7.1 Applications of thermodynamics principles
7.1.1 Nozzles
A device, in which kinetic energy and internal energy of a fluid are interchanged as a result of changing cross-sectional area available for flow, is termed a nozzle. It increases the velocity of the fluid at the expense of pressure. Nozzles and diffusers are commonly utilized in jet engines, rockets, spacecraft, and even garden hoses
A convergent nozzle is one in which with an incompressible fluid the nozzle area continues to decrease and velocity increases as the pressure decreases. On the other hand, for compressible fluids, the area first decreases and then increases so that the nozzle has a converging section followed by a divergent section. Such a nozzle is called a convergent divergent nozzle.
A converging duct is a nozzle for subsonic flow (M < 1).
A diverging duct is a nozzle for a supersonic flow (M > 1).
Nozzle
Convergent nozzle
Convergent-divergent nozzle
Shapes and sizes of nozzle
Nozzles come in a variety of shapes and sizes depending on the mission of the aircraft. Simple turbojets and turboprops have a fixed geometry convergent nozzle as shown in figure 7.2. Afterburning turbojets and turbofans often have a variable geometry Convergent-Divergent (CD) nozzle as shown on the figure 7.3. In this nozzle, the flow first converges down to the minimum area, or throat, and then is expanded through the divergent section to the exit at the right. The variable geometry causes these nozzles to be heavy, but provides efficient engine operation over a wider airflow range than a simple fixed nozzle. Rocket engines usually have a fixed geometry CD nozzle with a much larger divergent section than is required for a gas turbine.
Because the nozzle conducts the hot exhaust back to the free stream, there can be serious interactions between the engine exhaust flow and the airflow around the aircraft. On fighter aircraft, in particular, large drag penalties can occur near the nozzle exits. A typical nozzle-after body configuration is shown in the upper right for an F-15 with experimental maneuvering nozzles.
7.1.2 Diffusers
The diffuser is the gradually expanding passage following the test section in which the flow speed decreases and the pressure rises. The recovery of pressure from kinetic energy reduces the power needed to drive the tunnel: in the case of open-circuit tunnels the diffuser also reduces drafts in the laboratory. The pressure rise is less than that given by Bernoulli’s equation, because of losses due to skin friction and resulting growth of boundary-layer displacement thickness.
The cross-sectional area of a diffuser increases in the flow direction for subsonic flows and decreases for supersonic flows.
A converging duct is a nozzle for a supersonic flow (M > 1).
A diverging duct is a nozzle for a subsonic flow (M < 1).
Diffuser
Supersonic diffuser
Supersonic tunnels, in which a diverging diffuser after the test section would produce a further increase in Mach number, are equipped with a second throat at the end of the test section figure 7.5. The first throat is the one upstream of the test section through which the flow accelerates through the speed of sound. In the converging section leading to the second throat the flow is decelerated to slightly above sonic speed (obeying the one-dimensional inviscid compressible flow equations to a first approximation); in the diverging section downstream of the throat the Mach number rises again, until a shock wave or waves produce a reduction to subsonic speed. It may be shown that a shock wave in the converging portion of the second throat would be unstable, and in practice the second-throat Mach number is chosen large enough for the breakdown shock system to be located well downstream of the throat, to ensure stability under all operating conditions.
When the tunnel is started up, the second throat must be rather larger than the first throat in order for the latter to choke first, so that supersonic flow can be established in the test section. The inviscid-flow equations are not accurate during startup because a strong shock wave passes down the test section, reducing the speed below that of sound and usually causing massive boundary-layer separation. Therefore the second throat has to be significantly larger than the first, and to achieve the best possible supersonic diffusion in tunnels for high supersonic speeds the second throat is closed in after starting (reducing the local Mach number), by adjusting the shape of the walls. This is cost-effective only in large tunnels or if drive power is restricted.
Supersonic diffuser
Subsonic diffuser
The usual design rule for subsonic diffusers is that the total included angle of a portion of a circular cone with the same length and area ratio as the diffuser should not exceed 5 deg. This is well below the angle for maximum pressure recovery, which is nearer 10 deg., but at angles of more than about 5 deg. the boundary layer is close enough to separation for the flow to be unsteady. The 5 deg. rule will fail if the test section is unusually long, so that the boundary-layer thickness at entry to the diffuser is unusually large. (We expect the behavior of boundary layers in adverse pressure gradient to depend on some dimensionless parameter such as (δ/UCL)dUCL/dx, which must be small enough to avoid separation. Here δ and UCL are strictly the local boundary layer thickness and centerline velocity - related by Bernoulli’s equation to the pressure gradient, which is what really matters - but the evolution of δ is determined by its value at the beginning of the diffuser.)
Subsonic diffuser
7.1.3 Throttling valve
The critical flow nozzle allows the steam to reach sonic velocity at the throat of the nozzle at a very low upstream pressure. Due to the laws of physics the sonic velocity at the throat cannot be exceeded. As the upstream pressure increases the volumetric flow rate does not change, but the density of the steam increases with increasing pressure and thus the mass flow rate is proportional to the upstream pressure. A conical diffuser reduces the sonic velocity at the throat back down to the flow velocity in the pipe while recovering up to 90% of the upstream pressure.
Because of the pressure loss across the nozzle this type of meter can only be used in applications where steam is injected into a lower pressure area. Typical applications include humidification, surging tanks, atmospheric blanchers, steam eductors, steam injection into process ovens, and any other application where steam is exhausted through a manifold or nozzles to a lower pressure.
Throttling process
7.2 Compressor
A machine, used to provide gas at high pressure by the application of work from the external agency on the gas, is known as a compressor.
