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It is possible to charge refrigerant from an open drum a (8) When the drum is empty, close the valve on the
60° temperatures, although it is recommended that cooler and disconnect the drum. Remove the valve for
use with the next drum. Complete charging of the
leaktight connections be made to the charging valve. The
machine requires 1200 pounds of refrigerant.
charging valve is located on the side of the cooler. To
5. Adding refrigerant to bring refrigerant to standard
help in the charging procedure, each refrigerant drum has
level. When adding refrigerant, use the same procedures
a special type plug installed on the side of the drum.
that we have just discussed. Another method that can be
This plug is specially engineered for charging purposes.
used to add refrigerant is simply to allow the refrigerant
The charging connection on the drum consists of a 2-
to be drawn in as a gas. Let the drum rest on the floor
inch plug in which is inserted a smaller 3/4-inch plug.
and let the gas escape into the cooler while the machine
The 3/4-inch opening inside the drum is covered with a
is in operation or idle.
friction cap. The cap prevents leakage into or out of the
6. Removing refrigerant. In removing refrigerant
drum when the 3/4-inch plug is unscrewed.
from the cooler, the following procedure is
4. Refrigerant charging. To charge the machine
recommended:
with refrigerant, proceed as follows:
(1) By use of the purge recovery unit, inject air into
(1) The machine must be under a vacuum.
the machine until the pressure is 5 pounds gauge.
(2) Fit a 3/4-inch nipple into the standard globe
(2) Connect tubing to the charging valve on the
valve and close the valve.
cooler and allow the refrigerant to discharge into the
(3) Remove the 3/4-inch plug inserted in the 2-inch
refrigerant drum.
plug from the drum.
(3) Less loss of refrigerant will take place if the
(4) Place the valve with the nipple into the opening,
refrigerant is cold. Always allow space in the drum for
making sure that it is far enough in to push off the cap
refrigerant expansion.
inside the drum.
7. Troubleshooting. The steps to be taken in
(5) Place the drum in a horizontal position near the
detecting and correcting improper operation of the
cooler charging valve with the use of a hoist. The drum
centrifugal machine are outlined in table 19. Use the
should be high enough to allow the refrigerant to flow as
proper methods for making these service adjustments,
a liquid, by gravity, from the drum into the charging line.
repairs, and corrections as outlined in this chapter. All
Rotate the drum so that the valve is at the bottom.
settings, clearances, and adjustments must be made to
(6) Connect the two valves (drum and cooler) with
manufacture’s specifications. The manufacturer’s
a copper tube and fittings, making sure all the joints are
maintenance catalog gives definite clearances,
leakproof.
temperatures, pressure, and positions for adjustment of
(7) Open both valves and allow the refrigerant to
component parts. These tolerances must be set as
flow into the cooler. Operate the machine to maintain a
recommended for efficient operation; carelessness in
vacuum after the initial reduction to zero.
these settings can cause extensive damage to the
machine.
TABLE 19
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TABLE 19-continued
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TABLE 19-Continued
1. The refrigerant charge is approximately
Review Exercises
___________ pounds. (Sec. 9, Par. 1)
The following exercises are study aids. Write your
answer in pencil in the space provided after each exercise. Use
the blank pages to record other notes on the chapter content.
Immediately check your answers with the key at the end of the
text.
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2. Which component reduces the horsepower 11. The oil pump is driven from the
requirement per ton of refrigeration? (Sec. 9, _____________________. (Sec. 10, Par. 7)
Par. 2)
12. Which component does the pump lubricate first?
3. (Agree)(Disagree) The refrigerant flows through (Sec. 10, Par. 8)
the tubes in the cooler. (Sec. 9, Par. 3)
13. How is oil returned from the oil pump drive
4. The liquid refrigerant, from the condenser, gear? (Sec. 10, Par. 9)
enters the _______________. (Sec. 9, Par.
5)
14. How is the shaft seal actuated? (Sec. 1, Par. 10)
5. How much pressure is there within the
economizer chamber? (Sec. 9, Par. 5)
15. What purpose do the two holes in the inner
floating seal ring serve? (Sec. 10, Par. 11)
6. The suction gas is taken in by the compressor in
_____________ the shaft. (Sec. 10, Par. 1)
16. The automatic stop valve is set to open at
approximately ________________ pounds.
