Lecture 4 Insulated glass units

Transcription

1 ADVANCED DESIGN OF GLASS STRUCTURES Lecture 4 Insulated glass units Viorel Ungureanu CZ-ERA MUNDUS-EMMC

2 Introduction An insulating glass unit (IGU) is a structural transparent element aiming at providing superior building characteristics (reduce thermal losses, improve on the energy savings...improve transparency by reducing condensation on the warmer side).

3 Insulated glass units A multi-glass combination consisting of two or more panes enclosing an hermetically sealed air space. It takes advantage of the fact that air has a low thermal conductivity. The space is filled with dehydrated air or gas. The panes are connected by a spacer, using sealants to reduce water vapour penetration. The whole unit is hermetically assembled by a secondary edge seal The spacer contains a desiccant that absorbs humidity from within the air space

4 Insulated glass units Thermal and light Light radiation transmission Glass has very high transparency within the visible range of wavelengths (λ nm). Total solar radiation reaching the outer glass pane Non transmitted Transmitted Reflected Absorbed % % % 4% 55% Long wave radiation (thermal effect) Most energy from solar radiation is contained in the IR long wave radiation (55%). Therefore, the strategy for solar protection is to block as much IR as possible without reducing the transmittance in the visible spectrum. 4

5 Insulated glass units Thermal and light Heat transfer Heat transfer through a glass pane: From warm side to cold side (1) From the light radiation () 1 Heat transfer modes: Thermal conductivity Heat transfer within a body or between adjacent bodies. Thermal convection Heat transfer between the surface of a solid body and a surrounding fluid (liquid or gas). Thermal radiation Heat transfer resulting from a temperature exchange between two neighbour bodies at different temperatures. In the IR region. Heat transfer by conductivity Heat transfer by convection Heat transfer by radiation Emissivity (ε n ) is a characteristic of the bodies surface associated with thermal radiation. The lower the emissivity the lower the thermal radiation. For glass ε n = This value may be lowered by special coatings. 5

6 Insulated glass units Thermal and light Light radiation related standard parameters In an exposed glass pane the three types of heat transfer are present. The sum is expressed by coefficient U. convection radiation conductivity Total thermal transmittance (U) [U] = W/(m.K) U factor is the heat flux crossing 1 m of a glass wall for a temperature differential of 1ºC between inside and outside. The lower U value is, the lower are the heat losses. Thermal resistance (R) [R] = (m.k)/w R =1/U Total Solar Energy Transmittance (TET) [g] = dmless a.k.a. Solar factor (SF) or g value (g) in Europe; a.k.a. Solar Heat Gain Coefficient (SHGC) in the USA g factor is the ratio between the solar radiation that is transferred through the glazing (reaching the interior), and the total solar radiation reaching the outer pane. It is composed of (i) the direct transmittance, (ii) the part of the absorptance that is dissipated inwards and (iii) convection. The lower it is the less the solar gain is. Light transmittance tv value [tv] = dimensionless 6

7 Layout & components influening IGUs Number of panes, filling gas & low-e coatings In order to reduce coefficient U the thermal resistance of the glass element has to be increased. convection radiation conductivity It is not possible to change the convection properties but conductivity can be reduced by adding air space elements (preferably with a heavy gas: lower thermal conductivity) and heat transfer by radiation can be reduced by low emissivity coatings. Low emissivity coatings are sputtered or pyrolytic, transparent or metallic or metallic oxidic coatings that reduce heat losses by a combination of absorbtion and reflection. 7

8 Layout & components influening IGUs Example In order to reduce coefficient U the thermal resistance of the glass element has to be increased. convection radiation conductivity It is not possible to change the convection properties but conductivity can be reduced by adding air space elements (preferably with a heavy gas: lower thermal conductivity) and heat transfer by radiation can be reduced by low emissivity coatings. U thermal transmittance tv light transmittance g energy transmittance 8