The compressor is used for the following purposes:
- Transporting fluids
- Providing high pressure for carrying out reactions and separation processes
- For pneumatic instruments
- For transferring mechanical energy to a fluid for agitation, solid particle transportation, etc
7.2.1 Types of compressors
Variable area nozzle compressor
These types of Compressor are used when motive, suction, or discharge conditions vary and it is necessary to control the discharge pressure or flow. Control of this compressor is accomplished by a spindle, which regulates the motive gas flow through the nozzle.
Unlike a control valve where energy is lost, the spindle reduces flow without reducing the available energy. Control of the spindle can activated by temperature, pressure, flow or suction to motive ratio. Variation of the spindle travel can be achieved with any suitable actuator.
Fixed nozzle compressor
Fixed Nozzle Compressors have no regulating spindle, and are generally used where operating conditions are stable.
Single and double acting compressors
In the single acting compressor the air is admitted to one side of piston only while in case of double acting air is admitted to each side of piston alternatively so one side of piston performs suction stroke and the air on the other side is compressed.
Multi-stage compressor
In a single-stage compressor, the whole range of compression is accomplished in one cylinder, i.e. in one-step or stage. Contrary to this, if the whole range of compression (from initial to final pressure) is accomplished in a unit in which there are two or more cylinders in series, each compressing over only a part of total pressure range, then the compressing system is termed as multi-stage compression.
The advantages of multi-stage compression are:
- Reduction of driving power required for given pressure ratio
- Improved volumetric efficiency for a given pressure ratio
- Permits high delivery pressure with reasonable volumetric efficiency
- Size and strength of cylinders can be adjusted to suit volume and pressure of air
- Multi-cylinders give better mechanical balance and improved cooling during compression
7.2.2 Compressing devices
The different types of compressing devices are:
Centrifugal fans
A centrifugal fan has an impeller with a number of blades around the periphery, and the impeller rotates in a scroll or volute shaped casing, and it is this casing, which identifies the centrifugal fan. As the impeller rotates, air is thrown from the blade tips centrifugally into the volute shaped casing (snail shell) and out through the discharge opening, and at the same time more air is drawn into the ‘eye’ of the impeller through a central inlet opening in the side of the casing, thus creating a continuous flow of air through the fan impeller and casing.
The volute shape of the casing helps to transform some of the velocity pressure of the air leaving the impeller into useful static pressure to overcome resistance to airflow in the ducting system to which the fan is connected. In normal ventilation work, a centrifugal fan would be used for static pressures (system resistances) up to about 750 Pa(=N/m2). A point to note is that the air flow through a centrifugal fan cannot be reversed.
Centrifugal fan
The advantages are:
- Provides pulsation-free air 24 hours a day
- 100% continuous duty
- Quiet operation
- Energy efficient at full load
- Extended service intervals
- Reliable long life
- Improved air quality
Centrifugal compressors or turbo-compressors
Centrifugal compressor process simulation can be configured to compress a variety of process gases by varying the molecular weight. The default configuration is for dry air.
Air passes through a suction valve before entering the suction drum. If the process gas demand is less than the minimum recommended compressor flow (surge point), then the makeup gas will mix with the kickback flow. The gas then passes through a cooler, where the temperature is lowered to prevent excessive compressor discharge temperatures. Any condensate present in the gas will be knocked out in the suction drum before the gas enters the compressor.
The compressed gas is then drawn off from the discharge side of the compressor by users. In the event of a decrease in process gas demand by the users, a minimum compressor flow line (kickback or spillback) is provided to allow the recycling of gas to prevent compressor surging. A vent/flare line is also provided to prevent an over-pressuring of the system.
Centrifugal compressor
Axial compressors
The axial compressor is one of the main components of modern gas turbines used as drive for generators in the energy supply and as jet engines in aviation, respectively. For reasons of a reduction of weight and costs a reduction of the stage number by an increase of the stage pressure ratio is desired along with constantly high stage efficiency. This tendency increases the danger of flow instabilities, which means a considerable risk for the axial compressor and which may lead to aggravating damages.
An axial flow compressor generally contains several ‘stages’. Each stage consists of a rotor, which is mounted on a central shaft, and rotates at high speed, together with a stator, which is fixed to the casing. Both rotor and stator have many aerodynamically profiled blades designed to generate a pressure difference across the stage. The attainable pressure rise per stage is limited by the aerodynamic efficiency of the blade and many stages are required to build up a useful pressure difference.
The axial-flow compressor is not a positive-displacement device and will not operate for all flow conditions. At high loading and low flow rates it will surge as the flow pattern through the blading breaks down.
Axial compressor
Advantages of axial compressors:
- High overall π
- High η
- Small front area (High mass flux)
Reciprocating compressors
A reciprocating compressor is used to boost the pressure of the flow media for use as a seal gas by the flow compressor. This unit can be used to pressurize air for use in the loop. (Air is sometimes used for testing that does not involve in-line inspection tools.) Another reciprocating unit is used for pressurizing fuel gas from its supply pressure of under 50 psi to the pressure required by the flow compressor, nominally 170 psi. Each compressor system is self-contained with its own coolers, scrubbers, and filters.
The process gas to be compressed enters the suction of the first stage of the reciprocating compressor through a suction pressure control valve. The make-up process gas is mixed with the kickback flow before passing through the inlet cooler.
The process gas leaving the first stage of the compressor passes through an inter cooler before being compressed by the second stage of the compressor.
The compressed process gas is then drawn off by users from the discharge of the second stage. Excess gas may either by sent back to the suction of the first stage through the discharge pressure control valve, or may be vented.
Reciprocating compressors
Applications of reciprocating compressors
Refining (Hydro cracking, HDT, HDS, reforming, recovery, and recycle applications): Horizontal and balanced opposed compressors with unit power capability reaching 35,000 kW.
