COOLING TOWERS SECTION 1
OPERATION AND CONSTRUCTION
‘There are exhibits placed in the center of the book that will be referred to later in the program They should be removed and set aside now so
that they will be handy when needed
tọ
5
1 Á reñnery uses as much as 25 barrels of water for every barrel
of crude processed
A 200,000-barrel-a-day refinery might use as much as ——_ — barrs water
Of all the water used by a refinery, 80 to 90% is used as cool-
ant to absorb — _—— _ energy : 3 Look at tnis heat exchanger
COOL WATER
CooL, LIQUID
As the liquid travels through the pipe, the heat from the liquid is exchanged or transferred to the 7 4 Heat always travels from areas of higher temperature to areas
of (higher/lower) temperature
5, Water can absorb only so much heat
As water becomes hotter, its effectiveness as a coolant (in- creases /decreases)
6 After a while, the temperature of the water becomes so high
that it can no longer absorb _ from the hot
liquid
7 The water in this heat exchanger works as a coolant only as
long as it is than the liquid being cooled
8 Maximum cooling is achieved by constantly replacing hot
Trang 210 11 18 14 lỗ 16 1i
In order to provide for further cooling, two things are possible First, the hot cooling water can be discharged and
with fresh water
Or, the hot coolïng water can be and reused for further cooling
The amount of water needed for cooling in a large refinery is in the range of (thousands/millions) of barrels per day Drawing millions of barrels of water per day from the water
supply would be extremely
The amount of water needed is so large that many water sup- plies (would/would not) be able to provide enough
Arefinery must be careful about the quality of the water it discharges
Discharging millions of barrels of hot water per day might cause a — ——— — problem
Cooling the hot water enables the refinery to water over and over again
The best way to handle hot water is to (discharge it/cool and reuse it)
Hot water is cooled for reuse in special cooling
METHODS OF HEAT TRANSFER 18
Trang 321
22
23
As one section of the steel rod becomes hot, the rod (conducts/ does not conduct) the heat to the colder sections
In this example, the heat transfer from section A to C (occurs/
does not occur) by conduction
Conduction occurs when heat or thermal energy flows through a substance from a._== toa temperature region 24,,Suppose a hot bar of steel is placed in contact with a cold one 25 26 27 28, 29 30 COLD BAR HOT BAR POINT OF CONTACT A The heat energy from bar A (will transfer/will not transfer) to bar B
The cold bar becomes hot first at the point of Then, the heat is transferred through the bar by Another method of heat transfer is convection
WATER
BURNER
The burner heats the water in the vessel (all at once/at the
bottom only)
As the water at the bottom of the container gets hot, it be-
comes less dense
A volume of hot water weighs (more/less) than the same volume of cold water
As the water at the bottom of the vessel gets hot, it will (float
to the top/stay at the bottom)
Trang 431 Convection is the transfer of thermal or heat energy by actual within a substance
82 Heat transfer also occurs in another way
If you bring your hand near any hot object you (feel/do not feel) the heat from it
33 Usually, solids, liquids or gases have a high enough tempera-
ture to emit or yadiate_ energy
34 This method of heat transfer or flow is called (convection/
radiation)
Review
„ 85 There are three methods of heat transfer: radiation, conduc- tion, and
86 Heat flowing within a substance from a higher temperature region to a lower temperature region is being transferred by 37 This shows a flame heating the bottom opening of a duct & jo HOT AIR tứ COLD AIR
As it enters, the cold air absorbs thermal energy and becomes 88 As the air becomes hot, it rises and leaves the duct, taking its
Trang 5HOW COOLING TOWERS COOL WATER Results of Evaporation 40, 41 42 43 44 4ã 46 47 48 f 49 50 51 “ Some form of energy is required for any movement or change in matter
To boil water, a source of energy is needed
The molecules in any body of water move due to the heat
in them
The speed of the molecules depends upon the amount of heat energy in them The more heat, the the
molecules move
In any body of water, some molecules move faster than others
The molecules which move faster have (more/less) heat energy
Some molecules move fast enough to break away from the body of water and mix with the air
The molecules that break away first have a (higher/lower) amount of heat energy
As the molecules leave the body of water, they take their energy with them
The molecules that remain have a lower level of heat energy With a lower level of energy, these molecules move (slower/ faster)
In order for them to escape from the body of water, the slow- moving molecules have to ïm speed
Adding heat energy to the molecules will cause them to move Once moving fast enough, the molecules will escape This )
is evaporation - -ư_——
After partial evaporation, a body of water (is cooler/stays the same)
Cooling towers are designed to expose hot water to the air This (allows/ does not allow) partial evaporation of the water This partial evaporation (cools/does not cool) the water What Affects Evaporation
52
53
In order for water to evaporate, it (needs/does not need) to be in contact with air
Trang 654 5B 56 57 58 59 60
The more water molecules that leave a body of water at a given time, the (faster/slower) the rate of evaporation
These two basins contain the same amount of water
A B
The rate of evaporation is faster from basin (A/B)
