I have been working in the Gulf region for over 27 years in this industry, and it still strikes me that condenser auto tube cleaning is not being utilised extensively in District Cooling plants as well as standalone chilled water plants. The fact is that every water-cooled condenser in this region suffers from some sort of fouling, regardless of the type of water treatment and filtration used.

Cooling towers draw in particles from the atmosphere on an average rate of three pounds of solids per ton, per year, which would be 30,000 pounds of solids for a 10,000-ton plant. Some of these solids inevitably make their way to the condenser, regardless of filtration, water treatment and blowdown. In addition, most chilled water plants – be they District Cooling or standalone – utilise cyclone separators technology for condenser filtration. However, it has been found that the separation capability of a cyclone separator, regardless of maker, has not been effective when used in many cooling plants in this region. The reason is that the effective separating capability of a cyclone-based system is particle size >40 microns.

The main issue is that atmospheric contamination found in the Gulf results in a micron size in the range of 20-25 microns, sometimes even smaller – down to five microns. This means that although there is some particulate matter >40 microns, which can be removed by a separator, most particles by mass found in the condenser circulated water will be 20-25 microns. This has resulted in several operating issues; however, the most harmful would be chiller condenser fouling with solids, mud/sand and silt.

Let’s take a look at the effects of condenser fouling. In central chilled water plant applications, fouling of the chiller condenser tubes has substantial impact on the power consumption of the centrifugal compressor. Even with good water treatment programmes, it is common to find chillers that appear to be in good working order operating at a fouling factor of 0.0025 hr-ft2-F/Btu or higher – causing compressor power consumption to increase by 25% or more. Fouling occurs because cooling water contains minerals such as calcium and magnesium that precipitate to form deposits on heat transfer surfaces. Due to the reverse solubility of calcium and magnesium in water, the precipitation of these mineral deposits is more prevalent at higher temperatures; therefore, scale build-up usually occurs first on heat transfer surfaces. The most common scale in cooling water systems is calcium carbonate, but scale is also often composed of calcium phosphate, iron oxide or magnesium silicate. Several factors impact the formation of scale in cooling systems, including mineral concentrations, temperature and pH of the circulating cooling water.

Scale is an “insulator” and does not conduct heat well, because it has a relatively low thermal conductivity. As scale accumulates on heat transfer surfaces, such as the condenser tubes, it greatly reduces the efficiency of the heat transfer process. This causes the compressor of a liquid chiller to consume much more power and drives up the electricity bill. Even a minuscule amount of scale accumulation, which is undetectable by the human eye, can have severe impacts on daily operational costs. Additionally, scale accumulation contributes to corrosion of cooling water systems and heat exchanger tubes through a specific type of corrosion, termed “under-deposit corrosion”. In under-deposit corrosion, aggressive attack of the metal occurs beneath the scale layer, creating a crevice or pit in the metal wall. The deterioration of the metal occurs much more rapidly than general corrosion, elsewhere in the system.

Cooling water systems are also commonly plagued by biological growth that forms slime (biofilm) or algae on heat transfer areas. Bacterial growth occurs on the warm heat transfer surfaces and manifests as a slimy film on heat exchanger tubes. As bad as mineral scale is for heat transfer, biofilm is much worse – it is at least four times more insulating than mineral scale. As the biofilm grows thicker, the layer at the tube wall is depleted of oxygen, and sulphate-reducing bacteria generate corrosive by-products that aggressively attack the tube metal. This process is referred to as microbiologically influenced corrosion (MIC) and is a primary culprit for tube failure. These foulants reduce the heat transfer efficiency of even the best designed heat exchangers and also induce localised corrosion, leading to early equipment failure and forcing shut downs of the chiller.

