IMD

By Tony Stone

With clean water becoming an increasingly scarce commodity, and ever more costly, water sourcing and water treatment in air conditioning will become a critical sustainability factor.

Factory cooling towers
Photo credit: Birrup Fertilizers', Australia

Some disasters are manmade, others occur naturally.

Eskom’s failure to build capacity, because certain government ministers failed, in their ignorance, to grasp the realities of the then future, what is now today, have left this country with the problem of spiralling costs and the challenge of reducing its electricity usage - to avoid load shedding. According to Eskom, up to 35 percent or more electricity can be saved on heating, ventilation, air conditioning and refrigeration (HVAC & R) energy consumption. If strictly enforced, it will certainly put a damper on the industry, although it is an innovation opportunity.

SA’s looming water crisis

Gauteng’s water utility, Rand Water, states that South Africa, as a water-stressed country, is facing a looming water crisis. If the country continues at its present rate of consumption South Africa could run out of drinking water by 2025.

Of the nineteen water management areas in SA, eleven have water supply problems, and between twelve and fourteen million South Africans do not have access to safe drinking water.

Over the next 20 years demand for water in Gauteng is expected to increase by 30 percent, likely more. Urgent action is required by all role players. If the country wishes to avert a water shedding scenario, we need to invest in water security now. Currently, more than 98percent of South Africa’s available water supplies are allocated.

With limited options left for new dams and an ever-increasing demand for water, owing to a burgeoning population, urbanization and economic growth, we need to think innovatively about how we manage and use our water supplies.

Acknowledgement: Rand Water, Water Research Commission
----------------------------------------------------------------------------------------------------------------------------------------------------------

To make matters worse, along comes global warming with the world demanding that we all reduce our carbon footprint, except of course for a few industrious nations who selfishly cling to preserving as much of their status quo as possible. The sting in the tail though, for everyone, but more so for the HVAC & R industry in this country, are the effects global warming is having on rainfall patterns. In South Africa, water, already a scarce resource is going to become even scarcer. According to climatologists, the country will become drier from the west to the east. Civil engineers are already talking about, and designing, wastewater reuse systems. And then there is also grey water. Nevertheless, before long, the Department of Water and Environmental Affairs will be looking at the HVAC & R industry as a target for water savings. This of course poses some real challenges for the industry given that evaporative cooling is far more effective than air cooling, especially in the bigger applications.

AC facts

The South African Council of Shopping Centres (SACSA), in their 2009/10 Southern African Shopping Centre Directory, list 1535 shopping centres with a total of 17 million m2 of gross lettable area (GLA).

Buildings with evaporative water-cooled air conditioning systems use at least two litres of water per day for every square metre of GLA. This means more than 22 000 litres a day is evaporated into the atmosphere through the cooling towers of a shopping centre with an average GLA footprint of 11 000 m².

In one instance, on a particularly hot day, one large centre, with a GLA of 120 000 m², used 265 000 litres of water in just 19 hours.

Looking at SACSA’s directory, assuming a reasonable five percent footprint of evaporative water-cooled air conditioning systems, this would translate into 1.7 million litres of water evaporation per day or 52 million litres of water per month, enough to provide 8600 houses with 6000 litres of free water per month.

A respected member of SAIRAC, whose name we shall not mention, estimates the percentage of evaporative water cooled air conditioning systems to be as much as 30 percent. If this figure is accurate, taking into account open or closed systems, simple mathematics will extrapolate the water consumption loss figures involved somewhat more accurately than the five percent used above.

Acknowledgement: South African Council of Shopping Centres, Old Mutual Property Services
----------------------------------------------------------------------------------------------------------------------------------------------------------

The bad news for HVAC & R is far from over especially as far as both open and closed evaporative water cooled systems are concerned. Ground and surface water quality, in general, is being compromised by acid mine drainage and the wanton pollution of both air and water by a poorly educated populace and opportunistic industrialists. The Water Research Commission is on record as having stated that river pollution remains one of South Africa’s biggest water resource challenges.

Waste water treatment works, 93 percent of which, officially, have fallen into a sorry state of disrepair and/or management, pumping who knows what into our river systems, have, with agriculture as a willing accomplice, increased the levels of phosphates in the water, causing 58 percent of our dams and rivers to be infected with cyanobacteria, a harmful prehistoric bacteria that proliferates at an alarming rate in quiet waters and abundant sunshine.
Solid waste dumps, which appear overnight or have been built without the proper clay base and linings, leaching goodness knows what concoctions into ground water, leaves us with a rather dismal picture of what the water situation in South Africa will be.