(Sec. 10, Par. 12)
7. How are the wheels (impellers) protected from
corrosion? (Sec. 1, Par. 2)
17. Which oil pressure gauges are mounted on the
control panel? (Sec. 10, Par. 13)
8. Each bearing has ______________ large oil
rings. (Sec. 10, Par. 3)
18. How is the oil heater energized during
shutdown? (Sec. 10; Par. 14)
9. What prevents interstage leakage of gas? (Sec.
10, Par. 4) 19. (Agree)(Disagree) During operation the two
polished surfaces of the shaft seal are held
together with a spring. (Sec. 10, Par. 16)
10. Which end of the compressor will axial thrust
affect? (Sec. 10, Par. 5) 20. What type oil is used in centrifugal compressors?
(Sec. 10, Par. 17)
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21. The compressor gear drive (increases, decreases) 30. The motor furnished with the centrifugal
the motor to compressor speed. (Sec. 11, Par. 1) machine is __________ phase,
_________________ cycle, and has an
________________ rotor. (Sec. 13, Par. 1)
22. The grade of oil to use on a gear depends on
__________, ___________, and
______________.(Sec. 11, Par. 3)
31. The secondary drum control is used to adjust
the amount of resistance in the
___________________ of the motor which
regulates motor ____________________
23. When would you turn on the gear drive cooling (Sec. 13, Par. 3)
water? (Sec. 11, Par. 5)
32. Which switch is bypassed when the start button
24. Worn bearings in the gear drive will cause is held closed? (Sec. 13, Par. 4)
___________________. (Sec. 11, Par. 9)
33. What is the secondary function of the
25. Which coupling uses a spool piece? (Sec. 12, condenser? (Sec. 14, Par. 1)
Par. 1)
34. What prevents the discharge gas from directly
26. How is the hub expanded when it is to be hitting the condenser tubes? (Sec. 14, Par. 2)
installed on the shaft? (Sec. 12, Par. 2)
35. What precaution would you observe while
27. The angular alignment of a coupling is checked removing the water box cover? (Sec. 14, Par. 3)
with a _________________. (Sec. 12, Par. 3)
36. A burst rupture disc is caused by
28. Which instrument is used to check the offset __________________ (Sec. 14, Par. 6)
alignment of a coupling? (Sec. 12, Par. 4)
37. How can you determine the refrigerant charge
29. Which type of coupling can be lubricated while of the system? (Sec. 14, Par. 11)
the compressor is running? (Sec. 12, Par. 8)
38. What is indicated when the temperature
differential of the refrigerant and chilled water
increases? (Sec. 14, Par. 13)
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39. ________________ is prevented by the hot 48. When are large quantities of air normally purged
gas bypass. (Sec. 15, Par. 1) from the centrifugal refrigeration system? (Sec.
16, Par. 10)
40. Why is the liquid injector used in the hot gas
bypass? (Sec. 15, Par. 2) 49. When is water drained from the separator unit?
(Sec. 16, Par. 12)
41. What controls the amount of liquid refrigerant
flowing to the hot gas bypass? (Sec. 15, Par. 3) 50. The four safety controls that will stop the
centrifugal are _______________,
________, ___________, and
_______________. (Sec. 17, Par. 1)
42. (Agree) (Disagree) The high-pressure control on
the purge unit must be reset manually. (Sec. 16,
Par. 3)
51. Which safety control does not require manual
resetting? (Sec. 17, Par. 2)
43. Where is the weir and trap located on the purge
unit? (Sec. 16, Par. 3)
52. What is the differential for the high condenser
pressure control? (Sec. 17, Par. 3)
