Energy is released when air is mixed with fuel and ignited in the combustor. A common mistake is the belief that combustion increases the pressure of the gasses flowing through a turbine. In fact the heat-addition stage of a gas turbine cycle incurs a slight pressure drop to facilitate flow through the engine. For all intents and purposes, however, the combustion process can be considered as occuring at constant pressure, with an increasing volume to accommodate the temperature rise, as explained by the ideal gas law. This in turn results in an increase in the velocity of the gas flow (see gas laws). This is directed over the turbine’s blades, spinning the turbine and powering the compressor, and finally is passed through a nozzle, generating additional thrust by accelerating the hot exhaust gases by expansion back to atmospheric pressure.

Energy is extracted in the form of shaft power, compressed air and thrust, in any combination, and used to power aircraft, trains, ships, generators, and even tanks.

Theory of operation

Gas turbines are described thermodynamically by the Brayton cycle, in which air is compressed isentropically, combustion occurs at constant pressure, and expansion over the turbine occurs isentropically back to the starting pressure.

In practice, friction and turbulence cause:

a) non-isentropic compression - for a given overall pressure ratio, the compressor delivery temperature is higher than ideal.

b) non-isentropic expansion - although the turbine temperature drop necessary to drive the compressor is unaffected, the associated pressure ratio is greater, which decreases the expansion available to provide useful work.

c) pressure losses in the air intake, combustor and exhaust - reduces the expansion available to provide useful work.

As with all cyclic heat engines, higher combustion temperature means greater efficiency. The limiting factor is the ability of the steel, ceramic, or other materials that make up the engine to withstand heat and pressure. Considerable engineering goes into keeping the turbine parts cool. Most turbines also try to recover exhaust heat, which otherwise is wasted energy. Recuperators are heat exchangers that pass exhaust heat to the compressed air, prior to combustion. Combined cycle designs pass waste heat to steam turbine systems. And combined heat and power (co-generation) uses waste heat for hot water production.

Mechanically, gas turbines can be considerably less complex than internal combustion piston engines. Simple turbines might have one moving part: the shaft/compressor/turbine/alternator-rotor assembly (see image above), not counting the fuel system.

More sophisticated turbines (such as those found in modern jet engines) may have multiple shafts (spools), hundreds of turbine blades, movable stator blades, and a vast system of complex piping, combustors and heat exchangers.

As a general rule, the smaller the engine the higher the rotation rate of the shaft(s) needs to be to maintain tip speed. It turns out that tip speed determines the maximum pressure that can be gained in the turbine independent of size of the engine. jet engines operate around 10,000 rpm and micro turbines around 100,000 rpm.

Thrust bearings and journal bearings are a critical part of design. Traditionally, they have been hydrodynamic oil bearings, or oil-cooled ball bearings. This is giving way to foil bearings, which have become common place in micro turbines and APU’s (auxiliary power units.)

A cutaway example of an early jet engine showing the centrifugal compressor

Centrifugal compressors are often used in some small gas turbine engines, mainly because, at small size, all-axial units are less efficient than the equivalent CF compressor. Also because the rear axial stages become too small to be robust. Although some centrifugal compressors are capable of over 10:1 pressure ratio at a reasonable efficiency, temperature and stress considerations severely limit the pressure ratio that a CF unit can develop within the compression system of modern, high overall pressure ratio, gas turbine. Turboshaft engines often use an axial-CF or double centrifugal compressor unit to achieve a high overall pressure ratio.

Advantages of Centrifugal Compressors

Centrifugal flow compressors offer several advantages including simplicity of manufacture, relatively low cost, low weight, low starting power requirements, and operating efficiency over a wide range of rotational speeds. In addition, a centrifugal flow compressor’s short length and spoke-like design allow it to accelerate air rapidly and immediately deliver it to the diffuser in a short distance. Tip speeds of centrifugal compressors may reach Mach 1.3, but the pressure within the compressor casing prevents airflow separation and provides a high transfer of energy into the airflow. Although most centrifugal compressors are limited to two stages, the high pressure rise per stage allows modern centrifugal compressors to obtain compressor ratios of 15:1.

