Energy-effective strategies for ventilation

AHU
Energy exchange between exhaust and incoming air streams is the key to energy-efficient ventilation and good indoor air-quality without excessive use of energy.
Against the background of today’s environmental concerns, good indoor air quality and energy efficiency must go hand in hand. STEVE BATCHELOR explores the issues.There are many systems and product variations to satisfy modern building-ventilation requirements according to current air-tightness regulations and building restrictions. This wide variety of options showcases technical acumen from European, Scandinavian and British manufacturers, as well as several levels of energy efficiency and ventilation effectiveness. With all the options to consider when selecting the ideal strategies for clean indoor air quality, one common denominator remains — the explicit need to reduce allergies and air contamination. Poor air quality in offices, schools, day care centres etc. often means poorer work performance. For maximum performance and energy-efficiency, air-hygiene systems require high-capacity heat exchangers that can recover an estimated 87% of the energy that would otherwise be wasted by conventional extract ventilation. This is key in reducing power consumption. The last decade saw a 5.5% increase in energy use in the UK. Ideal solution Factors affecting air quality include air velocity, humidity, temperature and air pollution. The ideal ventilation solution is a system that brings fresh air indoors. People consume 20 to 30 kg of air per day (almost 25 000 litres). Most of us spend an estimated 90% of our time indoors. 50 years ago, allergies were relatively uncommon, but today it is all too commonplace. There are many signs that the air we breathe 21 out of 24 hours needs serious review. In its basic form, a fan is a mechanical device enabling movement of air, vapour and other gases in a given system. Fans are widely used for circulating air in rooms and buildings for cooling or heating people, motors, materials and products. Substantial investment in this area has made possible cutting-edge research. The current quality and performance categories for ventilating, heating and air-conditioning solutions can be grouped into categories of air tightness. Class C is three times as airtight as class D, which is the current UK industry standard. Products that meet the criteria for EN 12237 and DW 144 have an air leakage less than 0.08 l/s/m2 of duct area at 300 Pa. This is rare for fan technology but Scandinavian manufacturers have perfected this after years of R&D. Fans’ sound, airflow, design and efficiency are important factors to consider, but the most topical issue, with the new environmental standards, is how they contribute to energy-efficiency and whether they make an impact on fuel savings in larger environments. Additionally, there is little point in purchasing an energy-efficient mechanical system if it is not well maintained. Poor maintenance can reduce fan-efficiency by 50% or more. A technically correct ventilation plant enables greater airflow to be maintained without increasing energy consumption. There are, traditionally, three technical systems used for indoor ventilation. They are natural draught, fan-controlled exhaust systems and balanced ventilation systems that include supply as well as exhaust air fans. The balanced option can be improved by an energy or heat recovery system. Energy recovery Heating and ventilating a building accounts for a large part of the overall running and maintenance costs. To lower building overheads, air movement should be high performance and energy efficient. To achieve this, the system requires counter-flow heat recovery and demand control. Life-cycle costs of buildings are lowered because of drastically reduced energy bills and overall effectiveness and comfort. Heat recovery’s fundamental operation is to push out stale air, pull in fresh air and, with little or no mixing of the two air streams, transfer energy (hot or cold) from the outgoing air to the incoming supply. The fresh air arrives pre-heated or pre-cooled. To create an energy-efficient system, the unit requires high-quality filtration to ensure recycled air has been cleansed. When being installed in larger applications, the units require more power and a slightly longer installation process. These systems should be flexible for varying installation styles. Ideally, high-performance units should promise considerable savings on time and money throughout the entire specification, installation and ongoing maintenance process. As we already know, time is money and is just as significant to costs as energy. In larger structures, pollutants get trapped and build up, particularly in today’s super-insulated buildings. When pollutants accumulate, some concentrations can considerably exceed levels of outdoor pollution. Sufficient evidence exists to conclude that indoor air represents a major portion of the public’s exposure to air pollution, reinforcing the need for energy-recovery systems. Control Crucial to keep in mind, is that ventilation systems are only as effective as their control systems. In that vein, controls are considered a strategy for efficient heating, not just an accessory or commodity add-on. The process of selecting optimal temperatures, speeds and outputs, makes a highly significant impact on overall system performance. Energy recovery should incorporate a complete built-in controls package for simple plug ‘n’ play installation. An efficient control system ensures uncomplicated airflow management. When air-quality controllers offer more functions and options, the control panels should be easy to understand and administer. Intelligent controls offer bells and whistles, and as long as these extra options are designed for users to easily understand and operate, they, in effect, serve no higher purpose. Steve Batchelor is managing director of Systemair Ltd, Pharaoh House, Arnolde Close, Medway City Estate, Rochester ME2 4SP.
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