Feedwater pre-heating with a hotwell and vacuum pump

[JonRiley56]

All,

I have been pondering the above and have a couple of questions.

If I put a vacuum pump at the end of the condenser return (where it has come back thru the hull into the boat), and use that to draw the condensing exhaust away from the engine does it provide the impetus to get the condensate into the hotwell ?

Should I plan on taking the condensate thru a pre-heater loop before it enters the hotwell so that I essentially have a container of 150 degree water sitting on deck, or would it be better to have it pass thru the preheating pathway when it leaves the hotwell on its way to the boiler ?

If I use a float valve to either loop the feedwater stream back into the hotwell, or direct it to the boiler, what is happening in that feed line when it is looping ? Is the water in the preheater being converted to steam ? If so, is that an issue ?

any help would be appreciated, I am still reading.

jon

[barts]

I would use the vacuum pump to pull the condensate from the condenser into the hotwell. The feedwater heater should come after the feedwater pump; otherwise the hotwell can get too hot and the pump won't pump hot water (vapor lock).
The float valve bypass just returns the water from after the pump and before the feedwater heater back into the hotwell.

- Bart

[dhic001]

On 'Zeltic' there re two feedwater heaters after the feedpump, and both before the bypass valve. The first one is the feedwater line passing down and back through the inside of the exhaust line (before the exhaust exits the hull to the condenser), the second one is a loop of pipe around the outside of the (dry) firebox shell. The pipework then meets the tee that goes to either the feed checkvalve, or the bypass valve. As a result, the feed line doesn't try and make steam unless the engine is stopped and the bypass is closed. My bypass is manual, so I make a point of opening it when the engine isn't running. I've never seen a feed heater after the bypass point, and can't see a reason to do so.

Daniel

[preaton]

Daniel, it depends on your philosopy. If you have the heater after the bypass you will only be heating the water going into the boiler. Also the volume through the heater will be less so you heat the water more. On the other hand your hotwell really just becomes a feedwater tank.

Doing what you do and heat before the bypass will result is slightly cooler water going into the boiler but a hotwell. Good for driving the oxygen out of the water but if too hot then pump problems with water flashing into steam on the inlet side of the pump. Also your injector will need to come from another source of cold water.

So you win some, you lose some. Take your choice.

[JonRiley56]

Thanks for the replies.

If I am running in 60 to 70 degree water (F... ) and I have a reasonable amount of condenser length outside the hull, what temperature range would I expect at the return to the hotwell ?

I understand the point of ending up with a hotwell that is essentially a feedwater tank, but isnt part of the process separating oil from the feed stream so there is still a benefit ?

My understanding is that Steam driven Feedwater injectors do not like "hot water". If that is the case, and I use an injector, am I better off having the water feeding the injector colder rather than warmer ?

As far as the oil separation goes, will the oil rise to the surface faster in a a cold "emulsion" than a hot one ? Will I be more efficient in getting to the surface and not entraining it in the feedwater if the temps in the Hotwell/Feedtank are lower ?

I assume that using a "vacuum" pump to move the exhaust/condensate is more a function of having something that will pump air rather than actually pulling a vacuum, am I right ?

I have read of Power and efficiency benefits of pulling a vacuum on the exhaust side of the engine. If that is true should I have the pump before the discharge of the exhaust thru the hull for condensing rather than at the return to the hull after that process ?

If I use a steam injector, do I need an engine drive feedwater pump ? (obviously I will have a manual pump for emergency use)

Thanks in advance for the help !!! I will be flipping my hull in the next couple of weeks to remove the vestiges of the keel and do some measuring for prop shaft angle etc. After that I can begin the actual build.

jon

[Edward]

Dear JonRiley56

I can't offer an answer as to how hot your condensate will be after condensing it ; no doubt others will be able to .


Ideally the water you put into your boiler should be as hot as possible so it requires less energy to turn it into steam .
But if your feedwater is too hot the partial vacuum caused by either a mechanical pump or an injector "sucking" at the feed can lower the pressure (and therefore also the boiling point) enough to cause the feedwater to flash into steam before the pump/injector, so it won't go into the boiler .

So the water in your hotwell shouldn't be too hot . Also the colder your condensate return (within reason) the better the vacuum you will get which will help your low pressure cylinder produce more power .
The air pump/vacuum pump should be after the condenser . Put it before the condenser and you're trying to pump steam through the condenser , after the condenser and you're pulling out water and air .Initially a condenser by itself will create a vacuum , but it will soon fill with air and condensate. The function of the air pump/vacuum pump is to remove this air and condensate to the hotwell and maintain the vacuum in the condenser.


You are quite right about oil separation in the hotwell and another obvious advantage of condensing is that if your running on water which is unsuitable for use in a boiler you don't need to carry too much make- up feedwater on board .
I don't know if the oil in the hotwell will rise faster in hot or cold water , but I wouldn't rely on the oil being lighter than water and therefore rising to the surface to separate it with weirs etc . The oil droplets are VERY small and given the flow through the hotwell and the normal rocking motion of the boat the droplets are likely to remain in the water . You would be better off using some material filter , there are quite a few sugestions about this elsewhere in this Forum .


Regards Edward

[dhic001]

Doing what you do and heat before the bypass will result is slightly cooler water going into the boiler but a hotwell. Good for driving the oxygen out of the water but if too hot then pump problems with water flashing into steam on the inlet side of the pump. Also your injector will need to come from another source of cold water.