The faster the rate of evaporation from a body of water, the (faster/slower) the body of water will cool
Cooling towers are designed to provide the hot water with a _ surface-to-air contact
The hotter the water, the more (fast/slow) -moving molecules in it
Hot water will evaporate —————— — _ than cold water
Atmospheric pressure (exerts/does not exert) pressure on a
body of water
7 61, Atmospheric pressure (resists/does not resist) the molecules escaping from a body of water
- 62 It is easier for water molecules to leave a body of water at (high/low) atmospheric pressures
63 As air acquires moisture (water molecules), its humidity 64 Air can hold only a certain amount of water molecules If it
becomes water saturated, it will no longer water molecules
⁄ 65 The higher the humidity of the air in contact with the water, the the rate of evaporation
Review , #
66 Hot water evaporates ata (higher/lower) rate than cold water / 87 Which of the following affect the rate of water evaporation: /
a) humidity of the air
Trang 7
CONSTRUCTION OF COOLING TOWERS
68 In the early days of continuous processing, no attempt was
made to cool process water Water that was cool already was ® taken into the plant from the outside, then disearded when hot,
Z When a plant was loeated near a river or stream, the cool water 8 3 was taken into the plant upstream and released _ downstream -; „ 69 When a plant was not located near a river, the water was taken
from a pond Hot process water was returned to the pond từ, iN surface and cooled by surface exposure to the _ air, or atmosphere 70 The open pond cooling system was eventually modified HEAT EXCHANGER SPRAY POND COOLING SYSTEM In this system, hot cooling water is over the sprayed pond surface
71 By spraying the hot water, more water-to- — air
surface contact is reached
72 With a larger water-to-air surface contact, the rate of evapora-
tion (increases /decreases) increases
573 However, wind could blow away the sprayed water, resulting
in water and damage to nearby structures loss
é 74 The spray pond system was also modified
To reduce water loss due to drift, and to prevent property se
Trang 875 76 TH 78 79 80 The air contacting both of these ponds is at the same humidity level NO WIND HUMID AIR WIND A B
As evaporation occurs, the air in contact with the water becomes (more/less) saturated
As the air becomes more saturated, the rate of evaporation
The air over pond B never becomes saturated because it is
constantly being with new air
The rate of evaporation is more constant in pond (A/B) The rate of evaporation in the open pond, the spray pond, and the spray type cooler is greatly affected by the prevailing The rate of evaporation is also affected by the of the air Atmospheric Cooling Towers 81 82 84 85 ⁄ 86
Exhibit 1 shows an atmospheric cooling tower
This cooling tower, like a pond system, depends on the velocity and the relative _
of the air
Some of the wind entering the tower is carried upward, but
most of the wind blows straight the tower The wind flow through the tower is interrupted and changed
by the _and bars,
The louvers help direct wind and also prevent water The hot air and water vapors leaving the top of the tower
have to pass through the
Some atmospheric cooling towers have adjustable sections of
and drift to aid in the
control of air flow
Trang 1588 89 90 91 - 98, 98, 94 95 ⁄ 96: 97 98 99
The longer contact of water with air causes quicker and faster —_ — _ thanina spray pond
Due to evaporation and drift, there is some water a
which has to be replaced with makeup
As the quantity of air passing up and through the cooling tower changes, water loss due to evaporation and drift
The longer air remains in a cooling tower, the (more/less)
moisture it absorbs
the more moisture the air contains, the ‘faster/slower) it
accepts more moisture
In order to get maximum evaporation in a cooling tower, the air should pass through (quickly /slowly)
Look at this drawing
DIRECTION OF PREVAILING WIND —
IRí ÿ
More evaporation will take place in tower (A/B)
For best operational results, atmospheric cooling towers should be placed so that the prevailing wind blows through
the (shortest /longest) dimension of the tower
In an atmospheric cooling tower, a 10-mile-per-hour wind will cause (more/less) cooling than a 1-mile-per-hour wind
Without wind, an atmospheric cooling tower operates (more efficiently /less efficiently)
Atmospheric cooling towers are designed to operate best under the normal prevailing wind conditions at each site
If wind velocity is much higher than normal, there will bea higher than normal loss of water due to (drift /evaporation) High winds will cause water to be blown from atmospheric cooling towers Such towers are placed so that’ water blown
Trang 16100, Z 101 7 102 103 104 Whenever water is cooled by evaporation, there is always some water
When cooling water 10°F by evaporation, one percent of the water is lost due to evaporation In cooling 100 gallons of water 10°F, a tower loses gailon(s) of water
due to
Drift loss is usually about 0.2% of the water flow or about gallon(s) per 100 gallons
Makeup water is is used to replace water loss due to _, , or leaks
If there are no leaks and a cooling tower cools 100 gallons of water 10°F, there will be gallon(s) of water loss due to evaporation and gallon(s) due to drift Review 105 106 107 108 109, „ 110
An atmospheric cooling tower depends upon wind
and the relative of the air for effective operation The spray of water falling down the tower is broken up into Small droplets by _mmL._ bars,
