Economizer Question

[JonRiley56]

All,

I have a question about pre-heating feedwater. I have a small inline trensmission fluid cooler that I could put on my exhaust and route my feedwater through it, but I am thinking about doing something that will look a little neater. I have seen the desings where the economozer is in the upper part of the boiler, using the hot gases headed for the chimney to preheat the feedwater.

Is it concievable to put th economizer under the ashpan of the boiler between the boiler and the floor of the boat ? It would keep it from getting cruddy from soot etc, and would have the added benefit of helping cool the space between the boier and the floor. I know Fred had posted a picture a while back of a water cooled cooling plate under his boiler, but I dont think he routes the water to the boiler from there.

jon


P.S. - Position of the economizer aside, are you concerned about the water flashing to steam in the economizer itself ?

[DetroiTug]

Hi Jon,

I'm guessing that a preheating coil in proximity of the ash pan - that side of the process would not provide much benefit. It would only absorb some radiant heat (possibly). I think you'd find, the bottom of the firebox with an ashpan doesn't get very warm with cold air coming in.

On your particular setup (large boiler vs engine), I'm not sure an economizer is necessary, unless you're seeing considerable change in boiler pressure when taking on feedwater. One good gadget to have is an infrared thermometer. It can simply be pointed at surfaces and see the temperature levels of various components along the way.

Yes, economizers above the boiler in the smokehood are a bit problematic. When not taking on feedwater, they boil dry (why they need to be made of steel). Then when water is taken on, the first bit of cold water to hit the coil immediately flashes into high pressure steam, and a small relief valve set at considerable pressure, like I have on mine will start to pop off, and it this continues until the whole thing settles down. The harder it runs, the more it pops, and I always have to announce when I take on feedwater to crew aboard that are not aware as the noise is a bit startling. Economizers are a great use of waste heat and can greatly increase the efficiencies of the plant, but do so at some loss of operational smoothness. As it is with most steam tweaks.

Where you're condensing, an exhaust feedwater heater is a better route, if it's adequate.

I tried an exhaust heater on mine and I was getting too much water out the stack. I did notice up in Wisconsin, that the economizer alone was not as effective in the colder water, I may need to plumb the exhaust heater back in selectable for cold water running.

Here is a vid of Chief running hard.



-Ron

[JonRiley56]

Great Video, You are really moving !!!

[barts]

Typically, a feedwater heater heats water going into the boiler using exhaust heat. An economizer is in the flue gas exhaust path and also heats the water going into the boiler. Economizers may need to be made of steel if the boiler is over fired for its heating surface; I've had 1/2" copper tube as a feed water heater for many years, but then I don't warm the boiler using forced draft either.

This year I have one of those infrared gizmos, and I'll make some measurements of temperatures going in and out of both the feedwater heater and economizer.

- Bart

[fredrosse]

As Bart indicates, the "Feedwater Heater" is usually picking up heat from the engine exhaust steam. A coil of 3/8 OD copper tubing, 12 feet long, inside an exhaust pipe that is about 2 inches in diameter x 18 inches long is about right, 30 turns at 1-1/2 inch pitch circle. This will heat feedwater up by about 70F to 100F, to 150F or 180F outlet temp, if you are running with atmospheric exhaust. If you run condensing with a decent exhaust vacuum the heating of feedwater will not be quite as much, (because the engine exhaust steam temperature is lower with a vacuum) but it will still provide reasonable feedwater heating. This heat exchanger will generally not produce steam on the feedwater side.

The economizer, mounted in the boiler flue gas stack, can then heat up the feedwater another hundred degrees, to about 250F-280F before entering the boiler drum. Steaming economizers work OK also, generally because they are oversized, or they are exposed to a boiler running with a very hot fire. They have to be made of tougher stuff than copper, as they get very overheated before the feedwater flow is established. The economizer should be about 4x the surface area of the feedwater heater, a coil of 3/4 OD tubing, about 24 feet long will do for your boiler. 18 turns at 5 inch pitch circle, about 24-30 inch long coil. Carbon steel or stainless is OK for this service, the carbon steel performs better, and costs much less.

Each of these devices will reduce your boiler fuel input by about 10%, for a 20% reduction total.

The best way to start an economizer on our steamboats is to open a vent ball valve that dumps the economizer outlet to the sea on before startup, then after some steam pressure is established and you are ready to get underway, start pumping thru the economizer. It will groan and spit as the water hits metal surfaces that are probably way over 500F, but it will settle down fairly quidkly (30 seconds +/-), and then you shut the overboard dump and continue with the economizer in service.

You need to have a check valve and isolation valve between the economizer and the boiler, the overboard dump valve, plus a relief valve to keep the economizer from rupturing if it is isolated and heated by the fire. If you have individual isolation valves on your feedwater pump discharges, then each feed pump would need its own relief valve. If not, then the economizer relief valve will serve to protect the piping all the way back to the pumps.

[JonRiley56]

Thanks for the feedback ! I willl start with the exhaust heater and go from there.

jon


P.S. - Fred, Keep an eye out for a spare Lee's Mills Hat in the Margaret S, I made it home without mine.

[fredrosse]

Jon, found your hat in the forward compartment. Will be sending hat plus some other stuff soon.