9 Layout & components influening IGUs Spacer Spacer material: Steel/aluminum spacer Synthetic spacer Soft spacer Exterior - 10ºC Interior + 0ºC Exterior - 10ºC Interior + 0ºC Exterior - 10ºC Interior + 0ºC Aluminum Stainless steel Thermix (Stainless steel + plastic sheet) Steel spacer Synthetic spacer Soft spacer Edge seal material: Polyurethane edge seal Silicon edge seal Polysulphide edge seal aluminium spacer synthetic spacer drier 1 sealing (butyl) sealing (polysulphide, silicone) soft spacer (butyl) with integrated drier sealing (polysulphide, silicone)

10 Insulated glass units Flat glass units -Rectangular, triangular Curved glass units - Cylindrical, conical, free shaped Joanneumsviertel, Graz

11 Insulated glass units Noise is any type of sound that is disturbing, annoying or painful. Ambient noise consists of a multitude of sounds of different frequencies and intensities. To represent the volume perceived by the human ear, a logarithmic scale has been chosen for acoustic measurements. The unit of measurement is the decibel (db). The auditory threshold is the value of 0 db and the pain threshold has a value of about 10 db. Noise sorces: Airborne (e.g. Outdoor and indoor noise in buildings, internal inherent noise) Impact (internal noise, mostly footsteps) Structure-borne (equipment noise, building services) Effective sound control means controling the two physical effects of wave propagation: Noise insulation by reflection(sound insulation) the sound energy is not converted into a different energy form, but its direction of propagation is changed by reflection. Noise damping by absorption(sound absorption) sound energy is essentially converted into heat (dissipation).

12 Insulated glass units The sound insulation provided by a partition is defined by an index that represents the difference between internal and external noise (sound attenuation R). For each relevant construction partition element the parameter R must be such that the sound insulation it provides may meet the terms of the code regulation. These are established in terms of a normalized acoustic insulation (D n T). R depends on the sound frequency. The best behaviour of an insulating element is obtained when it provides insulation for the frequencies were the noise is stronger. By choosing apropriate materials and layout it is possible to tailor a glass pane for insulation for a precise type of sound. Window sound spectrum Sound profile 1 Sound profile

13 Insulated glass units Typical linear glass support The self weight of glass is transferred to the frame through setting blocks at the bottom glass edge. Lateral loads are resisted by clamping the glass between the frame system and clamping/pressure plate on the other side. Panels are fixed to the sub-structure at discrete points by clamps. The self weight is transferred through setting blocks and the lateral loads through low friction clamps.

14 Insulated glass units Watch out for building!

15 Insulated glass units External and internal actions External loads Wind loads Snow loads Dead loads Thermal loads Dynamic loads Hail loads Earthquake Bomb blast Impact loads Internal loads Internal loads Temperature difference T Meteorological pressure change p met Change of altitude H

16 Insulated glass units Durability and Service Life Expectancy Fogging Glass fracture Maintainability and Repairability Sustainability

17 Insulated glass units Durability and Service Life Expectancy Fogging Fogging of IGU s is caused by condensation of moist air that penetrates into the air space of IGU s through or around the hermetic seal of the unit. Seal failure is usually caused by: Prolonged water exposure of the perimeter seal Absence of functional weep holes to drain water leakage Discontinuities, poor bond or thin applications of the perimeter seals. To assess the susceptibility of IGU s to seal failures: Test by cycling through heating and cooling cycles (ASTM E-774) Units that pass the test are grouped in three performance levels: Class C (15% failure rate after 0 years) Class CB (15% failure rate after 0 years) Class CBA (5% failure rate after 0 years) The desiccant contained in the spacer helps condensation resistance by absorbing moisture built into the unit. Spacers with bent, welded, or soldered corners, rather than corners constructed with slip-in corner keys, are more reliable because they provide a stable surface for seal adhesion. Similar to IGU s seal failure, laminated glass can delaminate when the edge of the laminated glass is in contact with water over extended periods, causing the interlayer to debond from the glass surface. Building Envelope Guide Glazing by Nik Vigener, PE and Mark A. Brown

18 Insulated glass units Durability and Service Life Expectancy Maintainance and reparability The glazing seals between the glass and framing must be replaced periodically to maintain good performance. Properly installed silicone wet seals should last 10 to 0 years; gaskets 15 to 0 years. Fogged IGU s cannot be repaired.

