MER 439 - Design of Thermal Fluid Systems Heat Exchanger Analysis Professor Anderson Spring Term 2012

Outline (1) Heat Exchanger Types (2) Heat Exchanger Analysis Methods Overall Heat Transfer Coefficient fouling, enhanced surfaces LMTD Method Effectiveness-NTU Method

HX Classifications Recuperators/Regenerators Heat exchangers are devices that provide the flow of thermal energy between two or more fluids at different temperatures. Used in wide range of applications Classified according to: Recuperators/Regenerators Transfer processes: direct and indirect contact Construction Geometry: tubes, plates, extended surfaces Heat Transfer Mechanisms: single and two phase Flow arrangements: parallel, counter, cross-flow

HX Classifications

Concentric tube (double piped) Heat Exchanger Types Concentric tube (double piped)

Concentric tube (double piped) Heat Exchanger Types Concentric tube (double piped) One pipe is placed concentrically within the diameter of a larger pipe Parallel flow versus counter flow Fluid B Fluid A used when small heat transfer areas are required (up to 50 m2) used with high P fluids easy to clean (fouling) bulky and expensive per unit heat transfer area Flexible, low installation cost, simple construction

Shell and Tube Heat Exchanger Types Shell and Tube most versatile (used in process industries, power stations, steam generators, AC and refrigeration systems) large heat transfer area to volume/weight ratio easily cleaned

Compact Heat Exchangers Heat Exchanger Types generally used in gas flow applications surface density > 700 m2/m3 the surface area is increased by the use of fins plate fin or tube fin geometry

Cross Flow Heat Exchanger Types finned versus unfinned mixed versus unmixed

Heat Exchanger Types

Heat Exchanger Types

Heat Exchanger Analysis Overall Heat Transfer Coefficient LMTD Effectiveness-NTU

Overall Heat Transfer Coefficient The overall coefficient is used to analyze heat ex-changers. It contains the effect of hot and cold side convection, conduction as well as fouling and fins. Heat exchanger surfaces are subject to fouling by fluid impurities, rust formation, or reactions between the fluid and the wall.

Enhanced Surfaces Fins are often added to the heat exchange surfaces to enhance the heat transfer by increasing the surface area. The overall surface efficiency is:

Log-Mean Temperature Difference To relate the total heat transfer rate to inlet and outlet fluid temperatures. Apply energy balance:

Log-Mean Temperature Difference We can also relate the total heat transfer rate to the temperature difference between the hot and cold fluids. .

The log mean temperature difference depends on the heat exchanger configuration Th,in Th,in Th,out Th,out Tc,out Tc,out Tc,in Tc,in

LMTD Parallel-Flow HX

LMTD Counter-Flow HX DTlm,CF > DTlm,PF FOR SAME U: ACF < APF

LMTD- Multi-Pass and Cross-Flow Apply a correction factor to obtain LMTD t: Tube Side

LMTD Method Sizing a Heat Exchanger: Calculate Q and the unknown outlet temperature. Calculate DTlm and obtain the correction factor (F) if necessary Calculate the overall heat transfer coefficient. Determine A. The LMTD method is not as easy to use for performance analysis….

The Effectiveness-NTU Method Define Qmax for Cc < Ch Qmax = Cc(Th,i - Tc,i) for Ch < Cc Qmax = Ch(Th,i - Tc,i) or Qmax = Cmin(Th,i - Tc,i) Q = eCmin(Th,i - Tc,i)

The Effectiveness-NTU Method For any heat exchanger: e = f(NTU,Cmin/Cmax) NTU (number of transfer units) designates the nondimensional heat transfer size of the heat exchanger:

The Effectiveness-NTU Method

The Effectiveness-NTU Method PERFORMANCE ANALYSIS Calculate the capacity ratio Cr = Cmin/Cmax and NTU = UA/Cmin from input data Determine the effectiveness from the appropriate charts or e-NTU equations for the given heat exchanger and specified flow arrangement. When e is known, calculate the total heat transfer rate Calculate the outlet temperature.

The Effectiveness-NTU Method SIZING ANALYSIS When the outlet and inlet temperatures are known, calculate e. Calculate the capacity ratio Cr = Cmin/Cmax Calculate the overall heat transfer coefficient, U When e and C and the flow arrangement are known, determine NTU from the e-NTU equations. When NTU is known, calculate the total heat transfer surface area.

The Homework