Scavenge air and water mist catcher

[BjA]

Hi, I am not a seafarer. I am a mechanical engineering student, making a study on scavenge air.

I have no hands-on knowledge of how life is at sea, and how things are controlled in an engine room. Therefore, I have several times come across your site, and have enjoyed the vast knowledge you have about engine rooms. Thanks.
There are several things I have a hard time finding detailed (hands-on) info about. And, I was wondering if some of you would share your knowledge with me.
If you have answers for some of my questions, it would be very helpful if you also mentioned what type engine the info is used for (2 or 4 stroke, slow/medium speed, kW, route + whatever you know matters)
What I’m curious about is:
• How do you control the scavenge air temperature? Do you set the scavenge air temp…or the cooling water temp…or ??
• Do you fix the scavenge air temp at for example 40 degrees celcius, or does it follow the temp. of the seawater (of course some degrees above)
• How often does the water mist catcher extract water? Seldom, most of the time, only under some special conditions (Please note these conditions),… always…?
• Do you know how much water the WMC extracts?
• Is there an upper limit to how much water the WMC can extract per timeunit? Or is it unlimited?
• What temperature-settings are generally used for scavenge air temp?

Thanks a lot for whatever help you can give

[JollyJack]

Your questions bed questions.

Why control scavenge air temperature at all? Hot air, of course is not as dense as cold air. Cold air, being denser, contains more mass of air, and therefor more mass of oxygen with which to burn more fuel so you can get more power.

So OK, we agree that we need cool, dense air in the cylinder to burn more fuel. How cool? Air contains water entrained as vapour or droplets. If the air gets cold enough, it shrinks, squeezing the water droplets together until, at some point, there are enough all squeezed together to form bigger droplets......which are heavier squeezed together. The heavier globs of water cannot be supported as vapour or droplets, so they drop out of the air.

This is the "DEW POINT". There's a fine balance here. If you run the scavenge temperature below the Dew Point, you will have water pissing out of the air and filling your bilges with water through the scavenge space drains. Not a good idea.

If you run the scavenge temperature above the dew point, the entrained water vapour and droplets will vapourize during combustion. The problem here is that if the water condenses enough after combustion, it will assimilate sulphur from the fuel to form sulphuric acid, which doesn't do a lot of good to the stack and the economiser.

SO. Find the dew point of the air and run the cooler outlet slightly higher, so the water remains entrained in the air. The dew point will depend on where you are, of course, and the temperature of that particular bit of oggin you float on. The wavewatchers upstairs should give you the dewpoint, they have a gizmo for that. Keep the SW inlet valve to the cooler throttled so the aftercooler outlet is just above the dew point.

[BjA]

Thanks a lot for your comments JollyJack.

You write: " ...you will have water pissing out of the air and filling your bilges with water through the scavenge space drains. Not a good idea."
I have read that the bilge means "spaces in the lower part of the ship"...but have also seen this "drain place" referred to as "bilge tanks". Are they tanks or...? And how much water can they contain? Why is the water not pumped into the sea?
The "scavenge space drains" you refer to, could it be what I call "Water Mist Catcher drain"?
And I expect the comment "Not a good idea", means that your aim is to never have condensed water. Is it possible to avoid condensation...and in which situations can it perhaps be impossible.

You write:"...The wavewatchers upstairs should give you the dewpoint, they have a gizmo for that. Keep the SW inlet valve to the cooler throttled so the aftercooler outlet is just above the dew point."
Is this the dew point of the ambient air outside the engine room? And does this mean, that if the wavewatchers tell you the dew point is 25C, you throttle the seawater so the scavenge air becomes 26C?

