Transcript Essential Cooling Tower Treatment

Essential
HVAC Water
Treatment
Why Treat The Water?
•To Control Corrosion
•To Prevent Scale
•To Control Algae and Bacterial Growth
•To extend equipment life and efficiency
What’s Wrong with Water?
•Water is a natural solvent, it dissolves thing.
•Natural waters contain significant amounts of
minerals like calcium and magnesium.
•Natural waters typically contain significant amounts
of oxygen.
•Water, particularly warm water, is a great
environment for living micro-organism
What are Solids?
Any material that is or becomes dissolved in water
is referred to as a dissolved solid, or solids.
Common Dissolved
Solids
•Calcium and Magnesium
Salts, aka Hardness
•Iron
•Copper
•Sulfates
•Phosphates
•Silica and Silicates
More about Solids
The measure of all of the solids in a sample of
water is its Conductivity.
Typical Conductivity in parts of Missouri,
Arkansas and Kansas
St. Louis
430 micro Siemens
KCMO
480
Little Rock
440
St. Joe
700
Columbia
480
Lenexa
530
Cape Girardeau
500
Jonesboro
540
Another name for Conductivity
is Total Dissolved Solids. Both
are a measure of total mineral
content, but they differ in units.
TDS is expressed in parts per
million and Conductivity is
expressed in micro Siemens
Not All Solids Are Alike
Some solids are more problematic than others. Two of the most important
solids in water treatment are Calcium Carbonate and Magnesium
Carbonate, aka Hardness
Hardness
•Is naturally occurring in most all waters
•Account of nearly 40% of the Conductivity in natural
waters in the Midwest
•Is the primary component of white scale deposits
•Becomes less soluble as water gets hotter
Cycles of Concentration
When water evaporates is leaves it solids behind. If you completely
evaporate a sample of water and then refill the container with the same
water the mineral content doubles. This is called Cycling Up.
1 Cycle
More on Cycles
When water evaporates is leaves it solids behind. If you completely
evaporate a sample of water and then refill the container with the same
water the mineral content doubles. This is called Cycling Up.
As cycles increase the amount of solids
increase.
This means:
•The Conductivity increases.
•The total hardness increases
•The pH will go up as the alkalinity increases
•The risk of scale formation increases
Still More on Cycles
In a cooling tower system, evaporation is the main means to remove
heat. This evaporation causes the water to cycle up, or increase in
mineral content.
The cycles of concentration in a cooling tower system
are limited via bleed-off. The cycles limit is based on:
•The Conductivity of the makeup water
•The Total Hardness of the makeup water
•The Total Alkalinity (pH) of the makeup water
•The heat load or temperature differential of the system
•The type of water treatment chemistry used
Bleed Off
Bleed Off is the removal of high Conductivity water in favor of low
Conductivity makeup. Bleed off generally based on TDS or Conductivity.
This is because the maximum hardness or alkalinity of a given water is
relative to its Conductivity.
A Conductivity Controller and a Bleed Valve are generally
used to control cycles. It follows this process:
•Sensor continuously measures the Conductivity of the tower water.
•If the Conductivity is higher than the maximum, the bleed valve is opened.
•Bleed off removes high Conductivity water and forces the system to
makeup low Conductivity raw water.
•Bleed off continues as it dilutes the Conductivity is the system, until the set
point is satisfied.
Blowdown
 The most important part of boiler water
treatment
 Must include periodic skimmer and bottom
blowdown
 Proper skimmer blowdown: open valve for 8
seconds then close, repeat as necessary
 Bottom blowdown at least every week
 Automatic or needle valve on skimmer is the
best alternative to manual blowdown
Scale
Scale is the formation of mineral deposits. It can occur in any
area that experiences heat transfer or evaporation. Solids
precipitate when they reach the limit of their solubility; when
there is more of a solid than the water can keep dissolved
Common Scales
•Calcium / Magnesium Carbonate - Lime Scale
•Calcium / Magnesium Phosphate
•Calcium / Magnesium Sulfate
•Calcium / Magnesium Silicate
•Silica
Understanding
Scale Control
Scale Control works by changing the scaling solids to more
soluble solids, using water treatment chemistry
Assuming that the Conductivity is under control, scale
can be prevented by:
Converting the scale-forming particles into more soluble
particles
