The variable speeds Air Handling Units are used to serve air conditioning need for all area of
buildings
The Air Handling Unit comprises
- Variable Speed Supply Fan
- Chilled water coil with the 2-Way modulating control valve
- Duct mounted supply air pressure sensor
- Outdoor & re-circulating air modulating damper
- Carbon dioxide sensor.
- Supply and Return Air temperature sensors
- Supply air differential pressure switch
- Differential pressure switches for 2 set of filters
System Monitoring and Alarm
- Software alarms shall be generated at the operator workstation whenever the run status of the supply fan (with differential pressure switch) does not match the current command state.
- A failure alarm shall occur when the run status of the load shows no operation, and the load has been commanded to be on.
- An advisory alarm shall occur when the run status of the load shows operation and the load has been commanded to be off. All alarms shall be recorded in an alarm log for future review. Provide 15 seconds (adjustable) time delays before generating an alarm.
The sequence of Operation
a. Auto Mode:
When the AHU start is in AUTO mode (i.e. selector switch installed in the MCC must be in Auto Position), the unit is started and stopped from the BMS via a time schedule or BMS override command. When the start for the AHU is initiated, the control program residing in the controller follows the following sequence
Start-Up:
The following sequence follows with a preset time interval per interlock equipment start-up:
1) Check Supply fan trip signal – Normal State
2) Supply Air Damper –Open Position
3) Outdoor Air Damper –Open Position
4) Return Air Damper – Open Position
5) Once the above conditions are satisfied, AHU is enabled to start in Auto mode or using a plant enable button on the graphics in manual mode by the operator. Once enabled, BMS will automatically command the supply fan to start.
6) Supply Fan shall start, and it’s associated Interlock equipment in sequence. Through the signal from the Diff. Airflow Switch, if airflow is detected, the System will continuously run, if No airflow is detected by the DP Switch, the Supply Fan will de-activated and send an Alarm to the DDC – for “No Airflow” and shut down the whole system including its associated interlocks. If the Airflow switch signal is proved ‘ON’ then BMS will enable control loops.
b. Shutdown Mode:
When the shutdown command for the AHU is initiated, the control program residing in the
controller follows the following sequence.
1) Send Stop command to stop the supply fan
2) The outdoor air, return and supply air damper move to close
3) Move chilled water valve to close position
b. Manual (Hand) Mode:
When the AHU is the manual mode, the fans are started and stopped from the AHU control panel. Other control except for fan on/off control shall function as per the Auto mode.
c. Fire / Smoke Mode:
Fire condition is determined by the Fire Alarm Control Panel. AHU will automatically shutdowns the whole system with associated interlocks.
4. AHU Control
The control program, on the feedback of air handling unit operation, initiates the control
algorithm. This algorithm consists of three controls. Each temperature, pressure and ventilation control has its own control loop. The pressure control loop is used to modulate the speed of the supply air fan hence supply airflow. The control loops design to function as per the following explanation
a. Temperature Control loop:
The supply air temperature installed in the duct will relay the measured signal(temperature) to the DDC controller, the DDC controller compares this signal with set-point (adjustable by the operator from BMS central) and generates an analog output to the 2-way modulating cooling valve. Based on the difference between the two values, a proportional-integral program will determine the percentage of the cooling coil valves opening to achieve the desired condition. The default set-point value for the supply air temperature is 13ºC (Adjustable).
b. Pressure Control loop:
The supply air pressure sensor shall be installed in the duct will relay the measured signal (static pressure) to the DDC controller, the DDC controller compares this signal with the set-point (adjustable by the operator from BMS central) and generates an analog output to the variable frequency drive (VFD) of the supply air fan. Based on the difference between the two values, a Proportional-Integral program will determine the percentage of the fan speed to achieve the desired pressure. The set-point value for the supply air pressure for each AHU shall be adjusted.
c. Ventilation Control loop:
Demand control ventilation employs return air carbon dioxide controlling strategy.
A single carbon dioxide sensor sense carbon dioxide concentration in the return air duct and sent to the DDC controller, the DDC controller compares the signals with return air carbon dioxide concentration (Default carbon dioxide level difference value 400 ppm ).
Then DDC controller generates an analogue output to the outside air dampers and returns air damper to modulate, based on the difference between the values, the Proportional integral program will determine the percentage of the modulation of outdoor and return air dampers.
Minimum outdoor air quantity shall be governed either by building pressurisation requirement (Input from Building differential pressure sensor) or 20% of the Maximum outdoor demand of the AHU.
5. Alarms:
The following minimum alarms shall be generated on BMS
1) Filter Dirty Alarm: This is generated when the pressure drop on each filter exceeds the set value to indicate dirt accumulate at filters.
2) Fan Trip Alarm: A normally open “NO” volt free contact at the MCC panel when closed will generate an alarm at the BMS indicating that the fan is tripped
3) Fan Fail: In case the supply air fan fails to start or if the differential pressure switch across
the supply fan is not giving the signal according to the command due to any reason then an alarm shall be generated. In case of a fan fail alarm on the BMS, due to abnormal behaviour, the DDC controller will latch the alarm. The operator has to acknowledge (reset) the alarm on the BMS once the trouble has been checked and removed. The operator shall not be able to start the AHU until the alarm s acknowledged and reset.
4) Temperature High & Low: Temperature HIGH and LOW alarms shall be generated if the supply/return air temperature rises above or falls below the supply /return air temperature alarm limit.