RAFAEL CORREA Y LA CONSTRUCCIÓN DE SU LIDERAZGO

Initial Preparation

The test equipment, including the Argon carrier gas supply, was designed to be as compact and portable as possible. It was easily transported to a site in the back of a small family hatchback car. A general view of the test equipment is shown in Plate 1.

If starting the system from cold, it was necessary to allow a stabilisation period of about 20 minutes. During this time baseline drift of the column output reduced from unacceptable limits ( > 1cm per minute, but it was variable from test to test, even though operating conditions may have been identical ) , to more acceptable limits. If excessive baseline drift occured through the test period, it was a simple operation to re-zero the baseline using the chart recorder X facility.

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System Optimisation

A column temperature of 35 c and an Argon pressure of 2.5 bar was found to be the optimum balance of column operating conditions. This gave good resolution of all the chosen tracer gases, with a throughput time of 50 seconds for the Oxygen. The response times for the tracer gases were, Freon 12; 55 seconds, Freon 114; 60 seconds

and BCF; 90 seconds. These conditions differ slightly

from other workers' optimum settings (18), perhaps because of differences in the inter connecting pipework of the

modified chromotagraph. At these quoted figures i.e.

column temperature of 3 0 c and carrier pressure of 3 bar, it was found to be impossible to separate the output of Freon 12 from Freon 144.

It was a deliberate policy to reduce the levels of tracer gas released to as low as possible, because of the possible toxic and environmental effects described in section 2.4 . Therefore the maximum allowable scale setting of the gas analyser was used; this was not the absolute maximum scale possible, since at these extreme limits, the analyser showed signs of instability ( excessive baseline drift ) . At this setting the Oxygen

peak height went off the chart recorder scale. The

optimum gas analyser setting was 5 units, the chart recorder setting was 200 mV.

Injection Strategy

The injection strategy was to release two of the tracer gases used, Freon 12 and Freon 114, by inflating toy balloons with the gases. The amount of tracer gas released could be gauged with good accuracy by inflating to a known diameter, which by previous calculations, corresponded to a known tracer gas concentration. This was typically 18 cm and 25 cm for freon 12 and 114 respectively, for a-room volume of 60 m"3. The balloons could be exploded remotely by the use of an electrically operated hot filament. The balloons were placed in as central a location as possible. Because of the extremely low volume of BCF gas needed, ( typically 10 ml for a room volume of 60 irT3 ) , it was impractical to inflate the balloons to a usable diameter. Attempts to further inflate the balloon with Argon proved impossible, since during this operation BCF, would leak

from the balloon's mouth. Hence BCF was released by

manual injection into the test space, using a gas tight syringe, filled via a septum port fitted to the top of the gas bottle.

Mixing Strategy

To provide a uniform tracer gas concentration within the test space, small, 3 0 cm diameter, oscillating electric desk fans were used to promote mixing. After injection the mixing fans were switched on, with the doors to the test space fully closed. One fan (20) was found to be

sufficient for each zone ( between 3 0 irT3 and 56 m'3 ) with the exception of the hallway on site ( section 4.2 ). Two fans were necessary in this case, one upstairs, and one downstairs. This was because of the complex geometry of the zone, as compared to the others.

Different researchers allow different lengths of time to allow for sufficient mixing. For comparable zone volumes, Littler (5) allows up to 3 0 minutes, whilst Irwin (18)

deduced that 5 minutes was adequate. For very large

volumes, such as industrial warehouses (21) or aircraft

hangars (22) , the mixing period may be very long; up to several hours.

It was found during meaurements that 5 minutes was entirely sufficient to provide good mixing; any longer than this and it was noticed that tracer gas was leaking between zones, through cracks around the door, perhaps being exacerbated by the action of the mixing fans. If longer periods should be needed, it may be wise to seal up all the cracks around the door with adhesive tape, prior to mixing.

After the mixing period was over, the mixing fans were switched off, and a further 3 minutes were allowed - to enable pre-mixing equilibrium conditions to return. Then the interconnecting doors between the zones were opened, and measurements began. Irwin deduced that the errors in calculating the airflows between two zones due to non uniform mixing within the zones were approximately + 7%.

Problems of Mixing

If a tracer gas is released into a test space, there are three main types of mixing problem which can occur. These are mixing with fresh air entering the test space, mixing of air and tracer gas in the space and circulation of gas

within the space. Each of these mixing problems may

affect the measured tracer concentrations differently (23) .

Firstly, fresh air entering a space may hot be uniformly distributed, consequently the concentration of the air and tracer gas mixture will vary for different locations. Secondly, air infiltrating into a space and then

exfiltrating out again without mixing, can occur. This

does not effect tracer gas concentration and can generally be ignored.

The final mixing problem occurs if the physical volume of the space is not the same as the effective volume of the space. Effective volume is defined as the volume of the

space participating in the air exchange process. The

presence of cupboards or soft furnishings can lead to the effective volume being smaller than the physical volume. Conversely the presence of a suspended ceiling in the space of interest, having a significant air leakage will cause the effective volume of the space to be larger than the physical volume.

Sampling Strategy

As the mixing between the tracer gas and air can never be perfect, measurements made at a single point within the test space may not be a totally reliable indicator. This problem can be dealt with in practice by several means; Air is sampled at several points and then mixed together. This concentration of the mixture is then used in calculating the air flow rate.

The rate of decrease of concentration is measured at several points and the measurement point which shows a rate of decrease nearest to the average rate from all points is used.

The decrease in concentration is measured at several points and the average value of this decrease is used when calculating the airflow rate.

The first option, as described, is the method most widely used. To realise this in practice, sampling tubes were fed from the analyser to the required location. The ends of each individual tube was connected to a three way manifold, which in turn was connected to three sampling tubes of equal length ( in this way, the pump suction is equalised between the three tubes, allowing equal airflows through each ). The sampling tubes were positioned in a

central position within the test zone, this was

considered to be the most representative condition of the 43

room as a whole (24). The ends of the tubes from the manifold were positioned vertically within the test space, thus allowing for any tracer gas concentration gradients; either due to imperfect mixing, or to stratification with time.

2.7 Solution of Multiple Tracer Gas Continuity Equation