19 Insulated glass units Durability and Service Life Expectancy Sustainability IGU s have a shorter service life (most practitioners estimate it at 15 to 0 years) compared to monolithic glass, which, if not physically damaged, has an infinite lifespan. The energy savings afforded by IGU s usually pays for the replacement cost if the units last more than 15 years. On the downside, IGU s are typically not recycled: since they consist of a mix of glass, metallic glass coatings, sealants, and aluminum spacers, IGU s require significant and costly effort to separate the constituent materials. Furthermore, glass is manufactured from relatively inexpensive and abundant raw materials, which makes glass recycling unattractive. At the end of their service life, IGU s are generally discarded as general trash. Crushed glass is sometimes utilized as hard fill. Most glass manufacturing plants recover glass discarded during the float glass manufacturing process and combine them with other batch materials for subsequent production. Overall, the most promising strategy to limit the amount of glazing in the waste stream is find ways to extend the service life of IGU s.

20 Insulated glass units procedure Load distribution In case of double glazing, with panes of thickness h 1 and h, the distribution of external uniformly distributed loads (e.g. wind, snow, self weight) is essentially determined by the distribution of the stiffness of the panes, that is: Additionally, the distribution of external loads is determined by the insulating unit factor ϕ. h 1 s h ϕ = a 1 1+ ( a / a*) 4 b h1 δ1 = h + h 1 1 h δ = = 1 δ1 h + h The length a gives the actual dimension of the unit (e.g. in a rectangular unit the length of the short edge) while a* is the characteristic length of the unit, depending on the thickness of the glass panes (h 1 and h ) the gas space (s), and the shape of the unit (λ). Coefficient k 5 for calculation of the volume change Linear interpolation apply. For small deflections (linear theory) p*=0 may be considered. λ=a/b a p* (internal initial pressurization) = s h * 1 8,9 ( 1 ) h + h k ,0 0,019 0,019 0,019 0,018 0,017 0,015 0,011 0,008 0,007 0,005 0,9 0,04 0,04 0,0 0,0 0,00 0,017 0,01 0,009 0,007 0,006 0,8 0,09 0,09 0,08 0,06 0,0 0,00 0,015 0,010 0,008 0,007 0,7 0,05 0,05 0,04 0,01 0,08 0,0 0,017 0,01 0,010 0,008 0,6 0,04 0,04 0,040 0,07 0,0 0,07 0,00 0,014 0,01 0,009 0,5 0,050 0,050 0,048 0,044 0,040 0,0 0,05 0,018 0,014 0,011 0,4 0,059 0,058 0,057 0,05 0,049 0,04 0,01 0,0 0,018 0,014 0, 0,068 0,067 0,066 0,064 0,061 0,054 0,04 0,01 0,05 0,00 0, 0,077 0,077 0,076 0,076 0,074 0,071 0,06 0,048 0,040 0,01 0,1 0,086 0,086 0,086 0,086 0,086 0,085 0,084 0,081 0,077 0, ,095 0,095 0,095 0,095 0,095 0,095 0,095 0,095 0,095 0,095 h 0,5

