A turbine is a device that converts energy from a high pressure fluid to work. The work can be used to turn a generator to produce electricity or it can be used to turn a shaft that will propel a vehicle forward. The key parts of the turbines are the blades connected to the shaft. The blades capture the kinetic energy from the moving fluid and use it to spin the shaft. There are two types of turbines, reaction and impulse turbines. These different types use different methods to convert the moving fluid to turn a shaft.
In an impulse turbine, the fluid is pushed through a nozzle and then shot at a high velocity onto turbine blades. The shape of these blades determines how well the kinetic energy is captured and converted to spin the shaft. An impulse turbine is called an impulse because the fluid is shot at the blades in impulses. As the shaft spins a new blade is exposed to the nozzle and is shot, then the shaft spins and another blade is shot.
In a reaction type turbine, the fluid flows much more smoothly past the blades. The fluid does not have a drastic change in direction when it hits the blades. The fluid moves quickly past the blades, which causes the blades to move in the same direction as the fluid.
Remember that the turbine is part of an overall energy/power system. The turbine uses high pressure steam/gas to turn a shaft, which then produces useful work.
Steam turbines are simply turbines with steam as the fluid. Steam turbines are commonly used in power plants and in the Rankine cycle exam questions. Gas turbines are simply turbines with air as the fluid. Gas turbines are commonly used in diesel generators, aircrafts, trains and ships. Gas turbines are used in the Brayton cycle exam questions. Turbines are discussed more in the Combined Cycles section.
Remember that each piece of equipment fits into an overall energy/power system. See the combined cycle page for the overall system. The previous section discussed the turbine which uses high pressure gas/steam to create electricity. The boiler and steam generators are what produce the steam. The boiler and steam generator do not increase the pressure of the water. The increase in pressure is done at the feed water pump. These pieces of equipment simply convert water to steam.
Steam Generation System
It is important for you to understand the three different systems that comprise a steam generation system, the (1) Feedwater System, (2) Combustion System and (3) Steam System.
(1) The feed-water system describes the incoming fluid water to the boiler. It consists of a feed-water pump, water softeners to remove minerals that can damage boilers and de-aerators to remove oxygen. Feedwater is provided by a mixture of the water supply and condensate return. The important part of the feed water system is to be able to determine the entering enthalpy of the feed water, depending on the pressure and temperature of the incoming water. As previously discussed, water in the sub-cooled region has enthalpy values that are a function of temperature.
(2) The combustion system describes the fuel portion of the boiler. The combustion system consists of oxygen supply, typically provided by a fan or air is naturally induced, an ignition, a fuel supply and piping. It is important to be able to determine the total heat supplied by the fuel. Total heat is shown as “Q” is a function of the mass flow rate of the fuel, the higher heating value (HHV) of the fuel and the boiler efficiency. The HHV can be found in the Mechanical Engineering Reference Manual. Boiler efficiencies are a function of the losses and efficiencies in the system and on the exam you will either solve for the boiler efficiency or use the boiler efficiency to determine the output of the boiler.
The maximum amount of energy transferred is met if the entering condition of the 1st air stream exits the energy recovery device at the same conditions as the entering condition of the 2nd air stream. However, if one airstream has more air flow than the other, then the smallest airstream should be used.
(3) The steam system is the output portion of the boiler. It consists of the outgoing steam piping to the steam consuming pieces of equipment, which in the Thermal & Fluids field are turbines and feedwater heaters. The output of the boiler is either saturated steam or a super-heated steam and the values for this steam output can be determined from the saturated steam tables or the super-heated steam tables.
Efficiency of a boiler is found by dividing the output energy by the input energy. The output is found by determining the change in enthalpy between the feed-water and the super-heated steam. The input is determined by the mass flow rate of the fuel and the higher heating value of the fuel.
Controls are not included in this guide. One of the best free resources for learning about controls and other steam topics can be found on the following website. This guide provides the main topics that are covered on the PE exam, but there may be a few minor topics that may or may not be on the exam.
