Unit 7 Types Of Prime Movers And Heat Engines Flashcards
Prime mover
A machine that converts a naturally-occurring source of energy into mechanical energy
5 types of naturally occurring energy sources
Potential energy, kinetic energy, chemical energy, nuclear energy, solar energy
Another way to think of a prime mover is a machine that
Directly causes Motion in another machine
The prime mover is often called a
Driver
Electric motors are not considered Prime movers, because they do not
Convert a naturally occurring energy source to mechanical energy. Rather, electric motors are considered to be secondary Movers
As a group, prime movers include
Internal combustion engines:
- gasoline engines
- diesel engines
- gas turbines
External combustion engines:
- steam engines
- steam turbines
Wind turbines
Water turbines
Some prime movers are also
Heat engines
Heat engine
Converts heat energy to mechanical energy through a series of repetitive thermodynamic operations. Such as, combustion, compression, expansion, boiling, condensation and Cooling.
A heat engine may also be a complex system of
Various machines that, when working together, completes the necessary thermodynamic processes.
Rankine cycle heat engine
Complex system consisting of a boiler, a water pump, a condenser, a prime mover, and a heat source.
3 heat engine examples
Gasoline engine, diesel engine, gas turbine
What made steam engines obsolete
Steam turbines, combustion engines, and electric motors
Double acting steam engine
Steam pressure is alternatively applied to the top or bottom of the piston. The engine thus produces power on both the upward and downward stroke.
Expansive use of steam
Cutting off the steam Supply and allowing the steam in the cylinder to expand. Results in the most economical engine operation.
Steam engine Eccentric
A disk fixed on the crankshaft in such a way that the centre of the discs is eccentric or off-center with the centre of the shaft.
Steam engine crankshaft converts the
Reciprocating motion to a rotary motion
Steam engine crosshead
Guides the Piston Rod directly into the cylinder, without any side-to-side motion. Also transfers the reciprocating motion of the Piston Rod to the rotational motion of the crankshaft via the connecting rod
In most steam engines, the admission of steam stops
Before the end of the stroke, to allow work to be done by steam expansion. This develops a greater force on the Piston at the beginning of the stroke instead of at the end of the stroke.
Steam engine flywheel
Fitted to dampen or even out the changes of speed caused by varying steam pressure and loads
Steam turbines convert heat energy to mechanical energy. They do this by
Directing high-velocity steam onto shaft mounted disks with moving blades attached. The action of the Steam on the moving blades produces shaft rotation
In most steam turbines, the steam flows
In the axial Direction. Which is parallel to the shaft
Two basic types of steam turbines
Impulse and reaction. They differ in how the steam expands through the turbine
Reaction turbines
Expansion, pressure drop, and velocity increase of the steam takes place through both stationary and moving nozzles
Impulse turbine
High pressure steam expands as it procedes through stationary nozzles. This expansion and pressure drop creates Jets of high velocity Steam. The drop in steam pressure only occurs in stationary nozzles
Momentum
The product of mass times velocity
Impulse
Change in momentum over a period of time
Steam turbine bearings
Precision board with high-grade Babbitt, have a split sleeve and Oil ring. Can be removed without disturbing the wheel case cover
Steam turbine sealing glands
Consists of several segmental carbon rings, mounted adjacent to each other at either end of the shaft.
Steam turbine rotor and shaft assembly
The rotor is a carefully machined and balanced forged steel discs, pressed over a key on a shaft
Steam turbine blades
Stainless steel blades held securely in machined slots in the wheel Rim by Drive screws
Steam turbine blade ends
Shrouded to confine steam to the blade passage and to stiffen the blades against vibration
Steam turbine casing
Subject to exhaust pressure only
The condenser serves 3 important functions in condensing Steam
1- High vacuum that is produced increases the pressure drop in the turbine, and produces more work and higher efficiency
2- condensate provides a clean source of boiler feed water
- Remove air/non-combustible gases from steam/condensate (like a daerator)
More lubricating oil is supplied to the steam turbine bearings then that required because
Carry away the heat conducted along the shaft from the steam space and to maintain the bearings at a safe working temperature
Small turbines generally use _________ for shaft sealing
Carbon sealing rings
Carbon sealing rings
Contain graphite and are self-lubricating
Labyrinth glands
Usually used by High output machines to prevent The Escape of steam along the shaft. Offers a very narrow and winding path to the steam
turbine governor
Automatically regulate the speed and power output of the turbine at various load conditions. The governor automatically controls the steam flow through the turbine by adjusting the steam control valve.
