Web Hunter: Tools to use when confronting heat exchanger problems

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By Russ Kratowicz

PlantServices.com

Keywords: "heat exchanger"

The technology is available for evaulating and troubleshooting the different types of exchangers. You should know about the software and calculations you’ll need to document and design the units and Executive Editor Russ Kratowicz points you to free web resources that do just that.

Last month, this column featured industrial heat tracing. Look at this month’s exploration as an extension of this hot topic. Anyone who has studied thermodynamics learned that energy flows from hotter to colder objects. Of the three forms of heat transfer, conduction is probably the most common version you’ll deal with in a manufacturing plant. It’s through heat exchangers that we control that flow of thermal energy while we extract as much useful work as it can provide.

One should know the technology that’s available for evaluating and troubleshooting the different types of exchangers. You should know about the software and calculations you’ll need to document and design the units. So, why don’t you please join me for another romp through the digital morass we call the Web in search of practical, zero-cost, noncommercial, registration-free resources that you might be able to use when you’re called upon to discipline a recalcitrant or underperforming heat exchanger? Remember, we search the Web so you don't have to.

The basics

Before we launch into the more complicated Web discoveries, you might want to review some basic information regarding this month’s topic. Send your hot little mousie to www.answers.com/topic/heat-exchanger for a high-level primer on what they are, how they work and where they’re used. The content is on the left side of the screen and you’ll need to block out the annoying irrelevancies on the right side until you’ve scrolled down to about mid-document.

Trust, but verify

It might be necessary to evaluate the continued suitability of your heat exchangers when you change the process operating conditions. Even if operating conditions don’t change or become less severe, it would be wise to have tangible proof that an older heat exchanger, which might have no remaining design corrosion allowance, is still mechanically sound. In either case, you can perform the calculations to ensure that each exchanger element still complies with the various codes and guidelines that apply to exchangers and pressure vessels. So says Carmagen Engineering, Inc., Rockaway, N.J., an independent consulting firm that caters to the oil and gas industries, market segments that use many heat exchangers. The company has posted advice on the subject on its Web site. You’ll need to go to http://carmagen.com/newspub.htm and click on “The Carmagen Engineering Report.” Then, look for the entry titled “To Rerate or Not to Rerate — That Is The Question (Part II).” This article, written by Ray Chao and Doug Stelling, offers suggestions about which mechanical components should be evaluated and what you’ll need to do if the original heat exchanger drawings and calculations have gone missing. It’s a lot of work, but it’s also better to choose safe over sorry.

Spec it right

Heat exchangers involve so many variables it’s a wonder that any of them function well. Four temperatures, two flow rates, metallurgy-dependent heat transfer rates, possible phase transitions, insulating deposits collecting on heat transfer surfaces, and the list goes on. The only constant seems to be the exchanger’s physical size. Nevertheless, it’s possible to match the hardware to the job by using online information posted by The Chemical Engineers' Resource Page, Midlothian, Va. That’s the place to go to learn the 10 questions you should ask if you’re involved in designing or specifying one type of heat exchanger. The Web page presents an iterative approach to matching the thermal duty to the process demands. It’s not necessarily an easy calculation, and success hinges on making an accurate initial guess for the heat transfer coefficient and adopting something the article calls zoned analysis. If this is something you wish to tackle, make your way to www.cheresources.com and click on the “Free Articles” link on the left side. Then, select “Process Design” from the drop-down menu. Look for an article titled “Design Considerations for Shell and Tube Heat Exchangers” near the bottom of the next page. I hope it helps.

Heat load

The early stages of specifying a heat exchanger involve knowing something about how many BTU need to be transferred. The inputs include flow rates, temperatures and physical properties. In the old days, performing the appropriate math required an engineering hand calculator and too much time. But, that’s not the case if you pay a visit to www.freecalc.com/hxfram.htm, where you’ll find an input form that powers a heat exchanger sizing routine provided by Beacon Engineers Inc., Woodinville, Wash. The feature is designed for an exchanger configured for liquid-liquid, gas-gas, gas-liquid, condenser or evaporator service. Plug in the information you know, click the calculate button and the next screen reiterates your input plus gives you the heat load. At this point, you have the option of clicking to get a filled out data sheet. But, thanks to the folks at Intel and Microsoft, anyone can enter the numbers online and get a result in a jiffy but have no idea of the appropriateness, accuracy and precision of the math that goes on behind the scenes. It would be better if Beacon were more explicit about the underlying modeling technique.

Designing the hardware

Calculating the heat load is only the first part of the issue at hand. Keep in mind that most heat exchangers are pressure vessels and, therefore, need to be fabricated in accordance with recognized principles that ensure the safety of anyone in the vicinity. Those principles are enshrined in the ASME code, which Heat Transfer Consultants, Inc., Pleasant Hill, Calif., has reduced to a software package you can download for free. Called PVX – 2007, it’s used for designing and rating ASME pressure vessels, TEMA/ASME heat exchangers and interconnecting piping. As you’d expect with a freebie, the software is hobbled with a maximum internal pressure of 300 psi and temperature of 600°F, and external pressure of 10 psi, tube length of 16 ft., a limited selection of shell diameters and a disabled 3-D export capability. With that said, catch your download at www.htcsoftware.com and use it before its 30-day operating window closes.