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Sequence Of Operation For Variable Volume Type Fresh Air Handling Units FAHUs with Enthalpy Wheel Heat Recovery System

Sequence Of Operation For Variable Volume Type Fresh Air Handling Units FAHUs with Enthalpy Wheel Heat Recovery System. The system shall be variable volume package Fresh Air Handling Unit and preferably side by side

The unit shall consist of following:

Supply Side

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  • Intake motorized damper
  • Panel (Pleated) Filter
  • Bag Filter
  • Cooling Coil
  • Supply Fan (with VFD)
  • Wrap Around Heat Pipe
  • Enthalpy Heat Recovery Wheel
  • Intake and Discharge attenuators
  • Sensors and controls (refer to BMS Schematic Diagram)

Exhaust Side

  • 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 For Variable Volume FAHU

The fresh air handling units (FAHU’s) shall serve the fresh air requirements of recirculated air handling units (RAHU’s). BMS shall enable the start/stop operation of FAHU’s thru from DDC to VFC at starter control panel.

The Variable Volume fresh air handling units (FAHUs) shall operate under the dictates of one of the DDC controllers inbuilt time schedules (adjustable) to suit the operational requirement of the school and control in the following manner.

On FAHU’s start-up, the fresh air and exhaust motorised dampers with their respective positions confirmed by feedback signal open and shall be interlocked with the fresh (supply) and extract air fans respectively.

Duct mount temperature sensors located on fresh (supply) air duct shall modulate the 2 port valve of the cooling coil to maintain the fresh (supply) air set point temperature (of 12°C adjustable).

Fresh (Supply) fan speed shall modulate (thru 0-10VDC signal to VFD) according to the average space air quality (measured by CO2 sensors installed on the return/exhaust fan of RAHU it is serving) to reduce the load of the plant. This will ensure that the high volumes of outdoor air are not unnecessarily cooled. The carbon dioxide (CO2) level shall be maintained at 500 ppm (adjustable). An alarm shall be generated if CO2 level remains above 750 ppm continously for a period of 5 minutes. The actual fresh air volume delivered to the spaces will be measured by a multi-point velocity detector in the intake duct. The BMS is to review the outputted motorized (modulating) damper positional feedback.

The fan operation shall be proven when the differential air pressure switch signal is detected. When the proven signal is not detected, 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.

Once flow is established the system will allow its temperature control algorithm to operate and the exhaust fan shall be started.

A chilled water coil with a 2 port pressure independent control valve shall be provided for fresh (supply) air dehumidification and sensible cooling. The CHW valve shall be positioned closed (thru 0-10 VDC signal from DDC to valve actuator) when the air handling plant is not operating and when the BMS signals a low outside temperature.

The chilled water valve shall be fully open when the BMS signals a high outside temperature. The valve shall fully open when the fresh (supply) air fan is proven and the BMS signals an optimum cooling start operation. The valve position shall be modulated for supply air temperature control (supply air temperature setpoint is 12°C adjustable) when a normal operating period is signalled by the BMS and the supply fan is proven.

The CHW valve’s position is modulated in response to a PI control signal in order to obtain the required set point.

A variable volume fresh (supply) air fan shall be provided with VFD for commissioning and system operation flexibility to operate as variable volume. The fresh (supply) air fan shall be disabled when the BMS signals a shutdown period.

A hand/off/auto selector switch shall be located on the fresh (supply) air fan control panel. The fresh (supply) air fan motor shall be interlocked to this selector switch and to the extract fan fail and the damper status.

A smoke detection sampling probe shall be provided in the fresh (supply) air ductwork to monitor and ensure that no smoke will be supplied to RAHU’s. On sensing smoke (thru VFC to DDC) the fresh (supply) air fan (and extract fan) shall be stopped and an alarm shall be raised at the BMS central supervisor. The detector shall be manually reset by Fire Alarm System.

A variable volume extract fan shall be provided with VFD for initial commissioning and flexibility. The extract fan shall be disabled when the BMS signals a shutdown period. The fan operation signal shall be disabled if a supply fan fail signal is received by the BMS.

The extract and associated fresh (supply) air fan shall be interlocked so that they only operate in unison. A hand/off/auto selector switch shall be located on the extract fan control panel. The extract fan motor shall be interlocked to this selector switch, the supply fan fail and the damper status.

Monitoring of the cleanliness of the air filters shall be provided by differential pressure switch (thru VFC from DPS) across the filters. An alarm shall be generated to BMS in case the differential pressure across each filter bank exceeds the set-point decided during commissioning (adjustable).

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 12.0ºC (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.

An enthalpy wheel shall be provided to transfer heat and moisture between the fresh (supply) and extract air streams. The enthalpy wheel shall be disabled when the air handling plant is shut down .

The enthalpy wheel shall be enabled when the fresh (supply) air fan starter motor is operating (except for a night cooling or fan overrun operation). The wheel shall operate when the temperature differential between the outside air and return air is greater than a value of 2°C .

The supply air temperature set point is determined according to the strategy selected above.

Duct mounted humidity sensors shall be located in the common fresh (supply) and exhaust air ductworks for trend logging, fault finding and analysis purpose only. Theses sensors do not form part of the control sequence but are to be displayed on the BMS head end graphics.

Fresh Air Handling Unit Sequence Of operation Heat Recovery Wheel

Single Zone Variable Volume Type Recirculating Air Handling Units AHU

The system shall be variable volume package Fresh Air Handling Unit.

The unit shall consist of:

Supply Side

1 Intake motorized damper

2 Panel (Pleated) Filter

3 Bag Filter

4 Cooling Coil

5 Supply Fan (with VFD)

6 Intake and discharge attenuators

7 Sensors and controls (refer to BMS Schematic Diagram)

Exhaust Side

1 Exhaust motorized damper

2 Panel (Pleated Filter)

3 Exhaust fan (with VFD)

4 Intake and Discharge attenuators

5 Sensors and controls (refer to BMS Schematic Diagram)

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 school 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).

