Consulting-Specifying Engineer - April 2008 - (Page 50) East Valley High School in Los Angeles has 72-in.-wide perforated free-hanging linear panels on its ceilings. Heating capacity is 48 Btuh/sq. ft, and cooling capacity is 35 Btuh/sq. ft. Photo: Twa Panel Systems Inc. Based on the nominal heat gains in a conventional office space, and with 60% to 70% of the ceiling used as the radiant cooling surface, a 64 F surface temperature would be able to provide effective cooling to the occupants in that space. With an average ambient indoor air dry bulb temperature of 76 F, the corresponding maximum relative humidity at which condensation will start to form on surfaces colder than the dewpoint temperature will be approximately 64% relative humidity (RH) (dewpoint temperature of 63.5 F/ wet bulb temperature of 67 F). Considering that ASHRAE Comfort Standards require maintaining less than 60% RH levels for indoor comfort, there should be zero risk for condensation on large radiant cooling surfaces that operate at 64 F or higher. Also, testing of radiant cooling panels have shown that it would take more than 8 hours with a surface temperature held at 14 F below the ambient dewpoint temperature before visible drops were apparent. If the ventilation air system for the building can be designed to ensure that the indoor ambient RH is below 50% at all times (a requirement for good indoor comfort), then the lower limit on the radiant cooling dewpoint temperature would fall to around 57 F. This is a large change in potential radiant heat exchange rate when dealing with temperature to the fourth power. Lower the indoor relative humidity to 45% at an ambient air temperature of 76 F, and the dewpoint falls to 53 F. That’s a reasonable margin of safety to operate the radiant cooling surfaces down to 62 F. This can be observed on a psychrometric chart to see the envelope of performance that radiant cooling surfaces can work within, without risk of condensation. Outside air What about opening windows and infiltration of humid outdoor air during the summer? That should be a concern, but consider- ing that the amount of untreated humid outdoor air that mixes with the drier indoor air leads to a blended ambient RH, and the length of time it takes to form condensation, even light fogging of a cooled surface, then transient humidity spikes of a few hours can be tolerated. Condensation formation is a slow phenomenon, and can be responded to easily by conventional controls, so it’s easy to prevent damaging condensation formation by use of failsafe controls. To ensure that the condensation risk is minimized, there are relatively inexpensive condensation sensors that can be used to reset the cooling water supply up a few degrees to be slightly above the dewpoint to insure that condensation can’t form. There will be a slight loss of some radiant cooling capacity, but a good engineer always has a back-up plan, right? Use the required ventilation air supply as a second-stage cooling medium, using a re-cool coil for that zone to lower the ambient air temperature to deal with the room comfort for short periods of time. The ideal complementary ventilation air supply system that should be coupled with a radiant cooling system is a low-level air supply method like underfloor air distribution (UFAD) or low-level displacement ventilation (DV) terminals to make the most of the combined systems advantages. The basic premise here is that the radiant cooling system is providing close to 100% of the sensible cooling load in the space, therefore the ventilation air only needs to be supplied at virtually “room temperature” (i.e, around 70 to 72 F). If a low-level UFAD or DV system is used, lowering that small volume of ventilation air down to 67 F would provide an additional 10 to 14 Btuh/sq. ft of space cooling capacity. Using the ventilation air supply as your second-stage cooling source can provide a quick response to fast-acting solar transients at perimeter areas, as required. Ideally the building design team should try to reduce and eliminate those pesky fast-acting solar gain transients in the first place, but where they do exist, the air system can provide a fast-acting response. The HVAC designer will then have to ensure that the radiant cooling system is adequately sized for the “steady-state” cooling requirements, and then the air system would be sized for the basic minimum ventilation air supply plus whatever excess air is needed for any peak transient cooling loads. A variable air volume air valve could be used, or a re-cool coil to lower the air supply temperature of the basic ventilation air volume. If the room cooling was being provided by a traditional overhead all-air fully mixed air system, at least 1 cfm/sq. ft to 1.3 cfm/sq. ft of cooling air would need to be circulated. For the same heat gains in that same room, with most or all of the sensible cooling being done by the radiant cooling system, then the only supply air needed is the basic 20 cfm/person of ventilation air supplied at “room temperature.” That is less than 0.20 cfm/sq. ft in a normal office occupancy, maybe as high as 0.30 cfm/sq. ft in a densely occupied office space. As pointed out above, even that small volume of air, when supplied at low levels in a UFAD or DV style, can provide additional boost 50 Consulting-Specifying Engineer • APRIL 2008
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