Effect of sea water temperature on fresh water production

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Effect of sea water temperature on fresh water production

Postby hugerich » Sun Nov 10, 2013 9:46 am

I have a question regarding fresh water generators. I noticed a drop in the production of the FWG as we were moving into cooler waters, and reported it to the chief. He said that this is unusual as lower sea water temperatures actually increase the amount of water the FWG can produce, and suggested that the FWG needs cleaning. When I asked him to explain why he didn't have an answer so honestly, I'm sceptical. I would expect lower sea water temperatures to reduce the production rate, as although the condenser element will be cooler and may be able to condense more water, it takes more heat to evaporate the sea water in the first place. If chief is right, why is this?

Anyway, my solution was to just close in the HT bypass a little more. There is a superstition on here that doing this would cause all sorts of HT temperature engine alarms and be more trouble than it is worth, but once I closed it in by a turn or so our production increased drastically. I suspect that when someone tried closing it in more in the past they did it too quickly so the controller couldn't adjust in time. Chief has now changed his mind about cleaning the FWG as all is fine now, but was he right about the lower sea water temperatures increasing production?

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JK
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Re: Effect of sea water temperature on fresh water production

Postby JK » Sun Nov 10, 2013 11:39 am

Colder sea water decreases production but makes water with lower salinity.

You see it more dramatically with ROs

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Big Pete
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Re: Effect of sea water temperature on fresh water production

Postby Big Pete » Tue Nov 12, 2013 7:54 am

I think there are two opposing factors at work here:-

Inside the condenser of the evaporator you have both water and steam present, therefore there is a direct correlation between the temperature and the pressure inside the condenser. ( See T/S curves or steam Tables) When the Sea Temperature is lower, the condenser temperature and pressure should also both be lower (better vacuum). The pressure inside the evaporator is the same as in the condenser and the temperature at which the Sea Water evaporates will be the same as the temperature at which the Fresh water condenses, therefore, inside the evaporator the Sea Water will boil at at a lower temperature and require slightly less total enthalpy to boil but at the same time the temperature of the Sea water inlet being lower, it will still require a similar heat input to bring it to the boil.

It could be, that as a result of the colder ambient temperatures the jacket water temperature dropped, and the E.R. air temperature dropped causing greater heat loss from the Evaporator to the E.R.

With regard to the Jacket water bypass, I would always close the vacuum breaker, open the jacket water inlet and outle valves, and settle the evaporator down with the by pass full open and then only slowly shut it in until the difference in temperature between the inlet and outlet to the evaporator is optimum. Usually the makers recommend a difference of between 8 and 12 Degrees Celsius. The bigger the mass flow, the smaller the temperature drop across the evaporator and the higher the average temperature in hte evaporator, so more heat transfer and more steam production. Trying to force the output over the makers design will cause very violent boiling in the evaporator and droplets of water will "carry over" through the de-mister filter and into the condenser, increasing the salinity of the output.
Similarly, when shutting down the plant slowly open the Jacket Water bypass fully, and allow conditions to settle, before closing the Jacket water inlet and outlet valves, and .

I have seen, on some ships, a bypass valve on the the condenser Sea Water system, to control the temperature/ vacuum in the condenser. Reducing the S.W. flow will also raise the temperature of the S.W. coming out of hte condenser, which is of course the feed water going into the evaporator. Again the plant will usually be designed to operate with a temperature change of 8 to 12 Degrees Celsius across theopening the vacuum braker.


I am continually amazed by the number of people who do not understand the function of the vacuum breaker and shut down the Vap with this valve shut, so the vacuum inside the shell sucks both the evaporator and the condenser totally full of Salt water, which not only causes internal corrossion while the vap is shut down, but means the Vap has to run for a considerable time before all the salt is flushed out and and you can make water for consumption. On the old tube and shell type vaps it could take an hour or more before you flushed all the Salt out and you could make clean water.

I hope this helps.

BP
Last edited by Big Pete on Mon Nov 18, 2013 1:09 am, edited 1 time in total.
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Re: Effect of sea water temperature on fresh water production

Postby hugerich » Tue Nov 12, 2013 9:23 am

Very informative answers, thank you very much.

