Double Duct CV
Double Duct CV
System Description
The design is peak air and block cooling capacity.
This system is composed of a central, constant volume fan which delivers air via a hot and cold deck to the individual room mixing boxes. Return air is drawn from a common return air plenum; the return air from the associated interior and perimeter rooms is assumed to be uniformly mixed before being routed to the ROA deck.
System Simulation
A constant volume of air is delivered to each room as long as the supply fan is scheduled on. Once the drift temperature for the hour is known for all rooms, the program calculates the supply air temperature needed for each room.
If the room drift temperature rises above this hour’s cooling thermostat, the room supply air temperature will be controlled (by mixing hot and cold deck air) such that the room temperature is brought down and maintained at the room cooling thermostat.
If the room drift temperature falls below this hour’s heating thermostat, the room supply air temperature will be controlled (by mixing hot and cold deck air) such that the room temperature is brought up and maintained at the room heating thermostat.
If the drift temperature is within the deadband region, the room supply air temperature will be controlled such that the room temperature stays at the beginning drift temperature for this hour.
System Options
1. The value of minimum ventilation airflow will be proportional to the percentage defined by the outside air schedule. The value of the outside airflow may be greater than the nominal value if an economizer or nighttime purge is activated. The value of outside airflow may be less than the nominal ventilation value if the supply fan (entered as the main cooling fan) has been scheduled at less than 100%; the outside air percentage for a particular hour will be multiplied by the main cooling fan utilization percent for that hour. If the supply fan has been scheduled off for a particular hour, no outside air can be delivered to the rooms.
2. The supply fan is controlled in time clock fashion, i.e., if the schedule reads 80% for a particular hour, the fan will operate at full rpm for 80% of the hour and remain off the rest of the hour. Since the supply fan delivers conditioned air to the space, reducing the percent it can operate during a given hour will cause the cold deck to deliver a greater proportion of the supply air during the cooling mode or the hot deck to deliver a greater proportion of the supply air during the heating mode. If the fan is not allowed to operate long enough, the room temperature will be allowed to drift, the amount dependent on the magnitude of the unmet space load.
3. Specifying a supply air reset control will cause the program to reset the cooling supply air dry bulb upward based on the worst case cooling room and reset the heating supply air dry bulb downward based on the worst case heating room.
Application Notes
● The return/outside air moisture, i.e., the air routed through the ROA deck, is used by both the cold and hot decks. Consequently, when an economizer is specified, greatly increased heating loads can result since the ROA deck air will be controlled to the cooling supply air temperature dictated by the outside air reset schedule even if only one room in the system needs cooling. Specifying a supply air reset control will minimize the excess heating during unoccupied hours but will have minimal effects during occupied hours.
● Despite the differences in duct configuration, the airside energy requirements of double duct are identical to those of the multizone system if the system inputs are also identical.
● Humidity compensated controls can be implemented by using the humidity controller on the controls tab of the system properties to modify the default settings. By using the option supply air reset per maximum, for example, whenever a room relative humidity is greater than the design room relative humidity, the room's VAV dampers will continue to open until the room relative humidity is lowered to the design room relative humidity.
Alternatively, by specifying the option supply air reset per multizone maximum avg, whenever the return air relative humidity is greater than the design system relative humidity (which is the average of the design room relative humidities for the zones served by the system), supply air reset controls are deactivated and the main cooling coil leaving air temperature is depressed to its design value for that hour.