Operating Limits

Centrifugal compressors have the following operating limits:

* Minimum Operating Speed - the minimum speed for sustentation, below this value the compressor stops or goes to the called “Idle Speed”.
* Maximum Allowable Speed - the maximum design speed for the compressor, beyond this value the vibrations increase rapidly, becoming dangerous for the equipment.
* Stonewall or Choke - occurs when the velocity of the gas approaches its sonic speed somewhere in the compressor (it may occur at the impeller inlet or at the vaned diffuser inlet). It is generally not detrimental to the compressor.
* Surge - normally occurs at about 50% of design inlet capacity at design speed, is the point at which the impeller cannot add enough power to overcome the discharge pressure. This causes flow reversal (surge), high vibration, temperature increases, and rapid changes in axial thrust that can damage the labyrinth seals or even the driver.

Large centrifugal compressors are used for gas transportation in gas pipelines all around the world.

Description

Axial compressors are essentially a steam turbine reversed; instead of high-pressure gas flowing into the turbine and forcing it to rotate to provide power, in the compressor role, power is provided from an external source in order to spin the system and compress the gas.

Diagram of an axial flow compressor

A typical axial compressor has a rotor which looks like a fan with contoured blades followed by a stationary set of blades, called a stator. As the diagram illustrates, compressor blades/vanes are relatively flat in section. Turbine blades/vanes, on the other hand, have significant curvature. Each pair of rotors and stators is referred to as a stage, and most axial compressors have a number of such stages placed in a row along a common power shaft in the center. The stator blades are required in order to ensure reasonable efficiency; without them the gas would rotate with the rotor blades resulting in a large drop in efficiency. Improvements can be made by replacing the stators with a second set of fans rotating in the opposite direction, but these designs have generally proven to be too complex to be worthwhile.

Each stage is smaller than the last, as the volume of air is reduced by the compression of the preceding stage. Axial compressors therefore generally have a conical shape, widest at the inlet. Compressors typically have between 9 and 15 stages.

In a jet engine the compressor is powered by a turbine placed in the hot exhaust, using up some of its energy. In such a system axial compressors typically use between 60% and 65% of the engine’s power in order to run. This explains why jet engines are not used in cars; even when the car is standing still on idle, a turbine engine will still have to run close to full power, thus drastically reducing efficiency. In aircraft this is not an issue, since it is never on “idle” and its engine is always running close to full power for an entire trip.

Every thermodynamic system exists in a particular state. A thermodynamic cycle occurs when a system is taken through a series of different states, and finally returned to its initial state. In the process of going through this cycle, the system may perform work on its surroundings, thereby acting as a heat engine.

A heat engine acts by transferring energy from a warm region to a cool region of space and, in the process, converting some of that energy to mechanical work. The cycle may also be reversed. The system may be worked upon by an external force, and in the process, it can transfer thermal energy from a cooler system to a warmer one, thereby acting as a refrigerator rather than a heat engine.

The Carnot cycle is a special type of thermodynamic cycle. It is special because it is the most efficient cycle possible for converting a given amount of thermal energy into work or, conversely, for using a given amount of work for refrigeration purposes.

A Carnot cycle acting as a heat engine, illustrated on a temperature-entropy diagram. The cycle takes place between a hot reservoir at temperature T_H and a cold reservoir at temperature T_C. The vertical axis is temperature, the horizontal axis is entropy.

The Process
The Carnot cycle when acting as a heat engine consists of the following steps:

1. Reversible isothermal expansion of the gas at the “hot” temperature, TH (isothermal heat addition). During this step (A to B on diagram) the expanding gas causes the piston to do work on the surroundings. The gas expansion is propelled by absorption of heat from the high temperature reservoir.
2. Reversible adiabatic expansion of the gas. For this step (B to C on diagram) we assume the piston and cylinder are thermally insulated, so that no heat is gained or lost. The gas continues to expand, doing work on the surroundings. The gas expansion causes it to cool to the “cold” temperature, TC.
3. Reversible isothermal compression of the gas at the “cold” temperature, TC. (isothermal heat rejection) (C to D on diagram) Now the surroundings do work on the gas, causing heat to flow out of the gas to the low temperature reservoir.
4. Reversible adiabatic compression of the gas. (D to A on diagram) Once again we assume the piston and cylinder are thermally insulated. During this step, the surroundings do work on the gas, compressing it and causing the temperature to rise to TH. At this point the gas is in the same state as at the start of step 1.