This is definitely not good for the HVAC & R industry! It is however crucial that we be aware of these realities because we are going to have to work smartly, and adapt. One of the areas where we will need to be vigilant is water treatment, the degree of which will depend on the quality of water from our local water source. 

Nonetheless, as the wise people we are, we will leave nothing to chance, or assumption, because the simple reality, in water cooled systems, is that water quality has a profound influence on air conditioning efficiency, and costs.

Looking at water

The challenges in treating water used in air conditioning systems, which affects the efficiency of air conditioners, and water that is produced by air conditioning systems e.g. condensates, includes total dissolved solids (TDS), total suspended solids (TSS) and settle able solids (SS).

TDS is a measure of the combined content of all organic and inorganic substances contained in water, which, usually in molecular, ionized or micro-granular form, include, amongst others, calcium (Ca), chloride (Cl), potassium (K), sodium (Na), nitrates (NO3) and phosphates (PO4). The primary sources of these chemical elements include groundwater leaching, agricultural runoff, urban and storm water runoff, leaching of soil contamination and point source water pollution discharge from industrial or sewage treatment plants.

TSS, on the other hand, differs from TDS in size, and cannot pass through a sieve of two micrometers, and yet these are indefinitely suspended in the water.

Settle able solids are particulate matter of any size that is either insoluble or will only remain suspended or dissolved in water with agitation.

Water science is as complex as the element is itself. While we may think we know all there is to know about water, many questions still remain, and we discover every day that there is much more to learn.

Water treatment in HVAC & R depends on the type of system, the materials used to construct the system and the water source. This article only scrapes the surface of this complex subject and further reading is strongly recommended. We will also explore some of the finer issues of water treatment in later issues of RACA.

Looking at the technology

There are four types of air- conditioners – window, split, central plant and packaged air conditioners.

The majority of these systems, approximately 70 percent, are air cooled and the only water issues are the condensates that these systems produce, as a result of their cooling processes and also any closed water circuits included in installations. Smaller systems such as window and split air-conditioners do not produce enough condensate to be of concern. However, larger air cooled packaged and central air conditioners do produce quantities of condensate that need, by law, to be piped into the sewers – to be treated at the local waste water treatment works before it is reintroduced into the environment as final effluent.

Sampling the condensate is recommended so as to determine its constituent elements. Because water, oxygen and airborne chemical elements mix and react, despite air filtering taking place, corrosive chemicals, depending on location, are created and do have an effect, albeit over time. For example, being next to a busy highway where higher levels of carbon dioxide and sulphur dioxide are produced by passing cars, buses and trucks, especially diesel vehicles, even though low sulphur fuels are now available, carbonic acid and sulphuric acid are produced. Depending on the materials used to construct the evaporator, it may or may not corrode. 

Table 1: Corrosion Resistance (1) Good, (2) Be Careful, (3) Not Useable
Source: http://www.EngineeringToolBox.com

Note: A useful and more detailed reference table, detailing all the common metals, and their corrosion resistance to aggressive fluids such as acids, bases and more, can be found at http://www.EngineeringToolBox.com .

A Marley cooling tower
Photo credit: Quality Air Conditioning Company

Water cooled air- conditioners

Water used in an air- conditioning heat transfer process can be used once and then discharged or it can be cooled and reused in either an open [evaporative] or a closed [non-evaporative] recirculating system. Compared to once-through cooling, recirculating cooling systems save on costs through water reuse. However, each type of system has its advantages and disadvantages. For example, an open recirculating cooling system is inherently subject to more treatment-related problems than a closed recirculating cooling system due to increasing TDS and TSS caused by evaporating water.

Open recirculating cooling system

In an open recirculating cooling system, cooling is achieved through the evaporation of a small portion of the circulating water. This results in a loss of pure water from the system and a concentration of the remaining TDS and TSS, raising corrosion and/or deposition tendencies. In order to control this concentration the remaining water needs to be replenished with fresh water and also, a portion of the circulating water must be exhausted or “bled-off” to drain. Other disadvantages are:

• The relatively higher temperatures can significantly increase corrosion potential
• The longer retention time and warmer water in an open evaporative recirculating system increases the tendency for biological growth
• Airborne gases such as carbon dioxide, sulphur dioxide, ammonia or hydrogen sulphide can be absorbed from the air, causing higher corrosion rates
• Micro-organisms, nutrients, and potential foulants can also be absorbed into the circulating water.

