Heat Flow Unit Operations PDF

UNIT OPERATIONS UNIT 2 Heat and Mass Transfer 2. HEAT & MASS TRANSFER Heat flow from high region temperature to lower region temperature. According to principle of thermodynamic, whenever physical or chemical transformation occurs, heat flows into or leaves the system. Mechanism 1. C...

UNIT OPERATIONS UNIT 2 Heat and Mass Transfer 2. HEAT & MASS TRANSFER Heat flow from high region temperature to lower region temperature. According to principle of thermodynamic, whenever physical or chemical transformation occurs, heat flows into or leaves the system. Mechanism 1. Conduction When heat flow in body is achieved by transfer of momentum of individual atoms or molecule without mixing. This mechanism is based on Fourier’s law. Fourier’s law It states that the rate of heat flow through a uniform material is proportional to the area and temperature drop and inversely proportional to length of path of flow. M echani sm 2. Convection A. Forced convection When mixing of fluid is achieved by use of agitator or stirrer or pumping the fluid for recirculation, such process in heat transfer is called forced convection. In force convection, the stagnant films (film or surface coefficients) are of great importance in determining rate of heat transfer. Film coefficient is the quantity of heat flowing through unit area of film for unit drop in temperature. B. Natural convection Mixing of fluid is accomplished by the currents set up, when body of fluid is heated. Such process is known as natural convection. Fluid circulation caused by change in the density due to temperature difference in the fluid which depends on: Geometry of the system (size, shape and arrangement of heating surface). Shape of vessel in which the fluid is enclosed. This natural convection is observed when extracts are evaporated in open pans.  3. Radiation Radiation is a process in which heat flows through space by means of electromagnetic waves. Thermal radiation Heat transfer by radiation is known as thermal radiation. Various forms of emitters used for the supply of  Black body It is defined as a body that radiates Grey maximum possible amount of energy at given temperature. body Normally, hot bodies emit radiation. Stephen- Boltzmann law gives the total amount of radiation emitted by black body. The term gray body (or graybody or grey q =bAT4 body) is used in physics to refer to an q = energy radiated per ideal body that absorbs a certain second A = area of fraction of any radiating surface incident electromagnetic T = absolute temperature of radiation (EMR), such that this same radiating Surface b = constant fraction applies to When emissivity is equal to absorptivity then each wavelength. substance is considered as black body.  CONVEYOR Belt Conveyors A belt conveyor system consists of two or more pulleys (sometimes referred to as drums), with an endless loop of carrying medium—the conveyor belt—that rotates about them. A conveyor belt is the carrying medium of a belt conveyor system (often shortened to belt conveyor). Screw Conveyor A screw conveyor or auger conveyor is a mechanism that uses a rotating helical screw blade, called a "flighting", usually within a tube, to  CONVEYOR Pneumatic conveyors Pneumatic conveyors are continuous conveyors for bulk material in which material is conveyed in an enclosed tube system by means of compressed air or by  Chain Conveyors A chain conveyor is a type of conveyor system for moving material through production lines. Chain conveyors are primarily used to transport heavy unit loads, e.g. pallets, grid boxes, and industrial containers.  CONVEYOR Bucket Conveyors Bucket conveyors consist of endless chains or belts to which are attached buckets to convey bulk material in horizontal, inclined, and vertical paths. The buckets remain in carrying position until they are tipped to discharge the