andy digwire

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since Jan 06, 2014
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Recent posts by andy digwire

A crucial factor for those high pipes acting as an effective 'heat ramp' is their diameter, because such ramps work by letting hotter and cooler air slip past each other, in a convection circuit. As far as i understand, relatively warm air rises and cool air sinks as a a matter of convection. This is a very gentle forcing of air up or down depending on its relative temperature, it is relatively gentle but it can move air because there is little resistance involved when a corresponding body can move in the opposite direction. Concerning these ventilation pipes, relatively little convection might occur within them, and where it does occur it may mix and slow through turbulence.
Besides the local dynamics in the top pipes, they are part of a larger circuit with its own tendency for a flow of air through them in one direction or another. It seems intended/hoped that the overall flow will normally exit out bottom vents, and hence, enter through the top vents. I think on the warmest days the low lying air may be warmer than the air in the system - resulting in no overall flow. Without such flow some heat will still be able to move down the top vents by turbulent/mini-convection in the pipes, and by conduction, which might be enough to effective, but the only way i see to be confident of the design is by modelling it, or at least some ball-park heat conduction calculations.
10 years ago
Hiya Sean,
The natural circulation of air through the pipes would seem to depend on the lowest vents being warmer than the low lying air in the greenhouse. When that is the situation the air at the top might be sucked in regardless of its temperature iirc. This part of the system does look ambitious and experimental, its begs to be checked and calibrated by experiment or computer modelling or by someone very experienced in such a system before having any confidence that it is worth the time and expense of installing. The insulated mound itself is a much simpler and more reliable prospect. A less particular way of boosting heat in and out of the earth mound would be to build some significantly sized burrows into it, and manually open and close doors/vents to allow conduction and circulation as weather and conditions require.
10 years ago