Heat Exchanger Design
21
0
0
Full text
(2) PURPOSE OF A HEAT EXHANGER u To Heat or Cool a stream flowing from item of equipment to another. The steam may be a: ð A liquid ð A gas ð A multiphase mixture u To Vaporize a liquid stream u To Condense a vapor stream. 8/24/99. Heat Exchanger Design.
(3) SINGLE TUBE PASS, SINGLE SHELL PASS COUNTERCURRENT HEAT EXCHANGER. Shell Side Outlet Nozzle. Shell Side Inlet Nozzle. Tube Si Outlet Nozzle. ube Side et Nozzle. 8/24/99. Heat Exchanger Design.
(4) TYPES OF HEAT EXCHANGER SERVICE u Fluid heated by external utility ð Steam ð Hot oil or molten salt ð Combustion gas (furnace) ð Electricity (resistive, inductive, microwave) u Fluid cooled by external utility ð Cooling water ð Refrigeration u Fluid heated or cooled by other process stream 8/24/99. Heat Exchanger Design.
(5) TYPES OF HEATING AND COOLING CURVES Sensible Heat - Heating. Sensible Heat - Cooling. T (K). Pure Component Condensation or Vaporization. Multicomponent Cooling/Condensation. DUTY Q (kJ/s) 8/24/99. Heat Exchanger Design.
(6) CALCULATION OF COOLING CURVES u. Sensible Heat: Q = W Cp (Tin - Tout) (For constant Cp: Q = W (Hin - Hout) otherwise). u Latent Heat:. Q = Wλ. u Multicomponent Cooling Curves: Requires point-bypoint flash calculations. 8/24/99. Heat Exchanger Design.
(7) FEASIBLE COOLING CURVE PAIRINGS u Corollary of the Second Law of Thermodynamics: Heat can only be transferred from a higher temperature to a lower one. u For heat exchangers this means that the higher temperature cooling curve and the lower temperature heating curve cannot intersect. u When this condition is satisfied, the pairing of a heating and cooling curve is said to be feasible. 8/24/99. Heat Exchanger Design.
(8) FEASIBLE COOLING CURVE PAIRINGS T1in. Countercurrent Flow. Cocurrent Flow. T. T1out. T1out T2in. T2out T2in. Q. Q. INFEASIBLE COOLING CURVE PAIRINGS. T. Q. Q 8/24/99. Heat Exchanger Design.
(9) MAXIMUM HEAT EXCHANGER DUTY u Qmax, the maximum amount of heat that can be transferred in a heat exchanger, no matter how large it is, occurs when the heating and cooling curves either ð Intersect at one end of the exchanger or the other or ð Become tangent within the exchanger u For sensible heating with constant fluid heat capacities, the curves are straight lines. They will intersect at that end of the exchanger whose entering fluid has the largest WCp, call it Wcpmax.. 8/24/99. Heat Exchanger Design.
(10) MAXIMUM HEAT EXCHANGER DUTY. T1in. T2out T. T1out = T2in. 0. Q. Qmax. For this example, WCp2 > WCp1 8/24/99. Heat Exchanger Design. 10.
(11) MAXIMUM DUTY Qmax u For WCp1 > WCp2, the exchanger will “pinch” at the Fluid 1 inlet. Qmax = WCp2 (T1in - T2in) u For WCp2 > WCp1 the exchanger will “pinch” at the Fluid 2 inlet (as in the previous diagram) Qmax = WCp1 (T1in - T2in). 8/24/99. Heat Exchanger Design. 11.
(12) BASIC PERFORMANCE EQUATIONS FOR A HEAT EXCHANGER. For the fluid flowing in the positive z direction an energy balance on the section dz gives dT1/dz = (Ua./WCp1)(T2 - T1). where U = the overall heat transfer coefficient and a = the heat transfer area per unit length f the second fluid is a pure component either vaporizing or ondensing, then dT2/dz = 0 8/24/99. Heat Exchanger Design. 12.
(13) BASIC PERFORMANCE EQUATIONS FOR A HEAT EXCHANGER (Cont’d.) CASE 1: If the second fluid is a pure component either vaporizing or condensing, then dT2/dz = 0 CASE 2: If the second fluid is flowing in the negative z direction, I.e., in countercurrent flow, an energy balance on the section dz gives dT2/dz = (Ua./WCp2)(T2 - T1) 8/24/99. Heat Exchanger Design. 13.
(14) RATING SOLUTION FOR CASE 1 T1(z) = Exp[- η1 z] T1(0) + (1 - Exp[- η1 z] ) T2 where η1 = U a /WCp1 For z = L, A = aL and N1 = UA/WCp1 where N1 is defined as the number of heat transfer units (NTU’s) with respect to Fluid 1. Let T1out = T1(L) and T1in = T1(0). Then T1out = Exp[-N1} T1in + (1 - Exp[-N1]) T2 The exchanger efficiency E = (T1out - T1in)/(T2 - T1in) E = 1 - Exp[-N1] 8/24/99. Heat Exchanger Design. 14.
