Thermal System

Department of Mechanical Engineering, IUPUI

ME 414 Thermal-Fluid Systems Design Project 2: Heat Exchanger Optimization Instructor: John Toksoy May 6, 2005 Group Members: Luke Jones Justin Gast Mike Hughett 1

Problem Statement ‰

Design a heat exchanger given 80,000kg/hr of distilled water will enter at 35°C and leave at 25°C and transfer heat to 140,000kg/hr raw water entering from a 20°C supply.

ƒ Desired heat transfer rate = m& C p ∆T = 928.5 kW ƒ No baffles, neglect fouling, single pass. ‰

Optimize the weight, shell and tube pressure drops, and heat transfer of the design using the DOE capabilities of both Matlab and Minitab software.

2

Tools Utilized ‰

Matlab

ƒ Utilized the provided Matlab code to perform the heat exchanger analysis ‰

Minitab

ƒ Used in the selection of critical design parameters ƒ Provided tools needed to optimize Matlab heat exchanger design calculations

ƒ Aided in optimization ‰

Iterative optimization process

3

Where to Start? ‰

Input given values from problem definition

‰

Obtained desired to calculated heat transfer ratio of 1 by trial and error

‰

Ran DOE study using Minitab to find the main effects of the variables and their interactions

‰

Eliminated insignificant variables

4

Funnel Effect Shell ID, Tube OD, Length, Tube Material, Shell Thickness, Fluid Allocation, Layout Angle, Shell Thickness

Minitab

2-3 Critical Variables 5

Main Effects Plots

6

Design Decisions ‰

Counter Flow

ƒ Parallel Flow Not an Option Æ ‰

1.25 Pitch Ratio (rule of thumb)

‰

Square Pitch

ƒ Clean surfaces ƒ 90 degree layout angle ‰

Tube Material

ƒ Aluminum: ▲Heat Transfer ▼Low Weight ‰

Shell Thickness set to 1 mm (determined from hoop stress analysis) 7

Elimination from Evaluation ‰

After more Main effects plots were run, the 3 key variables discovered were: length, tube OD, and shell ID

‰

Next, a multi-level DOE was run in Matlab to determine good starting points for design optimization

8

Main Effects of 3 Critical Parameters

9

Heat Exchanger Optimization ‰

Analyzed Factorial Design to create Pareto charts of design parameters. This shows the weight each variable has on the design specification

‰

Verified that the statistical p-values were below 0.1

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Iterative Optimization DOE 1

DOE 2

+/- 20%

DOE 3

+/- 15%

+/- 10%

(Matlab

(Matlab

(Matlab

Check)

Check)

Check)

DOE 4

DOE 5

Matlab Results: Weight = 1051 kg

+/- 5%

∆P Tube = 978 Pa

(Matlab

(Matlab

Check)

Check)

∆P Shell = 914 Pa Q = 928.6 kW

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Cost Consideration ‰

While custom parts provide the most efficient heat exchanger design, manufacturing costs must be considered in the Total Cost of Ownership

TCO = Initial Costs + Maintenance + Repairs ‰

Using standard tube sizes greatly reduces initial costs, thereby reducing the TCO

Selected Material Sizes:

Standard Tube and Shell Size Optimization: ‰

Weight = 1005 kg

‰

Shell Diameter: 21.25 inches

‰

Heat transfer rate = 928.3 kW

‰

Tube Diameter: 20BWG ½ inch

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Tube Length: 3.477 meters*

ƒ Desired-to-calculated ratio of 1.00 ‰

Shell side pressure drop = 788 Pa

‰

Tube side pressure drop = 687 Pa

* There is no defined standard length

Even Better than the Minitab Optimization!! 12

Conclusions ‰

Heat Exchanger optimization was a success

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The standard tube and shell diameters provides the optimal weight, tube and shell pressure drops, and desired heat transfer

‰

One concern: the average tube velocity is 0.28 m/s for our optimal design, which is lower than the recommended velocity to prevent settling

ƒ Because distilled water is being used in the tubes, settling is unlikely ‰

TCO of our design is minimized:

ƒ Low material weight Æ initial costs minimized ƒ Low pressure drops Æ initial costs and operational costs minimized

ƒ Square pitch Æ maintenance costs minimized (time=money!!) 13

Questions?

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