Petrochemicals and chemicals balanced opposed compressors for: ammonia synthesis plants and urea synthesis plants with capacities up to 1,000 t/d per machine, polymer production plants, and liquefied gas storage plants. Special horizontal opposed hyper compressors for LDPE (low-density polyethylene) plants reaching over 3,500 bars.
Special service for corrosive and toxic gases.
Natural gas and oil fields horizontal balanced - opposed compressors for re-injection, gas lift, gathering, boosting, storage and treating services, both onshore and offshore. The compressors can be skid mounted in integral systems to form completely self-contained packaged units.
CNG compressors for bottling natural gas for automotive service; ‘Cubogas’ modular refueling stations (as many as 1,000 vehicles per day); turnkey stations for refueling as many as 2,500 vehicles per day.
Rotary compressors
The compression chamber is a toroidal channel in which an impeller rotates. The gas trapped between the vanes is centrifugally forced to the periphery of the chamber and swirls around the core before it is caught again by the next vane on the wheel, repeating the process all around the channel, and transforming the kinetic energy into pressure. A stripper separates the inlet from the outlet ports and helps in guiding the gas flow from suction to discharge. The clearance between the impeller and the stripper is very tight to limit the gas slippage.
The location, orientation, area of inlet/outlet ports, the geometry of the vanes (bending radius, height, penetration, angle), the shape and area of the chamber, and the shape of the channel impart different compressor characteristics (flow rate, pressure, efficiency), offering multiple solutions for dealing with varied process conditions.
Rotary compressor
The benefits of rotary compressors over reciprocating compressors are:
- Continuous and smooth operations
- Higher efficiency as a result of less power consumption
- Fewer moving parts leading to greater reliability
- Lighter weight and more compact size
- Quieter operation
Application
- Boosting and recycling of hydrogen mixtures and hydrocarbon gases
- Gas compression in industrial processes
- Gas phase reactor recycling
- Molecular sieve regeneration in gas drying processes
- Vent/Purge gas recovery
- Fuel gas boosting for GT feed
- For ventilation purpose in closed shed and other industries and also in furnaces
Jet compressors
Jet compressors utilize the energy of steam, gas, or air, under pressure, to circulate steam, boost low-pressure steam, and mix gases in desired proportion. In all of the processes where jet compressors are applied, they not only perform the primary function of compressing and mixing gases, but also take the place of a reducing valve and salvage much of the energy lost in the reduction of operating medium pressure.
Basically, jet compressors consist of an expanding nozzle, diffuser, and body. They are also equipped with manually or automatically controlled adjusting spindles for regulating the volume of flow through the nozzle. Manually controlled units are recommended for use when pressures do not vary. Automatically controlled units are recommended when capacities or discharge pressures vary. Units can be made of corrosion-resistant materials.
In operation, jet compressors utilize a jet of high-pressure gas as an operating force to entrain a low-pressure gas, mix the two, and bring the pressure to an intermediate point. Gases can be steam, air, propane, or others. When both fluids are steam, the unit is known generally as a Thermo compressor. These Thermo compressors are used as energy savers, recovering low-pressure ‘waste’ steam by combining it with higher-pressure steam to raise the discharge pressure so the mixture can be re-used in a process.
Jet compressors
Applications of Jet compressors
JET Compressor is an ideal device for:
- Gas recovery from marginal Wells
- Flare gas recovery / flare gas elimination
- Boosting capacity of mechanical Compressors
- Compression of overhead gas to MEA scrubbers
- Gas compression between two stages of a two stage Combustor
- Compression of waste gas into a flare header system
- Low pressure oil well boosting
7.3 Ejector-system
The operating principle of an ejector is basically to convert pressure energy of the motive steam into velocity. This occurs by adiabatic expansion from motive steam pressure to suction-load operating pressure. This adiabatic expansion occurs across a con-verging and diverging nozzle. This results in supersonic velocity off the motive nozzle, typically in the range of mach 3 to 4.
In reality, the motive steam expands to a pressure lower than the suction load pressure. This creates a low-pressure zone for pulling the suction load into the ejector. High-velocity motive steam entrains and mixes with the suction gas load. The resulting mixture’s velocity is still supersonic. Next, the mixture enters a venturi where the high velocity reconverts to pressure. In the converging region, velocity is converted o pressure as cross-sectional flow area is reduced. At the throat section, a normal shock wave is established. Here, a significant in pressure and loss of velocity across the shock wave occurs. Flow across the shock wave goes from supersonic ahead of the shock wave, to sonic at the shock wave and subsonic after the shock wave. In the diverging section, velocity is further reduced and converted into pressure. Motive pressure, temperature and quality are critical variables for proper ejector operating performance. The amount of motive steam used is a function of required ejector performance. The nozzle throat is an orifice and its diameter is designed to pass the specified quantity of motive steam, required to effect sufficient compression across the ejector. Calculation of a required motive nozzle throat diameter is based on the necessary amount of motive steam, its pressure and specific volume.
Ejector
Motive steam quality is important because moisture droplets affect the amount of steam passing through the nozzle. High-velocity liquid droplets also prematurely erode ejector internals, reducing performance. Operating a vacuum unit requires an ejector system to perform over a wide range of conditions. The ejector system must be stable over all anticipated operating conditions. Also, an accurate understanding of ejector system backpressure for all operating modes is necessary for stable operation. An ejector does not create its discharge pressure, it is simply supplied with enough motive steam to entrain and compress its suction load to a required discharge pressure. If the ejector backpressure is higher than the discharge pressure it can achieve, then the ejector ‘breaks’ operation and the entire ejector system may be unstable.
