5.72 A cold air chamber is proposed for quenching steel ball bearings of diameter D = 0.2 m and initial temperature T₁ = 400°C. Air in the chamber is maintained at -15°C by a refrigeration system, and the steel balls pass through the chamber on a conveyor belt. Optimum bearing production requires that 70% of the initial ther- mal energy content of the ball above -15°C be removed. Radiation effects may be neglected, and the convection heat transfer coefficient within the chamber is 1000 W/m².K. Estimate the residence time of the balls within the chamber, and recommend a drive veloc- ity of the conveyor. The following properties may be used for the steel: k = 50 W/m K, a = 2 × 105 m²/s, and c = 450 J/kg. K. Ball bearing -5m- Cold air Belt V Chamber housing

5.72 A cold air chamber is proposed for quenching steel ball bearings of diameter D = 0.2 m and initial temperature T₁ = 400°C. Air in the chamber is maintained at -15°C by a refrigeration system, and the steel balls pass through the chamber on a conveyor belt. Optimum bearing production requires that 70% of the initial ther- mal energy content of the ball above -15°C be removed. Radiation effects may be neglected, and the convection heat transfer coefficient within the chamber is 1000 W/m².K. Estimate the residence time of the balls within the chamber, and recommend a drive veloc- ity of the conveyor. The following properties may be used for the steel: k = 50 W/m K, a = 2 × 105 m²/s, and c = 450 J/kg. K. Ball bearing -5m- Cold air Belt V Chamber housing

Principles of Heat Transfer (Activate Learning with these NEW titles from Engineering!)

8th Edition

ISBN:9781305387102

Author:Kreith, Frank; Manglik, Raj M.

Publisher:Kreith, Frank; Manglik, Raj M.

Chapter6: Forced Convection Over Exterior Surfaces

Section: Chapter Questions

Problem 6.12P

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11. 30 tonnes of ice from and at 0°C is produced in a day of 24 hours by an ammonia refrigerator. The temperature range in the compressor is from 298 K to 258 K. The vapour is dry saturated at the end of compression and expansion valve is used. Assume a co-efficient of performance of 60% of the theoretical and calculate the power in kW required to drive the compressor. Latent heat of ice is 334.72 kJ/kg. Enthalpy Entropy of liquid Entropy of vapour kJ/kg Тетр. K kJ/ kg kJ/ kg K Liquid Vapour 298 100.04 1319.22 0.3473 4.4852 258 -54.56 1304.99 - 2.1338 5.0585 [Ans. 21.59 kW]
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