Upload
raven-thomas
View
46
Download
0
Embed Size (px)
DESCRIPTION
Thermal stress & Chromatics. Thermal stress & annealed glass. It is well-known that annealed glass can be vulnerable to thermal breakage when there is a differential in temperature across a sheet - PowerPoint PPT Presentation
Citation preview
Thermal stress & Chromatics
Thermal stress & annealed glass
• It is well-known that annealed glass can be vulnerable to thermal breakage when there is a differential in temperature across a sheet
• If the temperature difference is too big, stress at the edge of the glass will eventually set off breakage
Thermal stress & Chromatics
Chromatics products resist thermal breakage in 3 ways
• Under extreme stress Chromatics does not break, but may crack remaining glazed and in place
• Increased design strength – the temperature difference at which Chromatics becomes at any risk is some 25% higher than for annealed glass
• Stress relieving factor – the Chromatics process brings the temperature across a pane into equilibrium much more efficiently than a piece of opacified annealed glass, reducing the maximum difference experienced
Safe breakage of Chromatics
• Chromatics DOES NOT break!
• Under extreme conditions it may crack, but will remain glazed, intact & safe until it can be replaced – even if replacement can not be effected immediately
Stress Relieving Factor
A characteristic of a material by which the stress induced in it’s
surface is reduced when subjected to
temperature differentials
Stress Relieving Factor - Example
• Consider a piece of opacified float glass, which is heated in a frame so that the calculated centre-edge difference is say 60°C
• Annealed float glass has a SRF of 1.0 meaning that it does not modify the temperature differential at all
• That is: actual temperature difference = 60 x 1.0
SRF & Chromatics• Extensive work done at British
Glass, in conjunction with NSG Pilkington and verified by independent specialist JB Waldron has established an SRF for Chromatics of 0.62
• This means that Chromatics is MORE efficient than annealed glass at bringing temperature differences into equilibrium
Stress Relieving Factor - Example
• Consider a piece of Chromatics, which is heated in a frame so that the calculated centre-edge difference is say 60°C
• Chromatics has an SRF of 0.62 meaning that it modifies the temperature differential as shown below
• Actual observed temperature difference = 60 x 0.62 = 37.2 °C
Design strength
• Wired glass – 25 °C
• Cast glass – 30 °C
• Clear float glass – 40 °C•
• Heat-strengthened glass – 100 °C
• Toughened glass – 200 °C
Defined as that temperature below which 95% of panels of that type will not
break in 95% of cases
Design strength & Chromatics
• Determined using NSG Pilkington methods and analysis
• Verified by independent specialist JB Waldron
• Value = 50.5°C
Ranking of design strengths
• Wired glass – 25 °C
• Cast glass – 30 °C
• Clear float glass – 40 °C
• Chromatics – 50.5°C
• Heat-strengthened glass – 100 °C
• Toughened glass – 200 °C
What does this all mean?
• Type and thickness of the panel
• Its size (larger panels are more at risk because they can build up greater stress levels)
• Its aspect – does it face North or South, is it mounted vertically or at an angle?
• Where is the panel mounted? What is the diurnal temperature range and the solar power at that location?
• Its application – is it in a monolithic panel or double glazed? What type and colour frame is used? If a sealed unit, what gas filling has been used? Etc.
• Its colour
• Other factors – is the panel subject to a static, moving or no shadow?Are there mullions or canopies to consider?What is the temperature behind the panel?Is it insulated? Etc.
When considering whether a piece of glass will be thermally safe or at risk in a given location,
orientation and application,several factors must first be considered
What does this all mean?
• From these factors, it is possible to calculate a theoretical temperature difference across a panel
• This difference is modified using the value of SRF
• The result is then compared to the design strength
• If this is higher than the design strength, the panel is at risk of thermal damage – otherwise, it can be considered not at risk
Example• In a given location and application, a panel
of black Chromatics has a calculated temperature difference of 65°C
• The SRF for Chromatics is 0.625, so that the actual temperature difference will be 65 x 0.625 = 40.6°C
• This is lower than the design strength for Chromatics (50.5°C), so this panel should be considered NOT AT RISK
Implications in the real world
• OK – final calculated temperature likely to be below 45 °C
• Calculation should be carried out – final calculated temperature likely to be between 45 - 48°C
• Calculation should be undertaken with particular reference to static shadow – final calculated temperature likely to be 48 - 52 °C
Location Light Colours
Dark Colours
London
Frankfurt
Rome
Athens
Corfu
Prague
Moscow
Paris
Stockholm
Reykjavik
Conclusions• Chromatics is a unique product – can
be cut, is flat like annealed glass, but is not usually at any risk thermally
• Chromatics does not break – in an extreme case it may crack, but remain safely glazed
• Not at thermal risk in most European applications
Supplement John Brian Waldron
• Used to verify the calculations for Chromatics
• Former European Technical & Standards Manager, Pilkington PLC
• Chairman of various CEN & BSI committees
• CV available on request