Many turbine Governors work by
Sensing the turbine shaft speed, and then positioning a governor valve by a variety of mechanical means.
Most turbine Governors are mechanical or
Mechanical-hydraulic
Two types of turbine governing systems
Flyweight and oil pump
Flyweight governing system
Resolving weights move in accordance to changes in turbine speed. The change in the flyweight Position will change the governor valve position, which will then change the steam flow to the turbine
In a flyweight governing system, when turbine speed increases
The flyweights compress a spring until the increased spring Force balances the flyweight Force. The governor’s sleeve, which moves independent of the flyweight system, then moves upward.
Turbine overspeed trip
Relies on centrifugal force to release some latch, which in turn, closes the steam supply valve
Trip pin, also called an overspeed bolt
Spring loaded weight mounted in the turbine shaft, which senses overspeed
First step when starting a small steam turbine driving a feed water pump
Prepare the feedwater pump
First step for stopping a small steam turbine driving a feed water pump
Gradually reduce feedwater pump load
Condensers are heat exchangers that come in many forms. They are commonly used to
Condense steam into water, for reuse as boiler feed water. In refrigeration systems, condensers convert hot refrigerant gas to liquid, for reuse in evaporators
In Steam Plant use, condensers help reduce
Back pressure on Steam turbines. More energy can be extracted from a steam turbine that exhausts into a vacuum then one exhausting against back pressure
The largest heat exchanger in the Steam Plant is the
Condenser. It condenses the turbine exhaust steam back to water, which returns to the boiler as feed water
The primary purpose of a condenser in a steam power plant is to
Improve the overall efficiency of the plant
Another purpose of the condenser is to remove air
And other non condensable gases from the steam/condensate. This reduces their concentration in the system’s Downstream of the turbine and condenser. In effect, the condenser acts like a daerator
The largest single heat loss in the steam cycle is
Condensing the steam. Because the latent heat of the steam entering the condenser transfers to the cooling water, and then dissipates into the atmosphere via a cooling tower
For the turbine to extract the maximum amount of work from the steam
Condensing the steam must occur at the lowest practical pressure
Two main types of condensers
Contacts and surface
Contact condensers, also called jet condensers
Operate by bringing exhaust Steam and cooling water into direct contact with each other. The steam mingles with the cooling water, condenses, and the condensate leaves the condenser with the cooling water
Disadvantage of direct contact condensers
They require the cooling water to be chemically treated to avoid contamination of the condensate and maintain feed water purity. There are few in service
Surface condensers
Far more common. They have a barrier to prevent contact between the exhaust Steam and the cooling medium. Heat is transferred from the steam, through the separating surface to the cooling medium
Water-cooled condenser
Cooling water is pumped through small diameter tubes. The exhaust steam flows over and around these tubes. The condensate is collected from the bottom of the condenser shell
Ecological problems from higher temperature water
Algae or bacterial counts, or water that is too warm for Native fish to live and breed.
Cooling towers take heated water and
Reduce its temperature for reuse as a coolant
Cooling tower principle of operation
Heated water is pumped to the top of the tower. The water is distributed in the tower by spray nozzles and splash bars. This exposes a water two atmospheric air, aiding evaporation. Dry atmospheric air circulates through the tower, warms up, and carried away warm humid air, leaving the remaining water cool.