7.4 Heat conversion and power cycles
7.4.1 Heat engines
Heat engines convert heat energy into mechanical energy. Examples include steam engines, steam and gas turbines, spark-ignition and diesel engines, and the ‘external combustion’ engine or Stirling engine. Such engines can provide motive power for transportation, to operate machinery, or to produce electricity.
All heat engines operate in a cycle of repeated sequences of heating (or compressing) and pressurizing the working fluid, the performance of mechanical work, and rejecting unused or waste heat to a ‘sink.’ At the beginning of each cycle, energy is added to the fluid forcing it to expand under high pressure so that the fluid ‘performs’ mechanical work. The thermal energy contained in the pressurized fluid is converted to kinetic energy. The fluid then looses pressure, and after unused energy (in the form of heat) is rejected, it must then be reheated or recompressed to restore it to high pressure.
Heat engines cannot convert all the input energy to useful mechanical energy in the same cycle; some amount, in the form of heat, is always not available for the immediate performance of mechanical work. The fraction of thermal energy that is converted to net mechanical work is called the thermal efficiency of the heat engine. The maximum possible efficiency of a heat engine is that of a hypothetical (ideal) cycle, called the Carnot Cycle. Practical heat engines operate on less efficient cycles (such as the Rankine, Brayton, or Stirling) but in general, the highest thermal efficiency is achieved when the input temperature is as high as possible and the sink temperature is as low as possible.
The ‘waste’ or rejected heat (to the ‘sink) can be used for other purposes, including pressurizing a different working fluid, which operates a different heat-engine (vapor turbine) cycle, or simply for heating. Renewable sources of heat or fuels, such as solar or geothermal energy and biomass (as well as fossil fuels) can power heat engines. The following is a brief description of four types of heat engines, the Rankine, Stirling, Brayton, and the newly developed, and highly efficient, Kalina, that can be used or are being investigated for converting renewable sources of energy to useful energy.
Heat engine
Rankine cycle engines
The Rankine cycle system uses a liquid that evaporates when heated and expands to produce work, such as turning a turbine, which when connected to a generator, produces electricity. The exhaust vapor expelled from the turbine condenses and the liquid is pumped back to the boiler to repeat the cycle. The working fluid most commonly used is water, though other liquids can also be used. Rankine cycle design is used by most commercial electric power plants. The traditional steam locomotive is also a common form of the Rankine cycle engine. The Rankine engine itself can be either a piston engine or a turbine.
Stirling cycle engines
The Stirling cycle engine (also called an ‘external’ combustion engine) differs from the Rankine in that it uses a gas, such as air, helium, or hydrogen, instead of a liquid, as its working fluid. Concentrated sunlight, biomass, or fossil fuels are sources potential fuels to provide external heat to one cylinder. This causes the gas to alternately expand and contract, moving a displacer piston back and forth between a heated and an unheated cylinder.
Brayton cycle engines
Brayton cycle systems, which incorporate a turbine, also use a gas as the working medium. There are open-cycle and closed-cycle Brayton systems. The gas turbine is a common example of an open-cycle Brayton system. Air is drawn into a compressor, heated and expanded through a turbine, and exhausted into the atmosphere. The closed-cycle Brayton system may use air, or a more efficient gas, such as hydrogen or helium. The gas in the closed-cycle system, however, gives up some of its heat in a heat exchanger after it leaves the turbine. It then returns to the compressor to start the cycle again.
Kalina cycle engines
The Kalina cycle engine, which is at least 10 percent more efficient than the other heat engines, is simple in design and can use readily available, off-the-shelf components. This new technology is similar to the Rankine cycle except that it heats two fluids, such as ammonia and water, instead of one. Instead of being discarded as waste at the turbine exhaust, the dual component vapor (70% ammonia, 30% water) enters a distillation subsystem. This subsystem creates three additional mixtures. One is a 40/60 mixture, which can be completely condensed against normal cooling sources. After condensing, it is pumped to a higher pressure, where it is mixed with a rich vapor produced during the distillation process. This recreates the 70/30 working fluid. The elevated pressure completely condenses the working fluid and returns it to the boiler to complete the cycle. The mixture’s composition varies throughout the cycle. The advantages of this process include variable temperature boiling and condensing, and a high level of recuperation.
7.4.2 Internal combustion engines
An internal combustion engine burns a mixture of fuel and air. Typical IC engines are classified as Spark and Compression ignition engines.
The simplest model for IC engines is the air-standard model, which assumes that:
- The system is closed
- Air is the working fluid and is modeled as an ideal gas throughout the cycle
- Compression and expansion processes are isentropic
- A reversible heat transfer process characterizes the combustion of fuel and air
- Heat rejection takes place reversibly and at constant volume
The Otto cycle is used to model a basic Spark Ignition engine, while the Diesel cycle is the basic model for the Compression Ignition engine.
Spark ignition engine (Otto cycle)
The most common type is a four-stroke engine. A piston slides in and out of a cylinder. Two or more valves allow the fuel and the air to enter the cylinder and the gases that form when the fuel and air burn to leave the cylinder. As the piston slides back and forth inside the cylinder, the volume that the gases can occupy changes drastically.
The process of converting heat into work begins when the piston is pulled out of the cylinder, expanding the enclosed space and allowing fuel and air to flow into that space through a valve. This motion is called the intake stroke or induction stroke. Next, the fuel and the air mixture are squeezed together by pushing the piston into the cylinder. This is called the compression stroke. At the end of the compression stroke, with the fuel and the air mixture squeezed as tightly as possible, the spark plug at the sealed end of the cylinder fires and ignites the mixture. The hot burning fuel has an enormous pressure and it pushes the piston out of the cylinder. This power stroke is what provides power to the engine and the attached machinery. Finally, the burned gas is squeezed out of the cylinder through another valve in the exhaust stroke. These four strokes repeat over and over again. Most internal combustion engines have at least four cylinders and pistons. There is always at least one cylinder going through the power stroke and it can carry the other cylinders through the non-power strokes. The maximum efficiency of such an engine is emax = (Tignition - Tair)/Tignition where Tignition is the temperature of the fuel-air mixture after ignition. To maximize the fuel efficiency, you have to create the hottest possible fuel air mixture after ignition. The highest efficiency that has been achieved is approximately 50% of emax.
