The Doble model E monotube boiler provided steam at 750 PSIA and 750 F. It is a counter flow design with forced draft fire on top and exhaust flue at the bottom. Water enters the lower coil and progresses upward toward the fire at the top. The top coil exposed to the fire is radiant heated. The tubing is seamless cold drawn steel 575 3/4 feet long. When coiled and assembled it is 22 inches in diameter and 13 inches high. The boiler was cold water tested to 7000 PSIA. The boiler delivered sufficient steam to maintain 75 MPH in a large car weighing 4800 pounds and could be started from cold in 30 to 45 seconds.
Doble pioneered this type of boiler and the guidelines he established are useful today. It is necessary to insure good mixing of the fuel and air with minimum excess air. The combustion chamber should insure complete combustion at high temperature. In order to take full advantage of radiant heat transfer it is important to completely surround the combustion chamber with heating surface. To insure the best convection heat transfer it is important to maintain turbulent flow velocities of both the working fluid inside the tube and the combustion gasses as they pass over the tubes.
At full load the velocity of the water should be 8 ft/sec and saturated steam to be 75 ft/sec. These velocities maintain turbulent flow with minimal scaling of the boiler. From these velocities and the steam rate we can establish the cross sectional area requirements to achieve these velocities.
A = ([specific volume] [steam rate])/[velocity]
A = ([specific volume ft^3/lb] [steam rate lb/hr])/[velocity ft/sec]
Ain^2 = ([specific volume] [steam rate])/[velocity] ft^2 [in^2/ft^2] sec/hr / [sec/hr]
Ain^2 = ([specific volume] [steam rate])/[velocity] 144/3600
The cross sectional area of a tube is related to its inside diameter id
A = pi() id^2 /4
id in = sqrt(4((([specific volume] [steam rate])/[velocity]) 144/3600)/3.1415928)
The model F and bus steam generator were developed following the above guidelines. The drawing on the left illustrates the better design utilizing radiant heat by surrounding the combustion chamber with tubing.
In the F type and later boilers great use was made of radiant heat by surrounding the fire with superheating and vaporization sections. In the chart below you can see the advantage in heat transfer per sq foot made by the newer type boiler. The pounds of steam evaporated per sq foot per hour is almost doubled with these newer boilers.
Using the above formula the tubing size of the type boiler on the left to produce 800 lb of steam per hour at 850 PSIA and 850 F can be determined. These requirements were used in the design of the F type automotive boiler. The line in the chart below was used in Doble designs of F type and later boilers to figure the heating surface requirements. Using the line we see that to get 85% efficiency in this type boiler we need about 1 sq foot for every 4 pounds of steam produced. However in the F type and later boilers we might expect to get 10 pounds of steam per sq foot of heating surface.
In these type boilers there is a sudden drop in efficiency as the output is increased. The drop in efficiency is not due to flue losses as in the liner drop in efficiency of the E type boiler. At the highest output charted of the bus boiler there is 11% loss other then the 20% flue loss. The typical non-flue loss is 2% to 4% in all other cases. There is no data to indicate the cause of this sudden increase in non-flue loss but I would venture an idea that it might be due to quenching the fire. In these boilers Doble considered 50 ft/sec adequate for turbulent flow. However he over superheated the output steam and used a normalizer to bring the superheat temperature back into range. This would mean the superheated steam was less dense then expected and might even be in a laminar flow below the point of sudden drop in efficiency. A further increase in flow would result in turbulent flow and an increased heat transfer away from the firebox liner resulting in a quenching of the flame and incomplete combustion.
One last consideration the firing rate of the Doble boilers. In these types of boiler the fuel can be burnt at a rate releasing from 600,000 BTU/ft^3 of combustion space with 28. deg baume fuel oil, up to 1,100,000 BTU/ft^3 with various grades of light fuel oil, Kerosene, and gasoline.
The efficiency of the heat transfer is variable dependent on the fuel rate. The model E F and bus boilers all exhibit a common efficiency dependence on fuel rate.
In the above chart, made from Doble test data, the relation between fuel rate and efficiency can be seen. As the fuel rate increases the efficiency of the heat transfer decreases. This is for the most part due to increased stack losses. Another way to look at the data is that with increased demand for steam the amount of heat obtained per unit mass of fuel decreases. For more information on Doble tests and conclusions check out an article by Abner Doble published in Steam Car Developments and Steam Aviation. on Karl A Petersen's site.
id in = sqrt(4((([specific volume] [steam rate])/[velocity]) 144/3600)/3.1415928)
Water Velocity should be 8 feet per second at highest output.
Steam Velocity should be 50 to 75 feet per second at highest output.
[Combustion Space in cubic feet] = [Output BTU Rate] / [Efficiency] / [BTUs liberated per cubic foot]
[BTUs liberated per cubic foot] = 600000 to 1100000 depending on fuel type.
[Output BTU Rate] = [Steam Rate lb/hr] [BTU per pound added to feed water]
[Heating Surface Area square feet] =
from: (-0.01172 [Output BTU Rate] / 1000) / ([Efficiency] - 1.018646)
to: (-0.00772 [Output BTU Rate] / 1000) / ([Efficiency] - 0.880646)