Exam Flashcards
(167 cards)
1
Q
Project Phases of E.P.C Companies
A
1) Planning and Designing
2) Procurement
3) Construction - done by sub-contractors; but both the client (owner) and EPC firms have site representatives. Regular inspections will be done on the construction site.
4) Commissioning - Equipment is cleaned (blown, flushed, purged), pumps, meters, controls are energized, checked and adjusted. Process is first tested with water then with the actual chemicals (Start-up). This process is carefully planned and may take years.
5) Handover - to owner who will operate and maintain the plant. Training will be provided to the people who will be operating the plant.
2
Q
Operations
A
Plant Manager
- Technical
- Production
- Utilities
- Maintenance
- Admin
3
Q
Technical Section
A
- “Engineering” - analysis of operations, design of modification in existing plant, expansions, planning for shut downs/start ups, etc.
4
Q
Production Section
A
- Heart of the plant - running the plant makes profit.
- Costs the same if the plant is idle or at full capacity.
- Dominated by shifts (8 hr, 12 hr, etc.)
5
Q
Utilities Section
A
- May be blended with production
- Provides operation of utilities - steam, water, air and power.
- Normally operates with the same shift schedule as the production section.
6
Q
Maintenance Section
A
- Provides mechanical and electrical (control) services.
- Shift workers or contractors will be hired for larger projects
7
Q
Admin Section
A
- HR, Payroll, Accounting etc.
8
Q
Fine Chemicals
A
- High purity
- High Unit Value (>$10/kg)
- Often produced in small quantities (<1000 tons/year)
- E.g. Gold, heavy water, pharmaceutical products.
9
Q
Bulk Chemicals
A
- Lower unit value
- Typically low purity
- Produced in large quantities
- E.g. Fuel, fertilizers, paper
10
Q
Raw Material
A
- Could be a natural material that is extracted or recovered at the plant site; e.g. NaOH for making soap, NiCL, for preparing and electrolytic solution.
11
Q
Process - “creating” a product
A
- Common case is a reaction
- The vessel is called a reactor.
- The reaction converts the raw material to a product
- May have multiple stages with multiple reactions and processes.
12
Q
Purification
A
- Most chemical reactions produce more than one product
- Require pure products not mixtures
- Most of the products can be sold, those that cannot are termed as “waste products”
- Sometimes the purification process is a reaction that occurs to precipitate certain materials which can then be separated from a solution
- Sometimes it involves removing an unwanted solvent, commonly by heating.
13
Q
Batch Processes
A
- Shorter time periods.
- Can create a variety of products in limited quantities, e.g polymers.
- Less control required.
- Can be operated manually.
- Production can be done in stages with intermediate storage.
- Plant will not operate on a shift basis.
14
Q
Continuous Process
A
- Continuous flow of feed and product streams.
- Can create a specific type of product in very large quantities.
- Well controlled and monitored.
- Mostly automatic processes.
- Less storage space required.
- Plant operates on shift basis, 24 hours, 7 days a week.
- Production is stopped only for maintenance
15
Q
Two main types of reactors
A
Batch and Continuous
16
Q
Batch Reactors
A
- Used in a laboratory
- Reactants are placed in a test-tube, flask or beaker. Mixed together, often heated for the reaction to take place and are then cooled. The products are poured out and, if necessary. purified.
17
Q
Continuous Reactors
A
- Reactants are continuously fed into the reactor at one point, and products are withdrawn continuously from another point. There must be an equal flow rate of reactants and products.
- Large quantities of chemical can be produced.
- Operated hours, 7 days a week.
- Product has a more consistent quality than a batch reactor because the reaction parameters (eg, residence time, temperature and pressure) are better controlled.
- Produce less waste and require lower storage of both raw materials and products.
- Capital costs per tonne of product produced are consequently lower.
18
Q
Main Disadvantage of a Continuous Reactor
A
Their lack of flexibility.
19
Q
Tubular Reactors (Continuous)
A
- AKA plug flow reactors.
- Fluids (gases and/or liquids) flow through hollow tubes at high velocities.
- Mixing provided by turbulence.
- High turbulence is achieved in small diameter pipes (high velocity)
- As reactants flow along the tubes/pipes which may be heated or cooled, they are converted to products.
- There is little/no back mixing. The conditions are referred to as plug flow.
