rong điều kiện sản xuất hiện nay, nhất là trong các ngành công nghiệp thực phẩm, chế biến thức ăn, thủy sản, giấy, dệt nhuộm, thuốc lá, giải khát,… thiết bị trao đổi nhiệt (bộ trao đổi nhiệt) có vai trò quan trọng trong các nhà máy sản xuất. Chúng được coi là những lao động năng suất và hiệu quả. Vậy thiết bị trao đổi nhiệt là gì? Chúng được ứng dụng và phân loại như thế nào?
P1: IML/OVY P2: IML/OVY GRBT056-01 Kutz-2103G QC: IML/OVY GRBT056-Kutz-v4.cls T1: IML July 18, 2005 19:39 Source: Heat-Transfer Calculations Part Introductory Calculations Heat-Transfer Calculations opens with eight brief industrial heat-transfer calculations While they are short, especially when compared to the calculations that make up the bulk of this handbook, these opening calculations are not trivial And they deal with real life, which is the hallmark of the calculations throughout the book: A chemical processing technology that needs a thinner liquid film coating for better adhesive properties of a new product A new passive method of cooling an electronic assembly that uses a closed-loop thermosyphon During casting of an industrial metal component, a more efficient gating and risering system that minimizes the amount of metal poured to produce a good casting A furnace wall in a coal-fired power plant A reinforced-concrete smokestack that must be lined with a refractory on the inside The condenser in a large steam power plant The duct of an air conditioning plant A concentric tube heat exchanger with specified operating conditions Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website P1: IML/OVY P2: IML/OVY GRBT056-01 Kutz-2103G QC: IML/OVY GRBT056-Kutz-v4.cls T1: IML July 18, 2005 19:39 Introductory Calculations Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website P1: IML/OVY P2: IML/OVY GRBT056-01 Kutz-2103G QC: IML/OVY T1: IML GRBT056-Kutz-v4.cls July 18, 2005 19:39 Source: Heat-Transfer Calculations Chapter Multiphase Films and Phase Change Greg F Naterer University of Ontario Institute of Technology Oshawa, Ontario, Canada Problem A chemical processing technology needs a thinner liquid film coating for better adhesive properties of a new product The film flows down under gravity along an inclined plate Effective thermal control is needed to achieve a specified level of adhesive quality This goal is accomplished by changing the wall temperature at a certain point along the plate, which affects the thermal boundary layer within the film Before evaluating heat transfer based on the Nusselt number, the film velocity is needed to predict thermal convection within the film Then the growth of the thermal boundary layer can be determined The spatial profile of temperature remains constant, after the point where the edge of the thermal boundary layer reaches the edge of the film This point is needed for sizing of the processing equipment Thus, a thermal analysis is needed to find the film velocity, boundary-layer growth, and Nusselt number for evaluating the temperature within the film This processing temperature affects the adhesive properties of the film Consider a liquid film (ethylene glycol at 300 K) during testing of the chemical process The film slows steadily down along a flat plate in the x direction The plate is inclined at an angle of with respect to the horizontal direction The x position is measured along the direction of the plate It is assumed that the film thickness ␦ remains nearly uniform A reference position of x = is defined at a point where the wall boundary conditions change abruptly Before this point, the film, wall, 1.3 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website P1: IML/OVY P2: IML/OVY GRBT056-01 Kutz-2103G QC: IML/OVY T1: IML GRBT056-Kutz-v4.cls July 18, 2005 19:39 Multiphase Films and Phase Change 1.4 Introductory Calculations and air temperatures are T0 Then the surface temperature increases to Tn (downstream of x = 0) and a thermal boundary layer ␦T develops and grows in thickness within the film Part (a) Using a reduced form of the momentum equation in the liquid film, as well as appropriate interfacial and wall boundary conditions, derive the following velocity distribution in the film: u y y =2 − U ␦ ␦ where U refers to the velocity at the film-air interface Neglect changes of film velocity in the x direction along the plate Part (b) Consider a linear approximation of the temperature profile within the thermal boundary layer, specifically, T = a + by, where the coefficients a and b can be determined from the boundary conditions Then perform an integral analysis by integrating the relevant energy equation to obtain the thermal boundary-layer thickness ␦T in terms of x, ␦, U, and ␣ For U = cm/s and a film thickness of mm, estimate the x location where ␦T reaches the edge of the liquid film Part (c) Outline a solution procedure to find the Nusselt number, based on results in part (b) Solution Part (a) Start with the following mass and momentum equations in the film: ∂u ∂v + =0 ∂x ∂y u ∂u ∂u ∂p ∂ 2u + v =− + + g (sin ) ∂x ∂y ∂x ∂y (1.1) (1.2) From the continuity equation, the derivative involving u vanishes since the terminal velocity is attained, and thus v equals zero (since ∂v/∂ y = subject to v = on the plate) As a result, both terms on the left side of Eq (1.2), as well as the pressure gradient term (flat plate), disappear in the momentum equation Also, we have the boundary condition with a zero velocity at the wall (y = 0) and an approximately zero velocity gradient at the air-water interface The gradient condition means that ∂u/∂ y = at y = ␦ as a result of matching shear stress values ( ∂u/∂ y) on both air and water sides of the interface and a