This paper was originally published at GPD 2019 by Martin Teich, Christian Rehner and Fabian Schmid of seele GmbH.
The glass tube facade consists of 307 laminated safety glass tubes, 238 half tubes and 69 full tubes with a height of 9m and a diameter of 900mm. The aluminum profile connects adjacent pipes to each other. Each tube has a cut duplex stainless steel cover on the top and bottom, fixed with structural silica gel. The outer wall is located in the humid environment of Hong Kong, and the hurricane wind load is very high.
Due to the large enclosed air volume in the cavity, a closed cavity facade (CCF) system is used to pressurize the pipe. seele conducted extensive tests in Germany and Hong Kong to confirm the feasibility of the system. Semi-glass tubes are bent and laminated in Europe, shipped to Hong Kong, and are structurally bonded into full-glass tubes, and then installed in high-rise buildings in Hong Kong.
The closed cavity technology is a well-known and mature method used in multi-layer curtain wall systems. The purpose is to balance the pressure in the inner cavity of the curtain wall and prevent any condensation on the glass surface. Compared with the naturally ventilated double-façade system, the closed cavity facade system improves the acoustic performance, integrates the sunshade device, minimizes cleaning work, increases the floor space, and provides the largest building tenants. Comfort.
This article shows that CCF technology also has some great advantages for non-traditional applications. This article first outlines the general design principles of the enclosed cavity exterior wall. Compared with Europe, the author discusses key aspects of Hong Kong’s special geometry and climate zone. Finally, it shows how the volume flow of dry air passing through the tube affects the risk of condensation.
The closed cavity exterior wall uses pressurized air supply or ventilation systems, which are known from the concept of pressurized multilayer ETFEfilm mat structure. Dry, filtered and sometimes tempered air is blown into the system to control the condensation of the cushion structure. Mechanically driven vents or compressors, stainless steel pipes and valves are used as the air supply system. Filtering and removing any contaminants in the supply air is essential to ensure the execution chamber conditions. Designers can adjust system performance to a more forgiving or more efficient configuration. The tightness of components and pipes, usually in the range of 3 to 40 l/h∙m³, can meet the specific needs of most projects.
The CCF concept usually simplifies the glass elements to provide shading, light control and adjustment functions, and climate load management in one system. Most of the realized projects are high-rise buildings with office use. However, the CCF concept is also suitable for safe operation of double-layer curtain walls in challenging climatic conditions. With proper design, the technology can provide performance reserves and can even safely control extreme weather conditions and critical climate impacts. As shown in subsequent paragraphs, even extreme geometries of glass elements can be achieved.
The CCF design requires a system with an appropriate air flow rate to avoid condensation in almost all environmental conditions, handle loads caused by climate and other pressures, and minimize energy consumption. The challenge for development is to understand the effects of heating and cooling on special geometries, and to determine the flow of development to avoid congestion and heating.
In order to define the flow rate and the optimal inflow and outflow openings, different full-scale models and numerical calculations were used. CCF components are usually manufactured in clean rooms. The components are adjusted throughout the processing and supply chain until installation and commissioning to prevent unwanted substances from entering the cavity
The unique glass tube façade shown in Figures 1 and 2 is an attractive addition to the Hong Kong New World Centre, which is a hotel and office building in Kowloon. seele used 307 glass tubes and special LED lighting technology between the tubes to build the main facade of the New World Center. Part of the exterior wall is equipped with CCF technology, which is subject to special physical requirements. The entire building needs to withstand high wind loads and building movements.
The CCF system is integrated in 69 full pipes and half pipes. Two independent CCF supply systems respectively control the elements of the east and west facades to take into account different climatic boundary conditions. At the lower end of each pipe, a small vent is integrated to ensure pressure compensation. Mechanically driven ventilation, stainless steel pipes and valves are used to provide dry, filtered and temperature-regulated air. Filtering and removing any contaminants in the supply air is essential to ensure the execution chamber conditions.
When using exterior walls, especially CCF exterior walls, knowledge and understanding of local weather conditions play a key role in the correct size of technical equipment and system adjustments. In preparation for the project to be showcased in Hong Kong, seele conducted basic system surveys in Gersthofen (Germany) and Hong Kong to determine system parameters for safe and long-term operation.
An important survey at the beginning of each project is the evaluation of local climate data in order to evaluate the design process in detail. Therefore, the following section describes the most important climate differences between Gersthofen (Germany) and Hong Kong and their impact on system design.
Gersthofen in southern Germany is mainly affected by the mild and warm climate at the foot of the Alps. The climate classification of Köppen and Geiger describes it as a warm temperate, always humid, and warm summer climate. In contrast, Hong Kong is affected by a warm coastal climate. In summer, Hong Kong receives significantly more rainfall than in winter.
According to the Köppen Geiger classification, Hong Kong has a warm temperate climate with dry winters and warm summers. These climatic characteristics can also be seen in the analysis of representative weather data. The outdoor temperature in Hong Kong is significantly higher than in southern Germany. The annual average temperature in Gersthofen is 8.5°C, and that in Hong Kong is 22.5°C. In addition, the absolute humidity and vapor pressure in Hong Kong are significantly higher than in southern Germany.
