Chemical Processing - April 2008 - (Page 48)

>> PLANT INSITES Avoid exchanger errors always consider whether calculated duty requires correction When designing a heat exchanger, many engineers rely on a simple equation to determine the unit’s heat transfer capability: Q = U×a×Lmtd where Q is the duty transferred, U is the overall heattransfer coefficient, a is the exchanger effective area, and Lmtd is the log-mean temperature difference along the exchanger. this equation works fine as a rough estimate of exchanger performance for many services. however, in other cases, it can produce misleading results because one or more of its assumptions don’t hold. most of us at times forget these assumptions. they include: 1. true counter-current or co-current flow; 2. a linear relationship between duty and temperature >> Zone method 600 550 500 450 400 350 300 250 200 150 100 Zone 3 Zone 2 Two-phase hot Zone 1 Two-phase cold Temperature, °F 0 5 10 15 20 25 30 35 Duty, MBtu/hr Figure 1. Finding LMTDs for each zone enables calculation of a duty-weighted overall LMTD. Most exchangers aren’t true counter-current or co-current units. for the fluids (no complex phase change or no significant change in specific heat — isothermal phase change for fluid with no sensible heat duty will meet the requirements); 3. constant overall heat-transfer coefficient; 4. no flow or thermal maldistribution or bypassing; 5. for baffled exchangers (most shell-and-tube exchangers) the number of baffle compartments is relatively large (≥5); and 6. equal heat-transfer area in each flow pass. Failing to meet any of these requirements can lead us astray. the order of the assumptions in the list roughly reflects the extent to which they are violated. most exchangers aren’t true counter-current or co-current units. in contrast, few exchangers have unequal heat-transfer area in different passes. however, situations can arise that undermine any of these assumptions (as well as others). For nearly all design and evaluation purposes, you can use one of two approaches to account for the different situations. correction factors for geometry (aspect 1) are readily available. While less easily obtained, such factors also can 48 • April 2008 account for maldistribution (aspect 4), baffling (aspect 5) and area allocation (aspect 6). multiplying the basic Lmtd by the appropriate factor yields the corrected Lmtd. dealing with temperature profiles (aspect 2) and changing heat-transfer coefficients (aspect 3) is more complex. this requires zoned calculation methods. Figure 1 shows a duty/temperature curve for a heat integration service. Zoning divides the service into a number of sections — here, three. the break points between the zones occur at the points where phase change either starts (hot stream) or stops (cold stream). each zone requires separate calculations for corrected Lmtd, heat transfer coefficients, and overall exchanger performance. Using the simple equation would give an overall exchanger Lmtd of 154°F (before other corrections). cutting the exchanger into zones gives values of 92°F for zone 1, 112°F for zone 2 and 182°F for zone 3. this translates to a duty-weighted Lmtd of 109°F across the exchanger. modern design and rating software for rigorous exchanger calculations usually have zone method capability. specify the zone method when applicable — it’s not always the default. Keeping in mind the necessary corrections will keep exchangers from getting you in a hot spot. CP Andrew Sloley, contributing editor ASloley@putman.net www.chemicalprocessing.com http://www.chemicalprocessing.com

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Chemical Processing - April 2008 Contents From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor Is It a Tragedy or Comedy for Engineers? Better Understanding Boosts Mixer Scale-up Don't Err With Air Compressors Control Performance Supervision Enhances Revamp Process Puzzler Plant InSites Equipment & Services Product Spotlight/Classifieds Ad Index End Point

Chemical Processing - April 2008

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