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Heat Exchanger Design
Goals:
 Role of heat exchangers in chemical processing
 Basic concepts and terminology
 Types of heat exchangers
 Design methodology
 Sizing
 Design
 Rating
Purpose of a Heat Exchanger
 To heat or cool a stream flowing from item of equipment to another. The steam may be a
 A liquid
 A gas
 A multiphase mixture
 To vaporize a liquid stream
 To condense a vapor stream
Single Tube Pass, Single Shell Pass CounterCurrent Heat Exchanger
Types of Heat Exchanger Service
 Fluid heated by external utility
 Steam
 Hot oil or molten salt
 Combustion gas (furnace)
 Electricity (resistive, inductive, microwave)
 Fluid cooled by external utility
 Cooling water
 Refrigeration
 Fluid heated or cooled by other process stream
Calculations of Cooling Curves
 Sensible heat:
(For constant Cp: otherwise)

 Latent Heat:
 Multicomponent Cooling Curves: Requires pointbypoint flash calculations
Feasible Cooling Curve Pairings
 Corollary of the Second Law of Thermodynamics
Heat can only be transferred from a higher temperature to a lower one.
 For heat exchangers this means that the higher temperature cooling curve and the lower temperature heating curve cannot intersect.
 When this condition is satisfied, the pairing of a heating and cooling curve is said to be feasible.
Maximum Heat Exchanger Duty
 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,
 Intersect at one end of the exchanger or the other or
 Become tangent within the exchanger
 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, caiit it Wcpmax.
Maximum Duty Qmax
 For WCp>WCp2, the exchanger will βpinch: at the Fluid 1 inlet.
 For WCp2>WCp1 the exchanger will βpinchβ at the Fluid 2 inlet (as in the previous diagram)
Basic Performance Equations For a Heat Exchanger
For the fluid flowing in the positive z direction an energy balance on the section dz gives
where U = the overall heat transfer coefficient and
a= the heat transfer area per unit length
if the second fluid is a pure component either vaporizing or condensing, them
Case 1: if the second fluid is a pure component either vaporizing or condensing, then
Case 2: if the second fluid is flowing in the negative z direction, i.e., in countercurrent flow, and energy balance on the section dz gives
Rating Solution for Case1
Where
For z = L, A = aL and where N1 is defined as the number of heat transfer units (NTUβs) with respect to fluid 1. Let T1out = T1(L) and T1in = T1(0).
Then
The exchanger efficiency {tex inline}E=(T1_{\text{out}}T1_{\text{in}}/(T2T1_{\text{in}}){\tex}
Design Solution for Case 1
Note that
Substituting and rearranging gives
Where
which is the wellknown heat exchanger design equation.Rating Solution for Case 2
Without going through the details:
where
Design Solution For Case 2
The design solution is essentially the same for Case 2 as for Case 1, namely,
where now
Reprise: Assumptions
 The Cp's are constant over the temperature range involved. (Reasonable for most exchangers of practical interest.)
 U is constant over the temperature range involved. (Reasonable for most exchangers of practical interest.)
 Flow is pure countercurrent or pure cocurrent. If not, a correction factor F is required to adjust the LMTD.
F has been derived for most common heat exchanger configurations (multiple tube passes, cross flow, etx.).
Rating Calculations
 If U and A are known along with the W's and Cp's, use the appropriate performance equations solutions.
 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
using the appropriate correlations for hf1 and hf2
Sizing Calculations
 Choose a typical value for U based on the type of service. [Tables of typical values can be found most textbooks on heat exchanger design.]
 Determine the outlet temperatures based on the performance specifications and the appropriate energy balances.
 Calculate A from
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 sizing calculation.
 Determine the number of tubes required to provide the estimated area. Check the tubeside 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.
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