44. High head pressure indicates that
___________________. (Sec. 16, Par. 5)
53. How can you change (switch over) controllers?
(Sec. 17, Par. 6)
45. How is the air pressure in the condenser
released to the atmosphere? (Sec. 16, Par. 6)
54. The most efficient method of controlling the
capacity of the centrifugal is to
____________________. (Sec. 18, Pars. 1
46. What amount of water collected by the purge and 2)
unit is an indication of leaky tubes? (Sec. 16,
Par. 8)
55. What will occur if you add more resistance to
the rotor circuit of the drive motor? (Sec. 18,
47. When will a pressure drop exist across the Par. 3)
pressure-regulating valve? (Sec. 16, Par. 9)
56. When is suction damper control more effective
than speed control? (Sec. 18, Par. 4)
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57. What is the position of the drum controller lever 63. What is one of the most probable causes of high
during startup? (Sec. 19, Par. 2) condenser pressure? (Sec. 20, table 19)
58. What will cause the oil level to rise in the pump 64. Surging is caused by _________________,
chamber during an extended shutdown? (Sec. ________________, or
19, Par. 6) ________________. (Sec. 20, table 19)
59. The pressure within the machine during an oil 65. What would occur if the economizer float valve
replacement operation should be approximately stuck? (Sec. 20, table 19)
_______________ p.s.i.g. (Sec. 20, Par. 2)
66. What will cause a low "back of seal" oil pressure
60. (Agree)(Disagree) The 2-inch plug in the and a high seal oil pressure? (Sec. 20, table 19)
refrigerant drum prevents leakage when the 3/4-
inch plug is removed. (Sec. 20, Par. 3)
67. Noisy couplings are caused by
___________________,
61. How is refrigerant charged into the system as a ________________, or
gas? (Sec. 20, Par. 5) _________________. (Sec. 20, table 19)
62. How do you pressurize the system to remove 68. (Agree)(Disagree) A high oil level in the speed
refrigerant? (Sec. 20, Par. 6) gear will cause the gear to overheat. (Sec. 20,
table 19)
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CHAPTER 4
Water Treatment
WATER USED IN air-conditioning systems may create you will saturate the water and no amount of stirring
problems with equipment, such as scale, corrosion, and would cause any more salt to go into solution. But if you
organic growths. Scale formation is one of the greatest heat the water to 100° F., more salt can be dissolved into
problems in air-conditioning systems that have water- the solution. This dissolving action is known as direct
cooled condensers and cooling towers. Corrosion is solubility. But if you reaccomplish these steps using
always a problem in an open water recirculating system in calcium saturates instead of table salt, you would see
which water sprays come in contact with air. The more solids precipitate out of the solution as the heat is
organic growth we are greatly concerned with is algae or increased. This action is suitably called reverse solubility
slime. Since algae thrive on heat and sunlight they will and occurs in a water-cooled condenser cooling tower.
be a problem in cooling towers. As a refrigeration
specialist or technician you will save the military great 3. You will find that scale will form on heat
sums of money if you test and treat your equipment transfer surfaces when you use water containing even a
water. For example, if you allowed scale to reach the small amount of hardness. The pH value of the water
thickness of a dime in a water-cooled condenser, it would determines if the hard water will cause scale or corrosion.
cut the efficiency of the machine more than 50 percent. The pH scale is from 0 to 14. Neutral water has a pH
value of 7.0. Any reading under 7.0 is acid, while a
reading above 7.0 is base or alkaline.
21. Scale
4. Let us compare pH to temperature. A
1. When water is heated or evaporated, insolubles
thermometer measures the temperature of a solution,
are deposited on metal surfaces. These deposits usually
while pH measures the intensity of acid or base in a
occur on the metal in the cooling towers, evaporative
solution. As you know, pH means potential hydrogen.
condensers, or inside the pipes and tubes of the
When a hydrogen atom has lost its electron (H+ ), it
condenser water system which have a recirculating water
becomes a positive hydrogen ion. When a great many of
system. What causes scale? We can explain it in a
these hydrogen atoms make this change, the solution will
simple formula:
become highly acid and attack metals. When the
hydrogen atom gains electrons, the solution will be base
Ca (HCO3) + heat = CaCO3, + CO2 + H2O
and have a pH value from 7.1 to 14. A base solution
Calcium calcium carbon
contains more hydroxyl ions (OH-). Scale will form
bicarbonate + heat = carbonate + dioxide + water
when a base solution is exposed to a temperature rise,
providing the hardness is 200 parts per million or higher.
In this formula the calcium carbonate is the villain.
Notice the recommended pH for cooling towers in figure
Calcium carbonate is the chief scale-forming deposit found
69.
in air-conditioning systems, but magnesium carbonate and
calcium sulfate can also cause some degree of scaling.
