In the boiler or steam generating unit water is heated to the saturated liquid state, vaporized to the saturated Vapor State, and superheated. Below we see the relationship between volume, enthalpy and pressure, temperature of the water & steam. Both water and steam are referred to as the working fluid. Enthalpy is the heat energy content of that fluid. It is not possible to measure total enthalpy so it is measured relative to some point usually the triple point 32 F approximately. Below enthalpy is in BTU (British Thermal Units). Other heat units in common use are Calories = BTU's x 252, Joules = BTU's x 1055.1. The BTU is related to mechanical units, 1 Foot-pound = 778 BTU. In thermodynamics it is common to use H or h to represent enthalpy.
This heating process of the boiler steam generating unit takes place at a fairly constant pressure. In our ideal cycle we consider it as a constant pressure process. In the real application automated control maintains the pressure within a small range of a preset value. Below is a temperature enthalpy plot of several constant pressure lines. As heat (enthalpy) is added to water the temperature increases until it reaches the liquid saturation point. As we continue to add heat the temperature remains constant until all the water is evaporated. Vertical point on each pressure line. The difference in heat (enthalpy) between these two points is known as the heat of vaporization. It is interesting to note that the heat of vaporization decreases as the pressure increases until we reach the critical point pressure 3206.?? PSIA at which it is zero. Once all the water is evaporated the temperature continues to increase as the steam is superheated.
It is in the boiler that the heat energy to be converted to work is put into the steam. The heat Qb put in to a quantity of steam is:
Qb = Hsteam - Hwater
Hsteam is the enthalpy of the output steam and Hwater is the enthalpy of the water entering the steam generator at boiler pressure. To analyses we return to the General Energy Equation:
Q = U2 - U1 + W
Since the process may be considered as constant pressure process, where work is done by a change in volume of the feed water to output steam. Using a unit mass as our quantity Vsteam is the specific volume of the output steam and Vwater that of the feed water at boiler pressure p. Pressure p being constant. The work per unit mass may be computed as:
W = p (Vsteam - Vwater)
Letting U2 be Usteam the internal energy of the output steam and U1 be Uwater the internal energy of the feed water under boiler pressure p:
Qb = Usteam - Uwater + p (Vsteam - Vwater)
Or substituting the expressions for internal energy we get:
Qb = (Hsteam - p Vsteam) - (Hwater - p Vwater) + (p Vsteam - p Vwater)
Qb = Hsteam - Hwater
It is important to note that work done by the change in volume is included in the above expression.
In the boiler heat is transferred through Radiation from the fire and firebox liner to the tubes directly exposed to the fire, by convection from the hot burnt gasses, and by conduction within the tubes and support structure and again by convection from the tubes to the water. In the design of an automotive steam generator it is good to understand developments of the past. The best known steamer the Stanley used an upright fire tube design. Monotube steam generator units, some times called flash boilers, developed by Serpollet White Doble and others are the most practical for automotive use.
For larger drawing and further details click above.
Monotube steam generators are made up of one single continuous tube. The Water is forced into the tube end furthermost from the fire and travels toward the hotter and hotter areas of the generator. As the water travels through the tube heat is absorbed from the tube wall and it's temperature raises until it reaches the saturation temperature. At this point the water begins to boil. In this boiling area the temperature remains at the saturation temperature of the boiler pressure, almost constant, until all the water is vaporized into steam. Once all the water is vaporized the temperature continues to rise until it leaves the boiler.
It is impossible to completely control the point vaporization starts or where supper heating begins in monotube boilers. Due to the small quantity of fluid in these generators the pressure fluctuates with ever steam draw from the boiler. However it is the monotube flash type boiler that makes possible a practical steam automobile. These monotube steam generators are able to produce tremendous firing rates in a very small space. The key to designing a better monotube boiler is understanding the basic principals governing heat transfer within the boiler, the hydrostatic back pressure developed within the tube, and the proper combustion of the fuel.
The transfer of heat between the tube surface and the water is governed by the formula:
While the transfer of heat to the steam from the tube surface is
Cp is the specific heat at constant pressure of the
steam. Vs is the volumetric rate divided by cross section
area pi [ID]2/4. Tf is the average of the
wall temperature and the balk temperature of the liquid passing through the
tube. G is the mass velocity w/S, lb per hour per sq ft of cross sectional
area pi [ID]2/4. The heat transfer within the boiling area
is related to the formulas for water and steam but can not be predicted as the
boiling turbulence is to erratic.
The above formulas are not usable to predict actual performance of a boiler design but they give us some insight how tube diameter effects the heat transfer. Both formula relate heat transfer proportional to [ID]-1.8.
hid = [other terms] / [ID]1.8
The heat transfer from the combustion gases to the tube surface is also related
to tube diameter as the following formula predicts:
Only here G is a function of area of gas flow across the tube. The area that effects the heat transfer is the tube spacing. Here we see that as tube spacing is decreased the heat transfer from the hot combustion gas to the tube increases. The above formula along with the heat transfer of the tube wall
ht = ln([OD]/[ID])/(2pikL).
are used to predict the total heat transfer through a tube wall. Taking resistance as the inverse of conductance. The heat transfer network in simplest form is equivalent to the series resister network shown in the drawing to the right.
The S.E.S. boilers are the most advanced boilers to come out of the 70's eergy crisis. These boilers are capable of producing a tremendious output in a vary small space.
The Patteron boiler design incoperates some of the ideas of the S.E.S. boiler into a modular multi boiler design. My boiler is made up of several sections. Each section is an independently controled boiler. There is one common exhaust but each section has it;s own feed and fuel control. The internal circuit if modeled after the S.E.S. design. The Patterson boiler is a prefferal fired boiler having a large amount of radiant heated surface area compared to volume. It utilized small tubes to increase heat transfer while at the same time keeping them short preventing high hydralic back pressure. By using seperate boilers ganged together my boiler eliminates the problems asociated with multipath once boilers.