Four methods used to circulate air in cooling towers
Fans, convection currents, natural wind currents, induction effects from water sprays
Most of the temperature increase of the air and drop of the cooling water, is due to the
Latent heat of evaporation of the cooling water
The amount of water lost as water vapour leaving the cooling tower is due to
The amount of drift and the blowdown rate
After the water drops to the bottom of the cooling tower,
The water collects in a basin and is pumped back to the condenser
The rate of heat transfer in any cooling tower system depends on
4
Velocity of air and water during contact
Area of water in contact with air
Length of contacts time between air and water
Difference between Inlet water temperature and relative humidity of the air
Equipment that needs cool water to function properly
4
Bearings, Lube oil coolers, internal combustion engine cooling systems, compressor cooling systems, Etc
6 basic components of a cooling tower
Inlet water Distributing sprays, baffles, air moving equipment, Inlet air louvres, drift eliminators, cool water basin
2 air circulating methods for cooling towers
Natural draft and mechanical draft
Two kinds of natural draft cooling towers
Atmospheric Towers and chimney Towers
Atmospheric Towers
Air movement is dependant on atmospheric conditions. The sides have louvres to direct airflow and reduce water loss by mist. These only operate efficiently in locations with constant winds and large Open Spaces
Chimney cooling towers (hyperbolic towers)
Made with reinforced concrete. Air Inlet and water distribution and flil are similar to a mechanical draft Tower. Majority of tower height is purely chimney, to induce natural convection air flow
Chimney cooling towers are mainly used
In large generating stations
Mechanical draft cooling towers
Use one or more fans to move large quantities of air through the tower
Two subclasses of mechanical draft cooling towers
Forced draft and induced draft
Airflow in mechanical draft cooling towers can either be
Cross-flow or counterflow with respect to the falling water
Counterflow Towers indicate
The airflow is in the opposite direction of the falling water
Cross-flow Towers indicate
The airflow is perpendicular to the flow of water
Counterflow Towers
Occupy less floor space, but are taller to accommodate a given capacity. Has a low pressure drop in relation to its capacity. Also has a lower fan power requirement.
Cooling towers should be located
As close as possible to the systems they serve
Forced draft cooling towers
Fan is located at the base. There are no Louvred exterior walls.
Drift eliminators
Remove water entrained in the air
Induced draft cooling towers
One or more fans located at the top of the tower. Fans draw air upward against the downward flow of water
Dry cooling towers
Used where cooling water supply is unavailable or restricted. Uses a closed circuit method which eliminates contact between the water to be cooled and the coolant air. This eliminates water loss by evaporation and drift, and there is no makeup water required. Similar to an automobile radiator
Ice formation on the inside of cooling towers
Action must be taken, as this will jeopardize heat transfer. Heavy ice May overload the cooling tower structure, causing supporting members to break
Severe ice formation on louvres
May require that the fans be reversed for a period of time. This changes the pattern of falling water, and brings warm water in contact with the ice, so it can rapidly melt
Cooling tower Air flow control, with 2 speed or variable speed fan motors, can reduce
The amount of cold air that passes through the tower
When a cooling tower is shut down, the water basin
Must not freeze
Cooling tower “drift” refers to
When water particles become entrained with the flow of air leaving the cooling tower
Drift results in
3
Increased makeup water requirements and water treatment, reduction of cooling capacity
Drift must be eliminated or reduced to a minimum
Four factors which can cause excessive water
Missing louvres, incorrectly placed or plugged Splash bars, drift eliminators that are missing or out of place, over-pumping
If a fan motor fails to start there may be a
Control issue or a motor issue
The simple gas turbine has an
Upstream compressor attached to a downstream turbine, with a combustion chamber between them. The turbine draws air into a compressor and then discharging compressed air into the combustion chamber. There, fuel is added and burned, which further heats the air. The hot air expands through the turbine to provide Power about two-thirds of this power is used to drive the compressor
The output of a gas turbine increases when
