Heat Transfer

Heat ExchangerEngr. Rolie CastroHeat ExchangerA heat exchanger is used to exchange heat between two fluids of different temperatures, heating one and cooling the other, which are separated by a solid wall.Heat exchanger is an equipment made to transfer heat between hot and cold streams through a combined conduction-convection mechanism.Heat ExchangerTwo fluids, of different starting temperatures, flow through the heat exchanger. One flows through the tubes (the tube side fluid) and the other flows outside the tubes but inside the shell (the shell side fluid).

Heat ExchangerThe temperature of the two fluids will tend to equalize. The heat are simply exchanged from one fluid to the other and vice versa. The fluids can be either liquids or gases on either the shell or the tube side.

Heat ExchangerHeat exchangers with only one phase on each side can be called one-phase or single-phase heat exchangers. Two-phase heat exchangers can be used to heat a liquid to boil it into a gas (vapor), sometimes called boilers, or cool a vapor to condense it into a liquid, called condensers, with the phase change usually occurring on the shell side.Heat Exchanger ClassificationParallel Flowknown as concurrent heat exchangers, hot and cold fluids enter through the same point and leaves at the other end.

Use when large heat transfer rate is desired at the start of operationEx (Fermentation, Reactor)

6Heat Exchanger ClassificationCountercurrent FlowWith this type, hot and cold fluids enter at opposite ends of the heat exchanger.

Used when Large amount of heat is needed to be transferred7Heat Exchanger ClassificationCross-FlowTwo fluids flow at perpendicular to each other.

Design CalculationsPipe SizingOverall HTCArea of heat transferLength of heat exchangerNo. of tubesNo. of tube passesNo. of shell passesTube velocityHeat exchanger dutyDouble-Pipe Heat ExchangerHairpin Heat exchangerTwo concentric pipes with one fluid flowing through the center pipe while the other fluid moves cocurrently or countercurrently in the annular space.Design EquationGen. Equation:Q=U A TLMLMTD Method Countercurrent Parallel

Design EquationIf U is not constant but varies linearly with temperature.

Countercurrent

Parallel

Design EquationHeat given up by the hot fluidQhot= mh Cph ThHeat given up by the cold fluidQcold= mc Cpc Tc Note: Cp must be evaluated at mean temp.Steam is condensingQs= ms s Then: Q= Qhot = Qcold = Qs ExampleA counter-flow double-pipe heat exchanger is to heat water from 20oC to 80oC at a rate of 1.2 kg/s. The heating is to be accomplished by geothermal water available at 160oC at a mass flow rate of 2 kg/s. The inner tube is thin-walled and has a diameter of 1.5 cm. The overall heat transfer coefficient of the heat exchanger is 640 W/m2.oC. What is the surface area of the heat exchanger?The length of the tube that must be design is?NTU-Effectiveness MethodAppropriate if only the inlet temperatures of the fluid is known provided a mass flow of both fluid is given.Gen. Equation:Q = Cmin (Th1 Tc1)Cmin value:Cc = mcCpc and Ch = mhCphIf Cc >>> Ch then, Cmin = Ch If Ch >>> Cc then, Cmin = CcEffectiveness Value,

ParallelCounterflowEffectiveness Value,

ExampleWater flowing at a rate of 0.667 kg/s (Cp = 4.192 kJ/kg.K) enters a countercurrent heat exchanger at 308K and is heated by an oil stream entering at 383K at a rate of 2.85 kg/s (Cp = 1.89 kJ/kg.K). The overall HTC is 300 W/m2.K and the area A = 15.0 m2. Calculate the heat transfer rate and the exit water temperature.Use formula18

Shell and Tube Heat Exchanger most common type of heat exchangerOne fluid runs through the tubes, and another fluid flows over the tubes (through the shell) to transfer heat between the two fluids.Allocation of StreamsFactorTube-sideShell-sideCorrosionMore corrosive fluidLess corrosive fluidsFoulingFluids with high fouling and scalingLow fouling and scalingFluid temperatureHigh temperatureLow temperatureOperating pressureHigh pressureLow pressureViscosity Less viscous fluidMore viscous fluidStream flow rateHigh flow rateLow flow ratePhase ChangeNo phase changeWith phase changeKinds of Heat ExchangerDouble-PipeShell and TubeAir-finned Heat ExchangerPlate and Frame Heat ExchangerPhase-change Heat ExchangerBoilersCondensersShell and Tube Classification

Shell and Tube HE PartsTubesProvide heat transfer surfaceMay be seamless or weldedCommonly made of copper and steel alloys.

Tube Sizing : Birmingham Wire Gage (BWG)Shell and Tube HE PartsTubes SheetsShellTube and Shell NozzleChannel CoverBafflesPass Divider

Design EquationGen. Equation:Q=U A (TLM x FT)FT, (LMTD correction factor)a.k.a. Fudge Factor or Geometric FactorCaused by variation in flow patternVaries as the number of tube and shell passesLMTD Correction FactorLMTD Correction Factor

LMTD Correction Factor

LMTD Correction Factor

LMTD Correction Factor

Design EquationHeat given up by the hot fluidQhot= mh Cph ThHeat given up by the cold fluidQcold= mc Cpc Tc Note: Cp must be evaluated at mean temp.Steam is condensingQs= ms s Then: Q= Qhot = Qcold = Qs No. of Tubes and PassesNumber of tubes per pass

Number of Passes

33ExampleA 2-shell passes and 4-tube passes heat exchanger is used to heat glycerine (Cp = 2.324 kJ/kg-K) from 20oC to 50oC by hot water, which enters the thin-walled 2-cm diameter tubes at 80oC and leaves at 40oC. The total length of the tubes in the heat exchanger is 60m. The convection heat transfer coefficient is 25 W/m2.oC on the glycerine (shell) side and 160 W/m2.oC on the water (tube) side. Calculate the overall HTC in W/m2.oC.Determine the rate of heat transfer.What is the percentage increase in the consumption of water if the outer surface of the tube is fouled with coefficient of 0.006 m2.oC/W given that glycerine is to be processed at same flow rate and temperature?U = 21.622 | Delta T = 24.663 | A = 3.77 | Q=1849.57 WR=0.75 | S=0.67 | Ft = 0.92 Answer: 13.33 % increase34NTU-Effectiveness MethodGen. Equation:Q = Cmin (Th1 Tc1)Cmin value:Cc = mcCpc and Ch = mhCphIf Cc >>> Ch then, Cmin = Ch If Ch >>> Cc then, Cmin = Cc35Effectiveness Value,

ExampleHot oil is to be cooled by water in a 1-4 Shell and Tube heat exchanger. The tubes are thin walled and are made of copper with an internal diameter of 1.4 cm. The length of each tube per pass is 5m which consists of 30 tubes per pass, and the tube overall HTC is 310 W/m2.oC. Water flows through the tubes at rate of 0.2 kg/s (Cp=4.18 kJ/kg.oC). , and the oil through the shell at a rate of 0.3 kg/s (Cp=2.13 kJ/kg.oC). The water and the oil enter at temperature of 20oC and 150oC, respectively. Determine the rate of heat transfer in the heat exchanger and the outlet temperatures of the water and the oil. A = 26.389 | NTU = 12.8 | Rc = 0.76435 |E = 0.66Answer: Q = 54.8262 kW37End of Lecture