Consulting-Specifying Engineer - January 2009 - (Page 57) Equipment Lifecycles Minimize lifecycle cost of desiccant dehumidifiers Energy is the most costly item to consider when looking at the lifecycle of HVAC equipment. BY MIKE HAYES, PE, Munters Corp., Atlanta L ifecycle cost of any type of HVAC equipment consists of the installed cost of the equipment, including its commissioning, combined with the sum of its annual maintenance and energy costs over its lifetime. Energy is by far the most costly of these components. The other issues should not be overlooked because failure to install or maintain a piece of equipment can lead to its failure to perform the desired task efficiently. When purchasing a desiccant dehumidifier, the key factor is how much energy it will consume to achieve the desired dehumidification effect. According to exceptions accepted to ASHRAE 90.1-2004, desiccant dehumidifiers have been proven to be the most efficient dehumidifiers commercially available today. By reviewing ASHRAE 90.1 2004-3 at www. ashrae.org/technology/page/64, the specifier will note that the condenser reactivat- Reactivation air Wet air Dry air Pre-cooled outside air Drive motor This is a typical desiccant configuration for a wheel-based desiccant system. Source: Munters ed (site recovered heat) desiccant system is more efficient than energy recovery or a heat exchanger enhanced system at the vast majority of entering conditions and certainly at any condition a normal dehumidifier would see. Hourly analysis will be needed to estimate the energy usage of the equipment alternatives. This allows comparison of the alternatives at part load conditions as well as peak loads. By definition, part load is what the equipment will see more than 99.6% of its operating hours. Clearly, hourly analysis is necessary to quantify that operating expense. Another reason that space humidity should be evaluated at part load conditions is the industry has recognized that many traditional systems fail to provide adequate humidity control during those hours. The environmental impact of the energy used, as well as its cost, impacts the sustainability of the project. Desiccant dehumidifiers use a desiccant wheel to dehumidify the supply air. A dry desiccant wheel absorbs moisture from the supply airstream. The heat of vaporization of the moisture as it is absorbed increases the air temperature in a constant enthalpy process. Once the wheel has absorbed the moisture, the moisture must be driven off of the wheel during a “reactivation process.” Generally, this is performed by having a second airstream that has been heated to provide a low relative humidity. The majority of the energy consumed by such units is in reactivation: the energy needed to raise the reactivation air temperature high enough to remove moisture from the desiccant. The reactivation air is typically outside air, but exhaust air can be used as well. Consulting-Specifying Engineer • JANUARY 2009 57 http://www.ashrae.org/technology/page/64 http://www.ashrae.org/technology/page/64
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