21 Insulated glass units procedure Load distribution Internal pressure transmits the external loads (e.g. wind on pane 1) from one pane to the next ( Coupling Effect ) External load Part of the external load carried by pane 1 ϕ 1 Fd F d;1 acting on pane 1 ( ) ; 1 δ Part of the external load carried by pane ( δ1 + ϕ δ ) Fd ; 1 F d; acting on pane ( 1 ϕ) δ1 Fd ; ( ϕ δ1 + δ ) Fd ; The internal loads, given by the isochore pressure, are reduced by a factor proportional to the relative flexibility of the panes. Internal load Isochore pressure Dp Part of the internal load carried by pane 1 ϕ p Part of the internal load carried by pane ϕ p Climatic Load o i o i o i o i summer winter pressure suction

22 Insulated glass units Example 1 b = 1500 mm external load: wind w = 1, kn/m² internal load: Dp = ±16,0 kn/m² h 1 = 6 mm h = 6 mm s = 16 mm δ δ h 6 = = = h1 + h 6 = = 1 0,50 = a = 750 mm External load distribution: ϕ 1 0,50 0,50 = = = 4 4 L4 Insulated 1+ ( aglass / a*) units 1+ (750 / 94,1) 1 k 5 factor λ=a/b=750/1500=0,5 k 5 = 0,050 p* = 0 characteristic length a* a* = 8,9 insulating unit factor ϕ ( h + h ) k ( ) 0,07 0,5 λ=a/b p* (interna ,0 0,019 0,019 0,019 0, ,9 0,04 0,04 0,0 0,0 0 0,8 0,09 0,09 0,08 0,06 0 0,7 0,05 0,05 0,04 0,01 0 0,6 0,04 0,04 0,040 0,07 0 0,5 0,050 0,050 0,048 0, ,4 0,059 0,058 0,057 0,05 0 0, 0,068 0,067 0,066 0, , 0,077 0,077 0,076 0, ,1 0,086 0,086 0,086 0, ,095 0,095 0,095 0, ,5 s h h = 8, ,05 =

23 Insulated glass units Example internal loads ± ϕ p = ± 0,07 16,0 = ± 1, kn/m² + summer - winter external loads ( ) δ = ( 1 ϕ) δ w = ( 1 0,07) 0,50 1, 0, 56 ϕ kn/m² pane 1 1 F d ;1 = ( δ + ϕ δ ) = ( δ + ϕ δ ) w = ( 0,50 + 0,077 0,50) 1, 0, 64 1 F d ;1 1 = kn/m² pane External load F d;1 acting Part of the external load carried by pane 1 ϕ δ 1 Fd ( ) on pane 1 ; 1 Part of the external load carried by pane ( δ1 + ϕ δ ) Fd ; 1 External load F d;1 acting on pane 1 Part of the external load carried by pane 1 Part of the external load carried by pane F d; acting on pane ( 1 ϕ) δ1 Fd ; ( ϕ δ1 + δ ) Fd ; F d; acting on pane - - Internal load Part of the internal load carried by pane 1 Part of the internal load carried by pane Internal load Part of the internal load carried by pane 1 Part of the internal load carried by pane Isochore pressure Dp ϕ p ϕ p Isochore pressure Dp

24 Insulated glass units Example internal loads ± ϕ p = ± 0,077 16,0 = ± 1, kn/m² + summer - winter ( ) ( ) ( ) 554 external loads ϕ δ = 1 ϕ δ w = 1 0,077 0,50 1, 0, kn/m² pane 1 1 F d ;1 = ( δ + ϕ δ ) = ( δ + ϕ δ ) w = ( 0,50 + 0,077 0,50) 1, 0, F d ;1 1 = kn/m² pane o i o i o i o i summer winter pressure suction Lc 1: summer + suction Lc : winter + pressure Lc : summer + pressure Lc 4: winter + suction pane 1 pane q = 1,79 kn/m² q = 1,88 kn/m²

25 This lecture was prepared for the 1st Edition of SUSCOS (01/14) by Prof. Sandra Jordão (UC). Adaptations brought by Prof. Viorel Ungureanu (UPT) for nd Edition of SUSCOS 5

26 Thank you for your kind attention