A four day on-site investigation and report by the Building Research Establishment summarized below has proved the effectiveness of the Monodraught Windcatcher system under Summer load conditions.
The University of Hertfordshire had converted what was originally its old mainframe computer room into two lecture theatres, G105 and G111. The single storey, flat roofed building had previously been served by a separate air conditioning system. There were no openable windows in either lecture theatre. G105 is the smaller of the two with a volume of 458m3. G111 had a volume of 769m3.
The detailed monitoring and measurements were carried out from 10th to 13th August 1998. The external temperatures were very high for two of the days (approximately 29°C), while for the other two days it ranged from 18°C to 22°C.
The ventilation rates of the lecture theatres with the Windcatchers closed and sealed was determined to find a base comparison. Readings were also taken with the dampers closed and fully open and were measured on three days in G105 and four days in G111. The effect of night cooling from the units was also determined over two days. Flow visualisation studies were undertaken on the units using smoke as a tracer. A recording of these tests were made on videotape.
It was determined that the background ventilation of both lecture theatres (i.e. Windcatchers sealed) was relatively low. With the Windcatchers unsealed and closed the ventilation rate did not increase significantly. With the Windcatchers fully open the ventilation rate in G105 ranged from 1.24 ac/hr at 1.7 m/s wind speed to 5.2 ac/hr at 4.5 m/s. For G111 it ranged from 2.13 ac/hr at 2.6 m/s to 4.68 ac/hr at 4.1 m/s.
The effect of leaving the dampers to the Windcatchers open in one of the lecture theatres overnight demonstrated the night cooling effect of the units. A temperature difference of approximately 8°C was achieved in the lecture theatre overnight.
Ventilation Measurements
Ventilation measurements were carried out in both lecture theatres using the tracer gas decay method, Sulphur Hexafluoride (SF6). Three pairs of small mixing fans mounted in opposing directions in stands were placed in both lecture theatres. The purpose of the fans was to mix the incoming fresh air with the tracer gas inside the lecture theatre. The tracer gas concentration was sampled from three points around the lecture theatres and combined before being analysed by a B&K gas analyser.
The local wind speed and direction during the ventilation measurements were measured using an ultra-sonic anemometer mounted 10 metres above ground level at the top of a telescopic mast.
Internal and external air temperatures were measured every minute, using thermistors connected to Grant Squirrel data loggers. Two loggers were placed in each lecture theatre and the values averaged. The external temperature was measured by a data logger and thermistor located in a Stevenson screen (a white ventilated enclosure mounted 1 metre above ground level).
Flow Visualisation
To observe the operation of the Windcatcher units i.e. the air flow in and out of the units, a smoke machine was used to generate a dense white smoke which was released out through a purpose made four-way adapter which was placed underneath the unit. A videotape record of these flow visualisation tests was obtained along with some still camera photographs.
The flow visualisation study showed that there was a vigorous flow of air in through one or more segments of the Windcatcher and out through opposite segments, even when the wind speed was approximately 2 m/s. At approximately 4 m/s the effect was much more pronounced with a significant down flow of air through the Windcatcher towards floor level. There did not appear to be any evidence of short-circuiting of the fresh air.
When on an occasion the external doors to G111 were opened on a day when the wind speed was low (2 m/s), brief observations showed that air entered the lecture theatre and was exhausted through all four ducts of the Windcatcher system.
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Monodraught Windcatcher Units
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Windcatcher internal view
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Night cooling temperature
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Front of G111
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Flow visualisation moderate windspeed
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Flow visualisation low windspeed |
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Windcatcher Units
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B & K Gas analyser
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Mixing fans
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Ventilation rate |
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