HEAT EXCHANGERS By P M V Subbarao Associate Professor Mechanical Engineering Department I I T Delhi A Device which Enhanced the Utility of Fire!!! A True Mediator !!!
EARLIEST TYPES OF HX : COOKING • Primitive humans may first have savoured roast meat by chance, when the flesh of a beast killed in a forest fire was found to be more palatable and easier to chew and digest than the customary raw meat. • They probably did not deliberately cook food, though, until long after they had learned to use fire for light and warmth. • It has been speculated that Peking man roasted meats, but no clear evidence supports the theory. • During Palaeolithic Period, Aurignacian people of southern France apparantly began to steam their food over hot embers by wrapping it in wet leaves. • Crude procedures • as toasting wild grains on flat rocks and using shells, skulls, • or hollowed stones to heat liquids. • Introduction of pottery during the Neolithic Period. • A paste, toasted to crustiness when dropped on a hot stone, made the first bread.
Cauldron : A Formal Heat Exchanger • In 130BC. Hero, a Greek mathematician and scientist is credited with inventing the first practical application of steam power, the aelopile. • Simply a cauldron with a lid, the aelopile had two pipes that channeled steam into a hollow sphere. • The sphere, which pivoted on the steam pipes, had two nozzles situated on opposite sides of its axis. • Thus, the cauldron was fired, the water in it boiled, the steam was channeled into the sphere, and as the steam escaped through the nozzles, the sphere would spin. • It was a thought device and a novelty.
Milk Pasteurizers • The milk is piped into a pasteurizer to kill any bacteria. • The most common is called the high-temperature, short-time (HTST) process in which the milk is heated as it flows through the pasteurizer continuously. • Whole milk, skim milk, and standardized milk must be heated to 72° C for 15 seconds. • The hot milk passes through a long pipe whose length and diameter are sized so that it takes the liquid exactly 15 seconds to pass from one end to the other. • A temperature sensor at the end of the pipe diverts the milk back to the inlet for reprocessing if the temperature has fallen below the required standard.
Hot Fluid Thermal Structure HX Cold Fluid Convection HT Convection HT Rise in Enthalpy of Cold Fluid Drop in Enthalpy of Hot Fluid Mechanism of Heat Transfer Thermal Structure Acceptor Donor Basic Method of Heat Communication
Thermodynamic Perspective of HX. • The energy absorbed by cold fluid • The convective heat lost by hot fluid • Thermal Energy Balance:
Heat Transfer Perspective of HX. • How can A hot fluid loose thermal energy? • How much A hot fluid can loose thermal energy? • How can A hot fluid loose thermal energy? • How much A hot fluid can loose thermal energy? • What is the mutual interaction? • Generalized Newton’s Law of Cooling. • Overall Coefficient of Heat Transfer, U
Creative Ideas for Techno-economic Feasibility of HX. • For a viable size of a HX • How to maximize Effective area of heat communication?. • How to maximize Overall Heat transfer coefficient? • Should we decrease or increase Effective temperature difference?
Recuperation Regeneration Recuperation Vs Regeneration
Indirect Contact Hx Direct Contact Hx Direct Vs Indirect Contact
Multi Phase Boiling HX Single Phase HX Multi Phase Condensing HX Single Phase Vs Multi Phase HX
Tubular Hx Planar Hx Extended Surface Hx Geometry
Study of Simple Heat Exchangers Parallel Flow Heat Exchanger: Counter Flow Heat Exchanger:
Theory of Simple Heat Exchangers • Infinitesimal adiabatic Heat Exchanger model. • For Infinitesimal Heat communication between cold and hot.
Energy Balance in infinitesimal Heat Exchanger • Energy balance in an Infinitesimal adiabatic Heat Exchanger model. Energy Balance:
Local temperature difference for heat communication: Basic Counter Flow HX :A Perfect Heat Transfer Device
Newton’s Law for Combined Cooling & Heating A representative temperature difference for heat communication: Also called as Overall temperature driving force.
Discussion on LMTD • LMTD can be easily calculated, when the fluid inlet temperatures are known and the outlet temperatures are specified. • Lower the value of LMTD, higher the value of overall value of UA. • For given end conditions, counter flow gives higher value of LMTD when compared to co flow. • Counter flow generates more temperature driving force with same entropy generation. • This method is a capacity based design.
A Model Problem • A concentric tube heat exchanger is used cool the lubricating oil for a large power plant gas turbine. The flow rate of cooling water through the inner tube(di = 25mm) is 0.2 kg/sec, while the flow rate of oil through the outer annulus (do= 45 mm) is 0.1 kg/sec. • The oil and water enter at temperatures of 100 and 30 0C, respectively. • Calculate the value of LMTD for both Co and counter flow Hxs, if outlet temperature of the oil is 600C.
Parameters of Fluids • Inlet temperature of hot oil = 1000C • Outlet temperature of hot oil = 600C • Mass flow rate of oil = 0.1 kg/sec. • Specific heat of oil = 2.131 kJ/kg. • Inlet temperature of cooling water = 300C • Mass flow rate of cooling water = 0.2 kg/sec. • Specific heat of cooling water = 4.178 kJ/kg.
Conservation of Energy Rate of Heat lost by hot oil = Rate of heat gained by cooling water
Counter flow Model of Hx Hot Oil Cooling Water LMTD = 43.2 0C
Co flow Model of Hx Hot Oil Cooling Water LMTD = 39 0C
Comparison of Co-flow and Counter flow • Overall heat transfer coefficient, U = 37.7 W/m2K. • LMTD for Co-flow = 39 0C. • LMTD for Counter-flow = 43.20C. • Length of Hx for co-flow = 73.7 m • Length of Hx for Counter flow = 66.5 m