In atmospheric cooling towers, evaporation and drift are major causes of water
For best operational results, the atmospheric cooling towers
are placed so that the prevailing wind blows through the (shortest /longest) dimension of the tower
Atmospheric cooling towers usually operate hest at (high/ moderate) wind velocities
The towers are located so that water blown from them will not nearby buildings or equipment
Natural-Draft Cooling Towers 111
112
118
Heat in a furnace causes hot flue gases to flow through the stack causing a draft
Trang 17114 115 116 117 118 119 120 “121 122 123 124
The splash bars break the falling water into fine drops in order to provide better air-to- contact
Some of the heat in water transfers to the air As the air heats up, it becomes (lighter /heavier)
‘When it is light enough, the air _ in the chimney “ne heateu air is replaced with cold air that enters the tower
through the
Because of its design, a natural-draft tower (does/does not) depend as much on the wind direction as the atmospheric tower
The temperature of the air inside the chimney is always
than the atmospheric temperature This difference in temperature causes a constant to exist The natural-draft cooling tower (needs/does not need) drift eliminators Makeup water is needed to replace water loss due to leaks and
Louvers or baffles at times are installed around the air inlet of the natural-draft tower
nh louvers or baffles (can/cannot) control the amount of
air entering the tower
By regulating the amount of air entering the tower, the amount of cooling due to can be regulated
Mechanical-Draft Cooling Towers 125 126 127 128 129
Atmospheric cooling towers depend upon the natural flow of up and across the falling water
Natural-draft cooling towers create an upward flow of through the falling water
Exhibit 8 shows a forced-draft cooling tower The air flow through the falling water is produced by
The internal construction of a forced-draft cooling tower is similar to an atmospheric tower, but the sides are-
Trang 18130 131 132 133 134
The cooling of the water in all towers depends mainly on the amount and of air passing through
Fans used in forced-draft cooling towers should produce a large of air with a low velocity
Both mechanical-draft and atmospheric towers are provided
with = sto prevent water loss
due to air velocity -
If one or more of the fans is shut down, the cooling rate is
due to low -to-water contact
The degree of cooling can be adjusted by controlling one.or more of the — and the rate of —_———— fiow
Induced-Draft Cooling Towers 185 186 137 138 139 140 141 142 143 144
In a forced-draft cooling tower, the fans (push ‘pull) the air through the tower
Look at Exhibit 4 In the induced-draft cooling tower, the fan is located at the of the tower
As the fan rotates, it (pulls/pushes) the air through the tower The air is driven upward.from the top of the tower, where it can be carried away easily by the
This reduces the possibility of wet air reentering the at the bottom
Exhibit 5 shows two types of induced-draft cooling towers The tower with its sides open is the — type
In the counterflow type, the largest part of the tower has sides
In a tower with solid sides, the induced air travels most of the time in (the same direction as/an opposite direction from) the
falling water `
Both towers have movable side louvers to regulate the
intake
In both towers, the air volume flowing through the tower is
Trang 19i-Cell Cooling Towers
Large cooling towers are usually constructed in cells or sections which can be operated independently
The cooling capacity of a multi-cell cooling tower can be de-
creased by taking one or more _-_— out of service sections, or cells This is g lower side view of an atmospheric cooling tower
CELLS
The drawing indicates that the tower has — _— cells three 47 If a tower with only one cell needs cleaning or repairs, the
entire tower has to be ———— ——— _—- shut down
48, Any cell can be operated independently If one cell in a multi- cell tower must be shut down, the entire tower (must also be/
need not be) shut down need not be
Review
149 An atmospheric cooling tower depends greatly upon the
natural velocity for effective operation wind *
150 The natural-draft tower is constructed in such a way that it
causes its own draft
(151 Mechanical-draft towers depend upon motor-driven
: to force the air through them ‘fans
159 The induced-draft tower (pushes /pulls) air through the tower pulls 158, A tower with fans at the bottom is (an induced /a forced) a forced
-draft tower
154 All cooling towers cool water primarily by the : process of * evaporation
155 The rate‘of evaporation of water depends upon the water
surface-to-air contact and on the relative —————————— humidity of the air
Trang 20156 (67 159 160 161 162 163 164 165 166 167 168 169 170 171 172
The greater the water-to-air surface contact, the the rate of evaporation
The greater the water-to-air contact, the more is accomplished
- The splash bars in cooling towers break up the spray of water into smaller droplets as well as prolonging the water-to-
+ contact
If the air passing through a cooling tower is water-saturated to the maximum, there (will ‘wil] not) be any cooling due to
evaporation
If there is no evaporation taking place, but the air is cooler than the water, there will be (no,some) cooling due to con-
duction, convection, and radiation
In a cooling tower there is always some cooling of water due to conduction, convection, and
However, most of the cooling of water in a cooling tower is accomplished by
