Questions
1. A motor that draws 400kW at a 0.75 lagging power factor is placed in parallel with another that draws 110 kW at 0.8 leading power factor. What is their combined power factor.
2. A 250mm-diameter main carrying 3.5 kg/s of compressed air at peak periods services a section of a mine. The main is 2,500m in length, what would be the maximum pressure loss across it? The air in the column has a mean temperature of 35°C and the barometric pressure is 100 kPa. Average gauge pressure is 600 kPa. The Universal gas law can be applied and that the gas constant is 287 J/kgK.
3. Compare and contrast compressed air and electrical drives as power sources in mining. A stoped out area with a volume of 8,000 m3 acts as a receiver. It is pressurised to an absolute pressure of 5000 kPa during off shifts. The intention is to draw air from the receiver during the 4-hour drilling shift until the absolute pressure drops to 750 kPa. What is the rate at which the receiver boosts the main air supply during this period? Air temperature is 30°C, gas constant for air R=287 J/kgK.
4. A 200 mm diameter main services a section of a mine and carries 2.4 kg/s of compressed air at peak periods. If the main were 1,000 m in length what would be the maximum pressure drop across it? The air has a mean temperature of 30°C and the barometric pressure is 100 kPa. Average gauge pressure is 500 kPa.
5. A 25 mm diameter hose services a rock drill and carries 0.07 kg/s of compressed air at peak periods. If the hose is 30 m in length and the hose has, three meanders (half circles) along its length what would be the maximum pressure drop across the hose? The air has a mean temperature of 30°C and the barometric pressure is 100 kPa. Average gauge pressure is 500 kPa
6. The following data presented is from a nest of piezometers installed side by side at a single site.
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Piezometer
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A
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B
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C
|
|
Elevation at surface (m AHD)
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850
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790
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830
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Depth of piezometer (m)
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150
|
100
|
50
|
|
Depth to water (m)
|
37
|
42
|
26
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If A, B and C refers to the points of measurement of piezometers a, b and c, determine:
a) The hydraulic head, pressure head and elevation head at A, B and C
b) The vertical hydraulic gradients between A and B, and between B and C if they are spaced 200m apart