Saturday 20 February 2016

Multiple Zone Variable Volume Type Recirculating Air Handling Units AHU

Multiple 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 AHUs 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.

A hand/off/auto selector switch shall be located on the supply fan control panel. The supply fan motor shall be interlocked to this selector switch, the extract fan fail and the outside air damper proving end switch.

On a command to start the supply fan (thru a VFC from DDC to 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 and exhaust air air dampers (modulating) 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 supply fan control signal shall be modulated under PI control to obtain the minimum static pressure set points defined during commissioning. The index run VAV box shall be satisfied to have an inlet pressure of 150 Pa (adjustable). The controller shall modulate the supply fan speed utilizing 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 air 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 the encountered) and the fresh air and recirculating dampers will be modulated according to the average space air quality (measured by duct mount C02 sensors to maintain 500 ppm (adjustable) and an alarm shall be generated if the CO2 level remains at 750 ppm continously for a period of 5 minutes) 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.

The chilled water coil shall be provided with a 2-port pressure independent control valve for supply air dehumidification and sensible cooling. The CHW valve shall be positioned closed when the air handling plant is not operating.

No action is taken when the BMS signals a low outside temperature when the air plant is operational should this ever occur.

The valve shall be positioned to close when a fan failure signal is present. The valve shall fully open when the supply fan is proven and the BMS signals an optimum cooling start operation.

The CHW valve’s position shall be modulated in response to a PI control signal in order to obtain the required set point (design set point of supply air is 12°C) the greatest demand of dehumidification or sensible cooling control shall have priority. The position of the mixing dampers shall be controlled to supply the air to meet indoor CO2 levels.

If the supply air temperature rises above a set point of 25°C and 26°C during summer and winter respectively or below a set point of 12°C during normal operation the BMS shall give a supply air temperature high/low warning.

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 return fan control signal shall be modulated to produce a return air volume flow rate at a ratio of 90% (adjustable) of the supply fan speed or as per static pressure build up due to modulating VAV boxes in the system.

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 proving end switches.

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

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.
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HVAC is the technology of indoor and vehicular environmental comfort. Its goal is to provide thermal comfort and acceptable indoor air quality. HVAC system design is a subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer. Refrigeration is sometimes added to the field's abbreviation as HVAC&R or HVACR, (heating,ventilating and air-conditioning & Refrigeration) or ventilating is dropped as in HACR (such as the designation of HACR-rated circuit breakers). HVAC is important in the design of medium to large industrial and office buildings such as skyscrapers, onboard vessels, and in marine environments such as aquariums, where safe and healthy building conditions are regulated with respect to temperature and humidity, using fresh air from outdoors. Ventilating or ventilation (the V in HVAC) is the process of "exchanging" or replacing air in any space to provide high indoor air quality which involves temperature control, oxygen replenishment, and removal of moisture, odors, smoke, heat, dust, airborne bacteria, and carbon dioxide. Ventilation removes unpleasant smells and excessive moisture, introduces outside air, keeps interior building air circulating, and prevents stagnation of the interior air. Ventilation includes both the exchange of air to the outside as well as circulation of air within the building. It is one of the most important factors for maintaining acceptable indoor air quality in buildings. Methods for ventilating a building may be divided into mechanical/forced and natural types.