Boiler Basics: Waste Oil Generator System

November 2003

November -- we're beginning to see a bit of serious snow on the ground this month. It looks like it will be a good year for snowmobiling and dog sled races. 

Last year rumors were circulating that the famous Iditarod dog sled race held annually in Alaska might be moved to our neck of the woods -- due to global warming along the Pacific Coast. For several years in a row the race had to be postponed until late in the season due to unusually warm conditions. The race, which is normally held in late January to early February, was postponed until March for the last 3-4 years.

And while much of the Western U.S. is experiencing prolonged drought, and is draining the world's largest underground aquaifer dry, we continue to happily receive our annual fair share of snow & rain to keep the Great Lakes full.

It doesn't matter what the idiots of the world say about global warming -- it is real, and it is here to stay until greedy mankind puts a damper on emissions.

With that said, let's begin this month's project.

So far we have covered most of the basics for assembling turbines, so now we have to take a look at powering up the turbine with some sort of energetic gas. Since our focus this year is on waste oil to electricity, the obvious gas is going to be steam.

When considering boiler types, we can choose between either a chest or tube boiler. 

Chest Boilers

A typical chest or tank boiler consists of a tank of water heated to produce steam pressure in the operating range of the turbine. Since we ultimately need between 80-150 psi of steam, a tank boiler could prove to be extremely dangerous if it let go. So using a bit of wisdom, we'll forego the tank route and concentrate on the flash tube boiler system.

Flash Tube Boilers

Even though we engineer in safety valves and a reasonable amount of extra material, there is always the chance of something going wrong, and even a tube boiler will blow. Fortunately, the quantity of steam in a flash boiler is low and, aside from a split tube, the dangers are drastically minimized. With sufficiently heavy sheathing around the boiler tubes, there is little chance of a ruptured line doing any damage outside of the turbine enclosure.

Referring to Figure 1 we see a typical flash boiler system which is pretty much self-explanatory.

Figure 1 - typical flash tube boiler system

Boiler Operation

A heat source of around 80,000 Btu's enters a 4-inch to 6-inch casing which encloses a continuous loop of 0.125 inch to 0.375 inch (stainless) steel tubing.

Stainless is the preferred tubing type, but if cost is a problem, automotive steel brake line can be used -- although there is a tendency to rust. Copper is not a good choice due to the high heat involved.

The tubing can be shaped in a simple spiral, but the preferred method is to run the tubing length-wise with tight bends at the ends. Automotive tubing benders are low cost and work well in this application. A larger number of capillary tubes is more efficient than fewer large diameter tubes. Efficiencies as high as 90 percent are now achievable in modern tube boilers.

Beyond the tube boiler section you will need a stack leading to the outside -- remember, carbon monoxide kills!

O.K. Now for the action end of things. Since this is a closed loop system, we show a continuous chain of events from the outlet port of the water pump, back to its inlet. 

The water pump must be capable of overcoming the back pressure from the boiler, so a unit that can produce anywhere from 200-500 psi is sufficient. The volume is very small -- only 38 lbs./hp/hour of water. If water is around 12 pounds per gallon, this works out to be about 7 gallons per hour for a 2 horsepower engine.

Just past the water pump we need a check valve to control high pressure flares in the boiler. As steam exits the boiler, we need to monitor its pressure in order to control heat and water flow.

Along with the pressure sensor, we need to monitor the heat values of both the incoming and outgoing combustion gas. This allows us to control the steam generation efficiency of the system, and to signal water feed problems.

Beyond the steam pressure valve we see the necessary over-pressure safety valve (OPV) that opens if pressures rise beyond the prescribed limits. In our system it should open between 150-180 psi.

Just past the OPV we see our turbine, followed by a steam-to-water condenser. The condenser can be something as simple as a car radiator, or a simple tube-in-tube water flow condenser like those used on boat air conditioner systems. 

A more elaborate condenser, but definitely the smallest, most efficient design, is the jet condenser. If return steam pressure is sufficiently high, the steam is used in a jet nozzle to induce water from a reservoir to mix with the steam to transfer heat to the water. If steam pressure is low, water must be sprayed into the steam through pressurized jets. A small heat exchanger is then used to extract heat from the water.

From the condenser, we simply feed back to the water pump inlet.

So that completes this month's boiler basics section. Next month we'll cover the entire system, including the waste oil burner, to complete the goal we set for 2003. Until then, work on your own experiments and share the results with us and the rest of the world.

Ken Rieli