Open-plan aircon uncloaked article 01: How can effective open-plan heating and cooling be achieved?

by FM Media
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NICK ADCOCK from Lucid Consulting attempts to unlock the mystery surrounding open-plan air-conditioning and the typical issues that cause a reduction in occupant comfort and staff productivity.

There are many factors that affect the performance of an air-conditioning system in an open-plan office environment. Many installations, particularly in older building stock, present daily challenges for facilities managers as they juggle energy targets, running costs, occupant comfort, occupant expectation and the constantly changing internal arrangement through tenancy ‘churn’.

Temperature (or, more specifically, dry bulb temperature) is only one piece of the puzzle. A typical air-conditioning system will be set and control the dry bulb space temperature to 22 degrees Celsius ± 2 degrees Celsius; however, the occupant may feel discomfort within this temperature range if a number of other factors exist or are incorrect.
Supply airflow rate affects two factors: the actual dry bulb temperature and the perceived comfort. Supply airflow rate combined with supply air temperature relates to the system’s ability to meet its thermal loads and, hence, the resultant room temperature.
Perceived comfort is related to air movement within the space. The human body will always reject some heat. When air movement is high, the body is able to reject more heat, and, therefore, could feel cold even if other conditions are within an acceptable range. Likewise, a lack of air movement can manifest itself as a perception of warm conditions.

Independent of space temperature, exposure to radiant heat will regularly cause discomfort and temperature complaints in a commercial office space. These issues will arise with exposed glazed façades (no fixed external shading) on the east (in the mornings), north (on clear winter days) and on the west (in summer afternoons).
Radiant heat will be increased with glazing that has a low shading coefficient (clear or non low-E) or conductive performance (single glazing). An occupant that is permanently seated too close (less than 1200 millimetres) to the glazing will also feel the effects of radiant heat. For radiant heat control, I recommend occupant controlled blinds. The type of blind will depend on the orientation, weather profile (city) and façade design.

A key factor in the perception of comfort is the humidity ratio (relative humidity or RH) within the occupied space. A typical air-conditioning system will not ‘actively’ control the RH; however, in most temperate Australian capital cities, a typical system will maintain a space’s RH between 40 and 60 percent, and inherently maintain comfort.
Tropical climates will require active dehumidification systems to maintain the space within a comfortable range. As the RH falls, the ability for the skin to evaporate moisture will increase, creating a cool feeling and, conversely, as the RH rises, the ability to evaporate decreases, causing the body temperature to rise.

A further fundamental of the first principles of air-conditioning design is the notion of zoning. The correct zoning of an air-conditioning system will allow the system to react to a change in load via a dedicated temperature sensor within the zone. A thermal zone may be defined by the building interaction with external conditions (such as sun movement throughout the day) or by the patterns of internal loads (such as the movement of people or equipment).
For instance, the internal zone of an office will not be subject to building fabric loads (heat gain or loss through the façade) and will, therefore, have a fairly constant load through office hours. The western zone will, however, experience significant fluctuation of load, ranging from heating in the morning to full load cooling in the afternoon. Meeting rooms, boardrooms and breakout spaces are the classic examples of spaces that require a dedicated thermal zone due to the variance in load as people move in and out.
Supplementary cooling units are often implemented where the load within a zone is above the allowance or capacity of the base building. Supplementary units may be water-cooled (connected to the supplementary cooling loop) or air-cooled with an external condensing unit. In general, a meeting room with six or more occupants will exceed the capacity of the base building and require supplementary cooling.

In an open-plan office environment, the ability for a sensor to provide accurate information can be compromised by various fitout factors, resulting in the control system making incorrect adjustments. Typical examples include:

  • temperature sensors located behind joinery, in direct sunlight, adjacent to heat-generating equipment such as a printer
  • temperature sensors located out of the correct zone, or
  • poorly calibrated temperature sensors.

It is important to not only understand poor temperature sensor location within the space, but also to have the ability to interrogate the data. A building automation system graphical user interface will enable real-time monitoring of each sensor (and the system in general), including trend logging over a time period, so that rectification action can be taken.

The air-conditioning system type will determine its ability to maintain tight temperature control through a working day. Air-cooled split systems (similar technology to a domestic installation) will perform to a satisfactory level through most load scenarios; however, they may be limited in their ability to seamlessly react to load change or their ability to treat large quantities of outside air.
Inverter technology and modern controls packages have improved the application of these systems in a commercial environment, but careful design consideration is required. A chilled and heating hot water-based system will achieve increased controllability and energy efficiency compared to a ‘direct expansion’ type system with compressors, which simply stop and start to control their capacity.
Air handling and delivery exists in many forms (constant air volume, variable air volume, central air handling, chilled beams and so on); however, a ‘horses for courses’ approach is required in base building system design to ensure that system selection matches the building type and usage profile.

The final consideration is the ability to maintain air-conditioning equipment in an open plan office environment. Many system types will require maintenance access to equipment located within the ceiling or floor of the occupied zone (in lieu of a dedicated mechanical plant room). The equipment may include fan drives, variable air volume dampers, filters, control valves and the like.
It is important that technical staff have the ability to safely access the ceiling or floor to undertake routine maintenance or system tuning. One design solution is to create access paths in corridors (in lieu of over offices or workstations) with access panels or tiled ceiling sections. Where ceiling heights are in excess of standard safe ladder access, alternative design solutions must be considered.

Nick Adcock is Lucid Consulting Australia’s Victorian state manager.

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