- Reduces side reactions and increases the yield of the desired product.
- With constant flow rate, changes in time of the reaction are measure in terms of the position along the length of tube.
- Reaction rate is faster at pipe inlet.
- Reaction rate reduces as reactants flow through pipe due to the decrease in concentration of the reactant.
20
Q
Advantages if Tubular Reactors
A
- Easy to maintain and clean - no moving parts.
- Simple and good for rapid reactions.
- Low pressure drops.
- High conversion per unit volume.
21
Q
Disadvantages of Tubular Reactors
A
- Difficult to control - temperature and composition varies along the tube.
- Hot spots may occur.
22
Q
Fixed Bed or Packed Bed Reactors (PBRs) (Continuous Reactors)
A
- Gases flow through a fixed bed of solid catalyst.
23
Q
Advantages of Fixed Bed/Packed Bed Reactors
A
- Construction, operation and maintenance costs are low.
- Good for high pressure and high temperature systems.
24
Q
Disadvantages of Fixed Bed/Packed Bed Reactors
A
- Difficult to control.
- Channeling may occur.
- Temperature variations may occur.
- Side reactions possible.
25
Fluidized Bed Reactors (FBRs)
- Fine catalyst particles are placed on a distributor plate.
- When the gaseous reactants pass through the distributor plate, the particles are carried with the gases forming a fluid.
- The ensures very good mixing of reactants with the catalyst, increasing contact area and enabling good heat transfer.
- Rapid reactions reduces the variability of the process conditions.
26
Applications of Fluidized Bed Reactors
- Oxychlorination of vinyl chloride.
- Catalytic cracking processes.
- Oxidation of naphthalene to phtalic anhydride.
- Roasting of sulfide ores.
- Coking of petroleum residues.
- Calcination of limestone.
27
Applications of Tubular Reactors
- Steam cracking of ethane, propane and butane and naphtha to produce alkenes.
- Gasoline production.
- Oil cracking.
- Synthesis of ammonia from its elements
- Bioreactors and the oxidation reactions eg. oxidation of sulfur dioxide to sulfur trioxide.
28
Applications of Fixed Bed or Packed Bed Reactors
- catalytic reactions like the manufacture of sulphuric acid (Contact process, with vanadium oxide as catalyst
- Manufacture of nitric acid and the manufacture of ammonia (Haber process, with iron as the catalyst)
29
Advantages of Fluidized Bed Reactors
- Even distribution - good efficiency and contact between reactants and/or catalyst
- Can be controlled very well
30
Disadvantages of Fluidized Bed Reactors
- Expensive contruction and maintenance
| - Large pressure drop
31
Continouse Stirred Tank Reactors
- 1+ reactants are introduced into a reactor with an impeller and products are removed continuously
- Good mixing with uniform composition throughout
- Composition at outlet is the same as in the bulk in the reactor
- Steady state is reached when the flow rate into the reactor equals the flow rate out.
- Used in a lot of applications
32
Advantages of a Continuous Stirred Tanks Reactor
- Good mixing
- Easy to construct and maintain
- Low operating cost
- Easy to clean
- Efficient heat transfer
- Easy to control
33
Disadvantages of a Continuous Stirred Tank Reactor
Lowest conversion per unit volume
34
The Shell of a Tank Reactor
- Typically made of SS, carbon steel; a chemically-resistance lining.
- Consists of three parts: side wall, top dome, bottom dome.
- Sharp corners are avoided to ensure better handling operating pressure and to allow for better mixing.
35
The Bottom of the Tank Reactor
Has a discharge port and access ports for a sample line and a temperature indicator
36
An Agitator System of a Tank Reactor
Consists of a motor; a shaft; an agitator, often called an impeller.
37
Baffles in a Tank Reactor
May be used to create a turbulence (to break up the smooth swirling motion produced by the agitator) to increase mixing efficiency.
38
Total Mass Balance Equation
INPUT - OUTPUT = ACCUMULATION
if process is at steady-state there is no accumulation of mass within the process
INPUT - OUTPUT = 0
or
INPUT=OUTPUT
39
Percent Conversion
% conversion of A = (moles of A reacted) / (moles of A fed) x 100
40
Yield of C
Yield of C = (moles of A reacted to produce C) / (total moles of A reacted) x 100
41
When do you use pumps?
