Engineered Systems - March 2008 - (Page 48)

The Modern Unit Ventilator Operation of a VUV continued from page 46 FIGURE 2. A VUV with ducted air supply. time the control scheme is “constant volume, valve control.” A school may be slightly more susceptible to this than other building types due to the higher outside air fraction required by the population density. A UV would be no different than an air handler or a fancoil if the control was constant volume, valve control. Modern UVs have either face/bypass (F/BP), reheat, or some sort of O/A preconditioning. The authors prefer F/BP as the least costly from both construction and operating standpoint. The F/BP approach, when properly applied, will keep space humidity below 60%, even in high humidity regions of the country. Classroom noise. ANSI Standard S21.60 recommends that classroom equipment noise levels be kept below 35 dB in order to not interfere with student hearing. From a qualitative standpoint, 35dB is a very low number and a very quiet space. It is probably impossible to achieve with any system that has a single, large high-velocity outlet at ear level (UWUV). This requirement would probably also negate using anything with a compressor in the room, such as selfcontained DX or heat pumps. Curtains, acoustical ceilings, and carpet will help attenuate the equipment noise. The solution is to duct the air distribution and design the VUV unit with a large, slow fan and well-insulated cabinet. Additionally, noise from both UWUVs and VUVs will be significantly lowered when fan speed modulation is A big plus for UVs vs. a central system such as VAV is that an outage will only affect one room, not the whole building. However, a big negative is that UWUVs are hard to work on when things h do need maintenance. All work has to be done either kneeling or lying on the floor. i — Remove the old UWUV and block off the wall opening. Use an insulated window panel to install a new intake louver for use with the VUV. — Remove the UWUV and build bookshelves with an insulated plenum behind. Use the plenum for outdoor air intake from the existing wall louver into a rear plenum on the back of the new VUV. • Q: I would like to use a two-pipe changeover system because of the first-cost savings. Is this feasible in a school, or must I design a four-pipe system? A: By ensuring that a quick chilled water (CHW)/hot water (HW) automatic changeover function is incorporated in the design, the two-pipe system is an excellent choice. An operating benefit is also gained in that the HW loop can be kept at a much lower temperature than the four-pipe system because the coil in the VUV is considerably larger in a two-pipe unit. Depending on outdoor air temperature, the HW loop can be in the 90°F to 130° range, which results in significantly lower parasitic heat loss from piping compared to a conventional 180° loop. • Q: Why use face and bypass damper control instead of a modulating CHW valve? A: A face and bypass damper system provides superior temperature and humidity control and allows full flow of CHW through the coil to ensure that the air passing through the coil reaches dewpoint at the fin surface. With modulating valve control, on a day that is cool but humid, the valve will be positioned by the PID control loop so that the room drybulb temperature is satisfied but there might be little or no condensate formed at the coil because of the low flow rate. And in winter, unless the water flow is completely stopped, it is virtually impossible to freeze a F/BP coil. • Q: I am designing for the climatic conditions found in Florida and the other states on the Gulf Coast. Is the unit ventilator suitable for this application? A: Yes, with a variation. Economizer cooling is of no value in these regions. Specify a unit which has a dedicated outdoor air pretreatment coil included, in addition to the primary chilled water coil. This type of dual-path unit has been used in Florida since 1998 with great success. These units can handle up to 450 cfm of outdoor air. • Q: Is factory mounting of DDC an option? Can a condensate pump be included for rooms that are below grade? A: Factory mounting of controls is much easier in the VUV than in the UWUV because the layout of the components in the VUV provides better accessibility. A condensate pump can be factory-installed in the VUV. used as a primary control scheme, meaning that the UVs will operate at medium or low speed (lower noise) most of the time. Filtration efficiency. ASHRAE Standard 52.2 recommends a minimum filtration of MERV 6. Older UVs had 1-in. media filters with an approximate rating of MERV 2. The added static pressure drop associated with a higher filter rating may significantly affect airflow, and may be difficult to get with the low static fans in UWUVs. Again, the VUV has a fan that has more static capability and can meet the filter requirement. Freeze-ups. This big complaint and trouble was a result of several prob- 48 En gi neer ed S y stem s March 2008

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Engineered Systems - March 2008 Contents Editor’s Note Back2Basics HVAC Challenge Case In Point Commissioning Building Automation Energy Wiz HVACR Designer Tips Application Checklist Exemplary Design = Elementary Success The Modern Unit Ventilator On A Mission VAV Systems And Green Design – Part II Issues & Events Products Glossary Classifieds Advertiser Index Tomorrow’s Engineer

Engineered Systems - March 2008

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