Spark ignition engine
Otto cycle
Compression ignition engines (Diesel cycle)
In the Diesel, the fuel is not mixed with the air entering the cylinder during the intake stroke. Air alone is compressed during the compression stroke. The Diesel fuel oil is injected or sprayed into the cylinder at the end of the compression stroke. The Diesel Cycle differs from the Otto Cycle only in the modeling of the combustion process. In a Diesel Cycle, it is assumed to occur as a reversible constant pressure heat addition process, while in an Otto Cycle, the volume is assumed constant.
The four steps of the air-standard Diesel Cycle are outlined below:
- (1-2) Isentropic Compression (Compression Stroke)
- (2-3) Reversible, constant pressure heat addition (Ignition)
- (3-4) Isentropic expansion to initial volume (Power Stroke)
- (4-1) Reversible constant-volume heat rejection (Exhaust)
In diesel engines, compression ratios are as high as (22.5 to 1) and provide pressures of (500psi) at the end of the compression stroke. Through the compression process, the air can be heated up (1000 degrees F). This temperature is high enough to spontaneously ignite the fuel as it is injected into the cylinder. The high pressure of the explosion forces the piston down as in the gasoline engine.
Diesel cycle
7.4.3 Steam turbine
The steam turbine obtains its motive power from the change of momentum of a jet of steam flowing over a curved vane. The steam jet, in moving over the curved surface of the blade, exerts a pressure on the blade owing to its centrifugal force. This centrifugal pressure is exerted normal to the blade surface and acts along the whole length of the blade; the result of this centrifugal pressure plus the effect of change of velocity is the motive force on the blade. This motive power enables the turbine to drive the electric generator.
A steam turbine consists of a pair of blade rings consisting of a fixed ring of blades and moving ring. Both the moving and fixed blades are designed so that the jet shall not strike the blade but will merely glide over it in parallel direction. The fixed blades are fixed to the turbine casing and face the moving blades in opposite direction. The object of fixed blade is to receive steam jet discharging from the moving blade ring and to divert it to the next ring of moving blade by changing its direction. This diversion may continue over several rings of moving and fixed blades until the whole of K.E. of the steam is expanded.
Steam turbine
Single stage turbines
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Multistage steam turbines
Our Industrial Standard Multi-Stage Steam Turbine line offers you up to nine stages. And, to meet the variety of energy conditions in industrial environments, we offer 13 frame sizes that produce up to 6000 KW. These turbines operate at speeds up to 8000 RPM, with steam inlet conditions to 700 PSIG @ 825°F; and exhaust conditions to 300 lbs.
Exhaust nozzle
Gas turbine engines for aircraft have an exhaust system, which passes the turbine discharge gases to atmosphere at a velocity in the required direction, to provide the necessary thrust. The design of the exhaust system, therefore, exerts a considerable influence on the performance of the engine. The cross sectional areas of the jet pipe and propelling or outlet nozzle affect turbine entry temperature, the mass flow rate, and the velocity and pressure of the exhaust jet.
A basic exhaust system function is to form the correct outlet area and to prevent heat conduction to the rest of the aircraft. The use of a thrust reverser (to help slow the aircraft on landing), a noise suppresser (to reduce the noisy exhaust jet) or a variable area outlet (to improve the efficiency of the engine over a wider range of operating conditions) produces a more complex exhaust system.
Afterburners
In addition to the basic components of a gas turbine engine, one other process is occasionally employed to increase the thrust of a given engine. Afterburning (or reheat) is a method of augmenting the basic thrust of an engine to improve the aircraft takeoff, climb and (for military aircraft) combat performance.
Afterburning consists of the introduction and burning of raw fuel between the engine turbine and the jet pipe propelling nozzle, utilizing the unburned oxygen in the exhaust gas to support combustion. The resultant increase in the temperature of the exhaust gas increases the velocity of the jet leaving the propelling nozzle and therefore increases the engine thrust. This increased thrust could be obtained by the use of a larger engine, but this would increase the weight, frontal area and overall fuel consumption. Afterburning provides the best method of thrust augmentation for short periods.
Afterburners are very inefficient as they require a disproportionate increase in fuel consumption for the extra thrust they produce. Afterburning is used in cases where fuel efficiency is not critical, such as when aircraft take off from short runways, and in combat, where a rapid increase in speed may occasionally be required.
Afterburners
7.4.4 Jet engines
The main function of any aero plane propulsion system is to provide a force to overcome the aircraft drag, this force is called thrust. Both propeller driven aircraft and jet engines derive their thrust from accelerating a stream of air - the main difference between the two is the amount of air accelerated. A propeller accelerates a large volume of air by a small amount, whereas a jet engine accelerates a small volume of air by a large amount. This can be understood by Newton’s 2nd law of motion which is summarized by the equation F=ma (force = mass x acceleration). Basically the force or thrust (F) is created by accelerating the mass of air (m) by the acceleration (a).
Given that thrust is proportional to airflow rate and that engines must be designed to give large thrust per unit engine size, it follows that the jet engine designer will generally attempt to maximize the airflow per unit size of the engine. This means maximizing the speed at which the air can enter the engine, and the fraction of the inlet area that can be devoted to airflow. Gas turbine engines are generally far superior to piston engines in these respects; therefore piston-type jet engines have not been developed.