Mainly used to move fluids (liquid, slurries, gases) from one place to another.
If tank A is higher than tank B, a pump may not be required because the fluid will flow naturally by gravity. However, a pump may be installed to increase the flow rate. If tank A is lower or at the same level as tank B, a pump will be required.
42
Main components of a pump
Casing, inlet, outlet, component to move fluid
43
Casing of a Pump
Purpose is to contain the liquid inside the pump during operation and to hold the pump's internal parts
44
Inlet of a Pump
AKA the suction side. Liquid enters the pump through the inlet.
45
Outlet of a pump
AKA the discharge side. Liquid leaves the pump through the outlet.
46
Component to Move Fluid (component of a pump)
A component that physically moves the liquid through the pump. The type depends on the design of the pump.
47
What does the type of pump required depend on?
- Type of liquid that is pumped
- Viscosity of liquid
- Distance and height that the liquid must travel through the piping system
48
Two main categories of pumps
1) Positive Displacement Pumps - can be rotary or reciprocating
2) Centrifugal Pumps
49
Positive Displacement Pump
Uses a piston, diaphragm or some other device to push or positively displace controlled amounts of liquid. Good for precision, small volume and viscous fluids.
50
Centrifugal Pump
Uses centrifugal forces to move the liquid (pushes the liquid outward, away from the centre of rotation). Good for transferring low viscous fluids in large volumes.
51
Rotary Pumps
- Displaces liquid with a rotary motion using gears, screws, vanes, diaphragms etc.
Example is a gear pump
52
Gear Pump
- Rotary pump
- Comprises of two gears, suction port, discharge port and a casing.
- One gear is moved by a motor (known as the driver gear)
- Other gear (idler) moves because its teeth are meshes with the teeth of the driver gear
- Liquid enters by suction and is trapped in the spaces, as the gear rotates, the liquid is moved and displaced into the discharge port
- Used for slurries, metering, high pressure hydraulic liquids
- Limited capacity vs high pressure
53
Piston Pump
- Reciprocating pump
- Uses a reciprocating, or back-and-forth motion to move liquid by using a piston.
- Used mainly for gases, e.g. air compressors
- Piston moves back, suction inlet check valve allows fluid to enter cylinder.
- Piston moves forward, creates high pressure, closes inlet valve and forces fluid out through the outlet check valve
54
Diaphragm Pump
- Reciprocating Pump
- Uses a reciprocating, or back-and-forth motion to move liquid by using a diaphragm
- Used mainly for slurries, small volume and high viscosity fluid.
55
Centrifugal Pumps
- Pattern of the flow is always into the centre and out at a tangent to the circle.
- Used for large, low viscous fluid flows.
- Liquid enters at the center.
- Shaft and impeller rotates
- Rapid circular motion creates centrifugal force. It also creates a suction in the suction eye allowing more liquid to enter.
- The vanes on the impeller do not scoop the liquid, it flings it outward.
56
Multi-Stage Pump
- Series operation for increased pressure
| - Possible to have multiple impellers on a single shaft inside a single casing
57
Flow Control
Pumps are generally operated at a fixed speed but if flow control is require, it can be achieved by using 2 methods:
Method 1 : Install a control valve on the discharge side - meter runs at fixed speed.
Method 2 : Install a variable speed drive - energy efficient
58
Head of a Pump
Head is the height at which the pump can raise the water up. Water is used because it is the most common liquid.
Head is directly related to pressure - as pressure increases - pump will be able to push the water higher, therefore the head will also increase.
SUCTION HEAD = PRESSURE AT INLET
DISCHARGE HEAD = PRESSURE AT OUTLET
59
Total Head (Ht) is:
Ht = Hd - Hs
| where Hd is discharge head and Hs is suction head
60
Shutoff Head
(Total head without flow)
Max possible value because there is not flow, which means there are no frictional losses.