So you win some, you lose some. Take your choice.

Next time we do a decent run with Zeltic, you can come along Paul and see how hot the water actually gets. I've never had a problem with the pump due to hot water, but the injector won't pick up from the hotwell after even a relatively short run. I'm certainly happy with the heating I'm getting, and if it proves to not be sufficent long term, I'll send it round the firebox again.

As an indication of how hot the water is, without having measured it, on my second to last run, the then section of hose between the pipe coming out of the exhaust heater and going to the firebox coil sprang a leak. the water coming out was hot enough that i was unable to put my hand anywhere near the spray. Needless to say there isn't a section of hose there now (it was temporary anyway). remember the water gets heated again before it gets to the boiler itself.

Daniel

[JonRiley56]

All,

Thank you for the replies they are very useful. Point well made on the oil separation. I was intending to have a filtration step. As to the reluctance of steam injectors to handle hot water, I was pondering the tech data sheets on Penberthy injectors and came across something interesting, their ability to lift is related to pressure and temperature. The relationship listed is as follows, they quote the info for 20ft lift and then the temps for 3 ft lifts at varying pressure. (there is more detail there as well)

Low Pressure Injectors (25 to 140 psi)
Pressure Water Temp Lift
60 to 80 psi 74 F 20 ft
60 to 80 psi 120F 3 ft
100 psi 115F 3 ft
120 psi 105F 3 ft
140 psi 90F 3 ft


High Pressure Injectors (50 to 200 psi)
Pressure Water Temp Lift
80 to 120 psi 74F 20 ft
120 psi 120F 3 ft
140 psi 115F 3 ft
170 psi 105F 3 ft
200 psi 85F 3 ft

Based on this, it seems like the hotwell idea is best suited for use with a pump as opposed to an injector, particularly with respect to the ability to add water at low pressures.

I would actually like to have the ability to use both a steam injector and a pump. One thought I had was to have a tank of clean make up water on board that feeds the injector if I need it. (I will be running in saltwater, so dont want to pull outside water). I would then run the pump to move condensate from my hotwell/feedwater tank thru the preheater and into the boiler.

I like the idea of not dealing with the Schedule 80 piping requirement in the preheater if I preheat before it hits the hotwell. It would also take away the question as to whether the the injector, if I use one, can handle the lift and the back pressure from the pre-heater. (I tend to think that if the water makes it to the injector there will be no issue, so lift is the primary concern)

any thoughts would be appreciated.

jon

[artemis]

A few facts:
1. The suction side of an injector or feed pump should be below the bottom of the hotwell or other source of feedwater to prevent loss of suction due to the water flashing to steam on suction.
2. Air starts to be absorbed into boiler water in any open container (like the hotwell) above 50F. It stops being absorbed around 110F. At about 130+F the "oxygen" begins to be driven off from the water. 160F is about as hot you you want the water going to the feedpump. Don't use preheated water with an injector, it should draw from the "makeup" water carried aboard. Do use the injector to introduce makeup water to the hotwell (see diagram below).
3. Oxygen (or "air') entrained in water going into the boiler will cause rust or rust carbuncles on the inside of the boiler (water) surfaces. You don't see them so you don't know they're there. They cause pitting and decrease the strength of any ferrous material in the boiler. They also decrease the heat transfer from the fire to the water in the boiler, causing overheating and loss of strength in the boiler walls. Using a feedwater treatment will help keep the pH of the feedwater alkaline ("soft").
4. Schedule 80 pipe is not required until you reach the boiler stop valve.

The article below from Steamboats and Modern Steam Launches, November-December 1963, pgs. 32 - 33, is a description of an excellent system put together by Al Giles for his Crest. The system worked well until Al sold her in the 1980s, 20+ years later.

[JonRiley56]

Hi,

How do boats that are not running a condensing system deal with the oxygen issue ? If they are pulling right thru the hull and pumping it to the boiler there is no way for the "de-oxygenate"..............not sure if that is a word............the feedwater.

In order to do it "correctly", should they be pumping to a hotwell first ?

I received my copy of SBMSL last week so I looked at the schematic you posted and the corresponding text. Am I right in assuming that the heated water is being pumped into the boiler and "deadheading" against float valve through the "B" valve. Is that correct ?

I was operating under the assumption that one would be pumping into the hotwell after start up and when the water level in the hotwell got high enough.........the float coming up..........that would indicate that the water in the boiler had dropped enough to warrant replenishment and the valve would open to push water into the boiler.

The condensate is delivered to the hotwell via the vacuum pump on the inbound end of the condenser. A separate pump is actively pumping from the hotwell thru the economizer/pre-heater and back into the hotwell. This keeps the hotwell temps up and the O2 levels down.

As condensate moves in to the hotwell the float rises until it actuates the valve the directs the flow into the boiler.

I was hopeful that if I filled the system to the appropriate level on the water glass and in the hotwell, and understood the volume of water that would have to be "consumed" by the system to lower the level in the drum to the point that replenishment is needed that I could set the float valve appropriately to "automatically" replenish.

Obvioulsy you still need to monitor the system and the unaccounted for risk is water loss due to evaporation, forcing, promiscuous whistle use etc...........

Am I way off base ?