Air passing through the engine has to be accelerated; this means that the velocity or kinetic energy of the air must be increased. First, the pressure energy is raised, followed by the addition of heat energy, before final conversion back to kinetic energy in the form of a high velocity jet.
The basic mechanical arrangement of a gas turbine is relatively simple. It consists of only four parts:
- The compressor, which is used to increase the pressure (and temperature) of the inlet air
- One or a number of combustion chambers in which fuel is injected into the high-pressure air as a fine spray, and burned, thereby heating the air. The pressure remains (nearly) constant during combustion, but as the temperature rises, each kilogram of hot air needs to occupy a larger volume than it did when cold and therefore expands through the turbine
- The turbine, which converts some of this temperature rise to rotational energy. This energy is used to drive the compressor
- The exhaust nozzle which accelerates the air using the remainder of the energy added in the combustor, producing a high velocity jet exhaust
Jet engine
7.4.5 The compressor
In the gas turbine engine, compression of the air is effected by one of two basic types of compressor, one giving centrifugal flow and the other axial flow. Both types are driven by the engine turbine and are usually coupled direct to the turbine shaft.
The centrifugal flow compressor employs an impeller to accelerate the air and a diffuser to produce the required pressure rise. Flow exit’s a centrifugal compressor radially (at 90° to the flight direction) and it must therefore be redirected back towards the combustion chamber, resulting in a drop in efficiency. The axial flow compressor employs alternate rows of rotating (rotor) blades, to accelerate the air, and stationary (stator) vanes, to diffuse the air, until the required pressure rise is obtained.
The pressure rise that may be obtained in a single stage of an axial compressor is far less than the pressure rise achievable in a single centrifugal stage. This means that for the same pressure rise, an axial compressor needs many stages, but a centrifugal compressor may need only one or two
Compressor
An engine design using a centrifugal compressor will generally have a larger frontal area than one using an axial compressor. This is partly a consequence of the design of a centrifugal impeller, and partly a result of the need for the diffuser to redirect the flow back towards the combustion chamber. As the axial compressor needs more stages than a centrifugal compressor for the equivalent pressure rise, an engine designed with an axial compressor will be longer and thinner than one designed using a centrifugal compressor. This, plus the ability to increase the overall pressure ratio in an axial compressor by the addition of extra stages, has led to the use of axial compressors in most engine designs, however, the centrifugal compressor is still favored for smaller engines where it’s simplicity, ruggedness and ease of manufacture outweigh any other disadvantages.
Combustion chamber
The combustion chamber has the difficult task of burning large quantities of fuel, supplied through fuel spray nozzles, with extensive volumes of air, supplied by the compressor, and releasing the resulting heat in such a manner that the air is expanded and accelerated to give a smooth stream of uniformly heated gas. This task must be accomplished with the minimum loss in pressure and with the maximum heat release within the limited space available.
The amount of fuel added to the air will depend upon the temperature rise required. However, the maximum temperature is limited to within the range of 850 to 1700 °C by the materials from which the turbine blades and nozzles are made. The air has already been heated to between 200 and 550 °C by the work done in the compressor, giving a temperature rise requirement of 650 to 1150 °C from the combustion process. Since the gas temperature determines the engine thrust, the combustion chamber must be capable of maintaining stable and efficient combustion over a wide range of engine operating conditions.
The temperature of the gas after combustion is about 1800 to 2000 °C, which is far too hot for entry to the nozzle guide vanes of the turbine. The air not used for combustion, which amounts to about 60 percent of the total airflow, is therefore introduced progressively into the flame tube. Approximately one third of this gas is used to lower the temperature inside the combustor; the remainder is used for cooling the walls of the flame tube.
There are three main types of combustion chamber in use for gas turbine engines. These are the multiple chambers, the can-annular chamber and the annular chamber.
Multiple chambers
This type of combustion chamber is used on centrifugal compressor engines and the earlier types of axial flow compressor engines. It is a direct development of the early type of Whittle engine combustion chamber. Chambers are disposed radially around the engine and compressor delivery air is directed by ducts into the individual chambers. Each chamber has an inner flame tube around which there is an air casing. The separate flame tubes are all interconnected. This allows each tube to operate at the same pressure and also allows combustion to propagate around the flame tubes during engine starting.
Multiple chambers
Can-annular chamber
This type of combustion chamber bridges the evolutionary gap between multiple and annular types. A number of flame tubes are fitted inside a common air casing. The airflow is similar to that already described. This arrangement combines the ease of overhaul and testing of the multiple systems with the compactness of the annular system.
Can-annular chamber
Annular chamber
This type of combustion chamber consists of a single flame tube, completely annular in form, which is contained in an inner and outer casing. The main advantage of the annular combustion chamber is that for the same power output, the length of the chamber is only 75 per cent of that of a can-annular system of the same diameter, resulting in a considerable saving in weight and cost. Another advantage is the elimination of combustion propagation problems from chamber to chamber.
Annular chamber
7.4.6 Rocket engines
One of the most amazing efforts man has ever undertaken is the exploration of space. A big part of the amazement is the complexity. Space exploration is complicated because there are so many problems to solve and obstacles to overcome.
The gray areas to be confronted are:
- The vacuum of space
- Heat management problems
- The difficulty of re-entry
- Orbital mechanics
- Micrometeorites and space debris
- Cosmic and solar radiation
- The logistics of having restroom facilities in a weightless environment
But the major problem of all is harnessing enough energy simply to get a spaceship off the ground. That is where rocket engines come in.