In any other type of cooling tower, drift eliminators are needed to minimize water because of the wind
However, in a natural-draft cooling tower, drift eliminators
_(are/are not) used
Any cooling tower is subject to water losses produced by drift, leaks, and evaporation
These losses are compensated for by water In mechanical-draft towers, the volume of air passing through the tower can be adjusted by controlling one or more of the
Controlling the volume of air passing through the tower (controls/does not control) the amount of cooling
It is easier to control the amount of cooling in a(n) (atmos- pheric/mechanical-draft) cooling tower
Mechanical-draft towers (can/cannot) regulate or control the amount of air passing through the tower
They cannot control the relative of the air They cannot control the temperature of the
used for cooling
Trang 21nstruction Materials 3 76 TT 178 179 180 181 182 188 184 185
When iron is exposed to water and the oxygen in the air, it
- In a cooling tower, practically all parts are exposed to and
Iron and carbon steel are used to a very limited extent in cooling tower construction because they corrode or rust
(rapidly /slowly)
The best grades of California redwood are used because they resist corrosion caused by _ and
Metals which resist corrosion are used in certain parts of red-
wood towers Copper-coated nails resist
Cast-iron is used in anchoring members that hold the tower
on its concrete basin, but it does corrode and has to be occasionally
Brass bolts, washers, and nuts are used because they also corrosion and rust
Fir wood is used as well as redwood because it resists rot due
to moisture Like fir, synthetic materials such as tensile, fiber- glass, and other plastics resist moisture rot (poorly,’well) Although some wood resists corrosion and rot and has rela-
tively little expansion due to heat, wood swells when it absorbs water and when it dries
Expansion and contraction from either temperature change
or water content change can
Treating the tower wood with creosote increases its resistance to oth water-logging and moisture
Synthetic materials are (more/less) damaged by corrosion, water-log and rot than wood
Regardless of the material used in construction, cooling towers,
like other reñnery units, should be _ _ peri-
odically for structural soundness
Cooling Tower Mechanical Equipment
186 The fans on forced- and induced-draft towers are driven by
187
188,
electric
The pressure necessary to circulate the cooling water through
the plant cooling water system is provided by direct-acting steam or motor-driven _-. -
Trang 22189 The operator must lubricate equipment regularly and — _ all equipment daily for other conditions which require repair or replacement
CONDITIONS AFFECTING COOLING TOWER PERFORMANCE
190 The most important factor in any kind of cooling tower is how fast the water
191 Therefore, any condition which prevents water from evapo- rating _mWms _ the efficiency of the cooling tower
192 Air contains moisture or water vapor
On a damp day, the air holds (a lot of /very little) water 198 If the air surrounding a cooling tower is very humid, the water
in the cooling tower does not evaporate as much as it would if the air was
194 On damp, humid days, a cooling tower works (better than/ not as well as) it does on dry days
195 One factor that affects the rate of evaporation is the amount
of _ in the air in contact with the water
Absolute and Relative Humidity
196 Air becomes denser as the temperature decreases
Air is densest when temperatures are (very hot/very cold)
197 More moisture can be contained in air if it is less dense
Very cold air can contain (more/less) moisture than hot air
198 Temperature (is/is not) an important factor in measuring humidity
199 Here is one way to express humidity measurements
Suppose we have a humidity measurement that reads 1 pound
of water in 10 pounds of air This reading is expressed as (degree of saturation/weight per given volume)
200 Pound is an expression of a specific quantity
1 pound and 10 pounds are (relative/absolute; expressions of quantity 201 Humidity expressed as weight per quantity is (relative/ absolute) humidity 202 Temperature is not considered in measurements of absolute humidity
Trang 23
ecessary to know how much more _ the air water
: Suppose we have a humidity reading that says air at 85° holds f of the maximum it could hold at that temperature
degree of saturation bf This humidity reading is given as a percentage of maximum
RR humidity <x 2 given temperature
Humidity expressed as a percentage ci maximum humidity at
a given tempereture is (relative/absolute) humidity relative 206 A relative humidity reading (does does not) give an indica- does
tion of how much more water air can absorb :
207 If air holds all the water vapor it can hold at any temperature,
it is said to be saturated
208 The relative humidity of air at the point of saturation is
Jo 100
909 As relative humidity inereases, evaporation ————— ` decreases 210 The performance of a cooling tower (increases/decreases) as decreases
the relative humidity increases
211 Relative humidity is the (least/most) important variable af- most fecting the performance of cooling towers
Dry-and Wet-Bulb Temperatures
Trang 24'218 14 215 216 217 218
Because the wick is saturated with water, the thermometer it covers is called a -bulb thermometer
Evaporation has a cooling effect
If the water in the wick of the wet-bulb thermometer is evaporating, it will show a (warmer/cooler) temperature than the other thermometer