Keeping scale-forming particles apart or “dispersed”
through the use of polymers
Keeping the pH and alkalinity in check
Key Points of
Scale Control
Scale Control Hinges on a Few Key Issues
Understanding the mineral content of the tap
water
Proper and complete Bleed Off or Blowdown
Maintaining system water alkalinity in “Scale
Safe” zone
Consistent addition of scale inhibitors
Consistent addition of scale dispersants
The Effect of Scale on Your System
Thickness of Scale, in.
Extra fuel consumption, %
1/32
8.5
1/25
9.3
1/20
11.1
1/16
12.4
1/8
25.0
1/4
40.0
3/8
55.0
1/2
70.0
Corrosion Basics
Corrosion is the deterioration of a metal due to interaction
with its environment. Corrosion requires metal, oxygen
and water.
In cooling tower systems, the water contains significant amounts of
air. The oxygen carried by that air reacts with the metal at the
surface and starts a corrosion cell.
Water
oxygen
Iron
The key to
preventing this is
keeping the oxygen
from interacting with
with metal.
Corrosion Control
Since we can’t remove the oxygen from the tower water,
we must create a barrier between the metal and the water
and oxygen.
There are three basic kinds of barriers
•Passive
•Precipitated
•Film Forming
Water
oxygen
Iron
The protective layer
is thin and can be
very delicate.
More on Corrosion Control
The protective barrier prevents the oxygen from
contacting the metal. Thereby preventing corrosion.
Common Corrosion
Inhibitors
•Molybdate
•Phosphate
•Phosphonate
•Zinc
•Azoles
•Nitrite
•Sulfite, Oxygen Scavengers
Crevice and Underdeposit
Corrosion
When the metal is trapped under dirt, debris or scale it does not have
normal access to corrosion inhibitors and the bulk water. This sets up a
specially recognized corrosion cell. Generally, chemical treatment does
not prevent or cure these corrosion issues.
Hot Spots for Crevice and Underdeposit Corrosion
Under dirt collected in the sump of the tower or on tube sheet
Leaking threads
Good and bad weld joints
Under epoxy coatings
Under seals and gaskets
Other Issues in Corrosion
Aside from general corrosion, as previously described,
there are other forms of corrosion that are equally
important.
Other Forms of Corrosion
Galvanic - Corrosion between
dissimilar or incompatible metals
Stress Crack - Corrosion caused
by mechanical stress such as
vibration
Flow Related - Cativation, and
erosion
None of these
forms of
corrosion can
be controlled
chemically
Algae, Bacteria and Biocontrol
Cooling towers are a perfect environment for algae and bacteria growth
because of the warm temperatures, complete aeration and the availability
of nutrients from dust and debris taken from the air. Since the spores of
these organisms are always present in the air, it is impossible to eliminate
them from the water. Instead we use materials to stop their growth. This is
referred to as BIOCONTROL.
•When algae and bacteria are allowed to get out of control they can cause a loss of heat transfer,
reduce flow through the heat exchangers and cause corrosion of metal surfaces.
•The chemicals used to control the growth of algae and bacteria are called BIOCIDES
•Living organisms can become immune to biocides if they are exposed to the same material for
long period of time, this means that the biocides must be rotated through at least 3 different
chemistries periodically.
•Ideally biocides should be fed automatically, using a pump and timer to feed small amounts of
biocide every few days to maintain a clean system.
•Chlorine or bleach is commonly used to kill algae and bacteria in a tower, but chlorine is VERY
CORROSIVE and can remove the galvanized coating from the tower and can break down the
scale and corrosion inhibitors being fed to treat the system. Generally chlorination should be
avoided.
Biocontrol, Tower Maintenance
and the “L” Word
•Keeping a tower sump clean is the first step in biocontrol. Algae and
bacteria grow best when they have lots of dirt to grow on and in. Towers
are natural air-washers and pull dirt from the air continuously. Therefore
they must be periodically cleaned out. This cleaning not only helps keep
algae growth under control but also helps to prevent corrosion under the
dirt and help to prevent LEGIONELLA.
•Legionella is the bacteria that causes Legionnaires' Disease, a type of
pneumonia.
•Legionella is an air-borne spore so it is always possible to have it in your
tower water.
•Keeping the tower clean inside is as important as treating it to keep
Legionella under control.
•Biocide rotation and using an oxidizing biocide as part of your treatment