Properties and Significance

A generalized thermodynamic cycle taking place between a hot reservoir at temperature T_HC. By the Second Law of Thermodynamics, the cycle cannot extend outside the temperature band from T_C to T_H. The area in red ΔQ_C is the amount of energy exchanged between the system and the cold reservoir. The area in white Δ W is the amount of work energy exchanged by the system with its surroundings. The amount of heat exchanged with the hot reservoir is the sum of the two. If the system is behaving as an engine, the process moves clockwise around the loop, and moves counter-clockwise if it is behaving as a refrigerator and a cold reservoir at temperature T
The Temperature Entropy Diagram

The behavior of a Carnot engine or refrigerator is best understood by using a temperature-entropy (TE) diagram, in which the thermodynamic state is specified by a point on a graph with entropy (S) as the horizontal axis and temperature (T) as the vertical axis. For a simple system with a fixed number of particles, any point on the graph will represent a particular state of the system. A thermodynamic process will consist of a curve connecting an initial state (A) and a final state (B). The area under the curve will be:

which is the amount of thermal energy transferred in the process. If the process moves to greater entropy, the area under the curve will be the amount of heat absorbed by the system in that process. If the process moves towards lesser entropy, it will be the amount of heat removed. For any cyclic process, there will be an upper portion of the cycle and a lower portion. For a clockwise cycle, the area under the upper portion will be the thermal energy absorbed during the cycle, while the area under the lower portion will be the thermal energy removed during the cycle. The area inside the cycle will then be the difference between the two, but since the internal energy of the system must have returned to its initial value, this difference must be the amount of work done by the system over the cycle. Mathematically, for a reversible process we may write the amount of work done over a cyclic process as:

Since dU is an exact differential, its integral over any closed loop is zero and it follows that the area inside the loop on a T-S diagram is equal to the total work performed if the loop is traversed in a clockwise direction, and is equal to the total work done on the system as the loop is traversed in a counterclockwise direction.

The Carnot Cycle

A Carnot cycle taking place between a hot reservoir at temperature T_H and a cold reservoir at temperature T_C

Evaluation of the above integral is particularly simple for the Carnot cycle. The amount of energy transferred as work is

The total amount of thermal energy transferred between the hot reservoir and the system will be

and the total amount of thermal energy transferred between the system and the cold reservoir will be

The efficiency η is defined to be:

where

ΔW is the work done by the system (energy exiting the system as work),
ΔQH is the heat put into the system (heat energy entering the system),
TC is the absolute temperature of the cold reservoir, and
TH is the temperature of the hot reservoir.

This efficiency makes sense for a heat engine, since it is the fraction of the heat energy extracted from the hot reservoir and converted to mechanical work. It also makes sense for a refrigeration cycle, since it is the ratio of energy input to the refrigerator divided by the amount of energy extracted from the hot reservoir.

Carnot’s theorem

It can be seen from the above diagram, that for any cycle operating between temperatures TH and TC, none can exceed the efficiency of a Carnot cycle.

Carnot’s theorem is a formal statement of this fact: No engine operating between two heat reservoirs can be more efficient than a Carnot engine operating between the same reservoirs. Thus, Equation 3 gives the maximum efficiency possible for any engine using the corresponding temperatures. A corollary to Carnot’s theorem states that: All reversible engines operating between the same heat reservoirs are equally efficient.

In other words, maximum efficiency is achieved if and only if no new entropy is created in the cycle. Otherwise, since entropy is a state function, the required dumping of heat into the environment to dispose of excess entropy leads to a reduction in efficiency. So Equation 3 gives the efficiency of any reversible heat engine.

Efficiency of real heat engines

Carnot realised that in reality it is not possible to build a thermodynamically reversible engine, so real heat engines are less efficient than indicated by Equation 3. Nevertheless, Equation 3 is extremely useful for determining the maximum efficiency that could ever be expected for a given set of thermal reservoirs.

Although Carnot’s cycle is an idealisation, the expression of Carnot efficiency is still useful. Consider the average temperatures,

at which heat is input and output, respectively. Replace T_H and T_C in Equation (3) by <T_H> and <T_C> respectively.

For the Carnot cycle, or its equivalent, <T_H> is the highest temperature available and <T_C> the lowest. For other less efficient cycles, <T_H> will be lower than T_H , and <T_C> will be higher than T_C. This can help illustrate, for example, why a reheater or a regenerator can improve thermal efficiency.

Processes of Rankine Cycle

There are four processes in the Rankine cycle, each changing the state of the working fluid. These states are identified by number in the diagram above.

* Process 4-1: First, the working fluid is pumped (ideally isentropically) from low to high pressure by a pump. Pumping requires a power input (for example mechanical or electrical).
* Process 1-2: The high pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become a saturated vapor. Common heat sources for power plant systems are coal, natural gas, or nuclear power.
* Process 2-3: The superheated vapor expands through a turbine to generate power output. Ideally, this expansion is isentropic. This decreases the temperature and pressure of the vapor.
* Process 3-4: The vapor then enters a condenser where it is cooled to become a saturated liquid. This liquid then re-enters the pump and the cycle repeats.

