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Heating, Ventilating, and Air-Conditioning (HVAC) Natural Ventilation
Outdoor air is required for ventilation purposes, but can also be used to remove internal heat loads. When the outdoor air is cool (14-23°C) it can be used to cool the occupants. Increased air movement also has a cooling effect on occupants. This strategy should be used in conjunction with an adaptive comfort model. Natural ventilation can be successfully utilised as a form of free air-conditioning when the external temperatures are appropriate, however, external noise control can be a significant issue that needs to be considered. Some locations (eg adjacent to particularly busy roads/freeways) could preclude the use of natural ventilation. It is very important that these issues are considered early and appropriate noise mitigation incorporated into the design. For office spaces it will not be possible to achieve appropriate indoor adaptive comfort conditions for the entire year, however, this system may be used effectively in conjunction with a mixed mode air conditioning strategy and thermal mass. Prior to considering natural ventilation strategies consider the following issues:
Several strategies exist to make use of natural ventilation.
In Canberra, Australia, Arup have designed a building with a temperatures range from 19°C in winter up to 26°C in summer with no control of humidity. This naturally ventilated building has been well received by the occupants, who have expressed feeling healthier due to the improved indoor air quality. Air-Conditioning Design Conditions Based on guides set out by the Australian Institute of Refrigeration, Air-Conditioning and Heating (AIRAH), the base building mechanical services should comply with the following as a minimum: External Design Conditions:
For further information on locations within NSW please refer to the AIRAH guideline. Internal Design Conditions: Air-conditioning systems should generally be designed to provide 22.5°C ± 1°C. Calculated heat gains should include all simultaneous solar, infiltration, casual, lighting and machine loads. The following internal loads may be considered a typical minimal allowance: For a PCA Premium Grade Building:
For a PCA Grade A building:
Calculated heat losses should include thermal and infiltration loads only with no allowance for any internal heat gains. Systems The air-conditioning running and operating costs contribute significantly to the energy demands on the building. Choosing the right air-conditioning system can significantly reduce the energy output from the building. The following should be considered when reviewing an air conditioning system:
Temperature sensors are located to avoid direct solar gain, heat transfer through external walls etc. Systems should generally comply with the requirements of the Building Code of Australia, Volume One – Part F4. Standard industry air-conditioning systems include (a combination of systems is also often commonly found):
Variable Air Volume (VAV) A VAV system as the title suggests varies the amount of air supplied to the building depending on the cooling requirements. The building should be zoned to a minimum of north, south, east and west perimeters as well as an internal zone. The VAV system will provide cooling to these zones based on the cooling requirements within them. For example, early morning, the east zone will require more cooling than the west zone, therefore the amount of air supplied to the west zone will be reduced. A re-heat coil for the individual zones is used to control the temperature within them. All fans and pumps will have a variable speed drive so as to vary their duty depending on the requirements. The ability to reduce the duties result in reduced running costs. Constant Air Volume Systems (CAV) A CAV system as the title suggests supplies a constant volume of air to the space, varying temperature depending on the cooling requirements. As with VAV, a CAV system should be zoned to a minimum of north, south, east and west perimeters as well as an internal zone using a form of re-heat by either electric or hot water coils as described above. This system is not as energy efficient as a VAV installation as fans and pumps will operate at full design duty at all times i.e. no variable speed devices enabling a reduction in duties and therefore running costs will typically be installed. Displacement Systems Displacement systems supply air at a low velocity, generally 0.2m/s at low level into an occupied space. The cooler air supply forms a boundary layer at low level, which slowly rises replacing the air present in the room. At the heat sources, people and equipment, the air ascends and is removed at high level. The heat loads and pollutants are removed from the room rather than mixed as in traditional systems. The supply air temperature should be 1-3°C less than room temperature. A displacement system for a room 2.8-3m can typically remove a specific cooling load of 40W/m2. The displacement system conditions only the occupied space rather than mixing the whole occupied volume. This results in a smaller chiller size, which reduces running cost and plant space requirements. The fan power is reduced because of the low pressures, due to the low velocities. Chilled Beams/Ceiling Chilled water is run through tubes laid in the ceiling or factory assembled beams to carry off the room-cooling load. The cooling capacity is transferred to the indoor air through radiation with the remaining through convection. The radiation and convection shares depend on the design of the chilled ceiling system. For example a chilled ceiling which is integrated into a closed room ceiling contributes over 50% of the cooling directly to the body through radaitive exchange rather than cooling the air which then cools the body. Due to the ceiling removing a large proportion of the cooling load, the air volume supplied to the room can be reduced. (Air is still supplied to meet fresh air requirements and to remove to latent load). A typical chilled ceiling can achieve a specific cooling capacity of 60-90 W/m2. The reduction in air volume results in low noise levels, reduction in plant space requirements, lower energy costs and lower maintenance costs. A chilled ceiling system should be used in conjunction with a displacement system when cooling loads are too high for the displacement system alone. Reducing the flow rate through the ceiling can reduce the capacity of the cooling load. Therefore, the pumps should be variable speed drive and the pipework zoned to a minimum of north, south, east and west perimeter and internal zones.
Hybrid Ventilation The capability to avail of natural ventilation in the office area when external conditions are favourable should be investigated and costed on analysing the air-conditioning above options. The air-conditioning zones should be arranged so as the system shuts down or is automatically altered to operate in 'free cooling' mode (see above) on an occupant opening a window in that respective zone. Operable windows linked to the BMCS (Building Mangement Control System) with an open free area exceeding 5% of the total serving floor area should be allowed in the façade. A remote link to the BMCS indicating when external conditions favour natural ventilation should also be installed in each zone. Zoning Final air conditioning zones will be dependant on the chosen system but should at minimum be sectioned to independently control (water coil re-heat) a north, south, east and west perimeter (4m depth off façade) as well as an internal zone. Hybrid areas will require many sub-zones interlocked with operable windows to operate efficiently. Mechanical Ventilation Filtered outside air is mechanically supplied and extracted through a network of ductwork and air grilles. Room air is extracted and ducted to roof level exhaust plant for discharge to atmosphere. A negative pressure is maintained in these areas to prevent the migration of odours to the adjacent areas. Ventilation systems should be designed in accordance with the requirements of Australian Standard AS 1668.2. Where heated mechanical ventilation is required the outside air supplied to the space should be heated to maintain a set-point temperature within the space. Heating is to be by way of Low Temperature Hot Water (LTHW) heating coils served by the gas boiler (see section to follow). Mechanical Exhaust Plant or amenity exhaust air is extracted and ducted to roof level exhaust plant for discharge to atmosphere. A negative pressure is maintained in rooms served in order to prevent the migration of odours to the adjacent areas. Passive make-up air may be by way of door grilles or ceiling mounted transfer duct and grilles. Ventilation systems should be designed in accordance with the requirements of Australian Standard AS 1668.2. Heating LTHW generated centrally by heat pump or gas fired boiler located in a purpose built boiler plantroom should be used for primary space heating as a minimum and although electric re-heat is common, LTHW re-heat coils are more energy efficient. LTHW is distributed through copper pipework to various heating coils serving the air-conditioned and treated ventilation spaces. THW panel convectors may be used for underfloor heating or similar. The use of reverse cycle air-conditioning may be used where appropriate.
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