Below sequence of operation is for single zone Variable Volume type recirculating Air Handling Units AHU’s. The system shall be variable volume package Fresh Air Handling Unit. The covering of the sequence shall consist of:
Supply Side coverage of scope
- Intake motorized damper
- Panel (Pleated) Filter
- Bag Filter
- Cooling Coil
- Supply Fan (with VFD)
- Intake and discharge attenuators
- Sensors and controls (refer to BMS Schematic Diagram)
Exhaust Side coverage of equipment’s
- Exhaust motorized damper
- Panel (Pleated Filter)
- Exhaust fan (with VFD)
- Intake and Discharge attenuators
- Sensors and controls (refer to BMS Schematic Diagram)
Sequence of Operation
The Variable Volume re-circulating type AHUs for single zone will operate under the dictates of one of the DDC controllers inbuilt time schedules (adjustable) to suit the operational requirement of the building and control in the following manner.
On a command to start the supply fan (thru DDC to VFC at control panel) will be enabled and positive indication of this given by means of a differential pressure switch fitted across motor.
The fan shall be enabled when the BMS signals for the air handling plant to operate and the outside air, mixed air and exhaust air dampers are proven open. The fan operation shall be proven when the differential air pressure switch signal is detected.
When the proven signal is not achieved, following a 30 second start up period, a fan failure warning signal shall be sent to the BMS and the fan operation signal shall be removed. The fan operation signal shall be disabled when an overload relay in MCC has tripped.
The supply fan control signal shall be modulated (thru 0-10 VDC from DDC to VFD) under PI control to obtain the minimum static pressure set points defined during commissioning. The controller shall operate utilising the measured sensor value versus its set point.
The supply fan shall be disabled and a warning sent to the BMS if the supply air pressure rises above a limit of 1500 Pa (adjustable).
Dampers Modulation & Control
Once flow is established the system will allow its temperature control algorithm to operate. The system will maintain the minimum fresh air requirement (pre-set to ensure that negative pressure is not encountered) and the fresh air and recirculating dampers will be modulated (from 0-100% open) according to the average air quality (measured at return duct C02 sensors to maintain 500 ppm) to reduce the load on the plant.
This will ensure that high volumes of outdoor air are not unnecessarily cooled. The actual fresh air volume delivered to the space will be measured by a multi point velocity detector in the intake ductwork. If the CO2 level remains above 750 ppm continuously for a period of 5 minutes, an alarm shall be generated. Fan speed will be varied by the use of inverter/VFD drives via hardwire contacts (0-10VDC from DDC to VFD).
Indication of fan running is provided by means of a differential air pressure switch fitted across the fan which will alarm in the event of failure. Individual indication of “fan trip” and “switch not in auto position” will be provided through the DDC controller.
The supply air temperature set point shall be scheduled linearly when the BMS signals normal operation. The supply air temperature shall be fixed according to outdoor air temperature, typically at 12 ºC (adjustable) whenever outdoor air temperature is above 22ºC (adjustable) reset the supply temperature setpoint to 14.0ºC (adjustable).
If the supply air temperature rises above a set point of 24°C or below a set point of 10°C during normal operation the BMS shall give a supply air temperature high/low warning.
Space Condition Control
The space conditions will be maintained by the DDC controller modulating in sequence the cooling valve (thru 0-10 VDC from DDC to PICV) and the fan speed to the satisfaction of the return air temperature detectors. Positive feedback of valve and damper position will be displayed on the BMS.
Temperature control of the Return will be achieved with a combination of fan speed adjustment (air-side control) and waterside control via modulation of control valve (PICV) to optimize energy performance. Variable volume of air supply shall be achieved via VFD control. During occupied times the unit will start at 50% design air flow (adjustable) the cooling coil valve is positioned in response to a PI control signal acting on the temperature differential between the supply and its set point.
If the Return temperature cannot be achieved the supply air volume shall be increased in increments of 10% (adjustable) to meet space cooling load demand. Again the cooling coil valve is positioned in response to a PI control signal acting on the temperature differential between the supply and its setpoint. This increase continues up to 100% design flow. During commissioning the contractor is to ensure that the PI loop time constants are set to ensure that hunting does not occur due to over cooling of the space. Return temperatures setpoints are 22-24 °C during summer and 23-25 °C during winter.
If the relative humidity reported at the duct mounted supply air humidity sensor rises above its set point (60% maximum) and the supply fan is proven by the differential pressure sensors cooling coil has to operate in conjunction to dehumidify the supply air by cooling to 12.0°C (adjustable) The air side control is to begin at 50% air flow (adjustable) increasing in increments of 10% as described previously. However, relative humidity is uncontrolled and the reading is for monitoring purpose only.
The above temperature control mode shall be set up and commissioned for the specific project and the set points adjusted and suitable time delays applied to ensure hunting does not occur. At all times the outdoor air dampers will be modulated to achieve IAQ set point 500 ppm (adjustable) for CO2 level.
Supply and return humidity will be monitored by detectors and displayed at the BMS Supervisor. Pre and bag filters in the supply duct will have differential pressure switch fitted for indication and alarm purposes on the BMS.
Graphical User Interface Design
A graphical representation of the plant will be produced with all set points, alarms and time schedules displayed with simple mouse clicks. Access to the graphic will be through a system of site plans, plant rooms and systems.
All values are to be historically recorded at controller level so that locally any laptop or Portable operator’s terminal may retrieve the data as well as the network BMS Supervisor.
A fire alarm interlock (thru VFC to DDC) shall be hard wired into the control circuit of the AHU to ensure that it shuts down in an alarm condition. Moreover, a motorized smoke fire damper installed on the main supply and return air duct will be automatically closed by Fire Alarm Panel.