Hello, Another question; I have acquired the heat exchanger pictured. It was factory made as an oil cooler for a Mercruiser IO unit.
It is 8" long & 2" diameter with 1/4" id side fittings & 7/8" id end fittings. These measurements will fit well in my exhaust line & the fitting sizes are a match to the feed water & exhaust lines.
The unit is the same shape on the inside as outside. What would be my water lines go to a cylindrical device made of two tubes with about a 1/8" diameter difference. This can be seen poorly in the end view. There are no baffles. I have calculated the heated area of the water chamber as 25 square inches. The whole thing is made of copper & appears to be soft soldered together. I hope this description is understandable.
My question is whether this would work as a feed water preheater? If so, would there be an appreciable advantage in lagging it?
Yes, that will work as a feedwater heater. I use one on the exhaust of my steam pump and pick up about 45 degrees increased temperature of the feedwater. THis will vary on the water consumption of the boiler and the velocity of the feedwater. Fifty degree increase is better than cold water, but it should be more. Ideally the feedwater very near the boiling point is the best way to introduce it to the boiler. I use this and 50' of 3/8" economizer in the funnel and pick up another 50-60 degrees there. My feedwater runs about 180 where it enters the boiler and that seems to work pretty well.
There appears to be three different configurations of these exchangers. The one you have has an internal coil, the one I have that looks identical has two flat plates rolled up, and then another I seen looks like a firetube boiler inside. Two endplates with several parallel tubes.
On mine, I machined brass flare fittings and soldered them on so I could connect copper tubing to all four outlets. That omitted to the need to go from hose to copper.
Re: Lagging. Any part of the steam plant that is warmer than ambient water temperature and working to warm the boiler water or make steam or maintain the steam temp until it gets to the engine for expansion will benefit from thermal insulation/lagging. Beyond the exhaust feedwater heater out through the hull or to a condenser, it doesn't matter.
Depending on the temperatures involved, you may wish to preheat the water going into your hotwell from the condenser. Why? This will help remove dissolved air from your feedwater, and should reduce corrosion caused thereby. Note that this also allows one to oversize the condenser, which makes it work better in warm water or when partial fouling occurs. The water entering the boiler can and should be heated still further using a economizer in the boiler, which is typically extra tubing in the smokebox area that preheats the water prior to the boiler feed check(s).
Thank you Ron & Bart. My boat was operated for its first 10 or so years by its builder. During that time he fought with a condenser setup & finally gave up & ran on lake water. The keel condenser tubes are still in place but I feel if a man with considerable experience gave up, I should not even consider going there at present. There is a small makeup water tank that can be used if you decide to beach the boat or run into dirty water.
The boat will run primarily in a large, deep, clean, weed free lake in Northern Ontario. Most lake water in Ontario is slightly acidic.
There is an economiser in place with a check as it enters the boiler and another check as the feed water enters the economiser from the pump - is this correct?
Ron mentioned adding fittings to the preheater to go directly to copper rather than use rubber lines. What is the disadvantage of using rubber lines in the feed water system?
Re: Hose. Depending on how it is piped, the feedwater lines can potentially see boiler pressure and temperature if the check valve at the boiler inlet fails when the feedpump is bypassing.
I had that happen. It just went around the pump bypass and out the through hull pick up. Had there been a sea cock closed or a check valve beyond the pump, it would have been at boiler pressure. The result was the Perko Sea strainer got a nice backflushing. The Perko plastic cylinder body of the sea strainer would have probably exploded.
Plus, converting to flare fitting gets rid of the need for extra fittings i.e. hose to flare adapters.
"My question is whether this would work as a feed water preheater? If so, would there be an appreciable advantage in lagging it?"
In theory this would work OK for feed heating, however the small coil would be on the pressurized side of the feed pump, so the small tube would see something over boiler pressure. It appears that the fittings on the tube would be rubber with hose type clamps, usually not good enough for boiler pressure. If these small tube stubs can accept metal flared fittings, or metal compression fittings, then it could work OK. As mentioned above, if there is back leakage of hot water, then these connections could see both high pressure and temperature. On my plant all piping downstream of the pump is metal only, although the pump suctions are made from automotive coolant hose, rated for 15 PSIG/250F
There is such a surplus of available energy in the exhaust steam that lagging would not be necessary. Typically the exhaust steam enters the feed heater with, say 10% moisture, and condenses some extra steam in heating the feedwater, bringing the moisture up to perhaps 15%. There is still a large surplus of steam that needs to be condensed in the plant's condenser, or dumped to atmosphere if the plant has atmospheric type of exhaust.
fredrosse wrote:In theory this would work OK for feed heating, however the small coil would be on the pressurized side of the feed pump, so the small tube would see something over boiler pressure.
The one I have has two thin brass sheet sandwiched together and then rolled up. It has to be on the suction side of the pumps, anything over 10 psi would probably destroy it.
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The one I have has two thin brass sheet sandwiched together and then rolled up. It has to be on the suction side of the pumps, anything over 10 psi would probably destroy it.
-Ron[/quote]
Mine too is made with the two sheets rolled into a tube. I was thinking it could be on the suction side of the pumps & now it seems this can be done.
You both make good points on boiler pressure making its way into the feed water circuit. I will use copper on anything between the pumps & the boiler. Presently there is one rubber line that will need to be replaced.
Would it be prudent to have a ball valve right at the feed water penetration of the boiler in case of a failure in the feed water system? I read somewhere information on doing this & installing a safety valve in case the bypass & boiler valves were both off & the engine operated pump was pumping.
Jack Innes wrote:
You both make good points on boiler pressure making its way into the feed water circuit. I will use copper on anything between the pumps & the boiler. Presently there is one rubber line that will need to be replaced.
Would it be prudent to have a ball valve right at the feed water penetration of the boiler in case of a failure in the feed water system? I read somewhere information on doing this & installing a safety valve in case the bypass & boiler valves were both off & the engine operated pump was pumping.
Jack
Yes. All boiler connections should have a shut-off valve, which means a pressure relief valve on the feed pumps is required.
As Bart stated, all boiler connections should have a valve close to the boiler.
If the feed pump can possibly overpressure the piping with the boiler valve closed, then a relief valve is definitely required. On my system, the engine driven reciprocating pump has a relief valve, without that, something well over 1000 psi could be generated, and that is far above the pump's design pressure. However the hand pump does not have a relief valve, because I can stand on the pump handle and still not have enough pressure to damage anything.
Many pumps have limited pressure capability, such as centrifugal pumps, so if they are "dead headed" they only produce moderate pressures, and the piping can easily be designed to withstand the shutoff condition without over-pressurizing the fluid circuits. Not many small steamboats use centrifugal feed pumps, but larger steam plants do.
