A heat exchanger is a device that is used to transfer thermal energy between two or more fluids, a solid surface and a fluid, solid particulates and a fluid Typical applications involve heating or cooling of a fluid stream of concern and evaporation or condensation of single- or multicomponent fluid streams.
• Cross-flow Heat Exchangers Finned-Both Fluids Unfinned-One Fluid Mixed Unmixed the Other Unmixed
Compact Heat Exchangers• Achieve large heat rates per unit volume• Large heat transfer surface areas per unit volume, small flow passages, and laminar flow.
Shell and Tube Heat Exchangers• The shell and tube heat exchanger is the most common style found in industry.• As the tube-side flow enters the exchanger, flow is directed into tubes that run parallel to each other. these tubes run through a shell that has a fluid passing through it.• Heat energy is transferred through the tube wall into the cooler fluid.• Heat transfer occurs primarily through conduction and convection.
Shell-and-Tube Heat Exchangers: One Shell Pass and One Tube Pass Baffles are used to establish a cross-flow and to induce turbulent mixing of the shell-side fluid. One Shell Pass, Two Shell Passes, Two Tube Passes Four Tube Passes
Main Parts:1.Connections2.Tube Sheets3.Gaskets4.Head5.Mounting6.Baffles7. Shell8.Tube bundle
The Thermal Analysis: The fundamental equations for heat transfer across a surface are given by: Q = U A ΔTlm = w Cp(t) (t2 − t1) = W Cp(s) (T1 − T2) or W LWhere Q = heat transferred per unit time (kJ/h, Btu/h) U = the overall heat transfer coefficient (kJ/h-m2 oC, Btu/hft2-ºF) A = heat-transfer area (m2, ft2) Δtlm = log mean temperature difference (oC, ºF) Cp(t) = liquid specific heat tube side, Cp(s) = liquid specific heat shell side (kJ/kg-ºK, Btu/lb-ºF) w = tube side flow W = shell side flow (kg/h, lb/h)The log mean temperature difference ΔTlm (LMTD) for counter current flow isgiven by:
• A correction factor is applied to the LMTD to allow for the departure from true counter current flow to determine the true temperature difference. ΔTm = Ft ΔTlm• The correction factor is a function of the fluid temperatures and the number of tube and shell passes and• Correlated as a function of two dimensionless temperature ratios
• The correction factor Ft for a 1-2 heat exchanger which has 1 shell pass and 2 or more even number of tube passes is given by:• The overall heat transfer coefficient U is the sum of several individual resistances as follows: • The combined fouling coefficient hf can be defined as follows:
Area of Flow:• Shell side cross flow area aS is given bySpacing Required:• Spacing does not normally exceed the shell diameter• Maximum spacing is given by:
Shell side Film Coefficient Methods for SingleComponent Condensation in Laminar Flow:• Horizontal condenser sub coolers are less adaptable to rigorous calculation• But give considerably higher overall clean coefficients than vertical condenser sub coolers which have the advantage of well defined zones. The Nusselt Method: • The mean heat transfer coefficient for horizontal condensation outside a single tube is given by the relationship developed by Nusselt. • This correlation takes no account of the influence of vapour flow which, in addition to the effect of vapour shear, acts to redistribute the condensate liquid within a tube bundle.
The Kern Method:• Kern adapted the Nusselt equation to allow evaluation of fluid conditions at the film temperature• For horizontal tube surfaces from 0° to 180° the above equation can be further developed to give
• McAdam extended the Kern equation to allow for condensate film and splashing affects.• The loading per tube is taken to be inversely proportional to the number tubes to the power of 0.667. • This equation requires the film to be in streamline flow • Reynolds Numbers in range 1800 to 2100
Example:Problem : Design of a two-pass, shell-and-tube heat exchanger to supply vapour for the turbine of an ocean thermal energy conversion system based on a standard (Rankine) power cycle. The power cycle is to generate 2 MW at an efficiency of 3%. Ocean water enters the tubes of the exchanger at 300K, and its desired outlet temperature is 292K. The working fluid of the power cycle is evaporated in the tubes of the exchanger at its phase change temperature of 290K, and the overall heat transfer coefficient is known. FIND: (a) Evaporator area, (b) Water flow rate. SCHEMATIC:
ASSUMPTIONS: (1) Negligible heat loss to surroundings, (2) Negligible kinetic and potentialenergy changes, (3) Constant properties.PROPERTIES: Water ( Tm = 296 K): cp = 4181 J/kg K.ANALYSIS: (a) The efficiency is W 2 MW 0.03. q q Hence the required heat transfer rate is 2 MW q 66.7 MW. 0.03 Also 300 290 292 290 C Tm,CF 5C 300 290 n 292 290 and, with P = 0 and S = , from Fig. it follows that F = 1. Hence
q 6.67 107 W A U F Tm,CF 1200 W / m 2 K 1 5 C A 11,100 m2.b) The water flow rate through the evaporator is q 6.67 107 W mh cp,h Th,i Th,o 4181 J / kg K 300 292 mh 1994 kg / s.COMMENTS: (1) The required heat exchanger size is enormous due to the small temperature differences involved, (2) The concept was considered during the energy crisis of the mid 1970s but has not since been implemented.
REFERANCES:1. Design And Rating Shell And Tube Heat Exchangers , By John E. Edwards2. Engineering Data Book, By Professor John R. Thome3. www.pidesign.co.uk4. en.wikipedia.org