The faster evaporation occurs, the (greater/less) difference
there will be in the readings
Evaporation will occur faster if the air surrounding the wick is (moist/dry)
In dry air, the wet-bulb reading is always (lower /higher) than the dry-bulb reading
Suppose the dry bulb reads 90°F and the wet bulb reads 75°F 100% 90% 80% 70% 90 60% 3s CALS 50% 2 5 20 AAA wo 3 i ‘ L] 20% 5 a ễ 70 + 7 WA 20% = L an wi 4 L pt F |] = _u LZ L⁄ 5 3 ¬ a - wv Zee ⁄2 Ƒ A + ˆ i ⁄⁄4 || oA [| > 50 60 70 80 90 100
& DRY BULB TEMPERATURE
The chart shows that the relative humidityis —— _ Ø,
219 If the dry bulb reads 60°F and the wet bulb reads 60°F, the
220
relative humidity is —_ — _%
Trang 25A sling psychrometer also measures relative humidity GAUZE WICK ORY BULB
Tyo Ta0 lsat col sa eal ro eo eo Noon VGT 9T goi roi gọi
The operator whirls the sling psychrometer in the outside atmosphere after saturating the wiek with ` 22 If the atmosphere is not saturated, there will be
readings on the two thermometers
28, When the wet bulb reads lower than the dry bulb, it is because water has _ from the wick
24, Evaporation causes
25 Cooling tower performance is (highest /lowest) when wet-‘and % dry-bulb temperatures are equal
26 Even when the air is saturated in the tower, some cooling occurs by convection and conduction
Therefore, cooling towers do not depend entirely on to accomplish cooling
227 When the outside air is cooler than the water being cooled, some cooling oceurs due to radiation, " _ and , even though none occurs due to evaporation 128, "eventhough no evaporation occurs, if the air is cooler than the
_ water; heat is transferred from the water to the
229, The heated air then carries the heat with it out of the tower
by
230 Cooling towers are never 100% efficient
If the wet-bulb temperature is 65°F, that would be the mini- mum approach temperature of the water
The lowest possible water temperature after cooling with the above condition would be (60°F /above 65°F)
Trang 26Winter Operation 282 233 234 285 236 237 238 239 240 241, 22, 243, 245 246 Ai 248
The degree of cooling in cooling towers in part depends on the amount of air flowing through the
In addition, if the air is hot and humid, the degree of cooling is (more/less) than when the air is cold and dry
Low air temperature may result in too much
Water freezes at 32°F
In winter, air temperature well below 32°F may cause cooling
water to on parts of the cooling tower Because of more cold air contact, water broken up into small
droplets freezes (faster,‘slower) than if it was in a solid stream Ice formations may
splash bars the passages between The operator has to watch for formation of ice around the fan If ice builds up around the fan too much, it may shut off the flow of
Incold weather, moisture-filled air is likely toform on the fan blades and other moving parts
This may cause overloading of the fan motor and costly The operator needs to know how to prevent
‘formation in the tower and on moving parts
One way to control freezing is to limit the quantity of cold
entering the tower
In atmospheric towers, adjustable louvers can limit the intake of
To limit the intake of air in induced-and forced-draft towers, thế —————— can be sÌowed or shut down
Decreasing the pitch of the fan blades will also reduce the intake
If ice has formed in the fill deck, it can be melted by reducing
the amount of cold entering the tower
Trang 27
249 On a foreed-draft tower, the fan pushes the air into the (top “bottom) of the tower i at viên, bottom
250 Reversing the pitch of the fan blades causes air to be (pushed :_ 4 into/sueked out of) the tower sucked out of 251 Since the air in the tower is hot, reversing the fan (can/cannot) can
elt an ice buildup " 252 The quantity of air flowing through the forced- or induced- draft tower can be controlled:
by shutting off one or more _ —— fans
by changing the pitch or direction of rotation of fan
; and blades
by changing the of fan motors speed
Review and Summary
253 Cooling towers are needed in modern refineries to reduce the s temperature of cooling water Cooling water needs cooling so a that it can again be used in exchangers to: :
condense petroleum ; vapors
cool products enough so that they do not go off specification in storage due to heat; and to products to cool
pressure- or fire-safe storage temperature
254 Cooling towers depend mostly on the (conduction of heat `? ig from water to air/partial evaporation of water) partial evaporation of wat
.* +
255 Evaporation depends on water-to- contact air
256 The most important condition affecting the rate of evapora-
tion is the (temperature/relative humidity) of the air relative humidity .257, Atmospheric cooling towers depend primarily upon the pre- „ Vailing
for performance wind 258 The natural-draft tower is designed in such a manner that the
; heat of hot water causes a through it draft
ee
259 In foreed- and induced-draft cooling towers, the draft is
'® caused by motor-driven
fans
260 The draft in forced- and induced-draft cooling towers is
(easier/harder) to control than the draft in atmospheric easier Y\ tOWers
z 261 Redwood and fir is used in the construction of cooling towers
“because it resists wet rot
Trang 28292 263 264 265 266 267 268 269 271 272 2738 Ideally, metal parts of the towers should be highly to corrosion
Metal parts are coated with special — — toiïn- crease their corrosion resistance
The operator should keep alert to evidence of excessive and wet
Cooling towers perform best when the air passing through them: is cool.and (dry/wet)
Hygrometers and psychrometers are instruments used in