The exposed Rankine cycle can also present vapor overheating, which reduces the amount of liquid condensed after the expansion in the turbine.

Description

Rankine cycles describe the operation of steam heat engines commonly found in power generation plants. In such vapour power plants, power is generated by alternately vaporizing and condensing a working fluid (in many cases water, although refrigerants such as ammonia may also be used).

The working fluid in a Rankine cycle follows a closed loop and is re-used constantly. Water vapour seen billowing from power plants is evaporating cooling water, not working fluid. (NB: steam is invisible until it comes in contact with cool, saturated air, at which point it condenses and forms the white billowy clouds seen leaving cooling towers).

Variables

heat input rate (energy per unit time)
mass flow rate (mass per unit time)
mechanical power used by or provided to the system (energy per unit time)
η thermodynamic efficiency of the process (power used for turbine per heat input, unitless)
h₁,h₂,h₃,h₄ these are the “specific enthalpies” at indicated points on the T-S diagram

Equations

Each of the first four equations are easily derived from the energy and mass balance for a control volume. The fifth equation defines the thermodynamic efficiency of the cycle as the ratio of net power output to heat input.

Real Rankine cycle (non-ideal)

In a real Rankine cycle, the compression by the pump and the expansion in the turbine are not isentropic. In other words, these processes are non-reversible and entropy is increased during the two processes (indicated in the figure as ΔS). This somewhat increases the power required by the pump and decreases the power generated by the turbine. It also makes calculations more involved and difficult.

Variations of the basic Rankine cycle

Two main variations of the basic Rankine cycle are used in modern practice.

Rankine cycle with reheat

In this variation, two turbines work in series. The first accepts vapor from the boiler at high pressure. After the vapor has passed through the first turbine, it re-enters the boiler and is reheated before passing through a second, lower pressure turbine. Among other advantages, this prevents the vapor from condensing during its expansion which can seriously damage the turbine blades.

Regenerative Rankine cycle

The regenerative Rankine cycle is so named because after emerging from the condenser (possibly as a subcooled liquid) the working fluid is heated by steam tapped from the hot portion of the cycle. This increases the average temperature of heat addition which in turn increases the thermodynamic efficiency of the cycle.

Purpose

The purpose is to condense the outlet (or exhaust) steam from steam turbine to obtain maximum efficiency and also to get the condensed steam in the form of pure water, otherwise known as condensate, (condensate-not to be mistaken with usage of the word condensate in Natural gas condensate in petroleum industry), back to steam generator or (boiler) as boiler feed water.

Why is it required?

The steam turbine itself is a device to convert the heat in steam to mechanical power. The difference between the heat of steam per unit weight at the inlet to turbine and the heat of steam per unit weight at the outlet to turbine represents the heat given out (or heat drop) in the steam turbine which is converted to mechanical power. The heat drop per unit weight of steam is also measured by the word enthalpy drop. Therefore the more the conversion of heat per pound (or Kg) of steam to mechanical power in the turbine, the better is its performance or otherwise known as efficiency. By condensing the exhaust steam of turbine, the exhaust pressure is brought down below atmospheric pressure from above atmospheric pressure, increasing the steam pressure drop between inlet and exhaust of steam turbine. This further reduction in exhaust pressure gives out more heat per unit weight of steam input to the steam turbine, for conversion to mechanical power. Most of the heat liberated due to condensing, i.e., latent heat of steam, is carried away by the cooling medium. (water inside tubes in a surface condenser, or droplets in a spray condenser (Heller system) or air around tubes in an air-cooled condenser).

Construction

The adjacent drawing shows a typical construction of a condenser of about 200 MW capacity and the same is described here. The description is for a two part and one pass condenser. There are variations in fabricating designs depending on the manufacturers, the size of the steam turbine unit, and also some requirements dictated by the site conditions.