[D Winsor]

The engine manufacturer should have in their specifications or on the test certificate as to what the optimum scavenge air temperature should be to produce maximum horsepower. In my experience working on both medium and slow speed engines is roughly 40 deg C. which is well above the Dew point temperature in most environments. Depending on how the engine is designed the scavenge temperature is controlled by either manually or with some sort of automatic temperature regulator controlling flow of raw water through the cooler.
Some engine manufacturers, because of the high horsepower being developed, scavenge pressures up to 5 bar and scavenge temperatures up to 70 Deg. C, are now using the jacket water or a secondary Low Temperature cooling system to control the scavenge temperature in order to avoid producing thermal stresses in the combustion spaces and to insure complete combustion.
Ideally it would be desirable to keep the scavenge temperature above the Dew Point to minimize the production of condensate but it is much more critical to maintain engine exhaust temperatures below the maximum recommended values, because the higher your scavenge temperature the higher exhaust temperatures coming out of the cylinders will be which could lead to exhaust valve damage. In conditions like that, one would be obliged to reduce the engine engine load to bring the exhaust temperatures down to a safe range.

[JK]

higher temperatures of the intercooler will also start the turbocharger "barking"

have read that the bilge means "spaces in the lower part of the ship"...but have also seen this "drain place" referred to as "bilge tanks". Are they tanks or...? And how much water can they contain? Why is the water not pumped into the sea?

A deep bilge can hold tonnes of liquid. However, it is preferable to keep them dry and clean.

[JollyJack]

What they said

The engine room bilge tends to get oily and you can't pump oily water overboard to sea. The penalty for that can be quite severe, up to decapitation in the Persian Gulf (Dubai, for example). It's a bit less in other places, Carnival Cruises was fined $3 million US in USA a couple of years ago. These are extremes, of course. Oily bilge water is pumped through an oily water separator, which reduces the oil content to less than 15ppm, the water goes over the side and the oil is collected in a sludge tank. The sludge is disposed of by burning in an incinerator or pumping ashore to a reception facility. (There's some say that the sludge is mixed with heavy fuel and sold to ships, but that's just rumour, no oil company would ever cheat a sailor.)

[BjA]

Thanks a lot for your answers!

I'm still hoping for someone to give detailed info about some of the questions in my original post.
The reason is, that I have NO idea how things are done in a ships engine room. The things, that are everyday-subjects and obvious to you, are mysteries to me

And in general, I am very interested in whatever info you will share regarding scavenge air, Water Mist Catchers and related subjects.

Besides, I am very curious about this comment from JollyJack:

SO. Find the dew point of the air and run the cooler outlet slightly higher, so the water remains entrained in the air. The dew point will depend on where you are, of course, and the temperature of that particular bit of oggin you float on. The wavewatchers upstairs should give you the dewpoint, they have a gizmo for that. Keep the SW inlet valve to the cooler throttled so the aftercooler outlet is just above the dew point.

And one of my questions is:

Is this the dew point of the ambient air outside the engine room? And does this mean, that if the wavewatchers tell you the dew point is 25C, you throttle the seawater so the scavenge air becomes 26C?

[JK]

You will find that there will most likely be a thermostatic valve that will maintain water temperatures to the intercooler by either recirculating or going overboard. Temperature is MFR recommendations.
What you have not mentioned is outside air supplied to the ER. Sufficient air must be supplied to keep the ER under pressure when the engines are at maximum load.

[Big Pete]

Hi BjA, welcome to the site,

You have very honestly explained that you don't know anything about ships so I will try to give you some more back ground information.

As previously explained, the colder and more dense the charge air is, the more power can be developed by the engine without exceeding the maker's limits for the exhaust gas temperatures. However, if the exhaust temperature is too cold it causes thermal stress and cracking when it hits the hot components in the combustion chamber, so most manufacturers used to recommend temperatures around 45C with the engine at full load. Absolute minimum temperature 40 C. Ideally at part load the air temperature should be higher in order to improve the efficiency of the combustion, the thermodynamic cycle,(Carnot cycle) and reduce pollution by partly burnt fuel.

30 years ago ships had direct sea water cooling passing through the tubes in the charge air cooler, and the Lubricating Oil cooler and a Jacket water cooler. This was regulated by manually throttling in the Sea Water outlet valve to control the flow. Throttling the inlet valve could cause turbulence and erosion in the tube stack. Some ships were fitted with thermostatic Sea water valves but these did not tend to last long in hot sea water.

