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Design and Optimization of Thermal Systems Episode 2 Part 6

Tham khảo tài liệu 'design and optimization of thermal systems episode 2 part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Acceptable Design of a Thermal System 347 rate of the oil is given as 0.2 kg s and its inlet temperature as 90OC. The water is available at 20OC but its temperature rise is restricted to 12.5OC because of environmental concerns. The outer tube diameter must be less than 5 cm and the inner tube diameter must be greater than 1.5 cm due to constraints arising from space and piping considerations. The engine oil must be cooled to a temperature below 50OC. Obtain a feasible design if the length of the heat exchanger must not exceed 200 m. Redesign the system if the length is restricted to 100 m. Even though the fluid properties vary with temperature take these as constant for simplification with the specific heat at constant pressure Cp viscosity p and thermal conductivity k as 2100 0.03 and 0.15 for the oil and as 4179 8.55 X 10-4 and 0.613 for water all in S.I. units. Solution Several requirements and constraints are given as inequalities. Appropriate values may thus be chosen to satisfy these. Therefore the outlet temperature of the oil may be taken as 45OC the inner tube diameter as 2 cm and the outer tube diameter as 4 cm. These values satisfy the inequalities but may have to be adjusted if a feasible design is not obtained. Total energy lost by the oil Q mCph Thi - Th o 0.2 X 2100 X 90 - 45 18.9 kW where the subscript h refers to the hot oil i to the inlet and o to the outlet. The mass flow rate is represented by m and the temperatures by T Assuming zero heat loss to the ambient the energy lost by the oil is gained by the water. Therefore mwCp.w Two - Tw i 18 900 Since the temperature rise is restricted to 12.5OC m w 18 900 4179 X 12.5 0.36 kg s Let us choose the mass flow rate of water as 0.4 kg s which gives the outlet water temperature which satisfies the given constraint on temperature rise as T - w o 20 18 900 4179 X 0.4 31.3 C Therefore the log-mean-temperature-difference ATm is Th i - Tw o - Th o - Tw i 58.7 - 25 0C m ln Th i - Two Th o - Twi ln 58.7 25 . .