Principle of Steady-State Heat Transfer

Principles of Heat Transfer By Nur Hashimah AliasHeat TransferThe transfer of energy in the form of heat occurs in many chemical and other types of processes.Heat transfer often occurs in combination with other separation processes, such as drying of foods, alcohol distillation, burning of fuel, and evaporation.Why it occurs: because of a temperature-difference driving force and heat flows from the high to the low-temperature regionMode of heat transfer : Conduction, Convection and RadiationConductionOccurs when a temperature gradient exists through a solid or a stationary fluid (liq/gas)Heat is conducted by transfer of energy of motion between adjacent molecules.Specifically energy is transferred by free electron:metallic solidsExample ??

ConvectionOccurs within a moving fluid or between a solid surface and a moving fluid when they are at different temperatures.A fluid is forced to flow past a solid surface by mechanical, natural and free convection : warmer or coller fluid next to solid surface causes a circulation because of (a density difference resulting from the temperature differences in the fluid)Example:??? RadiationRadiation is the transfer of energy through SPACE by means of Electromagnetic Waves.No physical medium is needed for its propagationThe same law: transfer of light that govern the radiant transfer of heat.Solid and liquid tend to absorb the radiation transferred to them, so radiation is primarily important in transfer through space and gasesImportant in transport of heat to earth from the sunExamples:??

Heat Transfer

Fouriers Law of Heat ConductionFouriers Law of Heat ConductionFouriers Law of Heat ConductionExample 1Calculate the heat loss per m2 of surface area for an insulating wall composed of 25.4 mm thick fiber insulating board, where the inside temperature is 352.7K and outside temperature is 297.1K. Given that the thermal conductivity of fiber insulating board is 0.0048W/m.K. Answer: 105.1 W/m2Example 2Calculate the heat loss per m2 of surface area for a temporary insulating wall of a food cold storage room where the outside temperature is 299.9K and the inside temperature 276.5K. The wall is composed of 25.4 mm of corkboard having k of 0.0433 W/m.K.Answer : 39.9 W/m2Thermal ConductivityThermal ConductivityMechanism of thermal conduction1. Gases : the molecules are continuous random motion, colliding with one another and exchanging energy and momentum2. Liquid : Similar with gas (higher energy molecules collide with lower energy molecules) but the molecules are packed closely together. Hence, molecular force exerted a strong effect on energy exchange.3. Solid : two mechanisms a) applies to metallic solids, heat like electricity is conducted by free electron which move to metal latticeb) Present in all solids : heat is conducted by transmission of energy of vibration between adjacent atom.

Convective Heat-Transfer CoefficientConduction Heat TransferConduction Heat TransferHeat conduction in a flat wall: (a) geometry of wall; (b) temperature plot.

Conduction Heat TransferConduction Heat Transfer2. Conduction through a Hollow CylinderIn many industries, heat is being transferred through the walls of thick-walled cylinder, such as pipe that may or may not be insulated.

Heat conduction in a cylinder

Conduction Heat TransferConduction Heat Transfer21Conduction Heat TransferExample 3A thick walled cylindrical tubing hard rubber having an inside radius of 5 mm and the outside radius of 20mm is being used as temporary cooling coil in a bath. Ice water is flowing rapidly inside and the inside wall temperature is 274.9K. The outside surface temperature is 297.1K. A total of 14.65W must be removed from the bath by the cooling coil. How many m of tubing are needed. Given that the thermal conductivity k=0.151W/m.K Answer: 0.964mSolution for example 3Conduction Heat TransferConduction Through Solids in Series1. Plane walls in seriesThe temperature profile in the three materials A, B and C are shown.

Heat flow through a multilayer wall.

Example 5A cold storage room is constructed of an inner layer of 12.7 mm of pine, a middle layer of 101.6 mm of cork board and an outer layer of 76.2 mm concrete. The wall surface temperature is 255.4K inside the cold room and 297.1 K at the outside of the concrete. Use conductivities of 0.151, 0.0433 and 0.762W/m.k for pine, cork board and concrete respectively. Calculate the heat loss in Watt for 1m2 and the temperature at the interface between the wood and cork board.

Answer: -16.48 W, T2=256.7K2. Multilayer cylindersHeat transfer often occurs through multilayers of cylinder, e.g : heat is being transferred through the walls of an insulated pipe.

Radial heat flow through multiple cylinders in a series.

Example 6A thick-walled tube of stainless steel (A) having a k=21.63 W/m.k with dimensions of 0.00254 m ID and 0.0508 m OD is covered with a 0.0254m thick layer of an insulation (B), k=0.2423 W/m.K. The inside wall temperature of the pipe is 811K and the outside surface of the insulation is at 310.8K. For a 0.305m length of pipe, calculate the heat loss and also the temperature at the interface between the metal and the insulation.Answer: 331.7 W, 805.5KHeat Transfer ApplicationsApplication of heat transfer concept can be found in heater-treater (emulsion treating processes), stabilizer (multistage separator, distillation column, packed column), direct fired heater, heat exchanger, furnace, reboiler, etc. General heat transfer equation is:q = mCpTwhere q is the heat (BTU/hr) or (kW), m is the flow rate (lb/hr) or (kg/hr), Cp is the specific heat (BTU/lb-oF) or (J/kg.oC) and T is the temperature different (oF ) or (oC).

Heat Transfer ApplicationsHeat transfer for heater-treater equipment is:q = 1100 Qoil T [0.5(SG)oil + 1]where Qoil is the oil flow rate (BOPD) or (m3/hr), and (SG)oil is the specific gravity of oil relative to water.