- 13.164 kJ for the actual network and 21.296 kJ for the network providing optimal parameters for users.

- Exergy loss due to flow rate loss is 2.35%, approximately ten times lower than for the actual network.

- Exergy loss due to major losses is very low. Small diameter hose has the highest share of major loss, even if the length of the hose is much smaller

than the length of the ducts from the entry point of the network to the coal

face.

- Exergy loss due to throttling is significantly lower only 0.02%, as expected.

- An interesting rise is the acquired exergy much higher than in cases presented before due to lower temperature of compressed air T=13.52 .

while the temperature of the surroundings is T=23.16 in the crosscut

and T=24.3 at the coal face.

1. CONCULSIONS

Even if the studied network is only a small section of the much bigger

underground compressed air network, a number of useful conclusions can be

emphasized:

a) Exergy efficiency of the underground compressed air network is poor 70.32 % an it can be raised to 91.48% if some measures are taken.

b) Higher pressure and flow rate of compressed air at the inlet of the compressed air network, without optimization of the network, will lead to

higher losses and this will translate in poor economic efficiency since the

compressed air is an expensive form of energy.

The measures that must be applied in order to improve the efficiency of the

compressed air distribution network, according to results presented above are:

- Proper network maintenance in order to keep the flow rate loss as low as it can be;

- Use of hoses with uppermost available diameter that matches the pneumatic equipment.

- Proper cooling of the compressed air, that way on its evolution in the underground will acquire energy, since the surrounding temperature in

the mine works is usually higher than at the surface. This low temperature

has other positive effects too, like lower temperatures at the coal face, and

lower vapor content of the compressed air.

- Using orifice plates to regulate pressure on different sections of the compressed air network, with impact on the flow rate loss.

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