Engineering Answers

12. A 120 tons refrigeration system with COP of 5 has a condenser which is to be cooled by means of a cooling tower. Air enters the tower at 25oC db and 50% RH and leaves at 35oC db and 80% RH. Find the volume of air needed m3/hr. At 25oC and 50% RH: h = 50 KJ/kg, v = 0.86 m3/kg At 350C db and 80% RH: h = 108

11. A 100 BHP diesel engine has a brake thermal efficiency of 30% and jacket water loss of 34%. The engine is to be cooled by means of a cooling tower having a range of 27oC. Find the mass of water leaving the tower.

10. Air enters the tower at the rate of 60 kg/s. The entrance enthalpy is 86 KJ/kg and exit enthalpy of 53 KJ/kg. Find the temperature range of the tower if mass flow rate of water is 30 kg/s.

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32. An air-conditioning system is used to maintain a house at 65oF when the temperature outside is 85oF. The house is gaining heat through the walls and windows at a rate of 1250 Btu/min, and the heat generation rate within the house from people, lights, and appliances amounts to 350 Btu/min. Determine the minimum power input required for this air-conditioning system. A

31. A pressure difference of 500 Pa is available to force 20oC air through a circular sheet-metal duct 450 mm in diameter and 30 m long. At 20oC, p = 1.204 kg/m3 and take friction factor, f = 0.016. Determine the velocity.

30. Compute the pressure drop of 35oC air flowing with a mean velocity of 7 m/s in a circular sheet-metal duct 450 mm in diameter and 25 m long. Use a friction factor, f = 0.04, and Pair = 1.3799 kg/m3.

29. A coil has an inlet temperature of 70oF and outlet of 80oF. If the mean temperature of the coil is 120oF, find the bypass factor of the coil.

28. Compute the specific volume of an air-vapor mixture in cubic meters pre kilogram of dry air when the following conditions prevail : t = 45oC, w = 0.015 kg/kg, and Pt = 100 kPa.

27. A classroom that normally contains 40 people is to be air-conditioned with window air-conditioning units of 5 KW cooling capacity. A person at rest may be assumed to dissipate heat at a rate of about 360 KJ/hr. There are 10 light bulbs in the room, each with a rating of 100 watts. The rate of heat transfer to the classroom through the walls and the windows is estimated to be 15,000 KJ/hr. If the room air is to be maintained at a constant temperature of 21oC, determine the number of window air-conditioning units required.