5. You will find that it is very important to test for
solids in the water because solid content (hardness)
2. Causes of Scale . A rising temperature
determines the amount of scale formation. Hardness is
decreases the solubility of calcium carbonate and calcium
the amount of calcium and magnesium compounds in
sulfate. This is known as reverse solubility. Sodium
solution in the water. Water containing 200 p.p.m.
compounds such as table salt (sodium chloride), on the
hardness and a pH indication of 9 or above will enhance
other hand, have a direct solubility. Suppose you take a
the formation of scale. To avoid scale in cooling towers,
glass of water 80° F. and dissolve table salt into the
you must control hardness. The maximum p.p.m.
water. Soon
standards for cooling towers are
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titration directly measures the soap-consuming capacity of
a water. You will study this test in the following
paragraphs.
9. To begin the soap hardness test, measure 50
milliliters of the sample water into the hardness testing
bottle. Add the standard soap solution to the water, 0.5
ml. at a time, from the soap burette, shown in figure 70.
Shake bottle vigorously after each application and place it
on its side. If no lather forms, continue adding 0.5-ml.
portions of soap solution to a maximum of 6 ml and
place the bottle on its side. Now you must use the
formula below if you have a permanent lather to
Figure 69. pH scale.
complete the test. If a permanent lather does not appear,
see para 10. Hardness (p.p.m.)
100 p.p.m. for makeup water and 200 p.p.m. for bleedoff
(total number or ml. of standard
water.
= 20 X soap solution required for
permanent lather)
6. In cooling towers and evaporative condensers
10. If a permanent lather does not appear after
the water becomes harder due to evaporation. The term
adding 6 ml. of the standard soap solution,
used to compare hardness to the circulating water to the
makeup water is cycles of concentration. For example, 2
cycles of concentration indicate that the circulating water
is twice as hard as the makeup water. If the makeup
water contained 100 p.p.m., the circulating water would
contain 200 p.p.m. To avoid this damaging
concentration, you will find it is necessary to limit the
cycles of concentration. Bleedoff is an effective method
used for this purpose. The amount of bleedoff can be
calculated by using the following formula:
Cycles of concentration
= bleedoff hardness (circulating water)
makeup hardness
For example: if the bleedoff (circulating water) is 150
p.p.m. and the makeup is 50 p.p.m., the cycles of
concentration are 3.
7. There are many methods of treating water to
prevent scale. A few of these are:
• Bleedoff-regulate the amount of bleedoff water to
keep the cycles of concentration within tolerance.
• pH adjustment-maintain the pH of the water
between 7 and 9, as near 8 as possible.
• Add polyphosphates-keeps scale forming
compounds in solution.
• Zeolite water softening-exchanges a nonscale
forming element for calcium and magnesium
compounds.
Before we discuss water softening, we will introduce the
soap hardness test.
8. Soap Hardness Test. The soap hardness test is
used to measure total hardness. The presence of calcium
and magnesium salts, and to a lesser degree other
dissolved minerals, constitutes hardness in water.
Hardness can be best determined by soap titration. Soap Figure 70. Soap hardness test equipment.
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Figure 71. Accelator.
repeat the test with a new water sample. This time dilute
25 ml. of the sample water with an equal quantity of 13. Zeolite process. The zeolite process is usually
zero-hardness water (distilled water). Conduct the test as used for water which has low turbidity and does not
you studied previously. When a permanent lather has require filtration. Treatment may be given to the entire
been obtained, calculate the hardness as follows: supply at one point. This system is commonly used to
soften water for special uses, such as for the control of
(total number of ml. of standard scale. In such cases, the treatment units are located at
= 40 X soap solution required for points near the equipment requiring treated water.
permanent lather)
14. Turbidity is a muddy or unclear condition of
11. Water Softening. Hard waters are potable but
water which is caused by suspended silt, clay, sand, or
are objectionable because they form scale inside of
organic materials such a decaying vegetation or animal
plumbing and on metal system components. A
waste. Turbidity can be corrected by sedimentation,
temporary hardness can be caused by magnesium
filtration, or traps. In most cases the water supply and
bicarbonate. Hard water can be softened by two
sanitation personnel will supply you with usable, potable
different methods. The first is the lime-soda process
water.
which changes calcium and magnesium compounds from
soluble to insoluble forms and then removes these
15. Softening devices. Softening devices include
insolubles by sedimentation and filtration. The second
patented equipment such as the Accelator and Spiractor.
and most common is zeolite or base-exchange process.