The cooling tower for the West Ford Flat power plant, which is part of The Geysers, the world's largest geothermal power development.
Photo credit: R Tracey

Closed recirculating cooling system

A closed recirculating cooling system is actually a cooling system within a cooling system. As the water circulates within the system it is subjected to alternate cooling and heating, but without air contact or external influence. Closed systems are widely used in air conditioning, offer more advantages than disadvantages and have simpler water treatment requirements than open evaporative systems namely:

• Significantly smaller amounts of makeup water are required
• As such, higher quality water can be used for makeup water
• This greatly simplifies the control of potential waterside problems
•  Reduces the problem of scale deposits
• Makes the system less susceptible to biological fouling from slime and algae deposits
• Drastically reduces oxygen corrosion problems, because the recirculating water is not continuously saturated with oxygen

However, this type of system is not without its own problems. In particular, corrosion by anaerobic sulphate reducing bacteria [SRB], a subject which will be explored in more detail at a later date.

Rooftop Packaged HVAC Systems
Photo credit: S Katipamula and M Brambley, BSP

Water treatment

The optimal solution to water supply in air- conditioning is to have a reliable source of cheap, potable water – preferably of the highest quality. However, in South Africa, going forward, cheap water is unlikely. While potable water of the highest quality is available from certain sources, this is not the case across all the water utilities - given the latest Blue Drop Report. Nonetheless, we are theoretically just a few years away from this becoming a reality – if government delivers on its promises.

At the moment, water quality in South Africa is situational. Water supplied by a Blue Drop accredited source can be regarded as water of good quality. Otherwise it may well be a good idea to analyse the quality of your water and act accordingly.

Scale

As water evaporates in a cooling tower or an evaporative condenser, pure vapour is lost and dissolved solids concentrate in the remaining water. If this concentration cycle is allowed to continue, the solubility of various solids will eventually be exceeded. The solids will then deposit in the form of scale on hotter surfaces, such as condenser tubes. The deposit is usually calcium carbonate. Calcium sulphate, silica, and iron deposition may also occur, depending on the minerals contained in the water. Deposition inhibits heat transfer and reduces energy efficiency.
Deposition is prevented by threshold inhibitors that increase the apparent solubility of the dissolved minerals. The ratio of dissolved solids in the circulating water to that in the makeup water is called "cycles of concentration." With correct treatment, cycles of concentration can be increased so that less makeup water and consequently less chemicals are required

Slime and algae

Many types of antimicrobials are available for control of algae and biological slime in open cooling systems. Non-oxidizing organic materials (such as quaternary ammonium salts, other organic nitrogen compounds, and organo sulphur compounds) are frequently used. Some antimicrobials can be detoxified before discharge into the environment. Microbiological programs often employ a combination of non-oxidizing and oxidizing chemicals. Oxidizing chemicals include chlorine, hypochlorites, organic chlorine donors, and bromine compounds. Chlorine gas requires chlorination equipment and controls, which are not practical for most air conditioning systems. Chlorine and hypochlorites must be applied carefully, because excessive chlorine will increase corrosion and may contribute to deterioration of cooling tower wood materials and reduction of heat transfer efficiency.

Corrosion

Corrosion is a natural, and inevitable, process. However, it can be minimized, and even delayed. A study conducted by the National Association of Corrosion Engineers (NACE) in the US estimates the annual direct cost of metallic corrosion to the U.S. economy is 3.1% of the gross domestic product (GDP). The equivalent data for South Africa is not available but it would be reasonable to draw a similar conclusion.

Beyond the direct outlay of funds to repair and/or replace corroded structures, lost productivity and indirect costs adds to the overall cost.

The study also states that avoidable corrosion accounts for about one third of the total corrosion costs and concludes, with certainty, “these costs could be reduced by broader application of corrosion-resistant materials and the application of the best corrosion-related technical practices.”

