Chilled-Water VAV Systems - HVAC.Amickracing

Primary System Componentsweather, a concentration that provides “burst protection” is usuallysufficient for the chilled-water system (Table 13, p. 87). A concentrationthat provides “freeze protection” is only needed in those cases where noice crystals can be permitted to form (such as a coil loop that operatesduring very cold weather) or where there is inadequate expansion volumeavailable. Make sure to also use an inhibitor package to help resistcorrosion.The advantage of this approach is that it is predictable and relatively easyto maintain. However, antifreeze degrades the heat-transfer performanceof cooling coils and chillers, often increasing the size and cost of thesecomponents. In addition, it increases the fluid pressure drop through thecoils and chillers, impacting pumping energy use.• Preheat the outdoor air before it mixes with the recirculated air.Using an electric heater, steam coil, or hot-water heating coil to preheatthe sub-freezing outdoor air before it enters the mixing box decreases thetemperature difference between the two air streams, which improvesmixing effectiveness and reduces stratification.The advantage of this approach is that it is predictable and effective. Incold climates, a source of heat may already be needed in the centralizedVAV air-handling unit. However, because the heat source needs to belocated in the outdoor air stream, this approach may limit flexibility orincrease the cost of the air-handling unit. And, if a hot-water or steampreheat coil is used, they also require some method of freeze prevention(see “Heating coil,” p. 23).One common approach is to use a preheat coil with integral face-andbypassdampers. These dampers modulate to vary the amount of heattransferred to the air, while allowing full water or steam to flow throughthe coil tubes.• Use air-to-air energy recovery to preheat the outdoor air.As an alternative to the prior method, an air-to-air energy recovery device(such as a coil loop, fixed-plate heat exchanger, heat pipe, or wheel) canbe used to preheat the entering outdoor air during cold weather (see “Airto-AirEnergy Recovery,” p. 160).Figure 13. Air-mixing bafflesThe advantage of this approach is that it also reduces cooling and heatingenergy use, and can allow for downsizing of cooling and heatingequipment. However, such a device does increase the cost of the airhandlingunit and adds a pressure drop to both the outdoor and exhaustair streams, which increases fan energy use.• Use air-mixing baffles.This configuration of baffles (Figure 13), located immediatelydownstream of the mixing box, adds rotational energy and increases thevelocity of the air stream, which improves mixing (blending) to prevent orminimize temperature stratification.The advantage of this approach is that it works consistently and requiresno maintenance. However, it does increase the cost and length of the airhandlingunit, since distance is needed downstream for the air to finishmixing and slow down before reaching the filters. Also, the baffles add20 Chilled-Water VAV Systems SYS-APM008-EN