determining the of the air
If there is no difference in the temperature reading of a dry- and wet-bulb thermometer, the relative humidityis _— —_%, At 100% relative humidity, there will be (some/no) cooling due to evaporation
Even though there might not be any cooling due to evapora- tion, a cooling tower will still cool water slightly due to
radiation, and
&
270 In cold climates, where temperatures get below freezing, the operator must guard against theformation of
in the tower as well as the fans
A natural-draft tower (needs/does not need) drift eliminators
All other towers have
reduce loss of water due to wind to
Makeup water is needed in cooling towers to replace water loss due to leaks, and
SECTION 2
WATER CONDITIONING
PHYSICAL AND CHEMICAL PROPERTIES OF WATER
.1 HaO is a chemical symbol for water It shows that a water 2
molecule is a compound made up of two atoms of hydrogen and one atom of
Trang 29
ị a * 14 1 hã tử 8ubstance Sp Ht Substance Sp Ht Am 0.24 Kerosine 0.50
sie’ Aleohol 0.60 Lead 0.08
Ễ Aluminum 0.22 Lube Oil 0.45 Brass 0.091 Mercury 0.038 - 0.098 Steam 0.48 0.20 Stone 0.192 0.21 Tin 0.055 0.032 Water 1.00
Tce 0.51 Wood (avg.) 0.42
Tron (steel) 0.115 Zine 0.098
8 The drawings show different changes that water can undergo
ICE (SOLID) WATER (LIQUID) STEAM (GAS)
BELOW 32° F 32°F TO 212°F ABOVE 212°F These changes are (chemical/physical)
4, Whether water is a solid, liquid, or vapor, its molecules are still composed of two —————— atoms and one
atom,
5.eHeating or cooling water causes it to change state from one form to another Changing the state of water is only a change
6 A chemical change occurs only when atoms or molecules change from one substance into a
substance nentirely —_—_—
7, Water does not undergo chemical change easily Chemically, it is relatively (stable/unstable)
8: Water also has a good heat capacity That means that it has .a good capacity to absorb - 9 BTU stands for British Thermal Unit It is a measurement of quantity of energy
0 Specific heat is the number of BTU’s required to raise the tem-
Trang 3012 To raise the temperature of 2 pounds of kerosine by 1°F ¢ Tequires BTU 1 13 When any liquid or fluid is used as a coolant, it absorbs heat energy from the material being cooled
14 Asa coolant absorbs heat energy, its temperature rises ee 15 Raising the temperature of 1 gallon of kerosine a certain
amount requires (more/less) heat than raising a gallon of less water the same amount
16 A substance with a high specific heat can absorb (more/less) more heat per degree of temperature change than a substance with
a low specific heat
17 A substance with a high specific heat should make a (good/ poor) coolant
good
18 Water can dissolve many things Water is normally a (good/ poor) solvent |
good 1
19 Many tiny, insoluble particles, such as grains of sand, rust, and calcium carbonate, become suspended in water 1 Water can contain two kinds of solids: (1) dissolved solids and (2) solids suspended, or undissolve Review 20 Water is made up of two —_ atoms and one hydrogen atom oxygen
21 Chemically speaking, water is (stable/unstable: stable
22 Water makes a good coolant because it has a a l high specific heat
23 Because of its properties, water can and dissolve
carry various solids
24 The two types of solids that can be found in water are ,
and solids dissolved ; suspended
EFFECTS OF TOTAL SOLIDS ON COOLING
“25 The makeup water used in refineries comes from natural i
sources such as rivers, lakes, and wells
Such waters are likely to contain both ———_— _and dissolved - = fectly clear solids even though they may appear per- suspended
Trang 3126 Because the water circulates many times through pipes, ex- changers, cooling towers, and basins, it picks up (more/less) solids
27 .The total solids increase because the water
_, alittle bit of the substances it contacts 28," When water evaporates, it (takes/does not take) the solids with it
“29, After partial evaporation, the solids tend to concentrate in '“'the remaining water Cooled water leaving a cooling tower has (more.‘less) total solids per gallon than the hot water „ entering the tower
30, The circulating water in a cooling system has (more/less) total solids than the fresh makeup water
id: l
SIỂ So, the discarding of some cooling water and the addition of 2Ñ makeup water tends to keep the total volume of solids
: (up/down)
582 Suspended solids tend to settle out in sections of the cooling eo where the velocity of the water is (slowed /increased)
“Some dissolved solids are less soluble in hot water than in cold & ,water When the water beeomes hot, these dissolved solids
+, beeome solids
34 Caleium and magnesium carbonate are less soluble in hot is water than in cold water When cooling water goes through “gp heat exchanger, calcium and magnesium carbonate become ack solids
When water containing calcium and magnesium carbonate is boiled in a vessel, form on the sides and bottom of the vessel
6.The same thing happens when the water passes through a
heat
‘Deposits or scales formed in the tubing of heat exchangers will (increase/decrease) heat transfer
*98, The ‘buildup of deposits in an exchanger, regardless of the $3 cause or source of the deposit, is called fouling Fouling makes
, neeessary
ï periodie
đại Coarse, suspended solids also cause wear in narrow passages lệ -.:or turns in the flow This kind of wear is (erosion/corrosion) 40 If too much buildup of solids is permitted in a cooling tower,
Trang 3241 Tiny, microscopic plants sometimes thrive in cooling water
systems