ShellThe shell is the outer most body of the condenser providing arrangement for installation of tubes. The shell is fabricated from fairly thick carbon steel plates. Due to its large size the shell is sufficiently strengthened or stiffened internally with carbon steel plates to give sufficient rigidity for the shell proper. The shell also gives support to number of intermediate support plates for the long tubes, depending on the size of the condenser. These intermediate tube support plates also help to avoid the sagging of long length of tubes. These support plates have sufficient number of holes of suitable diameter drilled with the help of a jig in a suitable drilling machine to facilitate the easy threading of each and every tube during installation or during replacements. At the same time the intermediate tube support plates allow for the free movements of tubes in all directions particularly lengthwise due to expansion and contraction occurring during operation.The shell is connected to the outlet (exhaust) of the steam turbine by means of an expansion joint made generally of stainless_steel, flanged at both ends. The whole condenser is supported on heavy springs, mounted on steel sole plates at suitable places on the concrete foundation, normally with a slight inclination towards the outlet water box to assist complete water box drainage.

At the bottom of the shell where the condensate is allowed to collect, a sump is provided. This sump is common to both the halves but separated by a partition wall in the middle up to the height of the bottom row of tubes. This is to facilitate the measurement of conductivity of condensate on both sides independently. This is to detect contamination of condensate and from which half side it is.

On each side of the sump pipe connection with a flange is provided for connection to external pump for continuous removal of condensed water during normal operation. This small pipe is also provided with an expansion joint on the sump side to avoid the condenser movement coming on the rigidly mounted pumps.

The inside of shell and outside the tubes as a whole remains under vacuum under normal operating conditions. Inside the tubes the cooling or circulating water passes through.

Air zone

Inside the shell, a central or side portion longitudinally is separated by an outer shield except at the bottom. This partition is called the Air zone. This air zone is found in all the condensers irrespective of the manufacturers or the size. All the gases released in the condenser due to cooling are taken out via these air zone tubes.

From a suitable portion of this air zone inside the shell an air vent pipe is taken out and brought out of the shell for connection to an air extraction device.

Tube sheets

At each end of the shell, tube sheet of sufficient thickness generally made of muntz_Metal is provided, with holes for the tubes to be inserted and rolled. However at the inlet end each tube is also bellmouthed for streamline entry of water. This is to avoid eddies at the inlet of each tube giving rise to erosion. Some makers also recommend plastic inserts at the entry of tubes to avoid eddies eroding the inlet end. In smaller units some manufacturers use ferrules to seal the tube ends instead of rolling. To take care of length wise expansion of tubes some designs have expansion joint between the shell and the tube sheet allowing the latter to move longitudinally. In smaller units some sag is given to the tubes to take care of tube expansion with both end water boxes fixed rigidly to the shell.

Water boxes

The tube sheet at each end with tube ends rolled, for each half condenser is enclosed in a fabricated box known as water box, with flanged connection to the tube sheet. The water box cover is provided with minimum of two man holes on hinged covers.

These water boxes on inlet side will also have big size flanged connections for cooling water inlet at lower level for butterfly valves, small vent pipe with hand valve for air venting at higher level, and hand operated drain valve at bottom to drain the water box for maintenance. Similarly on the outlet water box the cooling water connection will have large flanges but at higher level for butterfly valves, vent connection also at higher level but drain connections at lower level. Similarly thermometer pockets are located at inlet and outlet pipes for local measurements cooling water temperature.

In smaller units of the size of about 5 KW, some manufacturers make the condenser shell as well as water boxes of cast iron.

Tubes

Generally the tubes are made of brass, aluminum brass, cupro nickel or titanium depending on the reliability requirement at site conditions. The lengths are fixed at about 20 ft (6m) (for the 200 MW device mentioned above), depending on the size of the condenser. The size chosen is based on transportability from the manufacturers’ site and ease of erection at the installation site. The outer diameter is limited to a maximum of one inch for ease of handling and ease of insertion through the shell tube holes and for rolling at both ends.

]]> http://koolkampus.com/engineering-notes-2/mechanical/condenser/feed/ http://koolkampus.com/engineering-notes-2/mechanical/fire-tube-boiler/ http://koolkampus.com/engineering-notes-2/mechanical/fire-tube-boiler/#comments Thu, 14 Sep 2006 17:52:46 +0000 koolkampus Mechanical Energy Conversion Systems http://koolkampus.com/engineering-notes-2/fire-tube-boiler/ A fire-tube boiler is a type of boiler in which hot gases from the fire pass through one or more tubes within the boiler. It is one of the two major types of boilers, the other being the water-tube boiler. A fire tube boiler can be either horizontal or vertical.

This type of boiler was used on virtually all steam locomotives in the horizontal “locomotive” form. It is also typical of early marine applications and small vessels, such as the small riverboat used in the movie The African Queen. It also has extensive use in the stationary engineering field, typically for low pressure steam use such as heating a building.

