altration in fresh water generator parameters

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altration in fresh water generator parameters

Postby H.E.D1986 » Tue Jun 20, 2017 9:42 pm

Iwant to know how we can change the parameters of fresh water generator such as mass flow or temperature changing to obtain more production

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D Winsor
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Re: altration in fresh water generator parameters

Postby D Winsor » Thu Jun 22, 2017 4:00 am

There are many factors that will ultimately control the output of a FW Generator. The FW Generating Capacity is directly dependent on the amount of available heating an cooling/water vapor condensing capacity within the generator, the capacity of the heat generating source to supply the heat necessary of optimal FW Generation (Engine Load), sea and feed water temperatures,(use of SW Recirc Systems to maintain SW temps in Cold Water) and the general condition of the Generator and it's controllers.

Keeping the FW Generator heat transfer surfaces as clean as possible (regular removal of scale) and keeping process controllers working properly such as feed regulators and heat source regulators functioning properly as the best means to keep a FW Generator operating a optimum capacity.

It is also suggested that controllers, such as temperature and feed regulators are set with the amount of dead band to provide fine gradual control with the minimum of control valve travel. As there is one thing I've learned that FW generators do not like fast or extreme changes in parameters.
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Big Pete
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Re: altration in fresh water generator parameters

Postby Big Pete » Sat Jun 24, 2017 9:46 am

It is possible to boost the output of a Fresh Water Generator, above the designed output, in the short term however, the output will then fall of with time.
The unit will have been set up in the factory for optimal production.

Output can be increased by increasing Heat Input by:-1) increasing the Jacket water temperature by raising the set point of the Jacket water temperature controller, obviously this will raise the temperature of the heating elements..
2) By throttling in the jacket water bypass valve, usually fitted between the inlet and outlet valves of the jacket water, at the FWG, for normal operation of the FWG there should be a temperature difference of between 8 and 12 Degrees C, increasing the flow will reduce the temperature difference and raise the mean temperature of the heating elements.

The evaporation Rate can also be increased by acid cleaning the FWG in accordance with the makers instructions, clean plates will transfer heat better.
Extra plates could also be added to the heater/evaporator, assuming that the FWG uses plate heat exchangers.

If the evaporator is producing less than the designed output of FW the usual reasons are scale or lack of vacuum. If the Brine temperature is lower than the instruction manual it is likely to be a scale problem if the temperature is higher and the vacuum less there is likely to be a vacuum problem. Check for air leaks, check the eductor pump performance, and that the overboard discharge is unobstructed, if there is no improvement, replace the eductor as these will wear out, an invisible amount of wear in the eductor throat will cause a loss of vacuum.
Having a great pressure at the eductor inlet will only generate a vacuum if the overboard discharge is completely unobstructed, I have seen people throttling the overboard discharge valve in order to increase the pressure at the eductor!!!!! They obviously did not understand that it is the pressure drop across the eductor that is important.
HOWEVER, forcing the evap/ FWG to produce more than it's designed output will cause fast and severe scaling and Fresh water output will collapse after a couple of days and the FWG will have to be taken out of service and Acid cleaned to restore production.

If you read the makers instructions you will see that the the surface temperature of the heating plates have to be maintained below a maximum temperature in order to prevent excessive scale formation and also only a maximum of 1/3 of the Feed water can be evaporated into steam and condensed into Fresh Water, or excessive scaling will occur. ( the greater the Brine Density the more scale formation)

If you have a problem with scale and the temperatures are correct it is likely that the automatic feed valve is not working correctly, these usually have a spring loaded valve that admits more feed water as the vacuum increases, and it is worth reading the makers instructions and opening up the valve and overhauling or replacing it as required. I found on one ship that the feed pipe had become corroded and when the ship's Engineers replaced it they threw away the old feed valve and fitted a ball valve because they thought they could control the feed manually, total insanity.

SO LONG as the FWG is operating within the makers limits of Temperature and Brine Density, scaling can be controlled by Chemical injection, although this is frequently done incorrectly.

The Chemical maker will give a table showing quantity of chemical to inject each day in proportion to Fresh water output per day.
The Chemicals work by maintaining a thin film on the surface of the heat exchangers that stops scale formation, so for the Chemical to be effective it has to be dosed 24 hours a day, 7 days a week.
The daily dose of Chemical should be added to a large plastic drum daily as part of the watch keeping routine,at the same time every day, and the drum topped up to the same level with Fresh water every day, this ensures a constant concentration of Chemical in the drum. Typical consumption may be half a litre of Chemical per day.
The Chemical is sucked into the FWG by the vacuum inside and usually enters the inlet pipe between the feed automatic feed regulating valve and the FWG body. In order to control the flow, the Chemical and water mixture will pass vertically through a clear Perspex block, with a tapered bore through it, a needle valve at the base and a small Black indicator ball inside the bore, and calibration marks vertically along the bore.
As liquid flows through the bore, viscous drag will lift the ball until the drag lifting the ball equals the effect of Gravity pulling it down. If the flow rate increases the velocity of the Chemical passing between the ball and the tapered bore will increase and so will the drag, causing the ball to move up, increasing the annular area between the ball and the bore for the chemical to flow through, so that the Viscous drag will remain constant, and the velocity of the Chemical past the ball will always remain constant.. The flow rate can be read off the scale. The actual flow rate does not matter so long as it is greater than the amount of Chemical to be injected!!
Setting the flow to between 30% and 70% of the scale on the flow meter will usually provide a steady constant dosage which is what is required. After a few days of adding the correct amount of Chemical and water to the drum the concentration of Chemical will become stable and the FWG should be correctly dosed.

Hope this helps, Water rationing is hell, especially when it is caused by people not understanding how a Fresh Water Generator works, and could be simply prevented..!!!!
Last edited by Big Pete on Tue Sep 26, 2017 1:49 am, edited 1 time in total.
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Re: altration in fresh water generator parameters

Postby George12 » Mon Sep 25, 2017 10:33 pm

This post is really helpful for me. Thanks for sharing and keep up the good work. :)
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