Then a central Coolers were introduced which used Sea Water to Cool Fresh water to about 40C, this Fresh water was then used to cool the charge air cooler, L.O. Cooler and either cool a separate Jacket water cooler or there was split circuit of hot water circulating around the engine cylinder heads and liners with a thermostatic valve delivering some of the water to the Low Temperature circuit to maintain a constant temperature of the jacket water.

Because the charge air was now always being cooled by water at 40C instead of varying from 0 C in the Arctic to over 30 C in the tropics, it became much easier to control the air temperature accurately, usually by bypassing water flow around the cooler, this also solved the problem of corrosion with hot sea water, and erosion.

The next step was two stage air coolers. In these hot Jacket water from the engine, maybe as hot as 95 C is used to heat (at low load) or cool (at high load) the air as it comes out of the turbo charger, the air then passes into a second stage air cooler, cooled by LT water and thermostatically controlled. This prevented undercooling the charge air at low load, on some the engines the LT cooling water is bypassed until the charge air pressure (proportional to engine load) reaches a certain value.

The Relative Humidity can be calculate using a simple Hygrometer ( football rattle) with wet and dry bulb thermometers, you swing this around until you have worked up a sweat then plot the 2 temperatures on a chart which will give you a value.
(This can also be done electronically now). The set point for the charge air temperature can then be set in accordance with the maker's instructions. The "wave watchers" are the deck Officers, based on the Bridge of the ship.

Water Mist catchers are mechanical devices fitted in the air flow downstream of the air cooler, they usually consist of baffles, similar to a half open Venetian blind, they deflect the water droplets down to the bottom of the manifold and there will usually be a weir to retain the water in the manifold before the 1st cylinder. There can also be a coalescing filter fitted.
On modern engines there will be at least one automatic (usually Float operated) drain valve where most of the water is expected to collect and additional manually operated drain valves along the length of the air manifold. Older ships had manual valves, and watch keeping Engineers (remember them?) would manually drain the valves every hour or more frequently if more water was being produced.

The quantity of water being produced will depend on the temperature and RH of the ambient air, the compression ratio in the turbocharger and the charge air temperature, and the mass flow of air, so it is highly variable.

Generally, waste water flows into the " Bilges " of an Engine Room. This is the space above the "Tank Tops" (The top of the Double Bottom Tanks, that the form the lower Box Girder of a ship's structure), but below the raised working Deck on which people walk around (Floor Plates). There is a lot of leakage and waste flowing into this area so it contains a mixture of Salt and Fresh water, Boiler water and cooling water chemical treatment, fuel lubricating and hydraulic oils, biocides from treating oil systems, possibly leaking Black or Grey water and a lot of other stuff. This is pumped into a Bilge Tank, usually part of the Double Bottom structure where it is stored until it can be disposed of by pumping ashore or through the Oily Water separator.


I hope the above fills in any blanks for you.

BP

[The Dieselduck]

I want to express my admiration and gratitude to those of you who share your knowledge so freely. Its really refreshing and inspiring. Thank you.

[JK]

Yes, when you read back over this thread particularly, you tend to realize how second nature a lot of things are. We all know what the bilge is, how our circ systems work, and the fact the ship moves through waters of different temperatures and the effects it has. You don't even consider that it is mysteries to others.
Thanks to the others that take the time to take the time to describe it for the people not so familar. What may not be realized is how long some of these posts may take to research and write, even if you are still working at sea as Big Pete and DWinsor are. They are still staring at the diesels and practicing the trade hands on everyday. A shout out for them!

[BjA]

Hi again. Sorry I haven't commented your last answers until now. I have been sick for a week.

Thanks again for your great answers (especially the very thorough answer from Big Pete)

It feels like I have a thousand loose ends.
Let me mention some of them:

Because the charge air was now always being cooled by water at 40C

The engine manufacturer should have in their specifications or on the test certificate as to what the optimum scavenge air temperature should be to produce maximum horsepower. In my experience working on both medium and slow speed engines is roughly 40 deg C. which is well above the Dew point temperature in most environments.

First of all: Is it a "mistake" or are the temperaturevalues just very "rough"...?
Because in Big Pete's text it is the temperature of cooling water and in DWinsor's text it is scavenge air!

