Being more sensible about air conditioning enhances energy efficiency

AC unit
Air-conditioning systems designed for high sensible cooling remove less water vapour form the air and achieve higher coefficients of performance.
Air conditioning can be provided more efficiently simply by designing it not to remove so much water vapour from the air — as PHILLIP ORD explainsOne of the effects of cooling air is to condense moisture from it. ‘That is why,’ explains Philip Ord of air-conditioning company Mitsubishi Electric, condensation forms on a glass of cold beer on a hot summer day. This change in the state of water from vapour to liquid requires the absorption of energy known as latent heat, which warms the beer’. Waste energy Philip Ord is product engineer for the company’s City Multi air-conditioning range. He is concerned that many air-conditioning systems in the UK waste energy by unnecessarily removing moisture from the air. By not removing too much moisture, the coefficient of performance (COP) can be increased substantially and the installed plant capacity requirement reduced He explains that this issue arises because, historically, cooling demand in buildings originated from the hotter regions of the world, such as the Far East, where high temperature and high humidity go hand in hand. In the UK, the relatively dry summers are associated with a low relative humidity. In the main, the cooling demand in UK offices comes from electrical equipment such as computers, photocopiers, fax machines, printers etc., which produce heat but not moisture. Cooling that removes moisture from the air is therefore not required. Sensible cooling Philip Ord asserts, ‘The most effective and efficient way to cool such rooms is by maximising the non-dehumidifying cooling. This is known as sensible cooling.’ The sensible cooling ratio of a fan-coil unit is calculated by dividing sensible cooling capacity by total cooling capacity (latent plus sensible). which is known as the sensible heat factor. (SHF). He explains that most direct-expansion fan-coil units have an SHF of around 0.6 to 0.75, which is due to them having been originally designed for humid climates and ideally suited to the cooling needs of such environments. For UK offices and other applications with high sensible cooling requirements, over 25% of the cooling capacity is wasted. The implication is that a condensing unit with a cooling capacity of 28 kW will deliver only 21 kW of useful cooling. Such a high-efficiency system with a COP of 3.5 for total cooling will have a sensible COP of just 2.65. Dramatic improvement Increasing the SHF of a fan-coil unit to around 0.85 will dramatically improve its effectiveness and efficiency for high sensible applications. Indeed, for an installation needing 210 or more of sensible cooling, one fewer condensing units will be needed. Is sensible cooling really sensible? Philip Ord certainly thinks so, referring to Mitsubishi Electric’s new R410A VRF system that is designed for high sensible-cooling operation to suite the UK demand. Mitsubishi Electric is at Travellers Lane, Hatfield, Herts AL10 8XB.
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