The Accelator is also used as a combined flocculation and
This process replaces soluble calcium and magnesium
sedimentation unit without softening. When this unit is
compounds with soluble sodium compounds.
operated before filtration to treat water with low
12. Lime-soda process. Lime-soda process plants are
suspended solids and low alkalinity, it may be necessary
essentially the same as water filtration plants. Lime and
to add lime or clay to add weight and prevent rising floc.
soda ash are added to raw water; the softening reaction
occurs during mixing and flocculation. The precipitated
16. The Accelator, shown in figure 71, is a
calcium and magnesium a removed by sedimentation and
suspended solid clarifier. Precipitates which are formed
filtration. An additional process, called recarbonation,
are kept in motion by a combination of mechanical
which is the introduction of carbon dioxide gas, is
agitation and hydraulic flow. Velocity of waterflow
frequently applied immediately prior to filtration. If the
through the system is controlled to keep precipitates in
raw water has high turbidity, the turbidity
suspension at a level where water passes through them.
is partial removed by sedimentation prior to the adding of
The accumulated
the lime and soda.
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with a swirling motion. The upward velocity keeps the
granular material in suspension. As the water rises,
velocity decreases to a point where material is no longer
in suspension. The contact time, 8 to 10 minutes, is
enough to complete softening actions. Softened water is
drawn off from the top of the cone. The size of calcium
carbonate granules increases during the process,
increasing the bulk of granules in the unit. The water
level of the cone is kept down to the desired point by
withdrawing the largest particles from the bottom. New
material must be added, which can be produced by
regrinding and screening the discharged material.
Softened water is usually filtered through a sand filter to
move turbidity. Advantages of the equipment are its
small size, low installation cost, rapid treatment lack of
moving parts and pumping equipment, and elimination of
sludge disposal problems. The unit is most effective
when hardness is predominantly calcium, there is less
than 17 p.p.m. magnesium hardness (expressed as
calcium carbonate), water temperature is about 50° F.,
and turbidity is less than 5 p.p.m.
18. Zeolite (ion exchange). Ion exchange is a
chemical operation by which certain minerals that are
ionized or dissociated in solution are exchanged (and thus
removed) for other ions that are contained in a solid
exchange medium, such as a zeolite sandbed. An
example is the exchange of calcium and magnesium, in
solution as hardness in water, for sodium contained in a
sodium zeolite bed. The zeolites used in the process of
ion exchange are insoluble, granular materials. A zeolite
may be classified as follows: glauconite (or green sand),
precipitated synthetic, organic (carbonaceus), synthetic
Figure 72. Spiractor
resin, and clay. Various zeolites are used, depending on
the type of water treatment required. Most zeolites
precipitate is called the sludge blanket. When the
possess the property cation, or base exchange, but anion
Accelator is operating properly, the water above the
exchangers are also available and may be used when
sludge blanket and flowing over the weirs is clear.
demineralization of water is required. In the course of
Operation depends on balancing the lift of particles by
treating water, the capacity of the zeolite bed to exchange
the velocity of upward flowing water against the pull of
ions is depleted. This depletion requires the bed to be
gravity. When the velocity of the water is gradually
regenerated by the use of some chemical that contains
decreased, a point is reached at which the particles are
the specific ion needed for the exchange. For instance,
too heavy to be supported by the velocity of the water.
when a sodium zeolite is used to soften water by
Continuous treatment builds up the sludge blanket which
exchanging the sodium ion for the calcium and
is drawn off as required. Operation of the equipment is
magnesium ions of hard water, the zeolite gradually
covered in detail in the manufacturer’s instruction
becomes depleted of the sodium ion. Thus, it will not
manuals.
take up the calcium and magnesium ions from the water
17. The Spiractor, shown in figure 72, consists of an
passing through the bed. The sodium ion is restored to
inverted conical tank in which the lime-soda softening
the zeolite by uniformly distributing a salt or brine
reactions take place in the presence of a suspended bed
solution on top of the bed and permitting it to pass
of granular calcium carbonate. In operation, the tank is
evenly down through the bed. The salt removes the
slightly more than half filled with 0.1 to 0.2 millimeter
calcium and magnesium taken up by the bed as soluble
granules. Hardwater and chemicals enter the bottom of
chlorides and restores the zeolite to its original condition.
the unit close to each other. They mix immediately as
Beds may also be regenerated with acid, sodium
the treated stream of water rises through the granular bed
carbonate,
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sodium hydroxide, or potassium permanganate, depending 7. Erosion-corrosion. Erosion-corrosion is caused by
on the type of zeolite being used. suspended matter or air bubbles in a rapidly moving
19. In addition to the problem of scale, the water. The matter can be fine to coarse sand, depending
refrigeration man knows that corrosion is a constant on the velocity of the water. Usually the greatest amount
problem. Let us now study corrosion, its causes, its of erosion-corrosion will take place at elbows and U-
effects, and its control. bends. Another place where erosion-corrosion takes place
is on the impellers of centrifugal pumps.
