Grantville Gazette-Volume XI
Page 32
Figure 6
Support Structure
Boilers are generally designed to operate in one position. Supports must maintain the boiler in the correct attitude for proper operation. Provision must be made for heat expansion and vibration. And the strength of supports must be sufficient to provide stability.
Design Parameters
Many things must be considered in the design of a boiler. The first consideration is what the use of the boiler will be. A residential heating system has very different requirements from a locomotive boiler. The footprint of the boiler (how much room it needs) and what support and supply it requires are also of import.
Next is how much power the boiler needs to supply, and how fast it needs to recover. The machinery the boiler will supply also dictates boiler design. Mobile or stationary operation must be considered. Materials and fuel are also important.
The following list contains things that should be considered for boiler design. In the sample design area we will use this list to define what we need for a specific boiler. (Note the list is not in order of importance):
1. Stationary or mobile
2. Fuel
3. Construction materials
4. Horse power to be produced
5. Steam or hot water, high or low pressure
6. Pressure needed
7. Fabrication method
8. Maintenance needs
9. Intended use
10. Safety systems
Sample Designs
For the sample designs we are going to look at three categories and an extra one just for grins. The first design will be for a residential heating boiler. The boiler needs to supply heat to a large multi-residence building and be low maintenance.
1. Stationary
2. City provided gas and electric
3. Cast iron
4. Large building so we will say 50 hp
5. Hot water, low pressure
6. 11 psi, supplied by water pump
7. Cast iron sectional
8. Very low, Water treatment, water makeup, auto fire on off control
9. Make hot water for heating
10. Low water control (2), Overheat gas shut off, pop-valves 15psi (2), water temperature burner control
This boiler is simple as it doesn't need to move, make steam, or operate machinery. Gas is run from the supply to a regulator, then to a burner below the boiler sections. Burning gas flows between the sections to the exhaust stack and transfers heat through the walls of the sections. Boiler dimensions are determined by the amount of heat each section can transfer, with the number of sections determined by the end hp needed to heat the building. According to my sources:
1 lb of gas contains about 21000 BTU/hr and 2545 BTU/hr = 1 hp.
So fifty horsepower is roughly six pounds of gas per hr. Please note that this is at 100% efficiency and I distrust the source of the thermal energy for the gas.
Cast iron boiler sections are widely different in size, but by fiat, each section is 98% efficient at heat transfer (very common for the industry) and about 2.5 by 2.5 by .3 feet in size. Each section also transfers about 2.5 hp/hr, so we will need 20 sections to do the job. Giving a dimension of 2.5 by 2.5 by 6 feet for the actual boiler, with another 1/2 foot in insulation around the sections, 8 inches for the frame holding the burners and supporting the sections, and 1.5 feet for the headers, total footprint of 3 by 8 by 5 foot (added a foot for gas regulator and controls.
Safety controls are two float valves that are attached to the sections and shut off the gas if the water level drops below the top of the section. Two pop valves that open if the pressure exceeds 15psi (note: pop valves need to have their vents piped to the ground.) And a thermostatic control that shuts off the gas if a preset water temperature is surpassed. Additionally there is a burner on/off control that keeps the water at the preset working temperature.
Maintenance on this type of system is low. Basically checking that the burners don't foul, and verifying that the water treatment chemicals, which condition water to stop mineralization, are present in the right amounts.
* * *
Next we have an industrial plant that needs steam for a number of machines. Or a boiler that can run a medium locomotive.
1. Stationary (Mobile)
2. Coal (12,000 BTU/lb)
3. Wrought iron
4. 150 hp
5. Steam, high pressure
6. 200 psi
7. Riveted, single pass, superheated, feedwater heater
8. Water treatment, water makeup, auto fueling
9. Industrial steam supply
10. Low water control (2) stationary, Gauge cocks (mobile) , popvalves, sight glass, Feedwater injector, feedwater pump, pressure gauge, and control valve.
This is a more complex boiler. It needs to provide steam on a fairly constant basis and needs replenishment of both fuel and water. First we calculate the amount of fuel to be burned per hour.
Coal gives approximately 12,000 BTU/lb, 1hp is about 2545 BTU/hr or 2.64lb of water evaporated an hr at 212°.F.
One hundred fifty horsepower is equivalent to 381,750 BTU/hr so our boiler needs to consume about thirty-two pounds of coal an hr (64 lb/hr? see note 2).
Using a grate area of 3 by 4 feet, the boiler needs to consume just less than six pounds every ten minutes. The inside of the fire box is 3 feet wide, 4 feet long and 4 feet high base to top of the crown. The firebox will extend 1 foot below the drum of the boiler, and the drum will be 4 feet in diameter. The crown sheet will be 12.5 square feet and the front sheet will be approximately the same (12.5 feet square). Each side will be approximately 12 feet square and the back of the inside wall of the firebox will be about the same. This gives the firebox an inside surface of 61 square feet, allowing that the firebox gathers 40% of the heat transferred by a boiler then the remaining 60% will be made up of the flues, and will need to be about 91 square feet.