program are the best practices in preventing Legionella growth.
New Tower Startup and White Rust
Prevention
New cooling towers with galvanized surfaces require a process known as
passivation to properly “set up” the galvanized coating for long-term
exposure to our water. Most tower manufacturers have at least some
statement in their O&M manual about “White Rust” or “galvanic passivation”,
but many contractors do not always include it in the startup procedure.
White Rust is the conversion of the zinc to zinc carbonate, a water soluble material. It looks like
little oval shaped white pimples on the wetted surfaces of the galvanized parts of the tower.
The passivation process must be started as soon as the tower is filled and must continue for at
least 90 days.
Passivation usually includes pH control, high bleed rates and a phosphate based inhibitor.
Generally, passivation is only required at new startup, so the additional feed equipment is usually
installed temporarily.
Properly passivated, a new galvanized coating can last 10 – 15 years. Without it the coating can
fail in as little as six months.
Closed Loops
Chilled Water, Hot Water, Heat Pump and Glycol Loops
Basic Terminology
What are we talking about?
Close System – Any water recirculated system that is completely sealed and does not
regularly or by design take on makeup
Fluid Cooler – A type of cooling tower wherein the primary heat exchanger is located
inside the tower. Tower or spray water cascades over the heat exchanger cooling the
loop water inside.
Heat Pump Loop – Most common type of closed loop system that is used with a fluid
cooler. Designed to provide a more highly controlled air temperature by being both
heated and cooled within a tight temperature range.
Chiller – A piece of HVAC equipment that is used to create chilled water. Commonly
part of a cooling tower/condenser system.
Glycol – A heat transfer fluid that can decrease the freezing point of a water solution.
The two most common types are ethylene and propylene glycol
Freeze Point – The temperature at which a water solution begins to form ice crystals
Burst Point – The temperature at which a water solution becomes solid and expands
Anaerobic Bacteria – Bacteria, typically slime forming, that do not require oxygen to
grow.
Why Treat a Closed Loop?
•
•
•
•
Water and Metal don’t mix!
•
The metal surfaces of your closed loop system are meant to last decades and they
can if they are properly protected. A chilled water system line can last 40 – 50 years
with proper treatment, but they can fail in as little as a couple years without
treatment. Hot water system lines have been known to fail in the first year without
treatment.
Hot Water Systems can become scaled.
•
Calcium deposits can form on heating surfaces in areas with high hardness. Proper
treatment can prevent any scale formation.
Anaerobic bacteria can destroy a closed loop system.
•
Even though the system is not exposed to sunlight or air, bacterial can still grow in it.
Glycols are more corrosive than water by itself.
•
The corrosion rate of carbon steels and copper alloys in a 30% glycol solution is 1015 times greater than in plain water.
What to watch for in a Loop
Closed loop systems should be the easiest to treat and they are, but when
things go wrong they are also difficult to correct.
Dirt and Particulate can damage pump impellers and seals
Excessive treatment levels can raise the pH of the loop water over a safe
level, yes a high pH can be as bad as a low pH
Normal Loop pH
should be between
9.0 – 11.0, unless
there is aluminum
in the system,
where it must be
kept in the 8.0 – 8.8
range
A low pH is bad as everyone knows. Acidic pH’s, those less than 8, can
cause accelerated corrosion. If the pH of a glycol solution is acidic it can
cause the glycol to breakdown which will push the pH even lower.
Bacteria in the closed loop is the hardest problem to correct. Anaerobic
bacteria excrete acidic wastes which breakdown the system metals, the
inhibitors and glycols. The slimes formed by many types of anaerobes can
insult the heat exchangers and reduce flow.
Iron oxides and copper oxides, aka rusts, have no place to go in a closed
system, therefore they must be kept at a minimum. If not they can clog
small diameter ports, contribute to the particulate problem mentioned
above and most importantly feed “iron-related” bacteria.
Propylene and Ethylene Glycols, Methanol / Ethanol, and
Antifreezes
Not all antifreeze agents are the same. There are many
differences but these are probably the most important.

