Fired Heater Design

Design of fired heaters is complex and best left to specialists. But there are times when a rapid preliminary estimate is required, and it is then that the website HEATERDESIGN comes to the rescue. It not only provides a nice overview of the thermal and mechanical design considerations of a fired heater but also comes loaded with nifty online calculators. Though the expressed purpose of the website is educational and it comes with a clear warning that the methods presented may or may not be suitable for actual design, this does not take away the utility of the online calculators for:

Flue Gas Composition and Properties

Tube Wall Temperature

Heat Loss

Flame Temperature

Acid Dew Point of Flue Gas

Pressure Loss in Ducts

Stack Draft

......and much more

Fouling Factors

While revamping a Chemical Process Plant, it is very rare to find a heat exchanger that is a bottleneck to achieve higher throughputs. In other words, heat exchangers more often than not have plenty of in-built design margins. This margin results from providing conservative fouling factors, in addition to the customary practice of designing heat exchangers for 110 % of the heat duty.

Consider the case of a simple condenser condensing organic vapours using cooling water in the tubes. For typical values of ho = 1000, hio = 400 and rd = 0.003 (all in British Units; Thanks to the pioneering text book of Kern, generations of chemical engineers are comfortable in British Units when it comes to heat transfer coefficients), the overall heat transfer coefficient U is 154. With improved treatment of cooling water and annual cleaning, it is possible to specify a less conservative dirt factor. With rd = 0.001, U leaps to 222, an increase of 44 %. Compounded with the 10 % design margin mentioned above, the heat exchanger has a margin of 58 %, good enough for most revamps.

There were times when conservative fouling factors were specified to hide the perceived inadequacies of empirical calculation methods and likely errors in physical properties. But with sophisticated software of HTRI and HTFS, which model the non-ideal flows on the shell side, estimates of clean heat transfer coefficients are more accurate. The case for conservatism in fouling factors no longer exists on this count.

Over sizing of heat exchangers can also be counterproductive. Lower velocities in an oversized exchanger will only promote scaling and accelerate fouling. The research work on fouling is scattered and not much has been done in compiling a meaningful database that can be used to choose economic fouling factors for cost-effective design of heat exchangers. Last year HTRI launched an industry-wide effort to examine the way heat exchangers are designed.

Pitting Resistance

Stainless Steels are vulnerable to corrosion by pitting. Higher the concentration of Chromium (Cr), Molybdenum (Mo) and Nitrogen (N) better the resistance to pitting. The Pitting Resistance Equivalent (PRE) Number is a good quantitative guide of resistance to pitting.

PRE = (%Cr) + (3.3x%Mo) + (16 x %N)

Higher the PRE Number, better the resistance.

Processing the Passion.

Last Saturday I delivered a lecture on ‘The Art of Process Design and Engineering’ at a Tech Fest organized by the Chemical Engineering students of Thadomal Shahani Engineering College, Bandra. While conluding the nearly three hour long talk, I enumerated the following minimum essential skills required of a good Process Engineer

• Understanding Chemistry
• Knowledge of Fluids and Materials
• Grasping the Principles of Flow
• Loads of Commonsense
• Feel for Numbers

Nanotechnology has clearly caught the fancy of young chemical engineers. Out of the 9 papers presented at the technical session, that I had the opportunity to judge, 4 were on Nanotechnology. Students used eye-catching visuals and made their presentations very confidently. But sadly the depth was lacking. Perhaps we should have debates rather than conventional presentations, in order to foster creative reasoning.

The excitement at the fest was however very palpable. The passion with which the students participated is encouraging for the future of Indian Chemical Engineering.

Life Savers

Rules of Thumb are life savers for Process Design Engineers.

Here are two excellent resources for Rules of Thumb freely available on the Internet

Heuristics in Chemical Engineering - Stanley M Walas

Experienced-Based Rules of Chemical Engineering

After this who needs computers?

News to Cheer !

Padmasree Warrior, Executive VP/Chief Technology Officer at Motorola has been included in Fortune magazine’s latest list of most powerful businesswomen. She is a chemical engineer with MS from Cornell University and a B.Tech from IIT - Delhi. She also has the distinction of becoming the first woman chief technology officer in the US. Now that is some news to cheer for Chemical Engineers, women and Indians in that particular order.

A New Curriculum?

It is Vijayadashami today and millions of Indians renew their oaths towards their trade and profession. And here is my bit towards my profession of chemical engineering.

The Board of Studies in Chemical Engineering, Mumbai University met last month to discuss restructuring of the syllabus for the undergraduate programme. As one of the two representatives from industry on this Board, I am often asked to suggest design and simulation software that should be included in the curriculum, so as to make it relevant for the industry. The curriculum today places more emphasis on application oriented courses like process engineering and process simulation at the expense of principle based subjects like fluid flow and heat transfer, in the mistaken belief that these produce more ‘industry-friendly’ students.

Universities should not sweat about tailoring their chemical engineering curriculum to meet the needs of a specific industry or business. What industry needs are engineers firmly grounded in chemical engineering principles and fundamentals, who can then be moulded to meet the specific requirements. Many Chemical Engineers would be applying a lot of fluid flow and heat transfer principles throughout their career and during our recruitment we test fresh candidates mostly on their knowledge in these two areas and they fare rather poorly on this. Process Simulation has lots of glamour value and everyone yearns for it. Instead of succumbing to the pressures of including this in the UG programmes, time and energy can be gainfully invested in the underlying principles of chemical engineering thermodynamics.