The basics
Rocket engines are fundamentally different. Rocket engines are reaction engines. The basic principle driving a rocket engine is the famous Newtonian principle that ‘to every action there is an equal and opposite reaction.’ A rocket engine is throwing mass in one direction and benefiting from the reaction that occurs in the other direction as a result.
Reaction process
This concept of ‘throwing mass and benefiting from the reaction’ can be hard to grasp at first, because that does not seem to be what is happening. Rocket engines seem to be about flames and noise and pressure, not ‘throwing things.’
A rocket engine is generally throwing mass in the form of a high-pressure gas. The engine throws the mass of gas out in one direction in order to get a reaction in the opposite direction. The mass comes from the weight of the fuel that the rocket engine burns. The burning process accelerates the mass of fuel so that it comes out of the rocket nozzle at high speed. The fact that the fuel turns from a solid or liquid into a gas when it burns does not change its mass. If you burn a pound of rocket fuel, a pound of exhaust comes out the nozzle in the form of a high-temperature, high-velocity gas. The form changes, but the mass does not. The burning process accelerates the mass.
How rocket engines work
The ‘strength’ of a rocket engine is called its thrust. Thrust is measured in ‘pounds of thrust’ in the U.S. and in Newtons under the metric system (4.45 Newtons of thrust equals 1 pound of thrust). A pound of thrust is the amount of thrust it would take to keep a 1-pound object stationary against the force of gravity on Earth. So on Earth, the acceleration of gravity is 32 feet per second per second (21 mph per second)
One of the peculiar rockets have is that the objects that the engine wants to throw actually weigh something, and the rocket has to carry that weight around. The Space Shuttle launch comprises of three parts:
- The Orbiter
- The big external tank
- The two solid rocket boosters (SRBs)
Solid-fuel rockets
Solid-fuel rocket engines were the first engines created by man. They were invented hundreds of years ago in China and have been used widely since then. The idea behind a simple solid-fuel rocket is straightforward... Here’s a typical cross section:
A solid-fuel rocket immediately before and after ignition
On the left is the rocket before ignition. The solid fuel is shown in green. It is cylindrical, with a tube drilled down the middle. When the fuel is lighted, it burns along the wall of the tube. As it burns, it burns outward toward the casing until all the fuel has burned. In a small model rocket engine or in a tiny bottle rocket the burn might last a second or less. In a Space Shuttle SRB containing over a million pounds of fuel, the burn lasts about two minutes.
The propellant mixture in each SRB motor consists of an ammonium per chlorate (oxidizer, 69.6 percent by weight), aluminum (fuel, 16 percent), iron oxide (a catalyst, 0.4 percent), a polymer (a binder that holds the mixture together, 12.04 percent), and an epoxy curing agent (1.96 percent). The propellant is an 11-point star-shaped perforation in the forward motor segment and a double- truncated- cone perforation in each of the aft segments and aft closure. This configuration provides high thrust at ignition and then reduces the thrust by approximately a third 50 seconds after lift-off to prevent overstressing the vehicle during maximum dynamic pressure.
The idea is to increase the surface area of the channel, thereby increasing the burn area and therefore the thrust. As the fuel burns the shape evens out into a circle. In the case of the SRBs, it gives the engine high initial thrust and lower thrust in the middle of the flight.
An ‘11-point star-shaped perforation’ might look like this:
Star-shaped perforation
Solid-fuel rocket engines have three important advantages:
- Simplicity
- Low cost
- Safety
They also have two disadvantages:
- Thrust cannot be controlled
- Once ignited, the engine cannot be stopped or restarted
The disadvantages mean that solid-fuel rockets are useful for short-lifetime tasks (like missiles), or for booster systems. When you need to be able to control the engine, you must use a liquid propellant system.
Liquid-propellant rockets
In most liquid-propellant rocket engines, a fuel and an oxidizer (for example, gasoline and liquid oxygen) are pumped into a combustion chamber. There they burn to create a high-pressure and high-velocity stream of hot gases. These gases flow through a nozzle that accelerates them further (5,000 to 10,000 mph exit velocities being typical), and then they leave the engine. The following highly simplified diagram shows you the basic components.
Liquid-propellant rocket
This diagram does not show the actual complexities of a typical engine (see some of the links at the bottom of the page for good images and descriptions of real engines). For example, it is normal for either the fuel of the oxidizer to be a cold liquefied gas like liquid hydrogen or liquid oxygen. One of the big problems in a liquid propellant rocket engine is cooling the combustion chamber and nozzle, so the cryogenic liquids are first circulated around the super-heated parts to cool them. The pumps have to generate extremely high pressures in order to overcome the pressure that the burning fuel creates in the combustion chamber.
The main engines in the Space Shuttle actually use two pumping stages and burn fuel to drive the second stage pumps. All kinds of fuel combinations get used in liquid propellant rocket engines. For example:
- Liquid hydrogen and liquid oxygen – used in the Space Shuttle main engines
- Gasoline and liquid oxygen – used in Goddard’s early rockets
- Kerosene and liquid oxygen – used on the first stage of the large Saturn V boosters in the Apollo program
- Alcohol and liquid oxygen – used in the German V2 rockets
- Nitrogen tetroxide/monomethyl hydrazine – used in the Cassini engines
7.5 Refrigeration and liquefaction
7.5.1 Plate heat exchangers in industrial refrigeration systems
The plate heat exchanger concept, with flow-trough channels formed by corrugated plates and the heat transfer taking through the thin plates, is an extremely efficient heat exchange technique.
The turbulent flow, coupled with fouling factors and high heat transfer coefficient, means that it is possible to operate with a small temperature difference in evaporation and chilled water temperatures. This in turn provides a good operational economy with high C.O.P. values.
