Energy conservation

I believe current practice in 'condenser' boilers is to use the flue gas to heat the water until the gas is much cooler that 350... of course you then need a fan to ectract the gas as it won't go up by convection.
Maybe a simple coil of pipe around the lower section of flue. This pipe could be the cold water feed to the heater.

Del

I was thinking the same before I read your post, Del. A pre-heater for the inlet water would increase the efficiency of the appliance by extracting heat from the flue gas that would otherwise just go up the stack.

The exhaust from many modern heating appliances leaves in a PVC pipe, along with the water which has been condensed from the water vapor that was produced from combustion. This stuff isn't very hot, and the last leg of the flue is usually horizontal or sloped down, to discharge the water.

Increasing the surface area for heat exchange would also help, although that would promote the accumulation of insolubles in a location with "temporary" hard water (that is, with dissolved bicarbonates of calcium and magnesium).

I remember seeing the flue of a hot water heater. It had a strip of metal in it that was twisted into a spiral. I assumed that this was to cause turbulence in the hot flue gases and increase the contact with the walls of the flue. Perhaps one could make the flue in the form of a spiral to increase the total contact with the water, or make multiple flues to increase the surface area.

A 4 inch burner with a 3 inch flue would seem to severely limit the amount of the bottom of the tank in contact with the flame, maybe make the burner larger without increasing the number of flame jets to maintain the gas consumption rate.

I just played around with some numbers. Assume that the tank is 5 ft or 60 inches tall. Instead of a single 3 inch flue use 9 one inch flues. In the case of the single large flue there will be about 565 square inches of contact with the water. With the 9 smaller flues there will be about 1,696 square inches of contact which would intuitively increase the opportunity for heat exchange between the hot flue gasses and the water. I picked the 9 one inch flues because they would have the same volume as the single three inch flue. This would seem to make sense as large boilers do not have a single flue but many, at least they did in steam locomotives.

Residential hot water heaters have an extension on the cold water inlet which releases the cold water at the bottom of the tank.

I don't know how well the flue gasses would travel through the smaller tubes and there is always the possibility that some form of voodoo math may have crept into the calculations.

This idea is the same as used in high efficiency steam engines, where it is called "an economiser".

Spencer.

<Caution>

It is important in many combustion-driven processes to keep the exhaust gas temperature above the dewpoint of sulphuric acid, around 135degC, so as to ensure the longevity of the materials comprising certain parts of the exhuast gas venting equipment. Economisers cannot simply be added to combustion processes without limit without considering the materials of construction of all parts. Brick chimneys are particularly prone to increased corrosion-based maintenance demands on this account, for example.

</Caution>

In some applications, they burn the natural gas (adding O2) directly in the water. This gets as close as possible to 100% energy recovery but you need a special system and cannot consume the water. It works well in some industrial systems where the produced water can be controlled properly. Nothing is perfect.

here is what the insides of a home high eff heater looks like. You'd be better off just buying this and getting it over with.

http://www.americanwaterheater.com/products/pdf/lpg100.pdf

Heat exchange rate is affected by 3 T's, time, temperature, turbulance.

Time: Increase time the hot gas and cold water are in contact; slow down flue gas passing through heater; curved flue gas tube through water; multiple 2 or 3 inch tubes in water to inrease time and surface area of heat exchange.

Turbulance: Small circulator in water to increase water turbulance; flue tube insert to increase flue gas turbulaance; multiple small tubes to increas flue gas turbulance.

Temperature: Increase the temperature difference between hot and cold surfaces.

Is this a residential application, with long periods of no activity with short periods of maximum usage? Or an steady state application that has a change of reasonable payback?

Also, in quest for efficiency, be aware of unwanted side effects such as the corrosive condensate mentioned earlier in the post.

Time is not a variable in heat exchange. The formula is Q=UAdeltaTlnm Heat transfered = Heat transfer resistance times the are times the log mean average temperature between in and out.

If you want the temperature difference real small, then you will need either more surface area of exchange or higher heat transfer rate. To increase area, you need more fire tubes. To increase transfer, you can make one side of the heater have more turbulance to assure more forced convection, but you reach a point that the forced convection can not get any higher. You could make the water side more turbulant by haveing a pump pump water around in the tank or add a mixer to the tank.

So, once you add a "turbulator" which most heaters have, you are done except for adding a pump or mixer.

There a number of things that are done on a large scale on steam boilers that could be adapted here. Economiser: run the flue gas through a water and air pre heater this could be a coiled tube around the flue as del said earlier. In some boilers they use rifled tube and finned tubes to make more contact with the heat to the water. Adding baffles to slow down the flue gas or a second pass flue tube would do this. At some point though you may need a ID fan and that take away more than it gives as far as efficiency goes. the balance of efficiency is critical on large boilers. I read once that Babcox and Wilcox never did get the whole boiler thing to working to the point where it was feasible till they invented the economizer and air pre heater to capture more heat from the flu gas.

pipewelder

Hi Wahid. I work with an insulating coating. We have coated boilers and steampipes and had excellent results. On a 267 degree F metal surface, a credit card thick application of our material dropped the surface temp to 155 degrees F. That was a 35% reduction in heat loss for thqat one.

We have other similar applications elsewhere that have resulted in 25 to 38% reductions in heat loss.

Should work for what it soumds like your project is.

Hal Skinner ceramatech_engineering@yahoo.com

www.ct-texas.com