HEAT LOSS OF GLAZING BARS

On the former ITT measurements are made with a box in a cold storage chamber. This box was closed, heated and provided with well insulated walls, except one. A glass plane with some bars formed the upper side. The whole was placed at an angle of 26° to imitate a glasshouse roof. At the outside air was led over the glass with a speed of 3–4 m/s perpendicular to the rods.

The inside-outside difference in temperature was 20°C.

The results of the measurements are shown in the table. The figures give the percentages of heat loss and are related to the projected surface of the bar and the glass with a pitch of the bars of 75 cm. The figures can not directly be compared with the results of the next experiment made by the institute TNO-IBBC at Delft.

After we passed through the energy crises the problem because more important and TNO-IBBC was requested to repeat the measerements on a small glasshouse with one span of 3.2 m and two compartments placed outside in more real conditions. In every compartment a hot box was fitted under part of the roof. In the box the airtemperature is the same as in the glasshouse. After checking the two hot boxes for the same heat loss with PD-glazingbars, provided with a plastic strip, some other types are compared with the PD-bars. The results are also given in the table.

Some comments must be made here.

1. At TNO the bars were always made airtight with putty if there was a risk of airleaks. We learned from the ITT-experiment that airleakage caused a remarkable and uncontrollable heat loss, so that we liked to avoid this in the TNO-experiment. This is the reason why especially some aluminium bars without a strip in the ITT row of figures have a bigger heat loss than in the TNO figures.
2. For the heat transfer by radiation it makes a difference whether the bar radiates to a cooling chamber ceiling or to the sky with its low temperature and whether the bar is made of an aluminium alloy or of some other material.
3. The figures are related to the surface of glass and bar, not to the whole glasshouse combination.

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