Engineered Systems - March 2008 - (Page 46)

The Modern Unit Ventilator Operation of a VUV • Full-flow chilled water coil. • Face and bypass dampers modulate by thermostatic control. The dampers are asymmetrical to compensate for the air pressure drop differential between face and bypass operation. • The design is return air bypass. The chilled water coil is positioned so that the humid outdoor air preferentially flows through the chilled water coil. Mixing of the outdoor air and return air is minimized by the use of parallel blade outdoor air/return air dampers. • The fan speed reduces as the room temperature reaches setpoint. If the fan motor is a PSC type, this is accomplished by stepping down through three fan speeds. If the motor is an ECM type, the speed reduction is through an analog signal. Design engineers should be aware of the very high aftermarket replacement cost of ECM motors and the potential burden imposed on the owner for future maintenance costs. • As the supply fan speed reduces, the outdoor air damper is opened to maintain the ventilation rate. Alternatively the outdoor air/return air dampers can be decoupled from the controller and responsibility for outdoor air volume given to a CO2 sensor. • 100% economizer function is included. This is an extremely important feature of the unit ventilator in terms of energy savings and is often not a function of other HVAC system designs. FIGURE 1. An under-the window-unit-ventilator (UWUV). under-the window-unit-ventilators (UWUVs) have long damper shafts that are prone to twisting and binding, and fresh air becomes inconsistent. The modern vertical-unit-ventilator (VUV) has a much better aspect ratio (length vs. width), which means that damper issues are solved. There is also the airflow difference frequently seen on old UVs when the air filter would load up. This concern can be addressed by the use of constant torque motors that will automatically compensate for changes in suction-side static. Two building system design issues deserve comment. The UWUVs typically have louvers that are close to the ground and they can be clogged by lawn clippings or snow drifts. Also, UVs typically have low static fans (unlike the high static fans of most air handlers) and they’re not capable of pushing excess/relief air out of the building. The VUVs typically have outside air louvers that are mounted much higher on the wall and are less prone to clogging. The relief air issues must be addressed with powered, not gravity, relief. The construction concerns involve installation details. Most UWUVs are set against the outside wall and sealed with a foam gasket against a block wall. The straightness, squareness, and levelness of an 8- by 2.5- by 1.5-ft section of rough construction will have a huge impact on whether fresh air enters as intended, or room air leaks into the outside air (O/A) chamber because of an incomplete seal. This lack of proper sealing of the O/A chamber is a prime reason why many UVs do not provide adequate economizer cooling. The VUVs have ducted connections to the O/A louvers. Poor air distribution and short circuiting of supply air. UWUVs have a single air outlet that is meant to make the air sweep across the ceiling to condition the whole room. This works very well in heating, as the outside wall and the roof are where the heat losses occur. It works adequately with some room configurations and poorly with others. And the concept can/will be terminated when the teacher piles books on top of the unit or hangs banners from the ceiling. A summer concern is the short-circuiting of conditioned air from the unit discharge back to the return because cold air drops. The shortcircuiting gets worse if the fan speed is reduced for noise or capacity reasons. The VUVs have ducted air distribution to ceiling diffusers and can easily accommodate any room configuration with no shortcircuiting. Humidity control. Humidity control, or lack of it, is a concern any FIGURE 1. An example of vertical-unit-ventilator (VUV). VUV Application FAQs • Q: I am planning to replace some UWUV units with the VUV type. How is this done? A: There are two methods: Continued on page 48 46 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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