Architect and Builder Magazine South Africa March/April 2014 | Page 65

Five chillers are located on the roof of the building, with total capacity of 3 megawatt, and are used to generate chilled water ROOF SERVICE LEVEL - CHILLERS fluctuation, acoustic noise and vibration. These requirements presented a challenge with regard to selecting the method of air conditioning the room, housing the high performance microscope. The four key environmental challenges are: 1. Electromagnetic fields to less than 0,01 T r.m.s. 2. oom set point temperature of 18°C to 20°C R with temperature changes to less than 0,2°C per hour. 3. irflow across the microscope column to less A than 0,076 m/s (i.e. 15 feet per min). 4. ir pressure changes to less than a few Pascal A per min. Conventional air conditioning systems ultimately utilize air to either cool or heat the conditioned space. The volume of air introduced into the space is a function of the heat to be removed from the room. To achieve even temperature throughout a conditioned space, using air as the cooling medium, requires high air change rates and a very low temperature difference between supply air and room set point. The disadvantage of using traditional air based conditioning, for election microscope laboratories, is that air movement can create noise and vibration as well as cold air streams. To illustrate the point, if a 300mm x 60mm strip of single ply toilet paper deflects more than 25mm, the air flow exceeds 0,1 m/s. In order to minimise the cooling load, as much of the heat generating equipment must be kept out the microscope room and housed in an adjoining dedicated equipment room. After extensive evaluation it was decided to implement a system utilising chilled water Sasol Infrachem radiant panels mounted on the walls of the microscope laboratory. These chilled panels act as a heat sink by absorbing heat by radiation (the reverse of radiant heating) and creating very slow convective ai ȁ