If the climatic conditions in the cavity are adjusted too slowly, a sharp drop in temperature or a rapid increase in the vapor of the outside air will cause condensation inside the glass tube and on the frame. In order to make a good assessment of the function of the CCF exterior wall, two parameters are relevant, which can be determined from representative weather data: the degree of temperature and humidity changes (rising and falling) and their probability distribution.
Figure 3 shows a comparison of the probability distributions of temperature rise and fall and vapor pressure changes between Gersthofen and Hong Kong. Corridors of 0 to 0.75 K/h, 0.75 to 1.75 K/h, and 1.75 to 3 K/h were identified to distinguish between non-critical, critical and very critical changes. In the investigation of humidity changes, the partial pressure of water vapor is divided into 0～125Pa, 125～250Pa and 250～375Pa according to the pressure change rate.
Partial pressure is another measure of the change in humidity in the air, because it provides an absolute value independent of the current temperature and gives an indication of the direction and amount of steam movement. Due to the rate of change of partial pressure, the actual amount of water input and output can be evaluated independently of temperature effects.
The critical temperature rises and falls more frequently in southern Germany than in Hong Kong. In addition, it is more prominent that a large temperature drop and rise have occurred in southern Germany, ranging from 0.75 to 3.75 K/h. As temperature changes are lower and less frequent, Hong Kong is less important. A comparison of the partial pressure of vapor and its changes shows that the air in Hong Kong is humid (as expected), and critical drops and rises in the range of 125-375 Pa/h are more frequent than in Germany.
The comparison concluded that Gestofen is more critical to temperature fluctuations and may condense more frequently due to rapidly changing weather conditions. Hong Kong is even more critical because the moisture content in the air is much higher, and when combined with the inside of the cavity, there is a risk of condensation under much lower temperature changes. This led to the recommendation that Hong Kong's CCF exterior wall should use pre-treated exhaust air from the air conditioning system. This has the advantage of significantly reducing moisture, otherwise it must be buffered by the machine technology of the CCF air handling unit.
In order to define the flow rate and the optimal inflow and outflow openings, seele constructed different full-scale models, as shown in Figure 4, to study different system parameters through long-term measurements. Test engineers continuously control the system during operation to monitor conditions and detect critical changes at an early stage.
The geometry and volume of the CCF facade influence the design of the ventilation system. In Hong Kong, seele constructed two geometric types: full pipe and half pipe. The diameter of the full tube is 900 mm, the height is 9 m, and the internal volume is 5.1 m³. Due to its geometry, the volume of the half tube is 50% of the full tube.
In various test series, seele measured the surface temperature of the inside of the full tube and half tube.
Figure 5 shows representative temperature measurements over a two-day period. The half tube shows a higher absolute temperature and a faster temperature rise than the full tube. This is due to the smaller air volume and the reflective coating of the half-pipe rear panel. Half pipes tend to have lower dew points and react faster to system conditions.
In the second step, the engineer compares the measured temperature with the dew point temperature of the climate inside the tube. According to the volume flow rate of the pipe adjusted with dry air under low pressure, record the higher or lower temperature difference between the surface temperature of the glass sheet and the main dew point temperature.
Figure 6 shows some selected results: the higher the volume flow, the lower the risk of condensation. Due to the smaller volume of half pipes, they are better regulated at the same air flow rate compared to full pipes. This has a positive effect on the dew point of the half pipe. At lower air flow rates of 20 l/h and 150 l/h, the difference between half pipe and full pipe is very obvious. In the case of a full pipe, the temperature difference is smaller and smaller than that of a half pipe, so the possibility of condensation is greater. A full pipe is a slower system and should be adjusted with a higher volume flow.
Combine the results with detailed planning risk analysis and appropriate air flow rates to consider the economic impact of building operations. The pipe diameter of the ventilation system is selected according to the defined airflow design to avoid unnecessary pressure loss. The choice of ventilation device depends on the pressure loss in the piping system and the required air flow rate. Therefore, the optimal air flow rate can reduce energy consumption during the life of the CCF system.
This article discusses several aspects of closed-cavity curtain wall system design and engineering. As shown in the figure, CCF technology can also be used as the preferred technology for the improvement of Hong Kong’s new world central tube exterior wall and non-classical exterior wall elements. The author compares the climate conditions in Central Europe (Germany) and Hong Kong. Although the air in Central Europe generally has a lower medium temperature and lower humidity, the temperature of the cavity in the CCF tube changes faster than in Hong Kong. But in Hong Kong, engineers must consider the high humidity of the outside air when designing the CCF system.
The CCF system provides a good solution to avoid condensation in the multi-layer facade system. However, it is necessary to consider the optimization of air flow, the reduction of energy consumption and operating costs. Therefore, system components and integrated sensors can be used for future optimization of system conditions within the overall integrated system optimization of the building.
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