Greater flow rate = lower head because friction will increase
61
Flow in Pipe Depends on:
- Size of pipe (diameter)
- Material of pipe (how smooth its surface area is e.g. PVC pipes are more smooth than concrete pipes, therefore water flows more easily in PVC pipes)
- Flow rate in the pipe (linear velocity - ft/s; volumetric flow rate - L/min; mass flow rate - lb/min)
- Properties of fluid (density, viscosity, presence of solids)
62
Velocity of fluid in pipe equation
Velocity = (volumetric flow rate) / (cross sectional area)
63
Most common types of valves
Globe, gate, diaphragm, butterfly, (check) ball valves, plug valves and pressure safety valves
64
Two general types of valves
Linear and rotary
65
Globe Valve
- Most common of five types
| - Plug is attached to a stem, which is moved linearly in a cavity with a globular shape
66
Gate Valve
- Flat or wedge-shaped plate that is moved into or out of flow path to control flow characterizes the gate valve.
- Widely used for manual on/off service, but a few designs are used to control flows.
67
Diaphragm Valves
- Linear motion valves with flexible diaphragms that serve as flow closure members
- Mainly used with difficult fluids such as corrosive liquids or slurries
- Valve body can be lined with glass, plastic or teflon
- Diaphragm is normally rubber, but in some cases is teflon, which requires a high closure force
68
Butterfly Valve
- Most common rotary-motion control valve
- Butterfly valves range in size from 1/2 inch to over 200 inches.
- Only cost-effective solution for the control valve application
69
Ball Valve
- Rotary-motion valve
- Part that closes flow is a sphere with an internal passageway.
- Most widely used control valve after globe valve.
- Advances in seal design and sealing material enable ball valve to offer tight shutoff
- Because of this, it is now widely used in on/off service for batch control
70
Check Valve
- AKA non-return valve or one-way valve
- Normally allows fluid (liquid or gas) to flow through it in only one direction
- Examples are diaphragm and swing check valves.
71
Plug Valves
- Have cylindrical or conically tapered "plugs" which can be rotated inside the valve body to control flow through the valve
- Simple and often cost-effective
72
Pressure Safety (Relief) Valve
- When pressure increases, force of spring is overcome, forcing valve open
- Fluid is released relieving pressure
- When pressure decreases, safety valve will re-seal
73
Different Types of Diagrams
Block flow diagram, plant layout, process flow diagram, piping and instrumentation diagrams, isometric drawings
74
Heat Transfer Methods
Conduction, radiation, convection
75
Conduction
Heat transfer by direct physical contact between a hot object and a cooler object.
- Rate at which heat is transferred depends on delta T an conductivity of materials.
- Good conductors are preferred when heat transfer is desired: metals.
76
Radiation
- Heat transfer through space from a hot object to a cooler object
- Amount of heat transfer depends on delta T, distance between 2 objects, and the nature of the objects.
77
Convection
- Convection is heat transfer of heat by heated fluid
- Natural convection: occurs as heated fluid expands and rises
- Forced convection: is when heated fluid is moved by devices such as a fan or pump
78
Factors Affecting Heat Transfer
- Delta T between two fluids (when delta T is increased, more heat can be transferred between the fluids)
- Surface Area (increasing surface area available for heat transfer increases amount of heat transfer)
- Type and thickness of material which heat is being transferred through (heat transfers faster through thinner materials)
- Fluid flow rate (greater flow through a heat exchanger, more heat can be transferred between fluids)
79
Rate of Heat Transfer
```
Rate = (k x A x delta T) / x
Where:
k=coefficient of heat transfer, depends on material
A=heat transfer area
delta T=temperature difference
x=thickness of material
```
80
Cooling
Process stream is contacted with colder stream to reduce temperature of process fluid
81
Different Types of Cooling
Air cooling, water cooling
82
Air Cooling
- Cooling can be provided by air but:
- its low heat capacity means that very large flows are required
- wide seasonal variation in air temperature also means large variation in flow throughout the year
- Air cooling is used relatively less when compared with water cooling
83
Water Cooling
- Most common
- Water has high heat capacity so small flows are required
- Temperature of cooling water supplied is relatively constant (little variation in flow makes it easy to control)
84
Types of Water Cooling Systems
- Once through
| - Recirculating
85
Once Through Water Cooling
- Water is drawn from body of water, contacts the process and is returned to its source.
- Lower temperature of source is better, since this allows for smaller equipment
- Cooling water is taken from bottom of lakes where it is coldest in summer
- Great Lakes is most common location for process industries
86
Recirculating Water Cooling
- Cooling water recirculates continuously
- Cooling tower operates by evaporating small portion of the cooling water, the rest is cooled.
