The life-cycle benefits of evaporative cooling

Not only are evaporative methods of heat rejection from air-conditioning systems smaller and quieter than air-cooled condensers, but they also offer much lower life-cycle costs.
For the appropriate air-conditioning installations, evaporative cooling offers much lower life-cycle costs than dry cooling — as well as a host of other benefits. Robert MacLeod-Smith explains.Evaporative cooling is the most energy-efficient method of rejecting waste heat from air conditioning, refrigeration and industrial process cooling systems. Cooling towers and evaporative condensers thus play an important role in reducing energy consumption by these systems and helping the UK meet its Kyoto Protocol commitments to reduce emissions of global-warming gases. Acknowledged Their energy-saving benefits are formally acknowledged by the inclusion of evaporative condensers on the UK Energy Technology List, thus qualifying for tax benefits under the Enhanced Capital Allowance Scheme, and by the recognition of the ‘free cooling’ capability of cooling towers in the revised Part L of the UK Building Regulations. The cooling capacity of cooling towers and evaporative condensers is dictated by the ambient wet-bulb temperature, which is invariably lower than the prevailing dry-bulb temperature that governs the performance of air-cooled alternatives. Thus the driving force for cooling with ‘wet’ systems is always greater than for ‘dry’ systems. As a result, cooling towers can achieve cooled water temperatures 6 to 12 K lower than equivalent dry coolers. In air-conditioning and refrigeration applications, the condensing temperature of the system can be reduced by a similar amount. Such reduced temperatures have great energy-saving potential as the electrical power consumption of a chiller or refrigeration compressor is reduced by 3% for every 1 K lower condensing temperature. Because evaporative-cooling equipment can also cool or condense closer to the ambient wet-bulb than air-cooled can approach the ambient dry-bulb temperature, this advantage can be extended further. Water-cooled plant can be designed with a condensing temperature as much as 15 K lower than an equivalent air-cooled system. Life-cycle costing These benefits are best revealed by life-cycle costing, which is a calculation method for evaluating how much a system will cost over its life. The purchaser of a cooling system generally has several options that will meet his operational requirements. The immediate problem is how to evaluate these alternative systems on an economic basis — and this is where life-cycle costing can be used. Often in the past , the evaluation process has been simple — the system with the lowest first cost was chosen. But rising energy costs as a result of Government policies to reduce energy consumption have increased the need for an evaluation method that considers the impact of future costs. Put simply, life-cycle costing is the economic evaluation of alternative systems by comparing the total owning, operating and maintenance costs over the useful life of the system. It can be used to choose between a wet or dry heat-rejection system or between different alternatives of cooling towers and evaporative condensers. There are three elements to life cycle-costing. • Initial cost • Total operating cost • Total maintenance cost. Initial cost includes all the necessary equipment costs plus the cost to install the components and make the system operational. This would include the equipment itself, installation, piping, wiring, controls etc. Recurring, or operating costs, result from the actual operation of the system and include energy and water usage and water-treatment chemicals Maintenance costs are those required to keep the system operational, including routine servicing, replacement parts, repairs, cleaning and disinfection and so on. To calculate the life-cycle cost of a cooling-tower system the following operational information is required. • Load profile • Typical annual dry- and wet-bulb temperatures • Operating strategy — the number of towers running, hours of running, control sequence etc. • Installed cost of cooling towers and water treatment for various options • Electricity and water tariffs • Water-treatment chemical costs • Routine servicing, spare parts and any major refurbishments for the cooling towers and water-treatment equipment • Life of the equipment • Tax rate • Cost escalation, % per annum • The interest rate or desired rate of return Cost comparison Using the above criteria, an accurate life-cycle cost comparison for the cooling-tower installation can be calculated, enabling the preferred cooling-tower construction and most effective water-treatment regime to be chosen. For example, the design team at BAC Balticare recently calculated the life-cycle cost of different cooling-tower options for the air-conditioning system at a major new London development. The project required eight cooling towers, with a maximum design capacity of 4500 kW per tower and a design water flow of 180 l/s per tower. Based on the projected load profile for the air-conditioning system a sophisticated model was used to calculate the annual evaporation rate from the cooling towers, and hence the operating and maintenance costs. Evaporation of water per tower worked out at 17 735 m3 per annum over the total annual operating hours. Taking into account this evaporation rate as well as bleed-off, windage and drift losses, and then adding the cost of make-up water supply and water treatment chemicals, the total water and chemical costs for each of the eight units as £12 475 per annum. In addition, the energy costs for each tower were calculated to be £601 per annum — taking into account the operating periods at both peak or low electrical tariffs and how this was split between the fan motors and the pan heater operation for freeze protection in winter. Life-cycle maintenance costs were calculated based on 30 years’ operational life and comparing two different types of cooling tower construction — a 316 stainless steel option with higher first cost but less maintenance and a coated steel option which requires re-coating during its lifetime. Expertise Drawing on the expertise of companies like BAC, detailed comparisons can be made (too lengthy to include here) which give designers and operators the owning costs of their wet systems over the expected equipment life. Longer-term considerations include commissioning, twice-yearly disinfection and cleaning, plus maintenance and water-treatment service visits, sampling, testing, analysis and reports form part of a total care programme — all of which services can be provided by an independent specialist. Finally, a system overview, again by an independent specialist, assures owners that the system is being properly and safely controlled — in line with all current health and safety requirements and legislation. Environmental issues need to be considered alongside life-cycle costings to determine the best type of heat rejection system. Not only do evaporative cooling systems have a lower life-cycle cost than air-cooled for medium and large installations, but there are also other environmental impact issues that favour the use of wet systems. Evaporative heat rejection equipment, for example, takes up less space and weighs less than air-cooled alternatives for the equivalent cooling capacity. Wet systems also emit less noise, have greater free-cooling availability and usually require a lower refrigerant charge. To conclude, life-cycle costing and environmental issues both have vital parts to play in the selection of the best heat-rejection system for air-conditioning, refrigeration and industrial process cooling applications. Robert MacLeod-Smith is with Balticare Ltd, Waters Meet, South Bucks UB9 4AF.
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