Air handling unit sequence of operations
Unlike the terminal sequence of operations, the air handling unit (AHU) sequence of operations consists of several
subsequences, so the AHU sequence of operations could be split over several controllers. The AHU sequence of operations has as attributes an airflow control subsequence and a temperature control subsequence. In addition, there is an optional filter monitoring subsequence which has Digital attributes representing differential pressure switch status and filter condition status as well as optional attributes representing filter alarm delay time and filter run time.
The airflow control subsequence has supply fan control, return fan control (optional) and minimum outdoor airflow control (optional) attributes. The minimum outdoor airflow control entity has attributes representing the minimum outdoor airflow rate, the minimum outdoor airflow setpoint, and the minimum outdoor damper control signal. The supply fan control entity, a subtype of the flow moving control subsequence, may be further classified as variable air volume (VAV). The VAV supply fan control subtype, which is also a subtype of the feedback control loop, adds the analog attributes of supply air static pressure setpoint, supply fan control signal, and a set of supply air static pressure reset requests (optional).The VAV supply fan subtype is further classified as either single or dual duct. If it is single duct, then an Analog attribute representing the supply air static pressure sensor is added. If it is dual duct, two Analog attributes representing hot and cold deck supply air static pressure sensors are added. The set of supply air static pressure reset requests is provided so that they may each be linked to the supply air static pressure reset request attribute of a terminal unit sequence of operations.
Also illustrates the return fan control, which is also a subtype of the flow moving device control sequence. The return fan control sequence may also be further classified as VAV. The VAV return fan subtype, which is also a subtype of the feedback control loop, is further classified into five subtypes as the return fan control signal is modulated to maintain the process variable listed below at a setpoint value:
• Building static pressure
• The difference between supply fan speed and return fan speed
• The ratio of return fan speed to supply fan speed
• The difference between supply air flow rate and return air flow rate
• The ratio of return air flow rate to supply air flow rate.
The temperature control subsequence consists of setpoint logic, cooling control (optional), heating control (optional), and economizer control (optional). The setpoint logic has two subtypes: single-zone setpoint control for single-zone AHUs and supply air temperature (SAT) setpoint control for AHUs that serve multiple zones. The single-zone setpoint control entity has two Analog attributes: zone temperature and zone temperature setpoint, as well as an optional setpoint reset schedule based on return air temperature (RAT). The reset sequence has a RAT attribute along with minimum and maximum zone temperature setpoints. The SAT setpoint control is classified as being either single duct, with SAT and SAT setpoint attributes, or dual duct, with cold deck SAT, cold deck SAT setpoint, hot deck SAT, and hot deck SATsetpoint attributes. The SATsetpoint control entity also has an optional SAT setpoint reset attribute. The SAT setpoint reset is abstract but there are two subtypes, one based on RAT and the other on heating and cooling requests. The RAT reset entity (the same type as the single-zone temperature setpoint reset schedule) has a RAT attribute along with minimum and maximum SAT setpoints. The heating and cooling request reset entity has sets of heating and/or cooling requests along with minimum and maximum SATsetpoints. The set(s) of heating and/or cooling requests are provided so that the elements of the set(s) may each be linked to a heating or cooling request attribute of a terminal unit sequence of operations. The
dual duct SAT setpoint control type has two optional SAT setpoint reset attributes, one for the cold deck and one for the hot deck.
The abstract economizer control subsequence has attributes representing the economizer status, economizer
control signal, minimum position, and low temperature limit
(optional). The economizer is classified into four subtypes in with the following additional attributes:
• Absolute temperature-based: outdoor air temperature and changeover temperature
• Relative temperature-based: outdoor air temperature and return air temperature
• Absolute enthalpy-based: outdoor air temperature, outdoor air humidity, and changeover enthalpy
• Relative enthalpy-based: outdoor air temperature, outdoor air humidity, return air temperature, and return air humidity.
The cooling and heating control subsequences are classified as either hydronic (chilled water for a cooling coil or hot water for a heating coil) or staged (one or more stages of refrigeration for a cooling coil and one or more stages of either electric heat or combustion furnace for a heating coil). The hydronic coil entity has a valve control
signal attribute and an optional “central plant run request” attribute. The staged coil entity has a set of on/off stage command attributes.
Central plant sequence of operations.
Chiller and boiler manufacturers often package the controls with their products, but the BAS may perform plant-level control, for example, equipment sequencing/scheduling, alarming, and possibly setpoint reset. The central plant control sequence consists of a subsequence representing the supervisory control strategy, an optional set of pump control subsequences, and an optional set of energy conversion equipment control subsequences. The supervisory control subsequence has attributes of plant type (boiler or chiller), supply temperature, supply temperature setpoint, return temperature, lockout temperature, run enable (optional), and a set of run requests (optional). The run requests are provided so that they may be linked to a boiler or chiller run request attribute of an air handling unit or terminal unit sequence of operations. The pump control subsequence is a subtype of the flow moving device control subsequence. It may be further classified as a pump feedback control subsequence, which is a subtype of both the pump control and the feedback control subsequences.
In addition, the pump feedback control entity has supply pressure, supply pressure setpoint, and pump control signal attributes along with an optional set of supply pressure setpointreset requests.
The energy conversion control subsequence is for supervisory control over a boiler or chiller with packaged local controls. It consists of the device type (derived from the central plant type), run command, lead/lag status (optional), alarm (optional), supply temperature (optional), supply temperature setpoint (optional), control signal (optional), run time (optional), and energy consumption (optional).