- Make-up water requires treatment to reduce solids accumulation and to prevent biological growth in system
- More complicated and more costly
- Typical temperature is 100 degrees F (40 degrees C) as opposed to great lakes temperature of 10-15 degrees C (50-60 degrees F)
87
Tube-in-Tube (Concentric)
Warm fluid circles a pipe filled with cold fluid. This cools the originally warm fluid and makes the originally cold fluid warmer.
88
Shell and Tube heat exchangers
- Most common
| - Cylindrical in shape, can be mounted horizontally or vertically
89
Shell and Tube Components
Shell is the cylindrical casing of heat exchanger.
Area within the shell and outside of tubes is called the shell side.
Tubes open into the headers.
One header directs flow into tubes, other collects fluid leaving tubes.
Tube sheets help support tubes. Also isolate headers from main part of shell, keeping two fluids separate.
90
Tubes
Shell and tube heat exchanger uses a bundle of small tubes instead of a single large tube in order to increase total surface area inside heat exchanger.
91
Baffles
Partitions called baffles are installed in a shell and tube heat exchanger.
Help support tubes, but their major advantage is they increase efficiency of heat transfer by directing flow of fluid through shell, which helps to minimize areas of little or no fluid flow
92
Multi-Pass Heat Exchangers
- Tube-side fluid flows past shell-side fluid ore than once.
| - To allow longer time for heat transfer between two fluids
93
Parallel Flow
- Shell-side fluid and tube-side fluid move in same direction.
- Amount of heat transfer is greatest where the fluids enter heat exchanger
- By the time fluids are near outlets, there is little or no temperature difference, so little or no heat is transferred
94
Counter Flow
- Most commonly used type of flow.
- Shell-side fluid and tube-side fluid move in opposite directions
- Temperature difference remains relatively constant throughout heat exchanger
- Heat transfer can take place the entire time they are in heat exchanger.
- Discharge temperature of fluid being heated can actually be higher than the discharge temperature of fluid doing heating
95
Cross Flow
- Shell-side fluid enters shell through shell inlet, and flows across tubes, somewhat perpendicular to flow of tube-side fluid
- Often found in heat exchangers that use steam or other vapours on shell side, which are cooled by the cooling fluid carried by tubes and condensed into liquids.
96
Fouling
- Build-up of deposits (called scale) on the internal surfaces of heat exchanger, caused by impurities in the fluids passing through a heat exchanger
- When fouling substances accumulate on the tube walls, they form a layer of material that reduces the effectiveness of heat transfer through the tubes.
- Fouling reduces the size of the tube openings and restricts the flow of fluids through the tubes
97
One major cause of scale formation
Calcium dissolved in cooling water.
| Also algae and bacteria can grow on heat exchanger parts and form slime
98
Corrosion
- Impurities can react with metal and cause corrosion products to build up.
- Eats away at metal and can cause a leak.
- Chemicals may be added to cooling water to prevent corrosion and other problems
- Can be minimized by attaching plates of metal, usually zinc, to the inside of a heat exchanger. Impurities that cause corrosion react more readily with zinc, leaving the heat exchanger not affected.
99
What is a boiler?
(AKA steam generator) is a pressure vessel in which water is converted to steam by the application of heat.
100
Uses of Boilers
```
Heating
-domestic heating systems
-industrial process heating
Electricity Generation
-drive steam turbine to generate electricity
Transportation
-steam locomotives
-marine transport
```
101
Classification of Boilers
(boilers are classified by heat source)
Coal: most common, abundance of coal and cheap.
Oil: expensive and commonly used in oil refineries.
Gas: most clean.
Electric: expensive, few large scale boilers.
Nuclear: use for electricity generation.
Wast Heat: waste gases from industrial processes.
Others: wood, solid waste (bagasse from sugarcane), sawdust, garbage.
102
A Modern Boiler Consists of:
- A circuit of metal tubes supplied with water.
- Combustion chamber where fuel is burned.
- Fuel, air, water, and exhaust systems.
103
Types of Boilers
Fire-tube boilers and water-tube boilers.
104
Fire-tube boiler
- Combustion gases are passed through tubes that are immersed in water - water outside the tubes boil to produce steam.
- Many small and medium-size unites are fire-tube boilers.
