For the 3/8” tubing we have 6 helical coils from 3 1/8” diameter center line to 8 7/8” diameter center line with 24 turns in a coil for 230 lineal feet of tubing and 23 square feet of surface area.

For ½” tubing we have 5 helical coils from 10” diameter center line to 15” diameter center line with 21 turns per coil and with a water wall 16” in diameter close touching for 32 turns.   This gives a total of 477 lineal feet of tubing and 62 square feet.  These ½” coils are spaced closer together than the smaller tubing because of the larger diameter of the helical coils and thus there is plenty of room for gas flow between the tubes.

Total heat exchange surface is 85 square feet.  These figures are for all sides of the water wall coil and so there will be effectively a little less square footage than that. 

When tested we were getting 300 degrees F stack temperature while getting combustion gas condensation on the inner coils.  We were boiling a little over 1 ½ gallons per minute of cold water with an exhaust steam temperature fluctuating from 300 to 500 degrees F depending on the adjustment of water flow from our variable speed water pump.  There was an 800 psi water pressure going into the coil stack when it was making maximum steam.  We tested water flow without any heat and the back pressure was just under 100 psi, which would be the skin friction of water flow.  Therefore steam flow was creating a lot of skin friction.  At one million btu we expect to boil 2 gallons a minute.

We purchased the stainless tubing in 600 foot rolls so the only welding was at the junction of the 3/8” and ½” tubing.  As we all know tubing is measured from the outside diameter and one has to subtract the .035 or whatever it is wall thickness twice to get the inside diameter.

The superheater was buried one layer in from the combustion gas annular passage.

We plan on welding a lot of fins to this center separator drum, which will be in the 3” diameter range, to give a nice swirl to the gases assisting in the even distribution of the gases to the inside of the helical coils.  The fins will be vertical at the bottom—the end where the fire is coming from—and each of the fin stacks as we go up will be slightly angled to that at the top they are completely horizontal.  The fins will secondarily transfer a great deal of heat to this central water/steam separator drum which will be used as a water level controller for the boiler, thus boiling out the water as fast as possible. 

Memo on the one million btu steam generator for sterilization work.

Tubing: 316 stainless steel
.035 wall thickness

Helical coils, measured to the center line for diameter

3/8” diameter tubing

1          3 1/8” 24 turns
2          4 3/8”  24 turns
3          5 ½”    24 turns
4          6 ¾”    24 turns
5          7 ¾”    24 turns
6          8 7/8”  24 turns

½” diameter tubing

7          10”      21 turns
8          11 ¼”  21 turns
9          12 ½”  21 turns
10        13 ¾”  20 turns
11        15”      19 turns

Water wall ½” tubing

12        16”      32 turns

This is a steam generator that we made in the shop this winter, the cold one of 2014.  It was designed to fit into a cylindrical volume that someone else had designed and made already that was located on top of a pellet burner of nominal one million btu’s.  For many reasons it would have been better, maybe only slightly better, if the combustion gases entered the coil stack from an 8” tube in the center and flowed out radially.  The combustion gases entry openings from the combustion chamber were already cast in ceramic and so we designed around that prototype.  Not too much difference was made, except that much more insulation is needed because this thing is hot on the outside.

Here we made the steam generator out of helical coils.  The first reason is because we had the room to bring a plenum up around the outside of the coil stack, albeit with a water wall on the outside to cool things down a bit, and to have gas flow from the outside in.  The second reason is because we had purchased stainless steel tubing in one thousand foot rolls and our coil winding technique meant that we did not have to do any welding between coils.  We just went up and back down and up again making only one weld where we went from 3/8 to ½” tubing.

When winding pancake coils we have only figured out how to make a double pancake coil and to do that a 12’ pigtail sticks out while winding and the coil winder people have to be agile enough to jump over this every time it comes around.  At least when we do a double pancake there is no welding on the inside of the coils but only at the periphery.  Also with pancake coils we would want the combustion gases coming from the top down so that the water can be pumped from the bottom up for counter-flow of cold to hot and hot to cold.

As you might imagine any burner can be designed with duct work and all so that the hot combustion products can be moved around anywhere ending up with flow from the top to the bottom.  The pellet burner people had concentrated on making a good burner and had not asked me for input into the ultimate steam generator design.  Thus we had to work with what had already been made.

Anyhow, we can wind helical coils with either a large or small central opening.  We are just making wild guesses as to gas flows.  When we make one of these things that works good we will hire the engineer to measure the gap between the tubes for each of the 11 helical coils.  Then the engineer will calculate how much the combustion gases will shrink, although there should be a better technical term for what happens as a gas cools.  Very roughly, air, and by extrapolation combustion gases, double in volume for each 500 degrees F of temperature.  We are starting out with 3,000 degree F combustion products and cooling down to 300 degrees F or so and thus a person can figure that there will be a considerable difference in how large the flow area needs to be for the gases to go through the tubes.

Our coil winding technique is such that we can precisely control the spacing between the tubing in all dimensions.  What we cannot control is the staggering of the tubing.  Thus sometimes a tube is lined up directly over a gap in the upwind coil and sometimes the gases have a clear shot through.  However, these coil stacks are designed for radial inflow or radial outflow.  This means that the hot gases enter the heat exchanger tangentially and flat thin vanes can be spaced anywhere between the coils to encourage this partially radial and partially circumferential flow, thus making for excellent heat exchange.  It all depends on how powerful the combustion fan is.

The choice of stainless steel for the tubing material was very casually made for this prototype.  The nice feature is that it can be over heated to red hot when one has difficulty pumping water into the coils.  At one time or the other everyone working in steam generation has trouble pumping water.  Either the water tank runs dry, the pump loses its prime, someone forgot to open a valve, someone forgot to close a bypass valve, or a wire shorts out; the possibilities are endless.  The downside of stainless is that one can never run city water through it.  Any little chlorine at all will cause pin hole leaks in the tubing.  I have two stainless coil stacks here in the shop inherited from other steam people.  When pressure tested to 3000 psi they look like the Fountains of Vesuvius, aesthetically pleasing but not practical.

We are confident that this steam generator will take 1,000,000 btu/hour of hot gases and boil 2 gallons per minute of water at about 90% heat exchange efficiency.  With controls the steam conditions can vary from saturated to 1000 psi and 1000 degrees F.   

We are not particular about how the steam is used; what we are doing is turning hot combustion gases into hot steam and efficiently and safely.  There is nothing to blow up and no possibility of shrapnel flying around with the monotube steam generator.  The four uses that come to mind are: a) making hot water for in floor building heating, b) making steam for sterilizing large areas, c) making steam to be used to heat air for either building heat of grain drying, d) making steam for power generation in a steam engine.  If the latter use is desired, then this unit will produce something in the 50-75 horsepower range with a steam engine of modern high efficiency design.