner surfaces of the tube are always covered with a then film of feed water, preventing formation of scale.


The heating medium (hot water/steam) circulates on the outside of the tubes in the heat exchanger. The vacuum is created by water ejectors connected to each effect.

A controlled amount of sea feed water is led to the bottom of each effect, where it is mixes with the brine from the previous effect and into the tubes in the heat exchanger, where it is heated.


The generated vapours enter a separator where the brine droplets from the wet vapour are separated. The dry vapour pass through the separator to the following effect where they condense. The remaining sea water which has been converted to brine, flows to the next effect as feed water. The brine is taken out and discharged overboard.


The latent heat in the vapours from the previous effect is used as a heating medium in the following effects. The process continues until the last effect where the generated vapours condense cooled by sea water. The condensate vapours flow from one effect to the next, and are retained in a collecting tank as distilled water.


If a low temperature evaporator is to be used for domestic purposes certain restrictions apply. Operation is not allowed within 25 miles of the coast or 50 miles of an estuary. Chromate jacket water treatment must never be used. The condensate must be treated in order to destroy bacteria. Care must be taken if chemicals are used to inhibit marine growth in pipework.



Vapour Compression


The boiler section is initially filled with fresh water. When the system is operating feed water is supplied via the level control valve. Hot steam is created in the boiler which passes over into the main section. Here the steam is mixed with a brine spray. Some of the steam is condensed and some of the brine spray is flashed off. The combined steam passes over to the vapour section via a scrubber. Flow of vapour occurs due to the action of the compressor which increases the vapour pressure increasing its saturation temperature. The compressed vapour has a tendency to condense out and latent heat is given up to the brine spray in the evaporator main section.


The produced condensate is pumped via the distillate pump, a proportion is fed to a desuperheater at the compressor inlet, a further proportion passes to the boiler section maintaining water level. Any excess is then delivered to distillate system via a heat exchanger were it is cooled by the exhausted brine.
A recirculation pump draws brine from the brine well, a proportion of this is fed back to the well to maintain a level, another proportion passes to the brine spray. Excess is passed out of the brine blowdown overboard.


The above description has been formed from interpretation of a not to great drawing. If anybody has actually sailed with this system I would be grateful for a more accurate description. Reverse Osmosis Osmosis describes the process whereby a fluid will pass from a more dense to a less dense solution through a semi-permeable membrane. It is very important to the water absorption processes of plants. RO is a process which uses a semi permeable membrane which retains both salt and impurities from sea water while allowing water molecules to pass. Filtration of up to 90% is possible thus making the produced water unsuitable for boiler feed without further conditioning. Improved quality is possible using a two or more pass system.



An experiment to determine this osmotic head is shown


The parchment paper acts as the semi-permeable membrane and allows the water molecules to pass but not the larger salt molecules.


Reverse osmosis is the process whereby a pressure greater than the osmotic head pressure is applied to a solution of high density. Fluid is forced from the high density side to the less dense side. For desalination plants the pressure is applied to sea water and the water is forced through the semi-permeable membrane.

The semi permeable membrane which is typically made of polyamide membrane sheets wrapped in a spiral form around a perforated tube resembling a loosely wound toilet roll.


Design of the cartridges is therefore such that the sea water feed passes over the membrane sheets so that the washing action keeps the surfaces clear of deposits. A dosing chemical is also injected to assist the action.


The two membranes sealed on the outer three edges, enclose porous under-layer through which the permeate spirals to central collecting tube



Pressurised feedwater passes lengthways through the tubular spiral wound membrane element. Freshwater permeate travels through the membrane layers as directed along a spiral bath into a central perforated tube, while brine is discharged out the end of the membrane element.


The fluid could be water and the solutions sea water. Under normal conditions the water would pass from the less saline solution to the more saline solution until the salinity was the same. This process will cease however if the level in the more saline side raises to give a difference greater than the Osmotic height.


For practical use to allow the generation of large quantities of water. It is necessary to have a large surface area of membrane which has sufficient mechanical strength to resist the pressurised sea water.. The material used for sea water purification is spirally wound polyamide or polysulphonate sheets. One problem with any filtration system is that deposits accumulate and gradually blocks the filter. The sea water is supplied at a pressure of 60bar, a relief valve is fitted to the system. The Osmosis production plant is best suited to the production of large quantities of water rather than smaller quantities of steam plant feed quality.



