Figure 6-3. VAV system—schematic and psychrometric chart.
132 ⏐ A LL -A IR HVAC S YSTEMS 6.8.2 VAV Design Considerations 18.104.22.168 Psychrometric Analysis. A typical arrangement of components for a VAV system with separate perimeter heating is shown schematically in Figure 6-3, with typical psychrometric per- formance for maximum and partial loads and for summer and win- ter operation. For cooling-only applications, air is delivered to a space at a fixed supply temperature that (at design flow rate) can offset the design space cooling load. The space thermostat reduces the volume of supply air when the cooling load is less than the design load. If some warm up is desired after system shutdown, a heating coil may be installed and manually or automatically actu- ated for warm up only. When system supply air is throttled while outdoor air volume is kept constant, tempering of the outdoor air may be necessary under cold winter ambient conditions. Equations 6-1 through 6-6 apply for fan gains, ceiling temperatures, and air mixtures. Zone ductwork and variable-volume terminals must be sized for the governing load factor, either sensible, latent, or venti- lation loads. The designer must determine the maximum system air volume from either (1) coincident system (block) load or (2) block sensible load plus all air requirements from governing latent or ventilation loads that exceed the volume required in any zone to satisfy the sen- sible load in that zone when the entire system reaches its peak sen- sible load. Thus, if a system load peaks at 4:00 p.m. and an east zone’s air requirement from sensible loads is less than the volume required by its latent or ventilation loads at that time, the latter vol- ume must be used for that zone and an appropriate temperature con- trol strategy applied. Actively controlling for nonsensible dominant loads is problematic, as discussed below. If building operators set thermostats below the values used for design calculations, more supply air will be required than the amount determined by the block load. Under these conditions, zones would experience a shortage of supply air if the system was designed based upon block loads only. Such an occurrence is possi- ble. Energy-efficient design practices suggest an indoor summer design temperature of 78°F [25.6°C], and some occupants may find that this setting provides marginal comfort, especially in the pres- ence of high mean radiant temperature. Attempts by the building operator to correct for such discomfort may involve resetting tem- perature sensors below design conditions, leading to the above sce-
A IR -C ONDITIONING S YSTEM D ESIGN M ANUAL ⏐ 133 nario. Designing to provide some flexibility in control, without drastically oversizing systems, is advisable.
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