- AKA "shel" boilers because the whole of steam-producing elements are housed within a single outer shell.
- Relatively small diameter (~3-4 in. or 7.5-10 cm)
- Tubes connected to tube sheets at each end of cylindrical shell
105
Water-tube Boiler
- Water is passed through metal tubes.
- Hot combustion gases pass over tubes - water inside the tubes boil to produce steam.
- Large boilers are water-tube boilers
- Because steam is in tube space, they are well suitable for high steam pressures
- Used when large amounts of high pressure steam are needed
106
Horizontal Return Tube
- Fire-tube boiler.
- Straight and connected at their ends by tube sheets.
- Water surrounding these tubes and contained in shell boils into steam.
- Space above water level serves for steam generation and storage.
107
Applications of Fire-Tube Boilers
- Many small-medium capacity boilers are fire-tube boilers.
- Max operating pressure is 250 psi (17 atm).
- Large diameter shells cannot withstand high pressure.
- Used in: small industries, hospitals, schools, commercial businesses, residential buildings.
108
Advantages of Fir-tube boilers
- Simple and rugged construction.
- Low capital cost.
- Small foot print.
- Low installation time.
109
Disadvantages of Fire-Tube boilers
- Large water volume and poor water circulation, which results in slow response to steam demand.
- Shells are large and cannot be economically built to withstand high pressure.
- Drums and joints are exposed to the furnace increasing likelihood of explosion.
- The P.T, and the amount of steam produced are not as high as for water-tube boilers.
110
Types of Fire-Tube Boilers
* May be designed for vertical, inclined, or horizontal positions and are classified:*
- Horizontal-return tube (HRT) boiler
- HRT with a refractory lined fire box
- Scotch marine or internal furnace boiler
- Packaged fire-tube boiler
111
Horizontal Return-Tubular (HRT) Design
* Fire-tube boiler*
- Most common design of fire tubes
- Size varies 4-15 ft diameter, 6-40 ft length
- Fire tubes are straight and connected at their ends by tube sheets
- Water surrounding these tubes and contained in a shell boils into steam.
- Space above the water level serves for steam generation and storage
112
HRT Boiler with a Refractory Lined Fire Box
* Fire-tube boiler*
- A variation of a horizontal return tube boiler has a firebox lined with refractory brick,
- Usually burn coal, but can also work on gas or oil.
- Hot combustion gases pass through tubes located in the upper portion of the furnace
- Water in outer shell is heated and converted to steam
113
Scotch Type Boiler
* Fire-tube boiler*
- AKA internal furnace boilers or Scotch marine
- Furnace and tubes are contained within the shell
- Furnace is a large tube made of metal
- Fire tubes run in length of shell at sides of, and above the internal furnace
- Size: 3-8 ft diameter, 4-20 ft length
114
Packaged Fire-Tube Boilers
Includes ancillary components like valves, gauges and are delivered and installed as a package.
115
Safety Issues with Fire-Tube Boilers
Regular inspection and maintenance is required to ensure that all the safety measures are taken into account.
116
Modern Water-Tube Boilers
- Are of bent-tube design
- Small diameter tubes are used; these can withstand higher pressure and temperature than fire tubes
- Can operate at pressures up to 5000 psi (340 atm) and temperatures up to 1000 F (540 C)
- Steam is collected in steam drums
117
Boiler Ratings
By:
- Capacity (pounds of steam generated per hour)
- Total heating surface (sq. ft)
- Horsepower (1 boiler hp = 33475 BTU/hour = 35316 kJ/hr)
- Pressure: below 900 psi; 900-1000 psi; 1200-1500 psi; 1800-2500 psi; 3500-5000 psi.
- Heat input capacity (amount of heat generated by burning fuel in the furnace (BTU/hr, J/hr))
118
Energy Efficiency
-Modern boilers have energy efficiencies from 70% to 96%, depending on design and the type of fuel used
119
Efficient Boiler Operation
- Burner controls should be always set to provide some amount of excess air, usually from 2% - 5%
- Excess air incurs a heat loss; it enters the combustion system at ambient T and leaves at stack T
- Reducing oxygen level in flue gas will reduce heat loss
120
Environmental Impacts: CO2
Source: complete combustion of carbon in fuel.
