Advances in technology mean changes in chemicals

HVAC SYSTEM ENGINEERS are making steady progress in their efforts to design cooling towers, chillers and steam boilers that operate at higher efficiency and lower cost. Recent innovations - film-type cooling tower fill, enhanced chiller condenser tubes, high-efficiency packaged boilers, and programmable logic controlled (PLC) systems - have made accomplishing these goals a reality.
These improvements in equipment design and operation have forced a change in chemical treatment programs used to condition water that flows through cooling towers, chillers and boilers of today's HVAC systems.
In years past, maintenance engineers reported many failures caused by mineral scale deposit, fouling, corrosion and micro biological growths. Recent advances in water treatment technology have made continuous, reliable and safe operation of this equipment possible.

Equipment evolution

A major shift in cooling tower design has been toward the use of film-type cooling tower fill instead of older, more conventional splash-type fill. Film-type fill is made of corrugated plastic layers that break the water flow into thin sheets. Water flows down the fill's surface, where it comes into contact with air flowing through the tower.
This design change improves the heat-rejection rate of die cooling tower over that achievable with splash-type fin. The honeycomb design of film-type fill makes it an ideal substrate for the growth of slime-forming bacteria and algae. It also is a filter trap for airborne debris. These foulants interfere with the flow of water through the cooling tower, and the result is a loss of operating efficiency.
Newer chiller designs include the use of enhanced condenser tubes. Also, older chillers are often retrofitted with newer condenser tube design. Enhanced tubes have spiral grooves cut into the waterside of the tube surface. This produces a rifling effect much like that inside a gun barrel.
Grooves cause water to spiral through the condenser tube, resulting in improved heat transfer, which translates into overall increases in chiller operating efficiency. Spiral grooves present many challenges for maintaining the integrity of the tube surface. The grooves create a nucleation site for the formation of mineral scale deposits. Severe scale deposits can fill in the grooves, resulting in a complete loss of benefit from the rifling.
Residual stresses in the grooves tend to promote corrosion and microscopic cracking of the tube wall. Likewise, any micro biological growths tend to promote active corrosion in the grooves by micro biologically induced corrosion.
Older low-pressure steam boilers are being replaced with smaller package units featuring high heat-transfer rates. These units are more susceptible to mineral scale deposits, such as calcium carbonate, that form in areas of high heat transfer.

Controller technology

Cooling towers, chillers and boilers are increasingly being controlled by PLCS. These systems were promoted as a way to reduce maintenance manpower. Now, smart control systems regulate the operation of plant equipment to enhance energy efficiency.
As a result, heating and cooling systems are operated intermittently or under constantly changing loads. They are turned down at off-peak hours or when electric power costs are higher. Cooling towers are operated at higher cycles of concentration to conserve water, and boilers are run with reduced blowdown to save energy. Maintaining water treatment programs during intermitent or cyclical operation of plant equipment is a real challenge for maintenance engineers.

Chemical treatment programs

Traditional water treatment programs for chiller condenser cooling tower systems relied on sulfuric acid for scale control and sodium chromate for corrosion protection. Although sulfuric acid is still used for this purpose, as of May 1990, chromates may no longer be used treating cooling water used in comfort air condition systems.
Because of the corrosive nature and hazards associated with the handling and storage of concentrated sulfuric acid, the trend is toward the use of non-acid treatment alternatives.
Phosphonates - organic phosphates - are used control scaling without the need for pH control with sulfuric acid. New phosphonates, such as phosphor butane- tricarboxylate (PBTC), are chemically stable against chlorine. They also prevent scale deposits under extreme operating conditions, such as high cycles of concentration in the cooling tower, or elevated pH levels above 8.5 to 9.0.
Phosphonates often are supplemented with polymers enhanced scale protection. Older polymers, such polyacrylates, are still effective in many systems but being replaced or augmented with co-polymers - made from two different chemical monomers - and terpolymers - made from three different chemical monomers.
These polymers work to solubilize, disperse or modify crystalline structure of scale deposits. Used alone or with phosphonates, they are a powerful weapon against foulants on heat-transfer surfaces. Their use often eliminates the need for sulphuric acid for pH control.

Removing suspended solids

Suspended solids that enter cooling water systems airborne debris often lodge in the honeycombs of film type fill. Removing these suspended solids from the cooling water is best accomplished by sidestream filtration of 1-5 percent of the total flow.
These filters are either cartridge-type spiral-wound elements or multimedia depth filters, which can remove solids down to 5 microns. Solids removal can be enhanced by using high-molecular-weight polymers or non-foaming wetting agents.
Micro biological growths such as algae, slime-forming bacteria and mold find suitable habitats on the surface of film-type fill and in the spiral grooves of enhanced condenser tubes. Traditional biocides, such as gaseous liquid chlorine, are still effective in many systems, but for added protection, they may be supplemented with bromide or non-oxidizing biocides, such as glutaraldehyde dibromonitrilopopionarnide (DBNPA), or isothiazolinc Bromine is available in convenient dry tablets or granules for easy application through a brominator feeder. Non-oxidizing biocides are effective against a broad spectrum of micro organisms, but they have the added advantage of rapidly breaking down into non-hazardous components that do not stay in the environment.
These are just some of the innovations in water treatment technology that are available to better protect new cooling towers, chillers and boilers from scale, corrosion and fouling.
Future trends will target finding low-dosage treatments that offer superior performance without being hazardous to the environment. Advances in computer technology also will help automate chemical feeding and testing tasks. The result will be heating and cooling systems that operate at higher efficiency and lower cost.

Safe Storage, Effective Application

OSHA AND ENVIRONMENTAL regulations require changes in the traditional procedures for chemical storage and handling. Suppliers are responding by offering products in returnable containers or by delivering directly into 250-500 gallon storage tanks, which include spill containment dikes and level warning signals.
Chemical feed and control systems have benefited from advances in computer technology. PLC-based controllers and data loggers feed chemicals in proportion to the amount of makeup water added to the system. This keeps chemical residuals within recommended guidelines and reduces dependence on the results of chemical tests and manual adjustment of chemical feed pump outputs.
Alternatively, suppliers offer products blended with a tracer additive. Its concentration is measured directly by the controller. Adjustments to the chemical pump output are made automatically based on the strength of the tracer signal.
Several devices can measure the performance of water treatment programs and give early warnings of potential trouble. Corrosion coupons remain the easiest and most common method of quantifying results, but more sophisticated devices - such as simulated heat exchangers, heat- transfer monitors and linear polarization resistance probes - are available.
Equipment inspection with fiber optic video cameras or eddy current tests are also helpful in determining the integrity of boiler and condenser tubes. Additionally, computer programs and simulation models are now available that help analyze system data to predict future results.

William F. Harfst is and independent chemical consultant and is president of Harfst and Associates Inc. in Crystal Lake, Ill. This article appeared previously in Maintenance Solutions Magazine.



First published November 2000