Consulting-Specifying Engineer - April 2008 - (Page 47) Where the local climate requires dehumidification during the summer, a series arrangement for the radiant panels can be used. Take the conventional low-temperature chilled water through the ventilation air handler’s cooling coil for dehumidification, temper the dehumidified cooled air with the heat recovery coil, and use that warmed chilled water as the source supply water for the radiant cooling system. Or, use a standalone refrigerant direct expansion cooling system for the ventilation air handler to provide dehumidification, and operate a standalone high-temperature chilled water system for the radiant cooling circuit. The standalone refrigerant ventilation air cooling option with a separate high-temperature chilled water circuit adds some costs, but this is generally a climate-dependent issue for warm, humid climate locations to enable more dehumidification to be done. The sub-cooling of the ventilation air and subsequent reheat can be handled by a properly designed energy recovery air handler with desiccant heat wheels and exhaust air heat recovery for the reheat function to keep the total net energy use as low as possible. Measure heat exchange Let’s start with basics: Radiant heat exchange is a sensible heating/cooling operation with no capability for airborne moisture removal or addition, which is a latent heat exchange humidity control operation. Radiant heat exchange rates depend on the amount of surface area “A,” which is at a surface temperature of “T.” A small radiant heat exchanger area requires a significant temperature difference “T” for a given fixed heat exchange load. A large radiant surface area requires a much smaller temperature difference “T” for the same fixed heat exchange load. Radiant heat exchange varies with temperature to the fourth power, and falls off by distance squared, and being infrared energy, operates at the speed of light. Radiant cooling is the creation of Radiant heating and cooling alone is not a complete comfort system for a healthy indoor environment. cooler surfaces around a warmer surface, such as a human body. This allows humans to transfer heat away from us to the cold surface at a higher rate, making us feel cooler. An example would be standing next to a large glass surface on a cold winter day. Radiant heat transfer energy always goes from hotter sources to cooler sources. Radiant heating and cooling alone is not a complete comfort system for a healthy indoor environment. The three basic human comfort parameters that must be addressed are radiant comfort (40% to 50% of the human comfort factor), fresh air/convection/air movement (30% of the human comfort factor), and humidity control (10% to 15% of the human comfort equation). Large area radiant cooling ceiling (radiant slab ceilings and full radiant ceiling panel ceilings) generally are operated at 64 F average surface temperature for effective radiant cooling. Because the extensive radiant ceiling has no thermal gradient differences across its face, convective air movement from falling cooler air does not occur. Cooler convective air patterns are associated with radiant The exterior of the Fred Kaiser building at the University of British Columbia, Vancouver, has exterior glass fritting that reduces the solar gain by 70%. This keeps thermal loads in the perimeter offices minimized in the summer to allow the radiant slab ceiling cooling to operate above the ambient dewpoint. The building has been operating with a nighttime operated fluid cooler cooling tower as the sole slab cooling source for the past 3 years. The cool nighttime air is used to generate slab cooling water to “recharge” the slab system for the next day of cooling. The thermal mass of the slab can maintain cooling all through the next day and only begins to rise above 65 F by late in the day when most occupants have left the building. Photo: Geoff McDonell This view of the passive suspended radiant heating/cooling panels shows them integrated into the ceiling at the Simon Fraser University Segal Graduate School of Business, located in downtown Vancouver, British Columbia. The suspended panels are dualcircuited, which allows them to be operated in either heating mode or cooling mode as required. The design surface temperature in cooling is 62 F, and even with high room occupancies (meeting room example here), there has been no evidence of condensation on the panels during the 3 years of operation. Ventilation air is supplied by low-level displacement ventilation air terminals fed from dedicated outdoor air units equipped with heat recovery sections. Photo: Geoff McDonell Consulting-Specifying Engineer • APRIL 2008 47
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