Yeah, we have much cooler ambient temperatures now and the HT outlet is cooler by a few degrees so that would have an impact.

It's funny you mention the vacuum breaker, because that is the same situation we have on here! Through my cadetship I was always told to open the breaker once the jacket water valves are closed and the shell has cooled down, so after shutting down the ejector pump. I was never told why, I always assumed it was to do with a vacuum putting undue stress on the seals for no good reason. The personnel on here however never seem to use the breaker, so much that when I came to shut it down the breaker was jammed shut, must have been closed for a very long time. Even now I can't get it free. I mentioned it to the other engineers and was told that it wasn't important and having a constant vacuum is a good thing!

Now, being a first trip engineer I am still learning many things, but after a couple of days shut down I see that the vacuum has dropped off and the sightglass is full of water. Then it strikes me, 100% vacuum, no air leaks, must have drawn up the water. Not good in port waters. Also I expect this is why the ejector pump mechanical seal has started leaking badly, there shouldn't be a vacuum on the discharge side of the pump.

Reading your post, you mention that you adjust the jacket water valves then close the breaker. However, I have always been shown that to start up you close the breaker then start the ejector pump, build up the vacuum then start with the jacket water valves. Shutting down you close the jacket water down, leave the ejector pump running to cool the shell, then once the shell is at sea water temperature stop the pump and open the breaker. Is there a better way than this? Thanks for your input.

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Re: Effect of sea water temperature on fresh water production

Postby Big Pete » Thu Nov 14, 2013 5:50 am

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.

B.P.
Last edited by Big Pete on Mon Nov 18, 2013 1:16 am, edited 2 times in total.
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Re: Effect of sea water temperature on fresh water production

Postby hugerich » Thu Nov 14, 2013 1:21 pm

Ramble all you want, this is gold! Noting it all down. See, this is the problem with the ship I am currently on, nobody has a clue about what they are doing the majority of the time so I generally don't bother asking anything technical as I either get a blank look or a load of nonsense. Just to set it in your mind what sort of ship I am on, last port state visit we were a hair away from being detained because all the engineers (including chief) didn't know how to start the emergency generator on hydraulic start, and neither could they work the emergency steering control properly without causing a load of hydraulic lock alarms (they didn't understand they should switch the pump they are not operating off before attempting to manually control the valve block). Then there is the general mess the engine room is in, I've tried to put things right but I find it just ends up a mess again in no time. It's good that there is somewhere like this I can get answers I can trust, so thanks for all your help.

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Re: Effect of sea water temperature on fresh water production

Postby Big Pete » Sat Nov 16, 2013 12:11 am

Hi Hugerich,

Glad this is helping you, and that you think my comments are Gold!
I started as a Cadet in 1971 so I have been around a long time. Most people at Sea just accept what they are told by the people senior to them onboard and never question things for themselves. That is as common with Europeans, Americans or Canadians as it is with Third World Crew. There is a lot of Urban Myth at Sea, and this is passed on fron one generation to the next. You are obviously asking questions and trying to verify the answers which is the way you become a good engineer, that understands not only what to do but how and why.
In a few years people will probably be describing your posts as "Gold".
I am afraid it doesn't get any better as you get older, more experienced and move up the ranks.

My present ship is 3 years old, and when I joined the Second Engineer was going to incinerate some rubbish, and told me he had to isolate the fire alarms for the incinerator room before he could run the incinerator. I asked him why. (Because the combustion chamber of an incinerator always operates under a vacuum, no smoke should leak out, any leakage will be air in) He couldn't explain why, it was just some thing he did.

After a lot of investigating I found that the incinerator was in a hermetically sealed steel room with 2 fire doors with rubber seals on them for access. There was also an EXHAUST FAN from the room, but no natural or forced air supply!

Apparently when the Ship first entered service the engineers wanted to run the plant with the fire doors open, but the Mate (Safety Officer) insisted that they should be shut, so it was operated with the doors shut.
The incinerator is fitted with a differential pressure switch that compares ambient pressure with the furnace pressure, obviously the air flow is proportional to this differential pressure.