Operating with Inlet air of high-density. Cold air at the compressor intake produces an increase in gas turbine output
Two basic types of gas turbines
Aeroderivative gas turbines, which are derived from the jet engines used in aircraft
Heavy-duty gas turbine, which are designed only for land-based applications
In the basic gas turbine, air is drawn in
Compressed, heated, and finally expanded through turbine blading
Aeroderivative gas turbine
Consists of two turbines. One is on the same shaft as the compressor. The output of this turbine is entirely absorbed by driving the compressor. The other turbine is free of the compressor. This second turbine Drives the propeller or the load
Small gas turbines are often used to produce power for
Portable and standby generators, fire pumps and compressors. Can also be used to generate electricity in stationery plants
A gas turbine is not self starting. Therefore, it must be
Rotated at 20 to 30% of its maximum speed before fuel is turned on. This rotation is done to give sufficient air compression so that, when fuel is injected, the gas turbine power will be able to drive the compressor and maintain the speed rise. Needs a starting motor
Simple gas turbine
Has no heat exchanger, regenerator, or intercooler
Open cycle gas turbine
Draws the air used to drive the turbine from the atmosphere, and Returns the air to the atmosphere after use
Closed cycle gas turbine
Recycles the working fluid back to the compressor Inlet
Single shaft gas turbine
Only has one shaft. The gas turbine and compressor are mechanically coupled together on the same shaft
The majority of gas turbines in industrial use are
Simple, open cycle, single shaft machines
Gas turbine advantages
5
High power to weight ratio Low installed cost Low maintenance and operating cost Minimum cooling water Rapid start up and loading
Gas turbine disadvantages
4
Low mechanical and thermal efficiency
High noise level
Limited types of fuel
NOx emissions
Gas turbine fuels
Limited to clean, liquid, or gaseous fuels. They cannot burn solid fuels
Gas turbine cooling water
Required minimum cooling water. Ideal for use wherever water is scarce or unobtainable
Because of the Simplicity of gas turbines
It eliminates many auxiliary is required in a steam plant
Gas turbine NOx emissions
High emissions due to high combustion temperatures and short residence time
The purpose of a regenerator is to
Improve the cycle efficiency by recovering some of the heat that would otherwise pass to waste with the exhaust gases
Regenerator placement
Placed in the air flow, after the compressor and before the combustion chamber. Because of the placement, exhaust gases from the turbine heat the compressed air before it enters the combustion chamber. The compressor works most efficiently with cold air. The heat recovered from the exhaust gas reduces the amount of fuel required to produce the same load
Combined cycle with gas turbine and steam boiler
Pressurize the boiler furnace with the air leaving the compressor. Then, hot gases pass from the boiler through a gas turbine to drive the compressor, and the alternator. The turbine contributes to the total plant capacity and overall plant economy. Allows for smaller boiler size
Air with high barometric pressure is
Denser than air with low barometric pressure
A change in either temperature or barometric pressure can dramatically change the output of a gas turbine because
The gas turbine power output depends on the mass of air flow, which is dependant on density
Changes in atmospheric pressure and temperature affect the performance of
3
Gas turbines, internal combustion engines, and boilers
Three methods of starting a gas turbine
Manual, semi-automatic, fully automatic
Gas turbine manual start
Operator starts all auxiliary systems, and raises the gas turbine RPM to the minimum Governor setting
Gas turbine semi automatic start
Operator only starts the auxiliary systems
Gas turbine fully automatic start
Operator pushes a start button. The turbine starts, and follows a timed procedure
Flyball type governors
Widely used with small turbine installations to control speed
Turbine High differential pressure air intake
Indication of dirty air Inlet filters
Gas turbine lubrication
Gear pumps are preferred, directly driven by the shaft
The gas turbine process is controlled by adjusting the fuel flow to
The combustor. Since very high temperatures cannot be measured directly, the temperature is calculated from the gas turbine and compressor discharge temperature
Gas turbine fuel pumps
Volumetric or centrifugal type
Gas turbine fire protection
Pressurized Halon or carbon dioxide nozzles are activated in the event of a fire
