5 System Loads and Autosizing
Autosizing prototype HVAC systems: Overview
Pre-defined prototype systems are autosizable with respect to zone- and system-level equipment and controls, and both levels are required in sequence to fully autosize a system. These two levels of loads analysis and autosizing can also be applied to a broad range of user-modified variants of prototype systems. While care must be taken to maintain desired zone-level autosizing links when modifying prototypes, any system, including any user-defined configuration, can be autosized with respect to system coils, fans, water loops, and plant equipment without special preparations.
For pre-defined prototype systems and variants thereof, a broad range of system elements are sized in each of two stages—first at the room/zone level, then at the system/plant level—with opportunity for user intervention between the two. ASHRAE Loads calculations are linked to target ApacheHVAC systems for both stages of the sizing process:
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Zone-level loads and sizing must be completed either manually or through the autosizing procedure described below. The resulting values for zone heating, cooling, and ventilation airflows, room unit water flow rates, and other controller settings are then assigned to the HVAC system for the spaces to be conditioned accordingly.
Zone flow rates and other controller settings can also be overridden or set manually.
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System-level loads and sizing scales the capacities and performance curves or data for coils, fans, water loops, chillers, boilers, DX cooling, air and water-source heat pumps, and similar equipment, to model appropriate performance and energy consumption. All of these components “see” the load during the system-level sizing run, and this applies to all ApacheHVAC systems, regardless of how and when they were created.
Equipment sizing and capacities can also be overridden or set manually.
Figure 5-1 : Systems setup and sizing toolbar in ApacheHVAC.
From left to right, the toolbar buttons in Figure 5-1 provide the following functions:
· System schedules and setpoints
· System parameters
· Zones tabular edit view
· Global system parameter assign
· Room and zone-level sizing
· System equipment and plant sizing
· System loads, sizing, and ventilation reports
Once an autosizable system (one with a gray system frame around it) has been loaded from the HVAC library and zones have been assigned to it, the toolbar buttons are listed above are used to access dialogs for systems setup and sizing, as described in the pages that follow below.
