Hi again Hugerich,
When I start an evap I always start the ejector/ feed pump first. Then close the vacuum braker. If you put heating on an evaporator that doesn't have a flow of feed water through it, you will boil it dry and create Hard Scale on the heating surfaces.
Once the correct vacuum, feed water flow and level have been established, I slowly open up the Jacket water inlet and outlet valves, wait for a few minutes for everything to settle down, start the distillate pump, check the condenser temperatures, Salinity , vacuum etc then slowly shut in the Jacket water bypass until the temp drop across the evaporator is correct.Then if I have a bypass on the Condenser S.W. line adjust that, then back to the JW bypass in a reiterative process, until everything is set up by the BOOK. (as I said previously, normally the temperature drop across any heat exchanger is 8 to 12 Degrees)
In order to produce the maximum amount of water without scaling you are aiming to evaporate 1/3 of all the feed water put into the evaporator, so correct operation of the automatic feed valve and cleaning of the feed filter are critical to the efficient operation of the plant. If you evaporate more than 1/3 of the feed water you will cause Hard Scale deposits on the heater elements even if you are using Vaptreat. If you evaporate less than 1/3 you are wasting heat by dumping a bigger volume of water at evaporation temperature.
I was on one ship where the engineers all assured me that the evaporator worked perfectly when I joined, it was a 20 ton/day job, when we sailed from Port it was producing about 10 t/day and after a week it was producing about 2 t/day!!
It appeared that at some point the Sea Water feed line had rotted out and they had made and fitted a new pipe, but they hadn't considered the feed valve to be important so they had thrown it away with the scrap pipe!
When I asked the Engineers how they could control the correct feed rate without an automatic valve they told me that it was "alright so long as you could see water in the sight Glass!!!"
It also turned out they were not dosing the feed water correctly, when I asked them how much Vaptreat they were putting in the Second Engineer told me he put in a couple of litres every few days and he had now idea of the correct proportion to add.
They weren't the brightest and best!
When shutting down, I always slowly open the bypass valve, close the jacket water inlet and outlet valves, allow the entire evaporator to cool down to ambient temperature, open the vacuum relief valve, and only after the evaporator shell vacuum has fallen off, do I stop the ejector pump,(This flushes out the concentrated Brine and again prevents the formation of Hard Scale.) if the Vap is below the water line you will have to shut the feed water valves to stop Sea Water gravitating into the Shell otherwise it is not a problem to leave it open.
With regard to Chemical Dosing I find that a lot of people get confused about this. The intention is to maintain a constant p.p.m. of Chemical in the Vap all the time the heating is on. The Vap Maker's Manual will tell you the designed Fresh Water output, multiply this by the figure given by the Chemical Maker to give you the daily dose. If you put less than this you will have scale formation but there will be no advantage in adding more, you are just pumping expensive Chemical into the Sea.
Obviously you want to avoid scale formation as this reduces the FW production and will force you to waste valuable manpower and De scaling Chemicals to clean the Vap in Port.
Usually the daily dose of Chemical is too small for it to be easy to meter it into the Vap at a constant rate, therefore it is diluted with water in a plastic drum and drawn into the Vap by the vacuum, passing through a flow meter and dosing valve.
On a Ship doing long deep sea voyages it is very easy to dose the Chemical at a regular time each day and top the dosing tank up to a fixed level with fresh Water (although I can see no reason why you could not use Sea Water). Thus maintaining a constant concentration of Chemical in the dosing tank, and therefore, so long as the dosing meter is always indicating the same flow rate, the daily dose will be metered into the Vap. evenly during the 24 hours.
It can be harder to maintain the correct dosing if the Vap is only run for short periods of time, but if the times of starting and stoping the Vap are accurately recorded in the log it should be possible to calculate the proportional daily dose of Chemical and top up the dosing tank wioth water to the normal level.
Incidently, for anyone who does not understand how the Chemical Dosing Flow Meters work, essentially they are a Force Balance. There is a little Black Ball "Floating" in a vertically mounted tapered Bore. The Ball floats at the height where it's weight is exactly balanced by the Vicous drag of the liquid flowing past it. The Bore is greater higher up the meter. The liquid flows through the annular space between the ball and the bore. If the ball is stable and then the flow is increased, the velocity of the liquid through the annular space will increase, creating more viscous drag and lifting the ball to the point where the increase in the annular area is sufficient for the velocity of the liquid to be the same as it was at the first position.
Essentially the speed of the liquid flowing past the Ball will always be constant, the Ball diameter is constant, so the flow will always be a function of the bore at that point.
Hope all these Senile ramblings help.
Last edited by Big Pete
on Mon Nov 18, 2013 1:16 am, edited 2 times in total.
It is always better to ask a stupid question than to do a stupid thing.