Schematic diagram of a “locomotive” type fire-tube boiler

Operation

In the locomotive type boiler, fuel is burnt in a firebox to produce hot combustion gases. The firebox is surrounded by a cooling jacket of water connected to the long, cylindrical boiler tube. The hot gases are directed along a series of fire tubes, or flues, that penetrate the boiler and heat the water thereby generating saturated steam. The steam rises to the highest point of the boiler, the steam dome, where it is collected. The dome is the site of the regulator that controls the exit of steam from the boiler.

In the locomotive boiler, the saturated steam is nearly always passed into a superheater, back through the larger flues at the top of the boiler, to dry the steam and heat it to superheated steam. The superheated steam is directed to the cylinders or a turbine to produce mechanical work. Exhaust gases are fed out through a chimney, and may be used to pre-heat the feed water to increase the efficiency of the boiler.

Draught for firetube boilers, particularly in marine applications, is usually provided by a tall smokestack. In all steam locomotives, since Stephenson’sRocket, additional draught was supplied by directing exhaust steam from the cylinders into the smokestack through a blastpipe, to provide a partical vacuum. Modern industrial boilers use fans to provide forced draughting of the boiler.

Another major advance in the Rocket was large numbers of small diameter firetubes instead of a single large flue (a multi-tubular boiler). This greatly increased the surface area for heat transfer, allowing steam to be produced at a much higher rate. Without this, steam locomotives could never have developed effectively as powerful prime movers.

Safety Considerations

Because the fire-tube boiler itself is the pressure vessel, it requires a number of safety features to prevent mechanical failure. Boiler explosion, which is a type of BLEVE (Boiling Liquid Expanding Vapor Explosion), can be devastating.

* Safety valves release steam before a dangerous pressure can be built up

* Fusible plugs over the firebox melt at a temperature lower than that of the firebox, therefore melting and dousing the fire in water should it overheat.

* Stays, or ties, physically link the firebox and boiler casing, preventing them warping

The fire-tube type boiler that was used in the Stanley Steamer automobile had several hundred tubes which were weaker than the outer shell of the boiler, making an explosion virtually impossible as the tubes would fail and leak long before the boiler exploded. In nearly 100 years since the Stanleys were first produced, no Stanley boiler has ever exploded.

Types of fire-tube boiler

* Cornish boiler has a single large flue containing the fire

* Lancashire boiler has two large flues containing the fires

* Locomotive boiler has a double-walled firebox and a large number of small flue-tubes. Larger flue-tubes carry the superheater elements, where present. Forced draught is provided in the locomotive boiler by injecting exhausted steam back into the exhaust via a blast pipe.

]]> http://koolkampus.com/engineering-notes-2/mechanical/fire-tube-boiler/feed/ http://koolkampus.com/engineering-notes-2/mechanical/water-tube-boiler/ http://koolkampus.com/engineering-notes-2/mechanical/water-tube-boiler/#comments Thu, 14 Sep 2006 17:47:04 +0000 koolkampus Mechanical Energy Conversion Systems http://koolkampus.com/engineering-notes-2/water-tube-boiler/ A water-tube boiler is a type of boiler in which water circulates in tubes which are heated externally by the fire. Water-tube boilers are used for high-pressure boilers. Fuel is burned inside the furnace, creating hot gas which heats up water in the steam-generating tubes. In smaller boilers, additional generating tubes are separate in the furnace, while larger utility boilers rely on the water filled tubes that make up the walls of the furnace to generate steam. The heated water then rises into the steam drum. Here, saturated steam is drawn off the top of the drum. In some services, the steam will reenter the furnace in through a superheater in order to become superheated. Superheated steam is used in driving turbines. Since water droplets can severely damage turbine blades, steam is superheated to 730°F (390°C) or higher in order to ensure that there is no water entrained in the steam. Cool water at the bottom of the steam drum returns to the feedwater drum via large-bore ‘downcomer tubes’, where it helps pre-heat the feedwater supply. To increase the economy of the boiler, the exhaust gasses are also used to pre-heat the air blown into the furnace and warm the feedwater supply. Such water-tube boilers in thermal power station are also called steam generating units.

Schematic diagram of a marine-type water tube boiler

The older fire-tube boiler design—in which the water surrounds the heat source and the gases from combustion pass through tubes through the water space—is a much weaker structure and is rarely used for pressures above 350 psi (2.4 MPa). A significant advantage of the water tube boiler is that there is less chance of a catastrophic failure: There is not a large volume of water in the boiler nor are there large mechanical elements subject to failure.