These tiny growths, often green in color, require light in order to grow They start growing on the walls of (closed/open) parts of the system
42 Parts of the growth break away from the walls and start floating in the water _ They become part of the
solids
48 They can plug narrow passages in the system and damage
wood in the :
44, These are biological, or living, substances To control or stop their growth they have to be
45 The biological growths are algae and slime There are a number of varieties; all cause _ to cooling systems 46 Cooling water, in addition to dissolved solids and suspended
solids, contains some dissolved oxygen and carbon dioxide
which are (solids/gases)
47 It is not the water in the system that causes corrosion, erosion,
and fouling
It is the dissolved — ,, suspended solids, and dissolved
KEEPING COOLING WATER IN CONDITION
48 Waters from different water sources are likely to contain kinds and quantities of solids and gases
49 The chemical analyses and the physical tests made on the
different waters are likely to be
50 The chemist prescribes what the operator must do to keep the cooling water in good `
51 The operator may be required to make a few simple tests and change his _ of the water as the test may indicate 52 Because each cooling system is different and the natural make-
up water is different, methods of treatment are
CONTROLLING SUSPENDED AND DISSOLVED SOLIDS
:ð8 If it is necessary for the operator to make certain tests on cooling water, his supervisor will arrange for the necessary instructions and —_—— ——_—— for the test : ‘54, Chemists use two measures to express the quantity of various
@ impurities in water: grains per gallon (gr./gal.) and parts per * million (PPM)
Trang 33' 55 Cooling water analyses usually are reported in PPM 50 PPM total solids means that in one million pounds of water there are pounds of suspended and dissolved solids
“56,
+ B6, There are both suspended and dissolved solids in the natural “makeup water These solids increase in the cooling tower water ; because of
and because of the treating
* chemicals that are added
-B7¿ Some small increase of both kinds of solids also comes from the basins, pipes, and wood in the cooling system
58." If there is so much suspended solid matter in water that it * can be seen easily, the water is turbid Turbid water has a (high /low) PPM of suspended solids, The solids are (more/less) likely to settle out
ˆ80, One way to settle out the solids would be to (speed up/ slow down) the flow of the water
61 One process for removing suspended solids from the water by -eausing them to settle out is called sedimentation
«The process uses a sedimentation basin which is large enough X3 r “pe
so that the flow of water through it is extremely
: 62 Then the particles of suspended solids can drop to the bottom of the basin due to (gravity /pressure) fins / 68 Clear water is drawn from near the (top/bottom) of the basin É '
‘64 If the basin is being used continuously, sediment must be ‘' removed from the bottom of the basin
165 If a battery of two or more sedimentation basins is used, the
flow through at least one can be completely
gee , »
66 If the flow is stopped completely, suspended particles settle + Jaster/slower)
Trang 34Mechanical and Chemical Clarifiers 68 68 70 71 72, 78 14 1 Mechanical clarifiers take up less space than sedimentation basins VARIABLE SPEED DRIVE CHEMICAL FEED INLET OUTLET LAUNDEAR ' |_ OUTLET TT =— HARD WATER INLET SLUDGE §LUDGE OUTLET SCRAPERS
Here, turbid water and coagulating chemicals are fed in through (the same inlet/different inlets)
The design of the clarifier causes the chemicals to be thor- oughly with the raw, turbid water
Aluminum compounds such as aluminum sulfate, sodium
aluminate, and iron compounds, such as ferrous sulfate and ferric chloride, are coagulants
When added to turbid water with other chemicals, they speed
up
Coagulants with other chemicals form a spongy floc which settles rapidly, taking with it sediments, algae particles, and other suspended
The chemicals coagulate the suspended solids so that they become heavy enough to settle quickly to the
of the clarifier tank
The solids pile up as sludge on the bottom of the clarifier tank A rotating scraper moves the sludge toward the center where it can enter the sludge
7
Because the chemicals speed up the settling, these clarifiers require (more/less) space than a sedimentation basin
Trang 35Filtration
76 Suspended solids can be removed from water by filtering the solids out Filters work by either gravity or pressure OPEN TO ATMOSPHERE SAND GRAVEL WATER INLET T=—— BACKWASH OUTLET FILTERED WATER OUTLET BACKWASH INLET Thisisa filter, 77 Note that the tank for this filter is completely enclosed BACKWASHING FILTER eR SS : FILTERED PUMP [ 4 WATER i er BACKWASH OUTLE filter "This isa
78.:Both gravity and-pressure filters can produce backwash
số Water can be forced back in the ————— —_dìirection 3¿z‡o normal flow,
19 ‘The solids trapped in and on the filter bed are backwashed or ty: flushed into the disposal system
80 Filters are usually installed in batteries of two or more so that
Trang 36Hardness 81 -88, 83 85
Hard water is water which contains dissolved Distilled water does not contain either dissolved solids and gases or suspended solids and gases It is therefore (soft/hard) Rain water is not entirely soft because it picks up some hard-
ness from thé solids and gases in the nee
Natural water from wells, lakes, and streams is often quite hard When the water is heated, some of the dissolved solids in the watér form insoluble sin the equipment The hardness itself does not cause corrosion in the system, but the deposits create a condition favorable tO