Effect: global warming
121
Environmental Impacts: CO
Source: incomplete combustion of carbon in fuel
Effects: smog, toxic to live organisms
122
Environmental Impacts: SO2
Source: combustion of sulphur in fuel
Effects: smog, acid rain
123
Environmental Impacts: NOx
Source: by-product of most combustion processes
Effect: Acid rain
124
Environmental Impacts: N2O (nitrous oxide)
Source: by-product of some combustion processes
Effect: global warming
125
Environmental Impacts: VOCs (volatile organic compounds)
Source: leakage and evaporation of liquid fuels
Effect: smog
126
Environmental Impacts: Particulates (dust, soot, fumes)
Source: unburned or partially burned carbon and hydrocarbons; also ash and dirt in fuel
Effect: smog
127
How do you know whether scaling is a problem?
- Flue gas temp is indirect indicator
- If flue temp rises it is possibly due to presence of scale
- Visual inspection of boiler tubes when unit is shut down
- Scale removal can be achieved by mechanical means or by acid cleaning
128
Blow Down
- Accumulation is controlled by blowing down
- A small portion of its liquid contents is forced out the bottom carrying the dissolved and suspended solids with it
- this can be done on an intermittent or continuous basis
129
Safety Valve
- The act requires that every pressure vessel have a “safety valve”
- designed to open and to discharge the pressure (i.e. vent the steam) when the pressure exceeds the maximum safe value for the boiler
130
Why is steam used for heating?
- Water is cheap, non-reactive, safe, and non-corrosive
- High enthalpy of vaporization
- Small amount of steam can transfer a lot of heat
- Piping and equipment are small and cheap
131
Condensing Steam vs Cooling Water
- When 1 kg of steam condenses it delivers: 2257 kJ
| - When 1 kg of water cools from 50 C it delivers 209.2 kJ
132
Electromagnetic Induction
-Production of an electric field across a conductor when exposed to magnetic field
133
Transformers
-Electrical device that transfers energy between 2 circuits through electromagnetic inductionq
134
Step-up Transformer
Takes small voltage and converts it into larger output voltage
135
Step-down Transformer
Takes large input voltage and converts it into smaller output voltage
136
Transformer Parts
- Two windings of wire that are wound around a common core to provide tight elecromagnetic coupling between windings
- Core material is often laminated iron core.
- Coil that receives the electrical input energy (ac source) is referred to as the primary winding, while output coil is called secondary winding and is connected to load source
137
Transformer Functioning
As AC current in primary winding oscillates, it causes flux to oscillate which induces voltage
138
Core Form Transformer
- Economical
| - High voltage applications
139
Shell Form Transformer
- Extra high voltage
- Labor intensive
- Less damage in transit
- More common for transformer applications
140
Windings (coils)
- Made of bare conductors
- Conductor material depends on application
- Must be electrically insulated to maximize current flow
- For small power transformers (low currents): coils are wound from enameled magnet wire
- For large power transformers (high voltages): coils are wound with copper strip conductors
141
Equation for ratio of voltages in relation to turns for Transformers
```
Vs/Vp = Ns/Np
Ns/Np = n
```
142
Transformer Current Law
*Ratio of currents is inversely proportional to ratio of turns*
Ip/Is = Ns/Np
Ip/Is = Vs/Vp
143
Transformer Power Law
```
*For transformer, no electric power -- 100% efficiency*
P in(primary) = P our(secondary)
Vp x Ip = Vs x Is
```
144
Power Losses Due to Eddy Currents Induced in the Iron Core =....