Using coal and a flue diameter of five inches (allowing for superheaters) each flue gives 15.7 square inches of surface per inch of flue. We will use a value of 15. The flue stack of five inch flues with one inch between flues allows a maximum of six flues wide in the nest. However we will use a stack: a layer of three wide, then a layer of 4 wide below, then a layer of 5, with a layer of 4 on the bottom. This will complete our flue nest. This gives sixteen flues with a combined surface of 245 square inches of surface area per inch of flue length. The flues will run to the front sheet at a slight upward angle to promote draft. We need 91 square feet, or 13,104 square inches to make up the rest of the absorption surface. This works out to about an additional four feet seven inches of flue.
Allowing for construction needs, our boiler works out to about 8 feet in length (had to add in the smokebox on front) The sides and top of the firebox need to be staybolted, and the entire assembly insulated. All of the above is to give us our dimensions so that we can work out our rough dimensions. Plugging them in to the formulas we find out how thick we need to make the plates that the shell and rods that the stays are made from.
From my Machinery's Handbook we find that the tension strength of wrought iron is 48,000 lb/in squared and the formula for the strength of materials is:
P=A*S where P=total pounds stress, A is the area of the material, and S is the working stress in lb per square inch.
Working this out gives us a thickness of .004+ to hold 200psi of pressure. However this is for a 1 by 1 inch sample and includes NO safety factors at all. To find the wall thickness for our boiler drum we need the formula of:
t=dP/2S giving (48*200)/(2*48000)
or .1 of an inch. . . It seems absurd, and is, because there is NO safety factor calculated in.
Safety factors are calculated from four factors in reference to the material under consideration. Elastic limit compared to the ultimate strength is the first of these factors
( a). Then the stress generated by the type of load incurred
( b). Next is the way the load is being applied to t
he device
( c). And last is the type of use the device is made for
( d). This also includes the fudge factor added in for materials quality and workmanship.
The equation is:
F= a * b * c * d or by the table (pp 331) 2*1*1*2.25-3 or 4.5 to 6.
this is multiplied to the formula for thickness and so we arrive at a shell thickness of .45 to .6 of an inch. In the post-Ring of Fire environment, I would probably push the fudge factor up some and run the wrought iron shell at .75 inch. A thicker plate, while more expensive, would be easier to make and stand up to rougher handling.
Now we come to the flat sections of the firebox. Using the shear number for wrought iron (40,000), a safety factor of 10, a plate thickness of .6 inch, and a working pressure of 200psi the formula:
L=1.89*t*[(square root of) S/P]L is the length between supports, t is the plate thickness, S is the stress number for Wrought iron, and P is the working pressure.
All of this gives us an L of just over five inches between staybolts (center to center). The diameter of staybolts should be one and a half to two times the thickness of the plate making up the walls to be stayed.
Joints are those places where two plates are joined together. Early boilers had lap joints where the plates to be joined were overlapped and riveted through. By the time of my handbook, longitudinal lap joints had fallen out of favor due to numerous failures caused by fatigue of the metal. The preferred joint was the butt joint where the plates to be joined are butted together between an inner and outer plate. This joint could be double riveted, triple riveted, or even quadruple riveted, depending on the job at hand. For this boiler we will use a triple riveted butt joint with the rivets (one inch in diameter) pitched (spaced center to center) seven inches apart with each line of rivets three inches apart. This gives us six lines of rivets along the joint of the drum of the boiler. The dome, mud ring, firebox ring, firebox to firebox, firebox to drum, sheet to drum and drum to smokebox joints will all be double riveted lap joints at a pitch of three inches and the rivet lines one and one sixteenth inches apart.
From the steam collection dome we will have a dry pipe with the control valve mounted below the dome. The valve will be operated from the backhead via a "stay pipe" running from the back of the valve to the back head with a control rod in the pipe. In the smoke box (attached to the front of the drum) the dry pipe attaches to the superheater header. From the header nine superheater pipes will run back into the top twelve flues and back out the same flue (basically making a long "U" into that flue) and connect to the collecting manifold of the superheater. From this manifold the steam is piped to its point of use.
The feedwater heater will be a heat exchanger in the smoke box from which the water will be piped to the injectors and pumps.
Safety controls for stationary mode are: two low water controls that are attached to warning devices (bells, whistles), sight glasses, pressure gauge, water injector, water pump, and pop valves. In the mobile mode, gauge cocks are substituted for the low water detectors as vibration makes the detectors unreliable.
* * *
The third application will be a marine boiler. This boiler will need to power a ship, assuming a paddle wheel, driven by two 26 inch cylinders with a 3 foot stroke. The foot/lb of the cylinders is figured by area of the cylinder heads, multiplied by the psi of the boiler output to calculate the total horse power. (This is just a rough estimate and ignores the torque component of the equation.) The cylinders have an area of approximately 1062 square inches, times 150 psi, gives 159300 foot/lbs. converting this number to horse power, (divide by 550 ft/lb a second) gives 290 hp consumption. The boiler needs to provide at least this much power. In order to allow for inefficiency we will calculate the boiler at 300 hp. 300 hp is roughly equivalent to 76,500 BTU/hr. Assuming coal fire @ 12000 BTU/lb. 300hp is 763,500 BTU/hr, coal consumption works out to 63.5 lb/hr (127lb/hr? see note 1). Please note this is under ideal conditions, actual "mileage" will vary
1. mobile
2. coal (12000 BTU/lb)
3. wrought iron
4. 300 hp
5. Steam, high pressure
6. 150 psi
7. Wrought iron riveted
8. Water treatment, cleaning
9. propulsion of small steam ship
10. Gauge cocks (mobile) , popvalves, sight glass, Feedwater injector, feedwater pump, pressure gauge, and control valve.
