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Cooling towers – making the right selection

Cooling towers have gained in popularity and have become the standard heat rejection method, says Georges Hoeterickx, and gives several pointers to be kept in mind while choosing packaged-type cooling towers.

| | Nov 17, 2012 | 12:24 pm
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Cooling towers have gained in popularity and have become the standard heat rejection method, says Georges Hoeterickx, and gives several pointers to be kept in mind while choosing packaged-type cooling towers.

Cooling towers allow designing of cost-effective systems with maximum system efficiencies. The aim of this article is to provide an overview of a few important criteria that need to be applied when selecting or specifying a cooling tower for a project. The scope of this article, however, is to focus on packaged-type of cooling towers and not field-erected cooling towers. Packaged-type cooling towers generally go from 100 tonnes up to 1,200 tonnes capacity per single cell.

DESIGN CRITERIA OF A COOLING TOWER – APPROACH

The size of a cooling tower depends on the flow, water inlet and outlet temperatures and the design wet bulb temperature. The difference between the water outlet temperature and the wet bulb temperature is called the approach. The smaller the approach, the larger the size of the tower. Cost-effective selections are based on a criterion using an approach of 4°C. A criterion using an approach smaller than 2.8°C is not economical, nor will it be certified by CTI. Selections using approaches more than 4°C result in higher condensing temperatures (reduced chiller efficiency and performance) without much savings on the cooling tower.

PERFORMANCE – CTI CERTIFICATION

While it is not easy for an expert to compare different cooling towers with regard to performance and confirm which one is correctly sized, the task will be even more challenging for non-experts. It is, therefore, strongly recommended to limit the choice of the cooling towers to those which are CTI-certified (Cooling Technology Institute). CTI is an independent organisation, which is well recognised around the world, and most global cooling tower manufacturers have their cooling tower ratings verified and confirmed by CTI as part of the CTI Certification programme. Selecting such cooling towers assures that you will receive the specified performance as well as fair play between all the different equipment manufactures.

Products which are undersized are still a common practice, and the final owner/user pays the penalty for the undersized cooling tower by having his system running at higher condensing temperatures, and as such lower efficiencies. Some manufactures do have certified products, but do not hesitate to offer non-certified products, too. It is, therefore, advisable to go to the CTI website (www.cti.org/cooling tower certification) to find the list of all cooling tower manufacturers and their respective certified products and models and make sure that you receive what you expect. In case of doubt, one can always contact CTI.

INTERNATIONAL BUILDING CODE

The International Building Code (IBC) is a comprehensive set of regulations addressing both the structural design and the installation requirements for building systems, including HVAC equipment. Compared to previous building codes that considered only the building structure and component anchorage, the requirements contained within the IBC address anchorage, structural integrity and operational capability of a component, following either a seismic or wind load event. Simply stated, the IBC code provisions require that the cooling towers and all other equipment permanently installed on a structure must be designed to meet the same seismic and wind load forces as the building they are attached.

Cooling towers should be certified by an independent approval agency to confirm they meet specified design conditions such as wind load and seismic forces. A common value for the wind forces used in the region is 2.8 kN/sqm or 110 km per hour wind velocity.

AXIAL OR CENTRIFUGAL FAN-TYPE COOLING TOWER

There are two types of cooling towers available – forced draft, using centrifugal fans and induced draft, using axial fans. The major advantages of the axial type fans are:

  • Much lower energy consumption, normally in the range of 50% less
  • Easier maintenance
  • Lower cost per tonne of heat rejection

Considering the benefits, induced draft axial fan-type cooling towers should always be the first choice. The only reasons why centrifugal fan-type units can or should be considered are applications where the cooling towers are installed inside the building and the fans need to handle an extra static pressure or when extreme low sound levels need to be achieved.

It is important to note that thanks to modern fan technology developments, axial fan units can be made available that meet very stringent sound criteria without being penalised on the power consumption of the centrifugal fan units.

The image, right, shows a typical super low sound fan. Such fans reduce cooling tower noise levels with nine to 15 dB(A). Further noise reductions can be achieved by installing water silencers (see Image 2) to eliminate the noise of the falling water.

KW FAN VERSUS TONNE HEAT REJECTION

When designing or selecting a cooling tower, one can minimise the first cost of the cooling tower and end up with a tower having high power consumption. In this regard, it needs to be noted that for example, two years ago, Kuwait’s Ministry of Electricity and Water issued a Code of Practice to be followed in order to rationalise the power consumption in buildings. For cooling towers they specify that the maximum power rating of the fan motors should not exceed 0.04 kW per tonne heat rejection. With designs available today, this target is relatively easy to achieve, even with CTI-certified products. However, there are manufactures who continue to offer lower cost axial fan models, which result in power consumption requiring 30% more fan power than a more efficient solution. Over a period of 10 years, this represents a significant amount of kWH and electricity cost.

TYPE OF CONFIGURATION – CROSSFLOW VERSUS COUNTERFLOW

The old style crossflow-type cooling towers are increasingly getting replaced by the more efficient and user-friendly counterflow-type cooling towers. This type is already the standard at all Middle East district cooling plants, and is now more and more used as a packaged tower.