Anyway, the most important thing for me here is if someone could tell me, if this is generally the case...that the central cooling water is close to 40C all the time.

Another thing: I am well aware that the dewpoint changes when the air is pressurized (like in the scavenge air). So I have combined two mystery-comments from Jolly Jack and Big Pete and figured out a possible explanation. Now I just need you to tell me if it's true
When you get the dew point info from the wavewatchers, do you then have a table from the enginemanufacturer, where you can match any wavewatcher-dew point with an engine load, and read the dew point of the scavenge air, at that load, in the table?
Please let me know if I'm way off...or dead-on

And finally: It would be fantastic if someone would briefly describe what changes are made in cooling water temp/scavenge air temp on a newer large ship on a summer trip from, say, Singapore to Rotterdam (through Suez), where the sea water temp in the tropical area is 32C, and the trip has unusually many all-day rainy days (but also non-rainy days).
I'm interested in:
Is the colling water changed: Never, a few times or constantly.
What would the cooling water temp. be (in degrees C)
Are there different conditions and settings (regarding cooling water temp.) in the different seas/oceans: Tropical, Read Sea, Mediterranean and Atlantic.
How much water would drain from the Water Mist Catchers in the different seas: Nothing, a little, Medium and A LOT?
Would it in some special cases be necessary to take special precautions because there was too much water draining? (for example reduced load or increased cooling water temp.)

Thanks a lot, and I will definitely go through every answer in details. It's so interesting to listen to your hands-on experience.

[BjA]

Is the colling water changed: Never, a few times or constantly.

Oops, I meant: Is the cooling water temperature changed:

[JK]

All cooling water in modern ships is thermostatically temperature controlled as mentioned before. SW is either recirced to the seabay/seachest or sent over the side.
The most you may do is close the overboard and open the manual recirc isolation valves when going into ice.

[D Winsor]

Further to your post I believe further clarification is needed

All medium and slow speed diesel engines has what's know as a "Closed Loop" jacket water cooling system similar to the cooling system in a car and raw sea water is not used as the primary method of cooling the cylinder jackets.

Many medium speed engines has either a single or two stage closed loop cooling systems with up to 12 thermostatic valves similar to the thermostat in a car to maintain the jacket water temperature at a constant 80 Deg C. Engines fitted a 2 stage cooling system the second stage is fitted with it's own thermostats set to open at 35 - 40 Deg C. The second stage system specifically removes heat from the scavenge air and the lubricating oil and each system has it's own circulating pumps. When the temperature on the primary jacket water system exceeds the thermostat set point cooler water is drawn in from the second stage an equal amount of hot jacket water is expelled back to the second stage cooling system. When the second stage cooling system thermostats open the lower temperature circulating water flows through either a tube or plate type heat exchanger transferring heat from the second stage cooling system to an open loop raw or sea water cooling system. The heat exchanger usually has sufficient surface area to provide effective cooling over a wide range of raw water temperatures from 20 - 40 Deg C.

When a ship transits an area where the raw water goes below 20 Deg C an automatic or manually controlled valve fitted to the raw water discharge of the heat exchanger will open redirecting a sufficient amount of warm raw water to the suction of the raw water circulating pump or the sea chest to maintain a constant 20 - 25 Deg C Raw water inlet temperature to the heat exchangers. When the ship passes through ice and slush 80 - 100% of the raw water is recirculated which also aids in the melting of any ice that may be drawn into the system.

Slow speed engines are usually fitted a single stage closed loop jacket water system and the jacket water is discharged directly to the heat exchanger, The temperature of the jacket water is again maintained with thermostats similar to that found on the medium speed engine or a PLC controlled three way control valve which mixes cool water from the heat exchanger with warm water that bypassed the cooler.

As with the medium speed engine an open loop raw water cooling system removes heat from the jacket water cooling system through a heat exchanger. However unlike the medium speed engine with a 2 stage cooling system raw water is supplied directly to heat exchangers to cool the engine lubricating oil and scavenge air. Again the raw cooling water from all the heat exchangers is recirculated back to the suction of the raw water circulating pump or sea chest to maintain a constant 20 - 25 deg C inlet temperature to all the heat exchangers.