Gas turbines experience a loss of ______ in power efficiency for every ______ hours of service
0.5%
1000
Four most common fuels used by internal combustion engines
Natural gas, gasoline, light fuel oil, heavy fuel oil
Natural gas engines
Can be two or four stroke cycle. Ignition can be a hot ignition tube or electric Spark. Governing is by throttling
Octane rating
Measure of the fuels resistance to premature detonation which causes engine knock
Gasoline engine
Can be two or four stroke cycle. Ignition is by Spark. Fuel is in liquid form and vaporizes when it is drawn or ejected into engine cylinder. Governing takes place by throttling the air and fuel mixture
Compression ignition engine meaning
Fuel ignites due to high temperature developed during compression of the combustion air charge
Internal combustion engines are grouped according to
The number of Strokes in a working cycle
Working cycles for internal combustion engines include
Two and four stroke cycle spark ignition and compression ignition
The four stroke cycle engine was developed before
The two stroke engine
A stroke
The complete movement of a piston in One Direction, corresponding to 1/2 revolution of a crankshaft
Intake stroke
Inlet valve is open while the Piston moves down. This draws a mixture of gasoline and air into the cylinder
Compression stroke
Inlet valve is closed and the Piston moves upward, which compresses the mixture in the combustion chamber. Near the upper end of the stroke, the spark plug is timed to ignite the mixture
Powerstroke
The mixture Burns and generates a high pressure which forces the Piston down words. This downward motion forces the crankshaft to turn and produce useful power. Near the end of this stroke, the exhaust valve opens to begin the removal of burned gases from the cylinder
Exhaust stroke
The exhaust valve remains open while the Piston moves upward. This pushes out most of the remaining four in the gases in preparation for the next stroke.
Poppet valves
Intake and exhaust valves
The four stroke engine is usually lubricated by
Oil in its crankcase, which is either pumped Under Pressure to the bearing surfaces, or splashed by rotation of the crank webs
Piston rings
Limit the amount of oil entering the combustion chamber, and keep the carbon deposits to a minimum
4 stroke spark ignition advantages and disadvantages
Good fuel economy, control at all speeds, and high torque at low speeds. However, they are more mechanically complex and heavier than a two stroke engine
2 stroke spark ignition cycle
Noted for its simplicity. Utilizes three openings or ports in it’s cylinder walls. The openings are covered or uncovered by the piston. There are no poppet valves and no camshaft
2 stroke spark ignition: intake and exhaust functions happen
Simultaneously during only a part of one stroke. The two stroke engine produces a power stroke for every crankshaft Revolution
A 2-stroke spark ignition engine is lubricated by
Thoroughly mixing a measured quantity of special lubricating oil with the gasoline in the tank
Why does a 2-stroke engine not produce twice as much power as a 4-stroke engine
Because it is less efficient at exhaust gas Scavenging, which is the removal of combustion products in the cylinder
2 stroke spark ignition engines operate best at
High engine speeds
Differences with diesel engine
2
1- fuel is injected at high pressure, directly into the engine cylinder and I finally atomized form
2- ignition takes place without a spark
4 stroke compression ignition cycle
Suction stroke, compression stroke, Power Stroke, exhaust stroke
2 stroke compression ignition cycle
Do not have an induction stroke, and cannot draw in adequate combustion air.
Two most common methods of air induction and exhaust Scavenging for 2-stroke compression ignition cycle engines
Superchargers and turbochargers
4 stroke diesel engine ignition
Use compression ignition, and have no carburetor. Fuel oil is delivered to the cylinder from a fuel injector pump and fuel injector nozzle, mixes with air in the cylinder
The majority of stationary industrial diesel engines are started
With the use of compressed air. The goal is to turn the engine crankshaft which will then produce a high enough temperature in the air charge to ignite the fuel when it is injected into the cylinder
Large diesel engines can be cooled with
Open or closed cooling
Open cooling system
Water that circulates within the engine is sourced directly from a Lake, River, Ocean, or pond. Open systems require considerable water treatment
Diesel closed cooling system
Engine coolant is recirculated, through a water or air cooled heat exchanger.