• General corrosion is the uniform loss of metal across the surface and is often combined with high-velocity fluid erosion, with or without abrasives.
• Pitting corrosion is the random loss of metal across the surface and is often combined with stagnant fluid or in areas with low fluid velocity.
• Galvanic corrosion occurs when two metals with different electrode potential are connected in a corrosive electrolytic environment. The anodic metal develops deep pits and grooves in the surface. A very common connection in piping systems is copper and iron/steel. In a connection like this iron/steel will corrode many times faster than iron/steel alone.
• Crevice corrosion occurs in the crevices of gaskets, bolts and lap joints. Crevice corrosion creates pits similar to pitting corrosion.
• Graphitic corrosion occurs with cast iron, losing iron in salt water or acids. It leaves the graphite in place but results in a soft weak metal.
• Concentration cell corrosion occurs where the surface is exposed to an electrolytic environment where the concentration of the corrosive fluid or the dissolved oxygen varies and is often combined with stagnant fluid or in areas with low fluid velocity.

Most hot-dip galvanized steel fabrications are found outside in the open air and are exposed to atmospheric wet and dry cycles. These cycles are crucial to the development of a series of films on the zinc surface known as patina, which is produced by oxidation over a long period, or by a chemical process. The patina is a stable, non-reactive film, in a way protective, unless exposed to aggressive chlorides or sulphides. It is a key component to galvanizing’s long life.

This image shows a sample of new galvanized steel, a sample covered with white rust corrosion and a sample of passivated galvanized steel
Photo credit: Association of Water Technologies, USA

The US based Association of Water Technologies (AWT) state that the corrosion of galvanized steel cooling towers may be referred to as white rust and the consequence of white rust can be premature failure of galvanized steel components.

White rust corrosion is often identified by the white, gelatinous or waxy deposit that can be observed. This deposit is a zinc-rich oxide, reportedly 3Zn(OH)2 • ZnCO3 • H2O and can be quite similar chemically to the protective zinc oxide typically identified as a dull-gray passive oxide. One critical difference between the two oxides is that the white rust oxide is porous and generally non-protective of the substrate, while the passive oxide is dense and non-porous effectively protecting the substrate from exposure to the environment. Corrosion control of galvanized steel, as with any metal, depends on forming and maintaining a stable and passive oxide layer.

The predominant chemistry parameter known to aggravate white rust is high alkalinity/high pH, and is further aggravated by low hardness (softened water). It was found that water treatment professionals have various methods of prevention, but that these methods were not always successful when alkalinity/pH and hardness levels were not maintained within the prescribed ranges.

The function of corrosion inhibitors is to control corrosion and deterioration. However the majority of corrosion inhibitors on the market are available under trade names and manufactured as proprietary products. A few examples are:

• A neutralizing amine condensate treatment will prevent corrosion. Water reacts with the surface of steel piping to form an insoluble iron oxide coating. This coating protects the rest of the steel from the water and consequently prevents further corrosion. When Carbon dioxide is present in a condensate system it reacts to form carbonic acid. This then reacts with and removes the iron oxide coating from the steel and leads to additional corrosion. This corrosion has been observed to cause deep grooves that run along the length of the pipe work. If left unattended the condensate pipe work will need replacing regularly, meaning high maintenance costs and loss of production. Clearwater technicians add a blend of amines to the condensate system, which neutralize the carbonic acid and eliminate corrosion.
• 12,3-Benzotrialole is a good corrosion inhibitor and it can be used together with many scale inhibitors and fungi disinfectants in circulating cooling water systems.
• 2-Mercaptobenzothiazole (MBT) can be used as the copper corrosion inhibitor in circulating cooling water systems. This due to the chemical absorption of MBT on copper surface, or chelation reaction between them.
• Methylbenzotriazole can be used as a corrosion inhibitor of copper and copper alloys. It also has corrosion inhibition for black metals. This product is absorbed on a metal surface to form a thin membrane to protect copper and other metals from corrosion of air and other harmful subjects. The membrane is uniform. When used together with MBT, the result is more effective.
• Hydrochloric acid can be used to clean black metals and is suitable for acid cleaning all types of high, medium and low pressure boilers, large scale equipment and pipelines. CAUTION: This work should be carried out by an expert given the corrosive qualities of hydrochloric acid.