Atmospheric solids (particles pulled into the cooling tower), scaling and biofouling represent a significant cost to open-loop condenser cooling systems due to the substantial efficiency loss, increased maintenance cost and shortened lifespan of the equipment. Several methodologies are commonly used to mitigate or reduce fouling in chilled water plants. Typically, these include off-line mechanical or chemical cleaning. Off-line cleaning methods require periodic shutdown of the chillers for condenser cleaning through hydro blasting, scrapers, nylon or metallic brushes or chemical cleaning. Automatic Tube Cleaning systems are the answer to significant savings for chilled water plants.

Automatic Tube Cleaning systems can solve condenser fouling and scaling problems that could cost the owner of the chilled water plant thousands upon thousands of dollars in unnecessary energy cost. The fact is that annual (which is most often the case) or periodic manual cleaning of a chiller’s condenser requires the chiller to be shut down and taken out of beneficial service; however, after the exchanger is cleaned and returned to beneficial service, the fouling and scaling start to develop almost immediately, lowering the condenser’s and chiller’s efficiency while also increasing the chiller’s kW/ton consumption until the next condenser cleaning, again generally 12 months later.

In the Gulf region, more than in any other region in the world, an automatic, online tube cleaning system makes sense, due to the atmospheric load on the condenser system and existing water quality; it will keep condenser tube surfaces clean, regardless of the available water filtration system and water treatment programme, and will bring the efficiency of the chiller condenser to its water-side heat transfer optimum design potential.

Ball Automatic Cleaning System

I would recommend a Ball-type cleaning system. My team and I have installed the system for clients across the Gulf region and in the United States, and the results have been outstanding. Sponge ball-type automatic tube cleaning systems were originally conceived by German engineers in the 1950s for application in the power generation industry. Since they are effective at maintaining the heat transfer efficiency of large-scale condensers, and applicable to once-through cooling systems that may have limited or no chemical treatment, they have been widely adopted in the power generation industry around the world. Globally, more than 15,000 sponge ball-type tube-cleaning units have been placed in service in the power generation sector alone, with 50% of total generating capacity in Europe and 40% of generating capacity in Japan. For liquid chillers, the operation of sponge ball-type automatic tube cleaning systems is based on the passage of elastomeric balls through the condenser tubes. The balls are slightly larger than the tube diameter and prevent the deposition of scale and fouling materials. The sponge balls are periodically injected into the cooling water inlet line and are circulated through the condenser tubes by the cooling water flow. The balls are designed and injected in a method that enables a uniform distribution of balls across the tube sheet. Since the diameter of a ball is slightly larger than the inner diameter of the tube, accumulation of deposits in the condenser tube is prevented by shear force between the ball and tube wall and the wiping action of the cleaning balls. The balls are constructed of material that is much softer than the tubes, preventing tube erosion.

The balls are collected at the outlet of the condenser in a ball trap, which includes a strainer or screen that allows water flow to continue but prevents the balls from escaping downstream. These ball-trap designs use perforated screens of unique geometry that require no moving parts and can guarantee that no balls escape to downstream processes. After the balls are accumulated in the ball-trap, they are returned to a ball collector by means of operating a recirculation pump, which supplies the necessary pressure differential for conveying the balls. The collector serves as a holding vessel between ball injections and also as a method for replacing the balls in the system.

The entire injection and collection process is fully automated and controlled by means of a single programmable logic controller. The controller also provides monitoring and alarm functions to ensure the system is continuously operating optimally.

Again, sponge ball-type tube-cleaning system technology has been a widely adopted best practice for optimising condenser performance in the power generation industry for decades. Advances in tube-cleaning system technology have resulted in more effective and reliable systems with enhanced scalability for the liquid chiller industry.

Automatic Tube Cleaning Systems (ball-type) deliver:

• Reduced kW/Ton consumption
• Reduced downtime and maintenance cost
• Reduction in water treatment cost
• Increased chiller capacity
• Increased chiller availability
• Increase in tube life and elimination of tube pitting
• Rapid investment payback

Dan Mizesko is President, Dalkia U.S. Chiller Services. He may be contacted at dmizesko@uscsny.com.