22. Corrosion
8. Good filtration installations will remove grains
1. In the refrigeration/air-conditioning field,
of sand and other matter that are large enough to cause
corrosion has long been a problem. Even in the modern
erosion-corrosion. To get rid of air tapped in a system, it
missile complexes, corrosion is prevalent. Corrosion is
is recommended that hand- or spring-operated bleed
very difficult to prevent, but it can be controlled. Before
valves be installed in the highest point of the water
we can control corrosion, we first must understand what
system. Purging the water system gets rid of the air
causes it.
bubbles that enter the system in the makeup water.
2. The effects of corrosion differ as to the type of
corrosion, such as uniform, pitting, galvanic, erosion-
9. Electrochemical corrosion. Electrochemical
corrosion, and electrochemical. We must understand
corrosion occurs when a difference in electrical potential
various ways of treating the system to control these types
exists between two parts of a metal in contact with an
of corrosion. Corrosion is generally more rapid in liquids
electrolyte (water). The difference in potential will cause
with a low pH factor than in alkaline solutions.
electric current to flow. The difference in potential may
3. Types of Corrosion. An air-conditioning system
be set up by two dissimilar metals, by a difference in
may have several types of corrosion in the water system.
temperature or amount of oxygen, or by the
Many of these types are undoubtedly familiar to you.
concentration of the electrolyte at the two points of
4. Uniform corrosion. One of the most common
contact with the metal. The anode is the point at which
types of corrosion encountered in acid environments is
the current flow is from the metal to the electrolyte; it is
known as uniform corrosion. This is caused by acids,
here that corrosion occurs. The cathode, which is usually
such as carbonic, which cause a uniform loss of metal
not attached, is the point of current flow from the
throughout the condensating water system.
electrolyte to the metal. This action is shown in figure
5. Pitting corrosion. Pitting corrosion is a
73.
nonuniform type, the result of a local cell action
produced when a particle, flake, or bubble of gas
10. Corrosion Inhibitor. The most common
deposited on a metal surface. The pitting is a local
chemicals used as inhibitors are chromates and
accelerated attack, which causes a cavity in the metal but
polyphosphates. These inhibitors alone serve only to
does not affect the surrounding metal. Oxygen
decease the rate of corrosion, but if other water
deficiency under such a deposit sets up an anodic action.
treatments are used in conjunction with them, corrosion
This area keeps producing such action until the
may be nearly stopped.
penetration finally weakens the structure and it falls,
developing a pinhole leak.
11. Chromates. Chromates are seldom present n
6. Galvanic corrosion. When dissimilar metals
untreated water; however, they may occur as a result of
which are capable of carrying electric current are present
industrial waste contamination. The chromates are used
in a solution, galvanic corrosion occurs. This action is
extensively to inhibit corrosion and are effective in the
similar to the electroplating process used in industry to
water air-conditioning systems in concentrations of 200-
bond or plate dissimilar metals. When two metals similar
500 p.p.m. at a pH of 7.0 to 8.5. Chromates are the
to each other are joined together, there is little reaction.
most commonly used corrosion inhibitors in chilled water
But the coupling of two metals from different groups
systems. For corrosion prevention the most favorable
causes accelerated corrosion in one of the two metals.
range is with the pH from 7.5 to 9.5, but scaling becomes
When using large amounts of copper in a system and a
a problem at the higher pH range. Consequently, the pH
few unions of steel, the steel will corrode at a rapid rate.
should be held near the lower range where corrosion
In such cases you should install nonferrous metal instead
protection is excellent. Because it is more economical,
of steel. Corrosion inhibitors reduce the corrosion rate
sodium bichromate (Na2Cr2O72H2O) is the most
but will not eliminate galvanic corrosion.
commonly used chromate compound. Sodium chromate
(Na2CrO4) is also used widely.
12. Chromate concentration is stated in p.p.m.
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