Types of water-tube boiler

* D-type boiler - This is the most common type of small-medium sized boilers, similar to the one shown in the schematic diagram. It is used in both stationary and marine applications. It consists of a large steam drum vertically connected to a smaller water drum (a.k.a. mud drum) via multiple steam-generating tubes. These are surrounded by walls made up of larger water filled tubes, which make up the furnace.

* Babcock & Wilcox boiler - this has a single drum, with feedwater drawn from the bottom of the drum into a header that supplies inclined water-tubes. The water tubes supply steam back into the top of the drum. Furnaces are located below the tubes and drum.

* Stirling boiler - This type has three upper drums connected to two lower drums by water tubes. These are mainly used as stationary boilers.

* Thornycroft boiler - A single steam drum is supplied by a single water drum via two sets of water tubes that arc around the boiler. Furncaces on either side of the water drum vent into a common exhaust, giving the boiler a wide base tapering profile.

* Yarrow boiler - This type has three drums in a delta formation connected by water tubes and is generally fuel oil-fired. Due to its three drums, the Yarrow boiler a has greater water capacity. Hence, this type is usually used in older marine boiler applications. Its compact size made it attractive for use in transportable power generation units during World War II. In order to make it transportable, the boiler and its auxiliary equipment (fuel oil heating, pumping units, fans etc.), turbines, and condensers were mounted on wagons to be transported by rail.

]]> http://koolkampus.com/engineering-notes-2/mechanical/water-tube-boiler/feed/ http://koolkampus.com/engineering-notes-2/mechanical/high-pressure-steam-locomotive/ http://koolkampus.com/engineering-notes-2/mechanical/high-pressure-steam-locomotive/#comments Thu, 14 Sep 2006 17:41:38 +0000 koolkampus Mechanical Energy Conversion Systems http://koolkampus.com/engineering-notes-2/high-pressure-steam-locomotive/ A High Pressure Steam Locomotive is a steam locomotive with a boiler that operates at pressures well above what would be considered normal. In the later years of steam, boiler pressures were typically 200 to 250 PSI (1.4 to 1.7 MPa). High pressure locomotives can be considered to start at 350 psi, when special construction techniques become necessary, but some had boilers that operated at over 1,500 psi (10.3 MPa).

Why High Pressure?

Efficiency in a heat engine depends fundamentally upon getting the temperature at which heat is accepted (ie in the boiler) as far as possible from the temperature at which it is rejected. (ie steam when it leaves the cylinder) This was quantified by Nicolas Léonard Sadi Carnot.

There are two options: raise the acceptance temperature or lower the rejection temperature. The former means raising steam at higher pressure and temperature, which is in engineering terms fairly straightforward. The latter means bigger cylinders to allow the exhaust steam to expand further - and going this direction is limited by the loading gauge - and possibly condensing the exhaust to further lower the rejection temperature. This tends to be self-defeating because of frictional losses in the greatly increased volumes of exhaust steam to be handled.

Thus it has often been considered that high-pressure is the way to go to improve locomotive fuel efficiency. However, experiments in this direction were always defeated by much increased purchase and maintenance costs.

High-pressure locomotives were much more complicated than conventional designs. It was not simply a matter of building a normal fire-tube boiler with suitably increased strength and stoking harder. Structural strength requirements in the boiler shell make this impractical; it becomes impossibly thick and heavy. For high steam pressures the water-tube boiler is universally used. The steam drums and their interconnecting tubes are of relatively small diameter with thick walls and therefore much stronger.

The next difficulty is that of scale deposition and corrosion in the boiler tubes. Scale deposited inside the tubes is invisible, usually inaccessible, and a deadly danger, as it leads to local overheating and failure of the tube. This was a major drawback with the early water-tube boilers, such as the Du Temple design, tested on the French Nord network in 1907 and 1910. Water tubes in Royal Navy boilers were checked for blockage by carefully dropping numbered balls down the curved tubes.

A sudden steam leak into the firebox is perilous enough with a conventional boiler- the fire is likely to be blasted out of the firebox door, with unhappy results for anyone in the way. With a high-pressure boiler the results are even more dangerous because of the greater release of energy. This was demonstrated by the Fury tragedy, though the reason for the tube failure in that case was concluded to be overheating due to lack of steam flow rather than scaling.

