A COMPARATIVE ENTHALPY APPROACH FOR CO- AND COUNTER CURRENT
CHAPTER ONE
1.O INTRODUCTION
1.1 BACKGROUND OF THE STUDY
According to the modern or dynamical Theory of heat: Heat a form of energy. The molecules of a substance are in parallel motion. The mean Kinetic energy per molecules of the substance is proportional to its absolute temperature. In description of heat, a molecule may consist of one or two or many atom depending upon the nature of the gas. The force of attraction between the molecules of a perfect gas is negligible. The atom in a molecules vibrate with respect to one another, consequently a molecules has vibration energy. The whole molecules may rotate about one or more axes, so it can have “notational energy”. A molecule has “translational energy” due to its motion, thus kinetic energy of a molecule is “the sum of its translational, rotational and vibrational energies. Summarily heat energy given to a substance e is used in increasing its internal energy. Increase in internal energy cause increase in Kinetic energy or potential energy or increase in both the energies. Due to increase in Kinetic energy of a molecules, its translational, vibrational or rotational energy may increase. In a nut shell “heat transfer is the science which deal with the rate of bodies called the Source aims receiver KERN, [2006:1]
1.2 MECHANISM OF HEAT Heat transfer is of three distinct way in which heat may pass from a source to a receiver, although most engineering application are combination of two or three method, which are conduction convection and radiation Conduction: Conduction heat Transfer is energy transport due to molecular motion and interaction. Conduction heat transfer through solids is due to molecular vibration. Fourier determined that Q/A, the heat transfer per unit area (W/m2) is proportional to the temperature gradient ∂t/∂x. The constant of proportionality is called the material thermal conductivity K. Fourier equation according to Colostate [2014:4] Q/A = -K ∂t/∂x …(1-1) The thermal conductivity K depends on the material and also some what on the temperature of the materials. Convection: Convection heat transform is energy transfer due to bulk fluid motion. Convection heart transfer through gases and liquids form a solid boundary results from the fluid motion along the surface.
Newton determined that the heat transfer/area Q/A, is proportional to the fluid sold temperature difference T2. if the temperature difference normally occurs across a thin layer of fluid adjacent to the solid surface. This thin fluid, layer is called a boundary layer. The constant of proportion is called the heat “transfer coefficient, h. Newton‟s equation: According to Colostate [2014:4] Q/A = h ( Ts – Tf) …(1-2) The heat transfer coefficient depends on the type of fluid and the fluid velocity. The heat flux) depending on the area of interest, is the local or area averaged. The various types of convective heat transfer are usually categorized into the following CONVECTIVE HEAT TRANSFER COEFFICIENT CONVECTION Types Description Natural Convection. Fluid motion induced by density difference Forced Convection Fluid motion induced by pressure differences from a fan or pump Boiling Fluid motion induced by a change of phase from liquid to vapour Condensation Fluid motion induced by a change of phase from vapor to liquid Source: Colostate [2014:4] RADIATION Radiation heat transfer is energy transport due to emission of electromagnetic wave or photons form a surface or volume. The radiation does not require a heat transfer medium, an can occur in a vacuum. The heat transfer by radiation is proportional to the fourth power of the absolute material temperature. The proportionality constant S or the stefom – Boltzman constant equal to 5.67 x 158 ml/m2k4. The radiation heat transfer also depends on the material properties represented by ℮, the emissivity of the material. Q/A = бT4 …(1-3) Source: Colostate [2014:4]
1.3 PROCESS HEAT TRANSFER.
Heat transfer been described as the study of the rate at which heat is exchanged between heat source and receiver usually treated independently. Process heat transfer deals with the rate of heat exchanger as the occur in the heat – transfer equipment of the engineering and chemical processes. This approach brings to better focus the important of the temperature difference between the source and receiver, which is after all, the driving force whereby the transfer of heat is accomplished. A typical problem of process heat transfer, is considered with the quantities of heat to be transferred. The rate of which they may be transferred be cause of the nature of the bodies, the driving potential the extent and arrangement of the surface separating the source and receiver, and the amount of mechanical energy which may be expanded to facilitate the transfer, Since heat transfer involves can exchanger in a system, the loss of heat by the one body will equal the heat absorbed by another within the confine of the same system.
1.4 HEAT TRANSFER PROCESS EQUIPMENT
Heat exchanger been one of the commonly used equipment in heat transfer a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall to prevent mixing o they may be on direct conduct. They are widely used in space heating, refrigeration, air conditioning, petro-chemical plant, petroleum engineers, and natural gas processing and sewage treatment.
Heat exchange is device that exchanges the heat between fluids of different temperatures that are separated by a solid wall. The temperature gradient or he difference in temperature facilitate the transfer of heat. Transfer of heat happens by three principle means: Convection, conduction, radiation. Conduction occurs as the heat from the higher temperature fluid passes through the solid wall. The biggest contribution to heat transfer on a heat exchanger is made through convection. To maximize the heat transfer, the walls should be thin and made of a very conductive materials. In a heat exchanger forced convection allows for the transfer of heat of one moving as heat is transferred through the pipe wall it is mixed into the stream and the flow of the stream removes the transferred heat. This maintain a temperature gradient between the two fluids. The double-pipe heat exchanger because on fluid flows inside a pipe and the other fluid flows between that people and another people. That surrounds the first “this is a concentric tube construction”. Flow in a double pipe heat exchanger can be co-current or counter current. There are two flow configurations co-current is when the flow of the two streams are in the same direction, counter current is when the flow of the stream is in opposite direction. 1.4.1 TYPES OF HEAT EXCHANGER In order to meet the widely varying applications, several types of heat exchanger have developed which are classified on the basis of nature of heat ex change process, relative direction of flow motion, design on constructional features and physical state of fluids. Nature of heat exchange process On the basis of nature of heat exchange process, heat exchanger are classified as follows.
i. Director contact heat exchanger: These type of heat exchanger are used predominantly in air conditioning condensing plants, water cooling, industrial hot water heating etc and it involves heat transfer between hot cold stream of two phases in the absence of a separating walls. Most direct contact heat exchange fills under the gas-liquid category where heat is transfer between a gas and begin in the form of drop, film, or spray. ii. Direct Contact heat Exchanger In this type of heat exchanger, the heat transfer between two fluids could be carried out by transmission through wall which separates the two fluids. On the other hand indirect – Contact heat exchanger. “The fluid stream remain separate and the heat transfer continuously through an impervious diving wall or into and out of a well in a transient, thus c, ideally there is not direct contact between thermally interacting fluids. This type of heat exchanger also referred to as a surpass heat, exchanger, it can e further classified into direct –transfer type, storage type and fluidized – bed exchanger. 1.4.2 RELATIVE DIRECTION OF FLUID MOTION Due to the relative direction of two fluid streams. The heat exchangers are classified into three categories. Parallel flows or unidirectional flow In parallel flow heat exchangers, the two medium enter the exchanger at sane bed, and travel in parallel to one another to the other side.
Download complete project material
