The Thermal Safety of Chromatics Products for the British Isles

THE THERMAL SAFETY OF CHROMATICS PRODUCTS FOR THE BRITISH ISLES

Summary

The British Isles are a relatively mild area as far as the risk of thermal fracture for opaque glazing is concerned (compared to other parts of the World): in all areas, Chromatics will be safe in most applications.

Introduction

The surface of the Earth receives radiative energy from the Sun, the power of which can be expressed in Watts per Square Metre (W/SqM or WM-2).

Figure one, below, shows the maximum solar power experienced in various parts of the World:

Figure one

A piece of Chromatics exposed to this radiation will naturally heat up, and if part of it is shaded, or behind a frame, differential heating across the panel may be set up, with resulting thermal stress. Such stresses can be calculated and risk of thermal breakage assessed.

Major factors in these calculations include:

1. The Solar Radiation Power (the higher, the greater the risk);
2. The Stress Relieving Factor for the panel (the higher, the greater the risk);
3. The Safe Break Temperature (the lower, the greater the risk).

The other major factor is DiurnalTemperatureRange. This is simply the maximum ambient temperature difference experienced in a particular geographical location in a single day. In areas where this is high, glass panels will be relatively cold in the mornings when the Sun comes up. This makes the risk of thermal breakage higher, whereas in locations with a low diurnal range, the risk is proportionately lower. Note that a high diurnal range does not necessarily mean a “hot” location: some tropical areas have little difference in temperatures between day and night (low diurnal range), whilst some areas in the far Northern latitudes have a very high diurnal range.

Figure two, below, shows the diurnal range around the World:

Figure two

By using these factors it is possible to calculate a “Basic Temperature Difference” for a given piece of Chromatics in a given colour in a given geographical location: this is then modified by a number of factors, including aspect, frame type & colour, type of backup, any shading etc. etc. Because of the multiplicity of these factors, it is important to recognise from the outset that it is impossible to give a general case answer, and say, for instance, that the products will be safe in one geographical area, and not in another. However, it is possible to give some general guidance, and to identify areas of high or low risk of thermal fracture.

Procedure for making a thermal safety check

1. Basic Temperature Difference (BTD) is calculated using the expression:

(For Single Glazing)

BTD = RI x α +DR x he

(he + hi)

Where:

BTD=Basic Temperature Difference(K)

RI=Radiation Intensity(W/m2K)

α=Absorptance of the glass

DR=DiurnalRange(K)

he=External Heat Transfer Coefficient

hi=Internal Heat Transfer Coefficient

1. A calculated temperature difference is then found by modifying the BTD according to any factors present in the application – for example, static shadow, blinds, different backup etc.
2. The final calculated temperature difference (FCTD) is then found by multiplying the value obtained in 6, above, by the Stress Relieving Factor. (0.625 for Chromatics)
3. Assessment of risk is now made by comparing the FCTD with the design strength (51°K for Chromatics)
4. If FCTD ≤ 51°K, then the application is deemed to be safe

If FCTD > 51°K, then the application is deemed to be at risk of thermal fracture

Factors to be used

Stress Relieving Factor:A Stress Relieving Factor (SRF) of 0.625 shall be used when making an assessment of thermal durability (80 Micron foil)

Safe Break Temperature Difference (Design Strength): A Design Strength (DS) of 51°K shall be used when making an assessment of thermal durability (80 micron foil)

The British Isles

Let us now consider the region in question: The British Isles (Figure three):

Figure three

Let us start by considering the Solar Radiation Intensity & the DiurnalRange experienced by the region (Figure four):

Figure four

As can be seen, much of the region experiences only medium levels of Solar radiation (orange & yellow areas) of 800W/SqM or less. Significant areas are only exposed to low levels (up to 750 W/SqM – green areas).

In terms of DiurnalRange , the whole area experiences a range of 10K or less

Thus, in broad terms, we can summarise the region into two distinct areas:

1. Lowland Britain, mainly the South & East, receiving a maximum of 775 W/SqM with a diurnal range of up to 10K
2. Highland Britain, receiving up to 800W/SqM, diurnal range of up to 9K

For these two regions, the basic temperature difference for Chromatics can be calculated:

Table One

(Assumed: he=10 hi=8, glazed perpendicularly)

By taking the calculation further, we can reach final temperature differences – if assumptions are made that will vary from building to building. Assume: no blinds, backup factor=23, shading factor = 1.301, frame factor =0.75)

Table Two

Comparing these numbers to the Safe Break Temperature (51K) it is clear that all of the region will be a “safe” area for Chromatics.

Specific locations

Consider these key specific locations in the region:

Definitions: (Risk is for the Chromatics Products, not for any other type).

■ OK – final calculated temperature likely to be less than 45K

■Calculation should be undertaken – final calculated temperature likely to be 45-48K

■Calculation should be undertaken with particular attention to staticshadow – final calculated temperature likely to be 48-52K

These figures are indicative only, and we recommend that a thermal safety check is carried out using data from a specific building design and location.

Page 1 of 5Document Created by AR Walker

April 2009