The book also includes brief discussions on the different types of boilers like firetube, watertube and condensing. A section on energy balance for boilers is also provided. Finally the boiler section includes information on pressure drop in steam piping. See the technical study guide to get more information.
An internal combustion engine is an engine where the combustion of fuel is used to produce hot gases which directly drive a form of mechanical work. An internal combustion system is opposite of an external type of combustion system. In an external type of combustion system, the heat from the combustion process is used to heat up another fluid, typically water to steam, which then drives a turbine for mechanical work. In this external type of system, the combustion gases are kept separate.
There are many different types of combustion engines, but there are a few types that you should be familiar with for the exam.
A reciprocating engine is an engine that has pistons which up and down within a cylinder. This engine is also known as a piston engine. These types of engines follow the basic process of in taking fuel and air, then compressing the mixture, followed by combustion. The combustion process releases heat and the combustion products are expanded and then exhausted to the atmosphere.
The next types of engines that you should understand for the exam are the two-stroke, four-stroke and six-stroke engines. This term describes the number of strokes it takes for the combustion engine to complete a cycle.
Two Stroke Engine
A two-stroke engine takes two strokes to complete a cycle. In the first stroke, the downward movement of the piston, fresh air is added to the cylinder and exhaust air is released from the cylinder. Then on the second stroke, the fresh air is compressed and ignited. The heat released from combustion expands the gases, which drives the mechanical work of the piston.
The book also covers the four stroke engine, compression ratio, BMEP and the Otto Cycle. See the technical study guide to get more information.
Heat exchangers are mechanical devices designed to exchange or transfer heat from a hot fluid to a cold fluid. Heat exchangers are used heavily throughout the Thermal & Fluids field. For example, a condenser and boiler both are heat exchangers. A cooling or heating coil is a heat exchanger that transfers heat from one fluid to another fluid.
Types of Heat Exchangers
There are many different types of heat exchangers, for example there are heat exchanger types that are based on their construction. These types include, shell and tube, plate frame and microchannel. But for exam purposes it is more important to understand the two flow classifications of heat exchangers, (1) parallel flow and (2) counter-flow heat exchangers. These two classifications describe the relationship between the direction of flow of the cold and hot fluids.
(1) Parallel flow heat exchanger: This heat exchanger has both the cold and hot fluids entering at the same end of the heat exchanger. At the beginning of the heat exchanger there is a large difference between the cold and hot fluids and at the end of the heat exchange the difference between cold and hot is reduced, refer to the figures below.
(2) Counter-flow heat exchanger: The counter-flow heat exchanger is opposite of the parallel flow heat exchanger. The cold and hot fluids enter at opposite ends. The figure below shows the counter-flow heat exchanger, notice the opposing directional arrows.
The book also includes LMTD, heat balance and feedwater heaters. See the technical study guide to get more information.
Cooling towers are mechanical pieces of equipment that function on the principle of evaporative cooling. Evaporative cooling is the process by which a liquid is cooled to a lower temperature by evaporating a small portion of the liquid into an airstream. Relatively dry air moves through a falling liquid and as the air moves it picks up water vapor from the liquid, thereby increasing the air’s moisture content. In order for the liquid to evaporate, the liquid needs a heat source to meet the latent heat of vaporization. This heat source is the sensible heat loss from the remaining liquid.
A cooling tower consists of two fluid flows, the air flow and the water flow. The water flow starts from the top of the cooling tower. Warm water is pumped to a series of nozzles. The nozzles’ purpose is to break up the water into tiny droplets to increase the surface area of the water that is in contact with the air stream. The droplets then fall through a fill material, which also serves to break up the droplets further to increase the surface area of the water. As the water moves downward it steadily decreases in temperature as heat is lost due to evaporation. Finally, the water collects at the basin, where it is sucked out and distributed to its required location.