I squared x R
145
Direct Current Transformers
- Batteries, fuel cells, solar cells
- Does not provide changing field needed for induction to occur
- Current in primary must start and stop to give the change necessary for induction
- Normally done with a magnetic make and break system at end of coil
146
Power Transmission
- Expensive
- High voltage and low current = long distance
- Power lost (resistive heating) is reduced
147
Heat Generation
- Transformers generate heat due to electrical and magnetic resistance
- Cooling is required
- Cooling can be achieved by a variety of methods
148
Cooling Methods
Small Transformers are self-cooled
-Natural convection and radiation heat dissipation
Larger Transformers
-Forced-air cooling
-Forced-oil cooling
-Water-cooling
Most common is to fill transformer with oil to exchange heat from the oil to atmosphere
-oil also acts as an insulator
Circulation of oil can be natural or pumped
149
Transformer Oil
```
Old transformers use oil that contained polychlorinated biphenyls-PCB's (C12H10-xClx)
-Environmental toxicity
-Human carcinogen
-PCB's are banned since 1980
Is refined mineral oil
-silicon-based oils
```
150
Liquid-Immersed Transformer
Conservator (reservoir) allows:
-for expansion and contraction of liquid
-for vent for release of pressure during explosions
Walls and fins provide heat dissipation
151
AC Transformers
Changing current of AC provides for changing magnetic field without a make and break mechanism
152
Electric Motor
- Converts electrical energy to mechanical energy
- Develops twisting effort called torque that tends to rotate shaft of motor
- Can be used to drive a machine to do work
153
Electric Motor Uses
At home: vacuum cleaner, laundry washer, sewing machine, blender, etc..
In industry: milling machines, grinding machines, pumps, agitators, conveyors, etc.
-Motors range in sizes (small fraction of horsepower to 5000 horsepower and even more (1 hp = 746 W)
154
Types of Motors
DC motors: relatively rare but used with few specialist applications related to low power and speeds, e.g. elevators, punch presses, etc...
AC motors: single and 3 phase varieties - simplest and most common type is induction motor
-Nowadays, most electric power supple systems are AC
155
Motor Ratings
Include:
- Voltage
- Current
- Power
- Speed
- Temp
- Frequency
- Torque
- Efficiency
156
Voltage Ratings
-Generally designed for supply voltages to be within +/- 10% of rated voltage
E.g. 115 V motor can be operated at full load on nominal 120 V supply and max voltage is 126.5 V (115 V + 0.1(115)V)
157
Current Ratings
- Given on motor name plate
- Running current is current motor draws when is loaded to rated horsepower, operated at rated voltage, frequency and temp
158
Power
- Horsepower rating of motor specifies amount of power available at motor shaft at rated speed
- Motor is capable of delivering more than rated horsepower
- Unless motor has service factor greater than 1.0, motor should not be operated with load greater than its rated power
- Doing so causes motor's operating temperature to exceed its design limits
159
Temperature Rating
- Designed to operate with a specified max ambient temp.
- Most designs are 40 C
- Max temp depends on type of insulating material used on motor windings
160
Four Classes of Motor Insulating Material
Class A: Max 105 C
Class B: Max 130 C
Class F: Max 155 C
Class H: Max 180 C
161
Rated Speed
- Speed is motor speed at rated horsepower
- Under light-load conditions, motor speed is greater than rated speed
- Under heavy load, it may be slower than rated speed
162
Frequency Tolerance
- For AC motors, frequency of source voltage is also specified
- Power is distributed at 60 Hz, therefore, AC motors are designed for 60 Hz (in North America)
- In Europe and many Asian countries, is 50 Hz, and motors have same designed frequency
- Most motors have frequency tolerance of +/- 5%, provding supply voltage is at rated values
163
Torque
```
Product force and distance from centre of rotation at which force exists
P=(TS)/5252
where:
P= power in hp
T is torque in lb-ft
S is speed in rpm
```
164
Efficiency of Motors
- Function of load on motor
- Designed to provide max efficiency at rated horsepower
- At half rated low, efficiency decreases about 10 %
- At no load, efficiency is 0
165
Motor Enclosures
- Structure that contains an electric motor to protect it from environment
- NEMA has developed standards for motor enclosures
- Size (height, length, shaft diameter, etc.) is indicated by frame number
- Considerable variation in frame number for given horsepower motor
- Variation is result of using different insulating core materials
- As better materials have been developed, a given frame has been able to house a motor with larger horsepower rating
- Larger frame number = larger horsepower rating
166
Motor Enclosure Classfication
Open Motors: ventilating openings which allow surrounding air to cool motor windings (coils)
Totally Enclosed: do not allow free exchange between surrounding air and air within enclosures
167
Totally Enclosed Motors
- No external fans to force air over external surface of enclosure
- Explosion of gas inside motor will not cause a like gas around motor to explode
- Waterproof motors
- Dustproof motors prevent explosive dust from entering enclosure
- Many have fins on exterior of enclosure to aid in transferring heat from motor windings to external air
- Larger ones may use circulating water or air as well as heat exchangers to air in cooling the motor
