Sequence Of Operation For Variable Volume Type Fresh Air Handling Units

Sequence of operation is very important part of testing and commissioning of equipment’s. This becomes more important when matter of mechanical systems is considered. SOA is technical document that is developed based upon the mechanical systems performance and operation requirements.

Sequence of operation is part of building management system BMS submission documents. BMS covers the monitoring and control requirements for all mechanical and electrical systems including chillers, AHU’s, pumps, electrical distribution and ELV systems.

Below is sequence of operation for Variable Volume type Fresh Air Handling Units FAHU’s wich are equipped with enthalpy wheel heat recovery system. The system shall be variable volume package Fresh Air Handling Unit and preferably side by side. The control of operation shall cover following parts of the HVAC system:

Supply Side of AHU

• 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 of AHU

• 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 building 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 Air Supply Fan Modulation

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.

Fresh Air dehumidification and sensible cooling using PICV Valve

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.

Requirements & Operation of Accessories for AHU & HVAC System

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.

Provision of Enthalpy wheel

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 duct works 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.