Saturday 20 February 2016

Constant Air volume Air Conditioning System

Constant Air volume Air Conditioning System  Air Handling Units AHU’s

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. Wrap Around heat Pipe.
  9. 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 constant volume full fresh air type AHU shall start/stop controlled operate under the dictates of one of the DDC controllers inbuilt time schedules initially set to 24 hours operation (adjustable) and control in the following manner.

On a command to start the supply fan 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 fixed control to obtain the required system flow rate defined during commissioning. The controller shall operate utilising a preset time clock (adjustable) to set back the unit flow rate during non-operational hours.

Fan speed will be varied by the use of inverter/VFD drives via hardwire contacts. Fan speed modulation is only to be utilised for commissioning purposes and at the change over from operational and non-operational time periods as defined by the time schedule.

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.

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. The Exhaust will run at the same speed of supply fan.

The supply air temperature set point to swimming pool shall be scheduled to 18°C (adjustable) to maintain room temperature of 28°C.

If the supply air temperature rises above a set point of 18°C or below a set point of 12°C during normal operation the BMS shall give a supply air temperature high/low warning.

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

The space conditions will be maintained by the DDC controller modulating in sequence the cooling valve based on PI control to the satisfaction of the supply air temperature sensors. Positive feedback of valve and damper position will be displayed on the BMS. 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.

If the relative humidity reported at the duct mounted supply air humidity sensor rises above its set point of 50-60% (adjustable) and the supply fan is proven by the differential pressure sensors cooling coil and heating coil are to operate in conjunction to dehumidify the supply air by cooling to 12°C (adjustable) with the heating modulating to maintain the space temperatures as defined above.

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.

During non-operational periods of the swimming pool as defined by the BMS time clock the AHU volumes are to be adjusted down to the set-back conditions with the control of the cooling coils as defined previously.

Room temperature and relative humidity will be monitored by sensors and displayed at the BMS System. Pre and bag filters in the supply duct will have differential pressure sensors fitted for indication and alarm purposes on the BMS. An alarm shall be generated to BMS in case the differential pressure across each filter bank exceeds the adjustable set-point decided during commissioning.

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.


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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.