Ethylene Glycol is toxic and is considered a hazardous material. Propylene
Glycol has a low toxicity and is actually used in cosmetics and cheap ice
cream.

Propylene Glycol is more viscous so it has more impact on head pressure .

Propylene Glycol has better heat transfer characteristics than Ethylene Glycol.

It takes a little more PG than EG to get the same freeze point.

Methanol can be used in place of glycols where the pump-ability is a major
issue. Methanol is less viscous than water so it will not cause head loss.

Methanol and Ethanol are highly flammable in their concentrated states

Glycols breakdown, either due to age, bacterial metabolization, or low pH. The
breakdown can happened very quickly and in most cases its not something
you can fix.
Start Off Right
Clean Loops are Happy Loops
 The key to successful closed loop treatment is starting off with a clean system
 New closed loop piping should always be flushed with an detergent or alkaline
cleaner, not just STP, prior to commissioning.
 Older systems with rusty or smelly water can be alkaline flushed, but this
should be followed by a biodispersent flush to remove bacterial slimes.
 Never put glycol in a system that is not absolutely clean. Glycol can act as a
solvent and bring more particulate and discoloration to the water.
 A little amber discoloration is normal and not an indication of corrosion or
other problems, but any foul smell is not a good sign.
Basic Water Softener
Operation
The regeneration sequence is as follows:
1.
Backwash for 15-25 minutes. Water should flow to the drain
at a rate of 10 – 40 gpm during this phase. This phase lifts
the resin bed in the tank and prepares it for…
2.
Brine Draw / Slow Rinse for 60 minutes. During first 20
minutes, the softener sucks brine from the brine tank and
forces it through the mineral tank. The remaining 40 minutes
is spent saturating the resin bed with a slow rinse of brine. All
of the liquid in the brine tank should be consumed during this
phase.
3.
Fast Rinse for 25-30 minutes. This rinses the excess salt
water from the resin bed in a reverse flow like the backwash.
Water should go to the drain at a similar rate at the
backwash.
4.
Brine Refill is the last stage and can be anywhere from 15 20 minutes
All water softeners operate on the
sample principle of cationic ion
exchange. That means that one ion is
traded for another in the water, in this
case Sodium is traded for
Calcium/Magnesium. This is done in the
“mineral tank” on the “resin”. The resin
are tiny amber beads that are in the
mineral or softener tank. Once the resin
is spent, it is regenerated using a brine
or salt solution to rinse away the calcium
and replace it with new sodium.
Basically high hardness water in, zero
hardness water out.
Troubleshooting a Water Softener
There are several things that can go wrong with a water
softener, but the four most common are:
 Soft water is coming out hard
 Salty water coming out of the softener
 The softener has stopped using salt
 Brine tank is overflowing all the time
 Brine tank is overflowing only during regeneration
Water Softener Troubles part 1
Soft water is coming out hard, based on a water test
Sometimes Hard
This can be caused by:
Always Hard
General this is due to a
mechanical issue with the
softener, such as a bad timer, a
blown resin bed or a broken
distributor.
•The regeneration control clock
is set to too many days between
regenerations
•The capacity setting on the
control water meter is set too
high
•The resin is failing and loosing
capacity
Water Softener Troubles part 2
System has stopped using salt
There are a couple of things to look for.
•Is the brine tank bridge – has the salt formed a hard crust on the
top with a possible open cavity underneath. Check by hitting the
top of the brine with something hard and blunt. If it caves in that
may have been the problem.
•Check for suction on brine draw line, if little or no suction
present then you may have a clogged or bad brine injector.
•If water is flowing into the brine tank instead of out then the
control valve needs to be rebuilt.
Water Softener Troubles part 3
Brine tank overflows
Brine tanks overflow in two way, all the time or just when the
system regenerates.
Continuous Overflow: the brine valve is worn out and not
shutting.
During Regeneration: the system is probably not drawing brine
but still refilling the tank so check the “stopped using salt”
issues.
Boiler Standby / Summer Storage
Proper Boiler Standby Storage Is
•Essential for the Longevity of the Boiler
•Prevention of Corrosion during offline periods
•Preventative Maintenance, but Low Maintenance
•On going during standby
Proper Boiler Standby Storage Is
NOT
•Shutting off the boiler at the end of winter
•Draining the boiler
•Something that is forgotten once started
•Costly or Time Consuming
Wet Storage
the Technique
•
Reduce treatment levels and blowdown heavily 2 days prior
•
Increase bottom blowdown to remove mud
•
Cool and drain, then wash down and refill (optional)
•
Fill boiler to header
•
Add oxygen scavenger, phosphate and alkalinity booster
•
Fire boiler at low fire until steam begins to form
•
Close header
•
Low fire boiler every 4 to 6 weeks to keep chemicals mixed and active
Dry Storage
the Technique
•
Reduce treatment levels and blowdown heavily 2 days prior
•
Increase bottom blowdown to remove mud
•
Cool and drain, then wash down
•
Open all drain plugs and inspection ports
•
Close heater and feedwater valves
•
Using a fan circulate air through boiler to completely dry
•
Place trays of desiccate at 10# per 1000 gallons of volume
•
Seal all drain plugs and inspection ports
Essential HVAC Water Treatment
Good Water Treatment
Starts with :
Good Water Treatment
Ends with :
Good Conductivity Control
Clean condenser tubes
Proper selection of treatment
chemicals based on makeup
water and operating conditions
Clean boiler tubes
Consistent Chemical Levels
for Maximum Protection
Consistent and timely
monitoring of the water
chemistry
Good Bio-control
Good heat transfer
Minimized corrosion of
piping and surfaces
Extended equipment life
Annual PMs, and Cleanings