Pretreatment and post treatment.

Sea water feed for reverse osmosis plant is pretreated before being passed through. The chemical sodium hexa phosphate is added to assist wash through of salt deposits on the surface of the elements and the sea water is sterilised to remove bacteria which could otherwise become resident in the filter. Chlorine is reduced by compressed carbon filter while solids are removed by other filters. Treatment is also necessary to make the water drinkable. Rochem disc tube reverse osmosis plant


Evaporator scale

There are numerous types of evaporators all working to produce pure water with concentrated sea-water as waste. This concentration effect can lead to the formation of damaging scales within the evaporator. Over concentration is usually prevented by having a continuous stream of sea-water passing through the unit thus maintaining a satisfactory dilution of the sea-water side of the evaporator. However, because of the high salt content, when sea-water is elevated to temperatures above 30 C scales can begin to form on heat transfer surfaces. Additionally as the majority of evaporators operate under vacuum there is a tendency for the make-up water side to foam, which can give rise to carry-over and contamination of the pure water stream.

Four scales which are principally found in evaporators are;
Calcium Sulphate (CaSO4)-1200ppm, scale formation is principally on density, remains in solution below 140oC and/or 96000ppm.The worst scale forming salt forming a thin hard grey scale

Magnessium Hydroxide Mg(OH)2
remains in solution below 90
oC

Magnessium Bi-Carbonate 150ppm
soluble below 90
oC, forms a soft scale, prevention by keeping operating temperature of evaporator below 90oC
Above 90
oC
breaks down to form MgCO
3 and CO2 and then Mg(OH)2 and CO2

Calcium bicarbonate Ca(HCO3)2 180ppm
Slightly solube, above 65
oC breaks down to form insuluble calcium carbonate forming a soft white scale. scale formation prevented by chemical treatment Ca(HCO3)2 = Ca + 2HCO3
2HCO
3 = CO3 + H20 + CO2

If heated up to approximately 80oC

CO3 + Ca = CaCO3

If heated above 800C

CO3 + H20 = HCO3 + OH
Mg + 2OH = Mg(OH)
2

Hence if sea water in the evaporator is heated to a temperature below 80oC calcium carbonate predominates. If it is heated above 80oC then magnesium hydroxide scale is deposited.


Sodium Chloride 32230 to 25600ppm -generally ignored
Soluble below 225000ppm forms a soft encrustation, free ions promote galvanic action. It is unlikely to precipitate and is easily removed


Supersaturation
This is where the concentration of dissolved salts exceed their solubility at the particular temperature encountered and precipitation begins to occur. When deposition occurs under these conditions heavy scale deposits can rapidly build up and lead to a loss of heat transfer efficiency. Scale deposition due to supersaturation is often localised in areas of elevated temperature such as heat transfer surfaces in heat-exchangers. This is because of localised over concentration of salts with respect to the temperature of the thin water layer at the surface of the metal. Scale deposition can therefore occur on heat-exchange surfaces even when the conditions in the bulk of the water are not scale forming.

Methods of controlling and minimising scale (evaporators)

Use low pressure evaporation plant-Operating at temperature below 80oC so that calcium carbonate scale predominates. That is a soft scale easily removed and not such a poor conductor of heat.

Use magnetic treatment-A unit consisting of permanent magnets, preceded by a filter., is installed in the evaporator feed line. The water passes through a strong magnetic field which alters the charge on the salts so that amalgamation of the salt crystals, formed during precipitation in the evaporator , is prevented and the salt then goes out with the brine.

Use flexing elements-a heating element made of thin gauge monel metal built like a concertina may be used. The advantage of such an element is that when pressure, and hence temperature , vary slightly the element flexes considerably thereby cracking off scale effectively and permitting longer running periods of the evaporator between shut downs. Care should be made not to submit the element to over pressurisation.

. Use continuous chemical treatment


Treatments

-Prevents scale formation below 80oC

- Mixture of different phosphates

-sludge (coagulent and antifoam) conditioners

-can be used with potable water.

-Prevent scale above 80oC

-safe with potable water.