National treasure

In recent years the National Gallery of Australia in Canberra has hit the headlines on numerous occasions – and not always for the right reasons. But with the installation of a new air-conditioning system this is about to change.

The gallery, which sits on the banks of Lake Burley Griffin, was designed in 1973 and opened in late 1982. The original building is a brutalistic concrete structure with a more lightweight temporary exhibitions extension, completed in 1998. The majority of the building is dedicated to the display of the nation’s art treasures, along with some on-site art storage and conservation areas that require continuous close temperature and, more importantly, humidity control.

Staff complaints about the air-conditioning system at the gallery were the main reasons for its bad press, to the extent that it attracted questions in the Australian government senate. However, one positive effect of this attention was the granting of funds to undertake a major overhaul of the air-conditioning system.

This project involved replacing the existing sprayed coil humidification system, following an audit commissioned to assess condensation, water ingress and health and safety concerns. That audit found serious issues with water ingress as well as major maintenance and occupational health and safety concerns associated with the original humidification system. Issues of condensation control and plant condition were obviously serious for a building housing delicate works of art.

Steensen Varming was brought on board to manage revamp of the air-conditioning system, including the replacement of the humidification system. One of the main constraints on the project was the existing critical multizone air-handling units. These four units handle air quantities of 19,000 litres/s, 38,000 litres/s, 48,000 litres/s, and 58,000 litres/s and are built into the building structure, which made maintenance and replacement of equipment difficult without major demolition work.

The original system was based on a sprayed cooling coil system of central humidity control with zone electric humidifiers and reheaters for trimming conditions to about 50 separate critical zones. The zone humidifiers had long been decommissioned as a result of energy costs, in the knowledge that acceptable control could be maintained centrally through the sprays saturating the air and using the cooling coil to dehumidify and maintain a constant dew point, a stable but energy intensive control strategy.

Energy consumption was a major consideration in the upgrade of the humidification system. Other factors taken into account were water spillage and the amount of chemical cleaning required for disinfection, which was very labour intensive as well as causing some concerns in the Conservation Department.

Going ultrasonic

A number of humidification and control options were analysed under a wide range of climatic and occupancy conditions. None of the original calculations were available, so very detailed psychrometric analysis was required, especially as the gallery was keen not to recommission the zone electric humidifiers. This analysis looked at providing conditions from the central air-handling units that then only required individual zone reheat control to provide room conditions within the critical condition target area. The target is for year-round internal conditions of 22°C +/- 0.5°C and 55% relative humidity +/- 3%, as well as very high air quality; however, the rate of at which the conditions change and the internal humidity are both more critical than temperature control when it comes to conserving the collection.

Steensen Varming assessed a number of options and initially suggested the use of atomisation – the process whereby compressed air and water pass through special nozzles – to humidify the required zones. This was trialled on the smallest critical air-handling unit. However, after analysing the results and addressing the risk of high pressure leaks and the take-up of the available droplet sizes within the constraints of the existing air-handling units, it was decided that ultrasonic humidification would be a better option. This decision was based on the quality of the humidification achieved, the close control provided by the system and the energy savings. In ultrasonic humidification, the droplet size is much finer and because this means water is absorbed more readily into the air stream, less space is required to install the systems.

The system of close control ultrasonic humidifiers installed is served by a reverse osmosis water filtration system, along with triple redundancy controls and a central humidification control strategy to satisfy about 50 separate zones.

Ultrasonic humidifiers function on the principle of generating a water mist that can be injected into a supply air stream. Electronic oscillations are converted to mechanical oscillations through a piezo electronic disk, which is immersed into the humidifier feed water reservoir. By focusing the high frequency oscillations at the surface of the water reservoir, a fine mist is produced and is quickly absorbed into the air stream, thus increasing its relative humidity.

Ultrasonic systems require high quality water supply, as demineralising is a big issue. There are a number of different methods for pretreating the water, such as employing a series of filters, however this can lead higher maintenance. Instead Steensen Varming opted to use reverse osmosis.

This can provide a 95-98% reduction in total dissolved solids and a 99.9% reduction in bacteria. The membrane is sensitive to chlorine, scale and the build up of suspended solids so pre-filtration and pre-treatment is required upstream of the reverse omosis membrane, along with regular cleaning. Steensen Varming has also written regular purge cycles into the reverse omosis controls sequence to ensure micro-bacterial propagation in minimised. The reservoirs in the humidification units also have a set drain-down cycle to ensure water is never stagnant.

Lessons learned

A disappointing aspect of the project was that the suppliers of both the ultrasonic systems and the water filtration were unable to provide quality advice on maintenance and cleaning regimes. After some investigation, it was found that such systems are often left to fend for themselves, with the false view that the high level of filtration and the water temperature will ensure no bacterial problems. However, this is not the case and therefore a systematic approach to water quality tests and cleaning was instigated in order to develop an effective, cost-efficient maintenance regime.

It was observed that because of the need to use carbon filters to protect the reverse osmosis membrane (carbon filters remove chlorine which is added to water supplies to control bacterial growth), bacteria could propagate. This bacteria could then traverse the reverse omosis membrane by effectively splitting through the membrane as it multiplies (bacteria multiplies by splitting in half).

The ultrasonic baths were also prone to some biofilm build up (a biofilm is a protective layer that colonies of bacteria produce once they become established) that was simply dealt with by regular draining and wiping with a broad based biocide. Suffice to say, even though some initial bacterial total plate counts were high in parts of the system, these have now been brought under control. And at no point did they threaten the air quality as the concentrations in the nebulised air were always below recordable levels and no points in the system ever recorded levels of Legionella.

Energy savings

Ultrasonic humidifiers have proved to be an excellent alternative in achieving precise humidity control where very specific environmental conditions is required.

As the humidification process is adiabatic, substantial energy savings can be achieved over equivalent systems that require boiling of the feed water (up to 90% less energy in production alone). In addition, low electrical demands and maintenance requirements reduce the cost of humidifying to below that of alternative adiabatic processes while still providing sensitive relative humidity control. Other occupational health and safety and maintenance issues can also be designed out when appropriate maintenance regimes are applied, leading to a generally cleaner system.

As the ultrasonic systems are able to pulse accurately to satisfy the load/part load requirements, the main savings are realised because the system is not required to constantly fully saturate the air and dehumidify using the cooling coil.

Overall, the improvements achieved by the system include:

• Gas – 15,774 GJ/year saving of gas or 41% of total gas consumption
• Electricity – more than 1 million kWhr/year saving or 11% of the total electrical consumption
• Annual energy cost savings of £54,000 or 15% of the total building energy cost.

As a result of these, and other similar savings in water use, chemicals and maintenance, the system has been showcased by the Australian government’s Greenhouse Office. And the Australian National Gallery has remained out of the headlines.