Recommended reading

Leading technology company General Electric has published an excellent book on the subject of water treatment in air conditioning titled: The Water Purification Handbook. This useful publication can be viewed electronically at www.gewater.com .

Corrosion inhibitors: an industrial guide by Ernest W. Flick (available at www.amazon.com) describes more than 1,100 corrosion inhibitors and rust preventives that are currently available for industrial use. Only the most recent information is included. The book is a valuable guide to those interested in preventing or alleviating corrosion. It lists the various products available giving company name, product category, trade name, product codes, a description of the product and its use.
-----------------------------------------------------------------------------------------------------------------------------------------------------------

In summary

In HVAC & R, water treatment, to summarise, should include and address, amongst others, as applicable, the following:

• Alkalinity builder to increase the pH of a system to optimum levels.
• Oxygen scavenger to remove corrosion causing oxygen from the system and a dispersant to remove the possibility of sedimentation or blockages within system.
• Anti-microbial water softener to treat hard water.
• Oxidising biocide e.g. bromine to treat inter alia open recirculating cooling systems.
• Bio dispersants and non-oxidising biocides to poison bacteria. These should be effective against Legionella as well as remove bio film.
• Cleaner/disinfectant for post commission cleaning and for disinfecting systems that have tested positive for Legionella (See note below).
• Copper inhibitor for closed systems that contain high levels of copper.
• Alkali pre-commission cleaner for hot and cold enclosed systems.
• Acidic pre-commission and post-commission cleaner for hot and cold systems.
• Corrosion inhibitors, scaling control and dispersants (see examples below). These treatments should also include corrosion inhibitors for down water (hot water) and red (rusty) water, for open and closed systems and medium to soft water areas. Cooling towers will need special attention, as will low temperature open recirculating cooling systems.
• Sludge inhibitor/dispersant for down water to prevents initial build up (threshold inhibitor).
• Antifoaming agent for boilers to avoid unwanted liquid and gas (bubbles) in the condensate system. This causes corrosion and loss of heat exchange efficiency.
• Antifreeze (monoethylene glycol) is used to lower the freezing temperature of a solution, enabling it to flow in sub-zero temperatures. It is primarily used in chilling systems but can be used in any system where water is exposed to low temperatures.

Note: Chlorine dioxide has been described as the "ideal biocide". It can be used in a wide variety of systems, across a wide temperature range and across a wide pH range. Typical applications include; drinking water purification, down water services, cooling towers, swimming pools and water features. It also eliminates micro fouling in water softeners and is an excellent disinfectant in combating Legionella contaminated systems.

The use of filters

To answer a key question asked by internationally recognised water and wastewater problem solver Paul Puckorius, “Should filters be used on cooling water systems?” the answer, in South Africa, given our current and developing water situation, has to be a resounding “Yes.” Filters remove TSS from the cooling water, and, as such, reduce many potential problems. Anyone with a cooling tower or closed cooling water system really does need to consider the use of filters. However, if you are not sure, simply answer the following questions:

• What is the quality of water being added to the cooling system?
• What is getting into the cooling system or being generated within the system?
• What is already in the system?
• What impact does water treatment have on TSS?

What we do know, from experience, is that filters substantially reduce TSS, thereby improving the operating efficiency, reducing cleaning and maintenance costs and increasing the life expectancy of all the system components involved. Filters are a proactive, cost-effective investment. However, it would be best to consider your water source, analyse your water input and output before choosing the most appropriate filter for your application.

A last word

Grey water reuse, waste water other than sewerage and industrial effluent e.g. bath and washbasin water is the subject of much debate at present. However, besides reuse at point of source, substituting clean water with grey water for flushing toilets, and so save the use of clean water for this purpose, the economic practicality of reticulating the collection of grey water and subsequent purification of it is simply too prohibitive. It would also not be practical to use grey water, without purification, in air conditioning. However, recent improvements in purifying grey water are increasing its usage, such as some of the large installations in Dubai.

Acknowledgements

• The Water Purification Handbook, Air conditioning and refrigeration systems, General Electric.
• Do All Closed Chilled Water Systems Need Water Treatment?, Paul Puckorius, www.process-cooling.com
• White Rust: An Industry Update and Guide Paper,  Association of Water Technologies, USA
• Filters for Cooling Water Systems, Paul Puckorius, www.process-cooling.com
• Water Treatment Chemical Products, Clearwater Technologies