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

Typical Conductivity in parts of Missouri, Arkansas and Kansas

The measure of all of the solids in a sample of water is its Conductivity.

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 AlikeSome 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

1 Cycle

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.

More on CyclesWhen 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 CyclesIn 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 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.

Bleed Off

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.

BlowdownThe 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

Heat Loss from Lime Formations in BoilersResults of test made by University of Illinois and US Bureau of Standards

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.

Iron

Wateroxygen

The key to preventing this is keeping the oxygen from interacting with with metal.

Corrosion ControlSince we can’t remove the oxygen from the tower water, we must create a barrier between the metal and the water and oxygen.

Iron

Wateroxygen

The protective layer is thin and can be very delicate.

There are three basic kinds of barriers• Passive

• Precipitated

• Film Forming

More on Corrosion ControlThe protective barrier prevents the oxygen from contacting the metal. Thereby preventing corrosion.

Common Corrosion Inhibitors

• Molybdate

• Phosphate

• Phosphonate

• Zinc

• Azoles

• Nitrite

• Sulfite, Oxygen Scavengers

A combination of inhibitors is usually the best approach

VCI Inhibitors are a new, novel chemistry designed to stop corrosion in both the wetted and air space surfaces in a off-line system

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 CorrosionAside 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 BiocontrolCooling 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 LoopsChilled Water, Hot Water, Heat Pump and Glycol Loops

Basic TerminologyWhat 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 10-15 times greater than in plain water.

What to watch for in a Loop

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

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.

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

Closed loop systems should be the easiest to treat and they are, but when things go wrong they are also difficult to correct.

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 RightClean 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 1Soft water is coming out hard, based on a water

test

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.

Sometimes HardThis can be caused by:

• 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 2System 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 3Brine 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

• 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

Wet Storagethe Technique

Dry Storagethe 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 Conductivity Control

Proper selection of treatment chemicals based on makeup water and operating conditions

Consistent Chemical Levels for Maximum Protection

Consistent and timely monitoring of the water chemistry

Good Bio-control

Good Water Treatment Ends with :Clean condenser tubes

Clean boiler tubes

Good heat transfer

Minimized corrosion of piping and surfaces

Extended equipment life

Annual PMs, and Cleanings