Major reasons opting for a counterflow-type cooling tower are:

  • Single water inlet per cell – easier piping and no balancing per inlet needed. This is a major issue with crossflow type of towers where the condenser water is fed to two open hot water basins at the top, which need to be balanced with expensive valves to ensure proper distribution over both sides of the cooling tower.
  • Easy access to the sloped basin for maintenance and operation makes this type popular with users.
  • The fill is completely encased in the unit casing, and as such has a maximum of protection.
  • No direct sunlight can come in contact with the water in circulation in the tower. This is an important feature, which reduces the potential for algae growth and development in the cooling tower, thus reducing water treatment and maintenance costs. In crossflow-type cooling towers, sunlight is in direct contact with the water at the air inlet sides and in the basin through the fan opening on the top.
  • Sand and dust are in the plenum beneath the fill and washed out of the water before the air enters the fill. On the other hand, in a crossflow-type, the air contaminated with sand and dust enters the fill directly at the air entry sides, causing rapid contamination and scale buildup in the fill.

HOW TO KEEP YOUR COOLING TOWER SAFE AND EFFICIENT

The above is the title of a guideline published by Eurovent a few years ago, and describes the required measures to be taken to ensure that a cooling tower operates safely and efficiently. However, it is more focused on the prevention of the development and spread of Legionella (LD) Bacteria.

A few of the major requirements for a cooling tower to minimise or avoid the development of LD bacteria in a cooling tower are as follows:

  • The basins must be designed for minimum water content and have (a) sloping bottom panel(s).
  • The water in the cooling tower should not come into direct contact with sunlight.
  • There should be no dead zones where debris can accumulate and become difficult to remove.
  • Spray system which is non-corrosive and easy to clean should be used.
  • There should be minimum drift losses.

When considering these important criteria it is obvious that the old style crossflow tower no longer meets any of these requirements, and high level designs have increasingly moved towards counterflow-type of cooling towers.

Crossflow towers have sunlight directly shining into the basin. The direct sunlight combined with the warm cooling tower water results in a strong formation of algae in the basin, increasing maintenance and water treatment costs. The result is shown in Image 3.

The area beneath the fill in a crossflow cooling tower is nearly impossible to inspect or access. Dirt will easily accumulate, also because of the low water velocities in this area, which will enhance the possibility for LD contamination of the cooling tower water.

Drift losses in crossflow towers are five times higher compared to counterflow, where Eurovent-certified values of 0.001% can easily be achieved. Besides the higher water losses with crossflow type of towers, the potential spread of contaminated droplets from a crossflow tower is five times higher, as with the modern counterflow cooling towers.

CONSTRUCTION MATERIALS

Cooling towers are made of different kinds of materials, such as galvanised steel, stainless steel or Fiberglass Reinforced Plastics (FRP) or a combination of these.

Considering the harsh conditions cooling towers have to operate in the region, galvanised steel, at times protected with some kind of coating, is a questionable practice, especially for the water basin components. Stainless steel offers an excellent alternative to this, and customers have the choice between SST 304 L or 316 L type. The 316 L type is more expensive but offers better corrosion resistance in high chloride environments.

The use of FRP should be handled with great caution for several reasons:

  • There are many FRP quality gradients in the market, and the cheaper ones are certainly not the better ones. Special attention needs to be paid to the flammability rate of these materials, as well as UV protection and resistance.
  • High quality FRP is not cheap but offers excellent characteristics with regard to corrosion and structural strength.
  • Some manufactures build only a steel frame, often from galvanised steel and use low-cost light-weight FRP panels as casing panels of the cooling towers. These structures are certainly not corrosion resistant, the panels don’t prevent sunlight from entering the fill section and have high flammability and no structural strength. Often, these towers are wrongly called “FRP” type of towers, creating a completely erroneous perception.

ORIGIN OF MANUFACTURING

It is advisable to know where the cooling tower is being made – at the manufacturer’s premises or outsourced to foreign subcontractors. In the latter case, assurance will probably be needed about the final quality and after-sales support, especially with regard to spare parts and replacements.

Often, cooling towers are shipped unassembled to jobsites and, then, assembled by a local labour force. In such a situation, never expect the same quality as you would get from a cooling tower assembled in the manufacturer’s own plant by skilled workers in factory conditions. When it comes to the practice used by some suppliers of outsourcing the complete manufacturing of cooling towers, it needs to be remembered that the savings made in shipping cost don’t offset the expenses and problems clients normally experience later, with units assembled on the jobsite by unskilled labourers in difficult conditions.

SUMMARY AND CONCLUSION

Selection of the right cooling tower for your application can be summarised as follows:

  1. Use a cost-effective approach (difference between water outlet temperature and design wet bulb) of 4°C.
  2. Select only CTI-certified models – you will be sure of receiving the right kind of performing unit.
  3. Make sure the tower’s structural design meets the right standards to withstand local conditions and that the product is IBC compliant.
  4. Whenever possible, select axial fan units. Consider super low sound fans when low noise levels are needed.
  5. Select or specify efficient cooling towers: 0.04 kW/tone.
  6. Opt for counterflow-type of cooling for the several obvious reasons listed earlier.
  7. In order to have a tower which offers you the minimum challenges to control LD development in your cooling tower, go for the counterflow-type.
  8. Either go for a full stainless steel cooling tower or a full FRP one. Don’t get misled by poor quality FRP materials and concepts.
  9. Opt for factory-assembled units and make sure you know where they are built. Visit the plant and inspect the towers while being assembled.

The writer is Director Business Development, Evapco Europe. He can be contacted at: hoeterickx_g@evapco.be


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