So what happened when the incinerator was run? It sucked all the air out of the room until there was a vacuum in the room and the air flow had reduced to the leakage past the fire doors. As the flow reduced the difference in pressure between the room and the furnace reduced and smoke blew back into the room, a potentially very dangerous situation that should be prevented by the operation of the differential pressure switch, but this did not operate. Eventually it would shut down on flame failure when there was not enough air for the oil fired burner. It was not effectively incinerating the load.
Unfortunatly nobody thought to list this design / construction fault as a Guarantee defect so the Safety Device (D.P. switch) was bypassed and they continued using the incinerator. It shuts down on flame failure (due to lack of Air) before it has properly incinerated the load.

My problem is that the man who was Chief Engineer on here for three years from new, is now my Super...
Last edited by Big Pete on Mon Nov 18, 2013 1:19 am, edited 1 time in total.
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Re: Effect of sea water temperature on fresh water production

Postby JK » Sat Nov 16, 2013 1:28 pm

that is actually quite scary to think about!

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Re: Effect of sea water temperature on fresh water production

Postby JollyJack » Sat Nov 16, 2013 9:15 pm

"It's always done that way"......o

Really irked me when I was sailing as Chief. I asked "why?", I asked to justify it, find the reason, rationalize it. If it couldn't be rationalized, then simplify. As a naturally born lazy bugger, I have found if you want to have a job done quickly and efficiently the first time, give it to a lazy bugger. He doesn't want to go back and do it again.
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Re: Effect of sea water temperature on fresh water production

Postby Big Pete » Mon Nov 18, 2013 1:25 am

I totally agree with Jolly Jack, one day I will probably have a Heart Attack when someone tells me "we always do it that way" after doing something particularly dangerous and stupid.
Since STCW came in a lot of people who in my opinion should have stayed as "Professional Thirds" operating on a "Monkey See, Monkey Do" basis, have been certificated and promoted to Senior Ranks where understanding should be required.
I agree with the Laziness bit as well, some lazy bastard that was told to drag a huge weight a long distance invebted the wheel.
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Re: Effect of sea water temperature on fresh water production

Postby hugerich » Mon Nov 18, 2013 9:29 am

"That's the way it always is", yeah, heard that one a lot. By the looks of things I can expect that a lot through this career! But thanks for all the input everyone, it is a little intimidating trying to change things with this being my first trip as an engineer. There are people with decades of experience telling me various inaccuracies or outright lies and showing me plenty of bad practice, hopefully I will be able to recognise when to listen up and when to be sceptical of what I'm being shown/told.

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Re: Effect of sea water temperature on fresh water production

Postby JollyJack » Mon Nov 18, 2013 8:24 pm

First trip? I'll pass on the advice I got when I joined Clan Line as 6th Engineer. Listen to the oilers, they know much more than you ever will about the ship.
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Re: Effect of sea water temperature on fresh water production

Postby JK » Tue Nov 19, 2013 7:29 am

well, that depends.
If you get a biker type from the union hall, you could be in trouble. I speak from experience on that!!
If you are in a company that has permanent crews, they might know the engineroom, or they could be a wiper only.

I will not speculate on how many decades since you have been a 6th engineer :lol:

One thing you will learn is how to read people.

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Re: Effect of sea water temperature on fresh water production

Postby JollyJack » Tue Nov 19, 2013 10:34 am

No Union hall, no bikers. Just a Serang with his chosen Tindal, donkeymen, firemen, oilers, wipers and topaz. Indian crews. Chapatties for smoko, Biryani for Sunday lunch, dhobiwallah for dhobi, ghingiwallah for general cleaning, bhindari for the crew, Chief Cook and Second Cook for Officers, 2 stewards for Engineers, 2 for deckies, 1 upstairs for the Old Man, Sparky and Chief, meals in the saloon, and you'd better be in Uniform!.....them wuz the daze!

You never, ever asked any of the crew to do a job, you suggested to the Serang that it might be a good idea if........ Of course, asking one of the watchkeepers on your watch, even as Junior on watch, was acceptable, there was an oiler, donkeyman and wiper to choose from.
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