tailieunhanh - HVAC Systems Design Handbook part 18

This chapter presents a basic overview of heat transfer fundamentals, particularly as they apply to HVAC. For a detailed, rigorous treatment, the reader should refer to a good college-level text on heat transfer or to the ASHRAE Handbook. | Source HVAC Systems Design Handbook Chapter 18 Engineering Fundamentals Part 3 Heat Transfer Introduction This chapter presents a basic overview of heat transfer fundamentals particularly as they apply to HVAC. For a detailed rigorous treatment the reader should refer to a good college-level text on heat transfer or to the ASHRAE Heat Transfer Modes Heat is transferred between any two bodies by one or more of three modes conduction convection and radiation. Thermal conduction refers to the direct transfer of energy between particles at the atomic level. Thermal convection may include some conduction but refers primarily to energy transfer by eddy mixing and diffusion . by fluids in motion. Thermal radiation describes a complex phenomenon which includes changes in energy form from internal energy at the source to electromagnetic energy for transmission then back to internal energy at the receiver. Radiation transfer requires no intervening material and in fact works best in a perfect vacuum. In accordance with the second law of thermodynamics net heat transfer occurs in the direction of decreasing temperature. In this text the Fahrenheit F scale is used or for absolute temperatures the Rankine R scale OR F 460 . 459 Downloaded from Digital Engineering Library @ McGraw-Hill Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Engineering Fundamentals Part 3 460 Chapter Eighteen Thermal Conduction For steady-state conduction in one direction through a homogeneous material the Fourier equation applies q -kA dt dx where q heat transfer rate Btu h k thermal conductivity Btu h ft F A area normal to flow ft2 dt dx temperature gradient oF ft The minus sign shows that heat flow takes place from a higher to a lower temperature. In HVAC calculations homogeneous barriers are never encountered even when the solid barrier is homogeneous .

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