The air flow starts at the bottom of the tower, where cold dry air is brought into the cooling tower where it comes into contact with the water droplets. As the air moves upward through the tower it picks up water vapor and slightly increases in temperature. Prior to exiting the cooling tower, the air must travel through the drift eliminators, which is a series of baffles. The purpose of the drift eliminators is to catch any suspended water droplets in the air stream and return them to the fill.
Characterizing Cooling Towers
The following section provides information on the different types of cooling towers used in the HVAC & Refrigeration field. This information is provided to give the engineer additional background on cooling towers.
Mechanical vs. Natural Draft Cooling Towers: There are two main categories of cooling towers: (1) Mechanical draft and (2) Natural draft cooling towers. Natural draft cooling towers move air based on the difference in buoyancy of the airstream inside and outside of the cooling tower. Mechanical draft cooling towers move air through the cooling tower by means of a mechanical fan. In the Thermal & Fluids field, both types of cooling towers are used.
Induced vs. Forced Draft Cooling Towers: Induced and forced draft cooling towers are both mechanical draft type fans and differ by the location of their fan. Forced draft fans blow air into the cooling tower and are located at the airstream entrance into the cooling tower. Induced draft cooling towers, on the other hand, have the fans located at the exit of the airstream for the cooling tower and suck air into the cooling tower.
Counter-flow vs. Cross-flow Cooling Towers: Counter-flow and cross-flow cooling towers are characterized by the relationship between the air flow and water flow. In a counter flow tower the air and water flow are at 90 degrees to each other. The water is falling downwards and the air is moving across from either left to right or right to left. In a cross-flow tower, the air and water flows have directly opposing directions. The water is falling downwards and the air is moving upwards, as shown in the below figure.
In a cross-flow tower, the air and water flows have directly opposing directions. The water is falling downwards and the air is moving upwards, as shown in the below figure.
The following figure is a schematic of a forced mechanical draft, counter flow cooling tower. The fans are located at the air inlets, near the bottom of the cooling tower. Also the air flow counters the water flow as the water drops downward through the fill material.
The following figure is a schematic of a forced mechanical draft, cross flow cooling tower. Since this cooling tower is forced draft, the fans are again located at the inlet of the cooling tower near the bottom. The air flows counter or perpendicular to the water as the water falls downward through the fill.
The following figure is a schematic of an induced mechanical draft, counter flow cooling tower. The fan is located at the exit of the cooling tower and air is sucked or induced through the cooling tower. This cooling tower is also a counter flow type, where air flows upward through the fill and counters the downward moving water droplets.
The following figure is a schematic of an induced mechanical draft, cross flow cooling tower. Again the fan is located at the exit of the cooling tower. This cooling tower is a cross flow cooling tower, where air flows perpendicular through the fill as it crosses the falling water droplets.
The book also includes Cooling Tower Performance equations, range, approach, water loss, makeup water and mass/energy balances. See the technical study guide to get more information.
A condenser is a device that transfers energy from one fluid to another through the process of changing one fluid from a vapor to a liquid. In thermodynamics, condensers are primarily used in the power cycle to take exhausted steam from the turbine and condense the steam.
There are multiple types of condensers but the most popular is a surface condenser. In a surface condenser, cooling water is circulated through tubes and steam is blown over the surface of these cool tubes. As the cooling water picks up the energy from the steam, the steam loses energy and is converted to a liquid. The condensate collects at the bottom of the condenser and then the condensate is pumped back to the boiler.
If you have a question on a condenser with steam, then you will most likely need your steam tables. The steam tables will help you find the properties of steam at various pressures and temperatures. These properties will then be used in one or a few of the governing equations below.
The first governing equation describes the concept that any energy lost by the steam is gained by the water. The energy lost will include the heat of vaporization and any change in temperature of the steam and will also depend on the starting point of the steam.
If the steam is superheated, then the steam must first be lowered in temperature to the saturation temperature, then the heat of vaporization must be removed and finally the condensate must be lowered in temperature to the final exit temperature. Most of the times, the exit temperature is the same as the saturation temperature, but you should make sure.
The book also includes the mass/energy balance equations for Condensers. See the technical study guide to get more information.