(15) DESIGN SOLUTION FOR CASE 1 N1 = - UA/WCp1 = ln {1 - E} = ln {[T2 - T1out]/[T2 - T1in]} Note that Q = WCp1(T1out - T1in). Substituting and rearranging gives Q = UA LMTD where LMTD = [(T2-T1out) - (T2-T1in)] ln {[T2 - T1out]/[T2 - T1in]} which is the well-known heat exchanger design equation. 8/24/99. Heat Exchanger Design. 15.
(16) RATING SOLUTION FOR CASE 2. Without going through the details: T2out = [Exp(N2-N1) - 1] T1in + [1 - N1/N2] T2in [Exp(N2-N1) - N1/N2] where N1 = UA/WCp1 N2 = UA/WCp2 T1out = Exp(N2-N1)[ 1 - N1/N2] T1in [Exp(N2-N1) - N1/N2] + N1/N2[Exp(N2-N1)-1] T2in [Exp(N2-N1) - N1/N2] 8/24/99. Heat Exchanger Design. 16.
(17) DESIGN SOLUTION FOR CASE 2. The design solution is essentially the same for Case 2 as for Case 1, namely, Q = UA LMTD where now LMTD = (T2out - T1in) - (T1out - T2in) ln {(T2out - T1in)/(T1out - T2in). 8/24/99. Heat Exchanger Design. 17.
(18) REPRISE: ASSUMPTIONS u The Cp’s are constant over the temperature range involved. (Reasonable for most exchangers of practical interest.) u U is constant over the temperature range involved. (Reasonable for most exchangers of practical interest.) u Flow is pure countercurrent or pure cocurrent. If not, a correction factor F is required to adjust the LMTD. Q = UA F LMTD F has been derived for most common heat exchanger configurations (multiple tube passes, cross flow, etx.). 8/24/99. Heat Exchanger Design. 18.
(19) RATING CALCULATIONS u If U and A are known along with the W’s and Cp’s, use the appropriate performance equation solutions. u If only the specifications of the exchanger (number of tubes, length of tubes, tube diameter, baffle spacing, baffle cut, etc.) are given, compute A from the geometry and U from 1/U = 1/hf1 + 1/hwall + 1/hf2 + rfouling using the appropriate correlations for hf1 and hf2. 8/24/99. Heat Exchanger Design. 19.
(20) SIZING CALCULATIONS u . Choose a typical value for U based on the type of service. [Tables of typical values can be found in Perry’s and most textbooks on heat exchanger design.] u Determine the outlet temperatures based on the performance specifications and the appropriate energy balances. u Calculate A from Q = U A LMTD. 8/24/99. Heat Exchanger Design. 20.
(21) RIGOROUS DESIGN. Determine the basic heat exchanger features such as tube diameter and wall thickness, tube length, baffle spacing, and baffle cut. Estimate the area required based on a sizing calculation. Determine the number of tubes required to provide the esti mated area. Check the tube-side fluid velocity. If below the acceptable range, estimate number of tube passes required. Calculate U based on the appropriate correlations and a reasonable estimate of the fouling resistance. Iterate until Uassumed = Ucalculated. Check the pressure drops and adjust design if unacceptable 8/24/99. Heat Exchanger Design. 21.
(22)
Related documents
There are many types of heat exchanger in industrial application such as double pipe heat exchanger, compact heat exchanger, shell and tube heat exchanger and plate
High performance, compact, low cost heat exchanger technology needed for.. increasing heating and cooling requirements in various
ABSTRACT:- Ground coupled heat exchanger (GCHE) systems are used for space heating/cooling across the globe. Hybrid GCHE systems are being preferred over unitary system due to
Effectiveness is says the overall performance of heat exchanger, the Effectiveness of the modified heat exchanger is high comparing with segmental heat exchanger,
First the plain tube double pipe heat exchanger was tested. At the beginning of series of tests, the hot water was circulated through inner tube and cooling
4 P ROPOSED APPROACH FOR DESIGN AND CONTROLLABILITY ASSESSMENT OF HEAT EXCHANGER NETWORK RETROFITS The approach for design and controllability assessment of heat exchanger
Both hot and cold fluids enter the heat exchanger at the same end and move in the same direction in parallel flow (co-current).. The heat exchanger also affected by hot
SHELL AND TUBE HEAT EXCHANGER(KERN'S METHOD) HOT FLUID-toluene SPECIFIC HEAT,J/KGK,CP: VISCOSITY,µ DENSITY ,Kg/m3,ρ; THERMAL CONDUCTIVITY,w/m2 k,K: MASS FLOW RATE Kg/s.
