Well done Ranger, spot on calculations.
You are sure to be pleased to find out that the pump will be more than adequate for the task.
As you can see, you have the capacity to even get to 2000rpm with a fair bit to spare.
Now all you have to worry about is building the boiler and a burner which can provide sufficient heat output to evaporate all that water in the same time frame.
I think you are going to need a pretty powerful one to meet the challenge.
Direction of water flow in a monotube boiler.
The reason for the reversed flow of water in this type of boiler is all to do with Effective heat transfer.
In a water boiler, Heat energy from the fire/burner is transferred to the walls of the water container (TUBE, BARREL, SHELL) by means of Convection (hot gasses) and by direct Radiation. (Closest to the heat source)
This heat is then transferred to the water initially by conduction through the material constituting the walls of the water shell/tube and then by convection within the water itself.
The amount of heat transferred through the shell/tube wall is dependent upon the thermal conductivity of the shell/tube material, the available heated surface area and the temperature difference on either side of the wall.
Other lesser factors, such as the surface contact time of the receiving media (water in this case) also play a role, however, we will ignore these for the purpose of this discussion.
HEAT will always travel from a HIGH TEMP side to a LOWER TEMP SIDE
. never the other way round.
If there is no temperature difference then there will be no heat transfer
regardless of the actual temperature, and the whole thing is said to be in THERMAL EQUILIBRIUM.
The higher the temperature difference, then the higher the potential heat transfer.
OK lets take a look at what this all means for your boiler: -
Lets first look at the flow of HEAT from your burner
.
The burner is effectively located at the bottom of a vertical fire tube with a funnel (chimney) at the top to allow spent combustion gasses to escape.
Within this fire tube there is a set of smaller water tubes, arranged in a spiral running from top to bottom.
Heat from the burner, in the form of hot gasses; passes up through the spiral of water tubes by convection and in doing so transfer some of their available heat to the spiral of water tubes by direct contact (conduction) and then, NOW MUCH COOLER, pass out of the funnel (chimney).
HYPOTHETICALY: - In a good boiler the temperature of the gasses at the burner end could well be 1200deg C or more and the temperature of those leaving the funnel (chimney) could be as low as 150deg C. Meaning a high proportion of the heat generated by the burner has been absorbed/transferred to some other media
. Hopefully to the water spiral
and hence to the water
. But there are other possibilities, such as heat loss through the walls of the fire tube to the outside atmosphere, but we will ignore these for this exercise.
Assuming that most of this transfer is actually to the water spiral, then the difference in temperature of the gasses, between those at the burner and those at the funnel is a good indicator of the thermal transfer efficiency of the boiler.
Ok so now we have a situation where the gasses at the bottom of the fire tube are considerably hotter than those at the top.
If you now think about the water flow within the spiral: -
If cold water enters at the bottom it will have a high temperature difference to the gasses surrounding the spiral, and hence high potential transfer of heat can occur
. So far so good.
As the water heats up, it is also travelling upwards, towards the colder end of the spiral (remember the gasses are cooler up here) and potential transfer of heat will reduce.
Worse still
at some point (indeterminate) inside the spiral, the water will achieve a temperature equal to that of the desired steam pressure (in your case 185deg C (365deg F) @ 150psi)
.. But, in this hypothetical example, the gasses at the top are only at 150deg C
. NOW the heat will flow back out of the steam towards the colder gasses
. And the pressure of the steam will drop.
REMEMBER
HEAT WILL ALLWAYS FLOW FROM A HIGH TEMPERATURE TO A LOWER TEMPERATURE.
This will not happen if you reverse the flow of the water
.. Think about it.
By reversing the water flow, you will always maintain a high gas temperature to low water temperature differential, and hence, you will always have heat flow into the water/steam.
In a conventional boiler (one with a pressure vessel) the steam is always in contact with the water it came from, which is at the same temperature, hence the heat cannot escape back to the gasses, since the water is between the 2. Neither can the heat escape from the water back to the gasses, since the gasses will always remain the hotter of the 2.
The majority of heat loss in this type of boiler is via the upper, outside, walls of the steam drum (those in contact with the atmosphere) which will make the steam in direct contact with the wall condense back to hot water again
. But it will be immediately replaced by more steam
thus maintaining pressure. Such losses can be minimised by proper insulation.
Also, with this type of boiler, feed water is normally injected at the bottom, or sometimes at mid-water level, and steam is taken from the top of the steam drum.
I hope you can understand this Ranger
it is a complex subject and not an easy one to explain in simple terms.
;D ;D
Perhaps now would be a good time to do some research into the complexities of steam and steam generation.
Here are a couple of good links that may be of help.
http://www.spiraxsarco.com
http://www.sparknotes.com/testprep/books/sat2/physics/chapter12section1.rhtml.
Although the first one deals with full size steam, the fundamental principles and steam tables are the same for model sizes.
Keep busy.
Regards.
SandyC