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 maiantain 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 abaove 750 ppm continously 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.

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 (airside 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 setpoint.

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.

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, plantrooms 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.

Variable Volume Type FAHU with Plate Heat Recovery

The system shall be variable volume package Fresh Air Handling Unit.

The unit shall consist of:

Supply Side

1    Intake motorized damper

2   Panel (Pleated) Filter

3   Bag Filter

4   Cooling Coil

5   Supply Fan (with VFD)

6   Plate Heat Exchanger

7   Intake and discharge attenuators

8   Sensors and controls (refer to BMS Schematic Diagrams)

Exhaust Side

1 Exhaust motorized damper

2  Panel (Pleated Filter)

3 Exhaust fan (with VFD)

4  Intake and Discharge attenuators

5 Sensors and controls (refer to BMS Diagrams)

The fresh air handling units (FAHU’s) shall serve the fresh air requirements of Kitchen area and other Back of House (BOH) areas such as plant rooms and store rooms.

The Variable Volume fresh air handling units (FAHUs) shall operate under the dictates of one of the DDC controllers inbuilt time schedules and control in the following manner.

On FAHU’s start-up, the fresh air and exhaust motorized dampers with their respective positions confirmed by damper feedback shall open and shall be interlocked with the fresh (supply) and extract air fans respectively.

Proof of fan start shall be sensed via the differential pressure switch fitted across motor.

Duct mount temperature sensors located fresh (supply) air duct shall modulate the 2 port valve of the cooling coil to maintain the fresh (supply) air set point temperature of 12°C (adjustable).

These AHU’s are distributing conditioned air via VAV units to the conditioned spaces. When the VAV modulating dampers start closing, the pressure in the supply duct rises. The supply air duct is provided with pressure sensor at 2/3rd distance, which gives 0-10vdc signal to DDC corresponding to increase in the duct pressure. On receiving the signal, the DDC gives a 0-10vdc to the fan motor VFD to reduce the speed. The supply pressure set point will be adjustable as per load requirement. The operator can adjust the supply pressure set point from BMS Workstation at any time.

A variable volume return fan shall be provided. The extract fan shall be disabled when the BMS signals a shutdown period.

The exhaust fan operation signal shall be disabled if a supply fan fail signal is received by the BMS. The fan shall be enabled when the BMS signals for the air handling plant to operate and the outside 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 detected, 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 extract and associated fresh (supply) air fan shall be interlocked so that they only operate in unison. A hand/off/auto selector switch shall be located on the extract fan control panel. The extract fan motor shall be hardwired to this selector switch, the supply fan fail and the damper proving end switches.

When the Kitchen Exhaust fan is activated (either within the kitchen or via BMS), an individual command to start the supply air VAV boxes damper will be motored to the fully open (design) position (in case of the VAV boxes are partially open to suit the room temperature requirement during night setback). Once the respective VAV damper position have reached the desired point, software shall provide the required confirmation to the DDC controllers and the fresh (supply) air fan and extract fan will be enabled. Therefore the operation of supply air VAV boxes is interlocked with Kitchen extract fan, such that if the kitchen extract fan run the make-up air system shall be available. When the Kitchen extract fan is closed, the supply air VAV boxes shall modulate to satisfy the room temperature requirement.

The supply air volume control set point shall have a minimum set point (adjustable) determined during commissioning, this set point shall be increased linearly when the zone temperature rises above the cooling set point of 12°C (adjustable). The maximum and minimum supply temperatures shall be defined as 22°C and 11°C. Variation of supply air temperature is to operate as follows:-

The BMS is to review the outputted motorised (modulating) damper positional feedback.

The fan operation shall be proven when the differential air pressure switch signal is detected.

When the proven signal is not detected, 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.

Once flow is established the system will allow its temperature control algorithm to operate and the exhaust fan shall be started.

A chilled water coil with a 2 port pressure independent control valve shall be provided for fresh (supply) air dehumidification and sensible cooling. The CHW valve shall be positioned closed when the air handling plant is not operating and when the BMS signals a low outside temperature.

The chilled water valve shall be fully open when the BMS signals a high outside temperature. The valve shall fully open when the fresh (supply) air fan is proven and the BMS signals an optimum cooling start operation. The valve position shall be modulated for supply air temperature control when a normal operating period is signaled by the BMS and the supply fan is proven.

The CHW valve’s position is modulated in response to a PI control signal in order to obtain the required set point.

A variable volume fresh (supply) air fan shall be provided with VFD for commissioning and system operation flexibility to operate as variable volume. The fresh (supply) air fan shall be disabled when the BMS signals a shutdown period.

A hand/off/auto selector switch shall be located on the fresh (supply) air fan control panel. The fresh (supply) air fan motor shall be interlocked to this selector switch and to the extract fan fail and the damper feedback.

A smoke detection device shall be provided in the extract (return) air ductwork. On sensing smoke the fresh (supply) air fan (and extract fan) shall be stopped and an alarm raised at the BMS central supervisor. The detector shall be manually reset by the Fire Alarm Panel.

A variable volume extract fan shall be provided with VFD for initial commissioning and flexibility. The extract fan shall be disabled when the BMS signals a shutdown period. The fan operation signal shall be disabled if a supply fan fail signal is received by the BMS.

Monitoring of the cleanliness of the air filters shall be provided by sensors measuring the differential pressure across the filters. An alarm shall be generated to BMS in case the differential pressure across each filter bank exceeds the set-point decided during commissioning (adjustable).

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 12.0ºC (adjustable) whenever outdoor air temperature is above 22ºC (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.


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