It also means that a plate heat exchanger becomes much more compact than a shell & tube heat exchanger for the same duty. The practical advantages are:
- Lower weight
- Smaller space requirements
- Lower refrigerant filling
The most common plate heat exchanger in industrial refrigeration is the Semi Welded Plate Heat Exchanger (SWPHE), which alternates welded channels and traditional gasketed channels.
The refrigerant flows in welded channels and the only gaskets in contact with the refrigerant are two circular porthole gaskets between the welded plate pairs. These gaskets are made from highly resistant materials, attached for easy replacement by a glue free construction.
The secondary medium flows in channels sealed by traditional elastomer gaskets.
The SWPHE is very flexible and variable and can be arranged in Twin or two-in one design, e.g. Desuperheater/Condenser, Oilcooler/Condenser. These features give the possibility to manufacture two duties in one frame at a lower cost, smaller volume and shorter footprint.
The plate heat exchanger concept allows the SWPHE to be opened and reclosed several times.
The SWPHE is not sensitive to temperature shocks and due to the turbulent flow freezing risks are small, but the flexible design will accommodate expansion and no damage will be caused should freezing occur.
The Semi Welded Plate Heat Exchangers are used as evaporators and condensers for refrigeration systems in a whole series of applications, e.g.:
- Dairy, brewery and wine yard production
- Marine
- Fishing vessels and fish processing
- Slaughter houses
- Chemicals and pharmaceutical industries
- Ice manufacturing, ice-skating rinks
- Food retail outlet
When the gasketed side is food approved it could be used in direct cooling of food liquids, e.g. NH3/beer, juice or water.
Other application like Heat Pumps, Organic Rankine Cycles and Absorption Systems could also request SWPHE for different duties.
Plate heat exchangers
7.5.2 Cryogenics
Cryogenic Separation is a distillation process that occurs at temperatures close to -170 degrees Celsius. At this temperature, air starts to liquefy.
Before separation can occur, there are specific operation conditions that must be achieved. Distillation requires two phases, gas and liquid. Air must be very cold for this to happen. For this instance, at one atmosphere, nitrogen is a liquid at -196 degrees Celsius. A pressure 8-10 times atmospheric pressure is required for this process. These conditions are achieved via compression and heat exchange; cold air exiting the column is used to cool air entering it. Nitrogen is more volatile than oxygen and comes off as the distillate product.
Air separation plants
A cryogenic air separation plant is expensive and large; the distillation column is several stories high and must be well insulated. Consequently, it only becomes economically feasible to separate air this way when a large amount is needed. Cryogenic separation is also capable of producing much purer nitrogen than either of the other two processes because the number of trays in the distillation column can be increased
Air separation plants
Natural gas liquefaction plants
In general there are two types of natural gas liquefaction plants, depending on the purpose for which they are required.
Baseload plants are in operation throughout the whole year. They liquefy gas up to 5 million tons per year per liquefaction line. These plants are designed to meet the basic requirements (baseload). The liquefied gas is transported by tankers to the consumer countries and is then offloaded into terminals. Here, it is re-evaporated and sent to the consumer by pipeline.
Peak saving plants are used to meet the requirements of peak consumption. During the summer months the energy supply companies accept larger quantities of gas than are required for average consumption. The surplus is liquefied and stored. In the winter months the liquefied natural gas (LNG) is used to meet the requirements of peak consumption by first pumping it up to the required discharge pressure, re-evaporating it in special heat exchangers, so-called vaporizers (Link P0075.htm) and feeding it into the consumer mains. The liquefaction capacities of peak shaving plants lie somewhere between 5 and 1000 kmol/h (around 2 to 420 m3/h).
Aside from the conventional liquefaction facility, Linde has also developed and engineered a concept of a skid-mounted transportable LNG Production Plant. This concept is technically and economically feasible for
- Natural gas from shut-in wells which have good delivery but insufficient reserves to be hooked up to a pipeline
- Associated natural gas from oil fields which is now flared
- Small liquefaction units for vehicular fueling
Thermal incineration for off-gas purification
Cryogenic processing of chemical plants and refinery off-gas streams is extensively used for the separation and purification of hydrogen, methane and carbon monoxide
Thermal incineration permits an environmentally safe and profitable removal of various combustible pollutants from the off-gases of chemical, petrochemical and other industrial processes.
As with this process a lot of pollutants can be converted at the same time it will in many cases be the only way to purify off-gases in a profitable and effective way.
Process description
The off-gas is preheated in a heat exchanger to a temperature dictated by the settings of the incineration reaction or by official regulations. Then the gas will be fed into the combustion chamber. As soon as the gas has been converted the clean gas will be cooled. If halogens are contained it may be necessary to treat the gas by a shock-cooling process to minimize dioxin formation.
Catalytic incineration process
The catalytic incineration process is used for the purification of industrial off-gases containing combustible pollutants at a rate of approx 10 g/Nm3. It is preferably used for off-gases with a low pollutant content for which it would be necessary to add fuel for a purely thermal combustion
The off-gas composition and temperature should not vary too heavily to exclude damages to the catalysts by thermal and mechanical overloading. The composition of the off-gas stream is crucial for the choice of the catalyst. If there is catalyst poisons such as phosphoric compounds or heavy metals the process may not be applied.
Process description
The exhaust air containing pollutants is preheated up to the ignition temperature of the catalyst. This is done by waste heat utilization from the clean gas. In stationary operation power requirement is in most cases limited to the exhaust air blower. The process data are determined by the reactor type and the catalyst applied.
Typical examples of exhaust air streams that can be treated by a thermal or catalytic process are:
- Residual gases from pharmaceutical plants
- Off-gases from tank aeration
- Process gases with odour load
- Exhaust-air with solvent load
Residual gases from pharmaceutical plant
Catalytic incineration for off-gas purification plant
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