by other impurities in the water Scale Control 86 87, 88 89, 90 91
Cooling water conditioning is necessary to prevent or minimize qcorrosion, fouling, and formation in heat
‘exchanger equipment
The dissolved solids which cause calcium and magnesium hardness also cause scale formation if the cooling water be- comes (hotter/colder)
Calcium and magnesium scale on heat exchanger tubes significantly reduces the transfer of through
the tube walls
Scale also reduces the flow of fluid through the heat exchanger shell, completely plugging some or all of the exchanger
if it is not removed
The formation of scale makes the equipment less efficient and makes costly — necessary
There are three widely used methods for conditioning water against scaling
Trang 3792, This is a cold-process softener CHEMICAL INLET MECHANICAL MIXER TREATED WATER UTLE Gry, HARD = WATER fe INLET SLUDGE BLOWOFF
The softening reaction is accomplished by feeding in chemicals such as-lime and soda ash and mixing them with the water dw bys means “98 This softener uses heat as well as chemicals ae HARD WATER INLET * CHEMICAL INLET TREATED WATER TO FILTERS SLUDGE SLUDGE BLOW-OFF ` is , There (is/is no) mechanical mixing hae
đi Heat speeds up the softening reaction
Trang 3896 97 98 99 100 101 102
If the processing rate for both hot- and cold-process units is the same, the size of the (hot/cold) unit can be smaller Heat makes the softening reaction more complete
For a given amount of processing time, the water from a (hot/cold) unit is likely to be softer
Because its reaction is faster and more complete, chemical costs for the hot-process unit are usually (higher/lower) than the costs for the cold unit
The amount of chemicals used depends, however, on the
hardness of the raw 2
Generally, the higher the hardness of the raw water, the
(higher/lower) the chemical costs to soften the water
Zeolites, chemicals that produce ion exchange, are also used - to soften water JON EXCHANGE UNIT REGENERANT TANK (SODIUM CHLORIDE BRINE)
“In this kind of softener, the water (intimately mixes with/
passes through) the zeolite
An ion is an atom that either has extra electrons or is missing some electrons
An ion exchange involves one substance collecting ions from, or giving up ions to, another substance
Trang 39108 104 105 106 107 108 109
Usually, the zeolite has sodium ions attached to it
When hard water passes through the zeolite, the sodium ions of the zeolite exchange with calcium or magnesium ions of
the raw
With calcium or magnesium removed, the water is now than it was
When all the sodium attached to the zeolite has been removed, the zeolite (can still.’can no longer) exchange ions
Suppose a sodium chloride brine is flushed through the
saturated zeolite
There (now can be ‘still can’t be) an ion exchange The zeolite now exchanges or ions with the brine
The brine gives up ions to the zeolite
With sodium once more attached to it, the zeolite is re- activated ~ Tt can (once again/still no longer) softeri water passed through 110 11 "112, "118 1 115 _for them to settle out and form scale 116 117 a
The zeolite can only be reactivated a certain number of times
Eventually, the zeolite must be
Zeolite softeners yield much softer water than is usually
needed in a-cooling system
Their use is generally limited to providing very soft boiler feed water and some of the watter in the
cooling system
Sulphuric acid also can be used in water to control scale Sulphuric acid is highly
Therefore, the quantity of acid added to the water has to be carefully -
The sulphuric acid acts on scale-forming substances to keep them dissolved in the water
This means that adding sulphuric acid to the water makes
scale-forming substances (more/less) soluble `
So long as these substances remain soluble, it is (easy /difficult)
Retarding the rate at which scale-forming substances settle out (speeds up/slows down) the formation of scale
Another way to control the formation of scale is to blow down or discard some cooling water continuously
Trang 40RROSION CONTROL
3 Cooling water corrodes by destroying the metal which it
1 Perfectly pure water is not corrosive
Cooling water corrodes because there are dissolved and gases in it “
) Dissolved solids in the natural makeup water become con- centrated because of which takes place in the cooling tower
i, Gases such as oxygen and carbon dioxide are picked up from the —_—— — in the cooling tower
Carbon dioxide causes the water to become acidic Acidic water is (corrosive/non-corrosive)
3 Oxygen in contact with iron causes oxidation or
\ These forms of corrosion are in part eliminated by using corrosion-resisting in construction of parts which come in contact with cooling water
” But, using expensive metals entirely is more costly than the water to lessen its tendency to corrode metal
„ One way to treat the water to prevent corrosion is to inhibit or stabilize the water chemically so that it will not attack
7, Another approach is to treat the water so that it deposits a thin, protective film on the . _ of the metal
thodic Protection
3, The tarnishing of polished metal is a form of
3 Oxidation of the metal occurs in dry air Water is not involved in the corrosion This kind of corrosion is called dry
) Because water is involved, the kind of corrosion which occurs in cooling systems is (wet/dry) corrosion
1 During wet corrosion, two reactions occur at the same time _Metal passes into the water and hydrogen passes out of the