]]> http://koolkampus.com/engineering-notes-2/mechanical/high-pressure-steam-locomotive/feed/ http://koolkampus.com/engineering-notes-2/mechanical/first-law-of-thermodynamics/ http://koolkampus.com/engineering-notes-2/mechanical/first-law-of-thermodynamics/#comments Sun, 10 Sep 2006 12:06:03 +0000 koolkampus Mechanical Automobile Thermodynamics Thermodynamics http://koolkampus.com/engineering-notes-2/first-law-of-thermodynamics/ The first law of thermodynamics is a generalized axiom of nature in relation to the conservation of energy. The most common enunciation of first law of thermodynamics is:

First law of thermodynamics

“The increase in the internal energy of a thermodynamic system is equal to the amount of heat energy added to the system minus the work done by the system on the surroundings.”

The first explicit statement of the first law of thermodynamics was given by Rudolf Clausius in 1850: “There is a state function E, called ‘energy’, whose differential equals the work exchanged with the surroundings during an adiabatic process.”

The mathematical statement of the first law is given by:

where dU is the infinitesimal increase in the internal energy of the system, δQ is the infinitesimal amount of heat added to the system, and δW is the infinitesimal amount of work done by the system on the surroundings. The infinitesimal heat and work are denoted by δ rather than d because, in mathematical terms, they are inexact differentials rather than exact differentials. In other words, they do not describe the state of any system.

The integral of an inexact differential is path dependent, i.e. it depends upon the particular “path” taken through the space of thermodynamic parameters while the integral of an exact differential depends only upon the initial and final states. If the initial and final states are the same, then the integral of an inexact differential may or may not be zero, but the integral of an exact differential will always be zero. The path taken by a thermodynamic system through state space is known as a thermodynamic process.

Revesible Process
An expression of the first law can be written in terms of exact differentials by realizing that the work that a system does is equal to its pressure times the infinitesimal change in its volume. In other words, δW = pdV where p is pressure and V is volume. For a reversible process, the total amount of heat added to a system can be expressed as δQ = TdS where T is temperature and S is entropy. For a reversible process, the first law may now be restated:

In the case where the number of particles in the system is not necessarily constant and may be of different types, the first law is written:

where dNi is the (small) number of type-i particles added to the system, and μi is the amount of energy added to the system when one type-i particle is added, where the energy of that particle is such that the volume and entropy of the system remains unchanged. μi is known as the chemical potential of the type-i particles in the system. The statement of the first law for reversible processes, using exact differentials is now:

Force Function
A useful idea, introduced by Willard Gibbs in 1876, is that quantities such as internal energy U and Helmholtz free energy A may be regarded as a kind of force-function. For example, the energy gained by a particle is equal to the force applied to the particle multiplied by the displacement of the particle while that force is applied. Now consider the first law without the heating term: dU = pdV. The pressure p can be viewed as a force (and in fact has units of force per unit area) while dV is the displacement (with units of distance times area). We may say, with respect to this work term, that a pressure difference forces a transfer of volume, and that the product of the two (work) is the amount of energy transferred as a result of the process.

It is useful to view the TdS term in the same light: With respect to this heat term, a temperature difference forces a transfer of entropy, and the product of the two (heat) is the amount of energy transferred as a result of the process. Here, the temperature is known as a “generalized” force (rather than an actual mechanical force) and the entropy is a generalized displacement.

Similarly, a difference in chemical potential between groups of particles in the system forces a transfer of particles, and the corresponding product is the amount of energy transferred as a result of the process. For example, consider a system consisting of two phases: liquid water and water vapor. There is a generalized “force” of evaporation which drives water molecules out of the liquid. There is a generalized “force” of condensation which drives vapor molecules out of the vapor. Only when these two “forces” (or chemical potentials) are equal will there be equilibrium, and the net transfer will be zero.

The two thermodynamic parameters which form a generalized force-displacement pair are termed “conjugate variables”. The two most familiar pairs are, of course, pressure-volume, and temperature-entropy.

Thermodynamics and Engineering

In thermodynamics and engineering, it is natural to think of the system as a heat engine which does work on the surroundings, and to state that the total energy added by heating is equal to the sum of the increase in internal energy plus the work done by the system. Hence δW is the amount of energy lost by the system due to work done by the system on its surroundings. During the portion of the thermodynamic cycle where the engine is doing work, δW is positive, but there will always be a portion of the cycle where δW is negative, e.g., when the working gas is being compressed. When δW represents the work done by the system, the first law is written:

Very occasionally, the sign on the heat may be inverted, so that δQ is the flow of heat out of the system:

Because of this ambiguity, it is vitally important in any discussion involving the first law to explicitly establish the sign convention in use.

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