For this boiler we will use a small Scots marine boiler. This is an internal firebox three pass boiler, that uses exhaust steam to promote draft.
The burner tube has a short grate to allow ash dump and air injection to the fire. The combustion chamber will be twelve inches in diameter and three feet long. This gives a surface of just less than 1300 square inches for the firebox equivalent. Assuming this as 40% of the transfer area, we need an additional 2000 square inches of surface area. Each tube layer in a Scots marine boiler is of lesser diameter than the layer closest to the combustion chamber. This reduction of tube size in each pass allows the heat transfer to remain even in each pass of the exhaust traveling through the boiler. The tube reduction also helps regulate the resistance to the flow of the exhaust gasses. Allowing 1000 square inches for each of the remaining passes we arrive at a mid bank of three tubes three inches in diameter and a top bank of four tubes 2.25 inches in diameter. There are no stayed surfaces in this boiler. The drum is twenty-four inches in diameter. Here we hit our friend t= (d*P)/(2*S). Where d is the inside diameter, P is the pressure, and S is the stress factor (including safety factor of 10). The equation becomes:
75.4*150 / 2*4000, or 11310 divided by 8000.
Becoming 1.414 inches of wall thickness. The end sheets should be of the same thickness to ease construction. The combustion tube is set by chart to 1/2 inch of wall thickness, and the tubes to a thickness of 1/8th inch.
The end covers have a divider that creates a chamber so as to redirect the flow of exhaust gasses in to each pass of tubes and out the stack. Steam is collected in a dome with a below-the-dome mounted control valve mounted in dry pipe.
Control of the valve is through a stay pipe from the valve through the back head of the boiler. Exhaust steam is injected in to the combustion tube to promote draft.
Safety controls: sight glasses, pressure gauge, water injector, water pump, pop-valves, and gauge cocks.
* * *
Last, how do you power a locomotive used in a factory where any fire will cause an explosion? Gunpowder, flour, chemical, refineries, and many other products have significant hazards with an open flame present.
The solution is to have a locomotive that uses no combustion to produce its work. This locomotive has a large tank that is rated for high pressure and is charged with superheated water. The superheated water will form steam in the space within the tank not filled with water. As the steam is used, the water level lowers until there is no more usable pressure in the tank, whereupon a fresh charge is put in the tank. A stationary boiler separate from the factory is used to produce this charge.
* * *
Shear values, welding standards, and construction standards are per the Machinery's Handbook, 11th ed. 1942. This handbook has been continuously updated since the first edition and is currently in its 27th edition. The most modern edition available to Grantville residents would be the 25th or 26th eds.
Notes
1) accident report
http://www.ntsb.gov/Publictn/1996/SIR9605.pdf
CFR49 part230 Form 4 and figures included
http://www.washingtonwatchdog.org/documents/cfr/title49/part230.html#230.1
and here are a few other sites of interest.
http://gesswhoto.com/rice-hill.html
http://afu.com/steam/
http://caselaw.lp.findlaw.com/cgi-bin/getcase.pl?court=us&vol=266&invol=521
http://www.catskillarchive.com/rrextra/toc.Html
2) Throughout the writing of this article I have felt ill at ease with the fuel consumption numbers I h
ave been getting. The number seemed very low in terms of fuel used per hour. As luck would have it, I have a point of reference for the numbers. When I was seventeen, I sometimes had the opportunity to "guest fire" a locomotive here in the west. The locomotive in question developed about 2295 hp and required nine hours to make the trip. During the trip it would use about four and a half tons of coal up hill, and only half a ton coming back. Running the numbers:
hp * 2545 = BTU/hr (5840775) Divided by 12000 (487lb/hr @ 100% efficiency).
Compared to, total coal used (10000 lb) divided by nine hrs (1112 lb an hr @ actual efficiency) shows an efficiency of about 44%, a number not out of line for a locomotive built in 1924 and allowing for the level part of the run. My conclusion is that the fuel use numbers should be multiplied by two, to allow for actual efficiency as opposed to Ideal efficiency.
Adventures in Transportation:
An Examination of Drags, Carts,
Wagons and Carriages
Available in the 17th century
Written by Karen Bergstralh
The intention of this piece is to acquaint the 1632 aficionados with the types of wheeled and non-wheeled land transport available for moving heavy and bulky items. Considering that the up-timers will have a variety of old horse-drawn wagons and a few buggies, I have not limited the scope to things available in the 17th century. I have not covered specialized military transports such as artillery carriages.