Outline (1) Heat Exchanger Types (2) Heat Exchanger Analysis Methods Overall Heat Transfer Coefficient fouling, enhanced surfaces LMTD Method Effectiveness-NTU Method (3) Homework #4 March 1, 2002

Introduction 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 March 1, 2002

HX Classifications March 1, 2002

HX Classifications March 1, 2002

Heat Exchanger Types Concentric tube (double piped) March 1, 2002

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 March 1, 2002

Heat Exchanger Types Concentric tube (double piped) 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. March 1, 2002

Heat Exchanger Types Shell and Tube March 1, 2002

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 March 1, 2002

Heat Exchanger Types Compact Heat Exchangers 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 March 1, 2002

Heat Exchanger Types Compact Heat Exchangers March 1, 2002

Heat Exchanger Types Cross Flow finned versus unfinned mixed versus unmixed March 1, 2002

Heat Exchanger Types March 1, 2002

Heat Exchanger Types March 1, 2002

Heat Exchanger Applications March 1, 2002

Heat Exchanger Analysis Overall Heat Transfer Coefficient LMTD Effectiveness-NTU March 1, 2002

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. March 1, 2002

Fouling Factors Heat exchanger surfaces are subject to fouling by fluid impurities, rust formation, or reactions between the fluid and the wall. March 1, 2002

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: March 1, 2002

Enhanced Surfaces March 1, 2002

Log-Mean Temperature Difference Need to relate the total heat transfer rate to inlet and outlet fluid temperatures. Apply energy balance: March 1, 2002

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. March 1, 2002

LMTD Parallel-Flow HX Apply energy balance: Assume: insulated no axial conduction DPE, DKE negligible Cp constant U constant March 1, 2002

LMTD Parallel-Flow HX March 1, 2002

LMTD Parallel-Flow HX March 1, 2002

LMTD Counter-Flow HX DTlm,CF > DTlm,PF FOR SAME U: ACF < APF March 1, 2002

LMTD Counter-Flow HX DTlm,CF > DTlm,PF FOR SAME U: ACF < APF March 1, 2002

LMTD- Multi-Pass and Cross-Flow Apply a correction factor to obtain LMTD t: Tube Side March 1, 2002

LMTD Method SIZING PROBLEMS: 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…. March 1, 2002

The Effectiveness-NTU Method If the inlet temperatures are unknown then the LMTD method requires iteration. Preferable to use the e-NTU method under these conditions. 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) March 1, 2002

The Effectiveness-NTU Method Now define the heat exchanger effectiveness, e. The effectiveness is by definition between 0 & 1 The actual heat transfer rate is: Q = eCmin(Th,i - Tc,i) March 1, 2002

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: March 1, 2002

The Effectiveness-NTU Method March 1, 2002

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. March 1, 2002

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. March 1, 2002

Homework Number 4 COMPLETE PARTs 1&2 BEFORE CLASS ON THURSDAY. WE WILL USE CLASS/LAB TIME ON Tuesday TO WORK ON PARTS 3 and 4. Part 1: Review Chapter 11 of Incropera and Dewitt. Work through Example Problems 11.3, 11.4, 11.5. Part 2: To ventilate a factory building, 5 kg/s of factory air at a temperature of 27 oC is exhausted and an identical flow rate of outdoor air at a temperature of -12 oC is introduced to take its place. To recover some of the heat of the exhaust air, heat exchangers are placed in the exhaust and ventilation air ducts and 2 kg/s of ethylene glycol is pumped between the two heat exchangers. The UA value of both of these crossflow heat exchangers is 6.33 kW/K. What is the temperature of the air entering the factory? March 1, 2002

Homework Number 4 Part 3: Calculate the UA value for the cross flow heat exchanger specified above. The heat exchanger is 1m high by 1 m wide by 1 m long (in flow direction) and contains a tube bank heat transfer surface. The ethylene glycol flows through the tubes and the air flow around them (i.e. figure 11.2B in Incropera and Dewitt). The tubes are made of 1" diameter thin walled copper and are mounted in an aligned pattern (3 inches apart center to center – Review tube bank heat transfer data in Chapter 7 of Incropera and Dewitt). Use the data for internal tube heat transfer coefficient given in the Project 4 technical specifications. Part 4: Calculate the pressure drop for the ethylene glycol and the air. Use the data for ethylene glycol friction factor given in the Project 4 technical specifications. March 1, 2002