A Few Typical Solved Problems

• A plant receives a quotation from a boiler vendor for a finned tube superheater.

Information on process data and tube geometry details is given. Using heat transfer correlations presented in the Appendices, the U is estimated (overall heat transfer coefficient) and the adequacy of surface area is checked in addition to gas-steam side pressure drops and tube wall temperatures. To predict its off-design performance, two methods are discussed, one being the NTU method and the other the conven- tional method. (A similar analysis for evaporator, economizer, and air heater is given.) These examples will educate plant engineers and enable them to perform similar calculations and rectify any flaws in the component design before it is ordered.

• Using fuel data, furnace dimensions, and tube geometry details of an oil- and

gas-fired steam generator, the complete performance of a steam generator is evaluated at full and part loads. One example starts from the economizer exit and works backwards all the way to the furnace, and another method starts from the furnace end and works through to the economizer exit. These examples will help plant engineers to check independently if the boiler supplier’s surfacing of furnace, superheaters, boiler bank, economizer, fuel consumption and efficiency is reasonable and as guaranteed.

• An example shows how in a natural gas-fired steam generator the feed water temperature entering the economizer may be reduced to improve fuel utilization and boiler efficiency. The feed water entering the economizer is used to preheat the make-up water. One must ensure that the feed water is cooled to slightly above the water dew point of the flue gas.

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• Performance of radiant and convective superheaters is evaluated to see which option results in flat steam temperature over load. The effects of excess air and flue gas recirculation on their performance and steam and tube wall temperatures are also evaluated. It may be noted that radiant superheaters are more prone to overheating and tube failures and hence these examples will be informative and help plant engineers be careful when they evaluate a high-pressure, high- temperature steam generator design for possible purchase.

• An example shows the performance of plain and finned tube economizers in an oil-fired steam generator in both a counter flow and parallel flow arrangement and the resulting lowest tube wall temperature (to avoid acid dew point corrosion) and boiler efficiency. Plant engineers sometimes think of changing the economizer configuration to avoid acid corrosion problems and this example explains the implications.

• An example shows how one can evaluate the complete performance of a HRSG using field data and tube geometry details and check whether the original design is reasonable or not. Often HRSGs do not operate at design gas flow or steam flow conditions and hence this example will be helpful to plant engineers who can relate field data with design guarantees even if the steam generation or flue gas flows are significantly different in operation from those shown in the proposal.

• An example shows what happens to the superheated steam temperature and tube wall temperature when saturated steam in a waste heat boiler is taken off from the drum for process heating. The HRSG supplier may not have envisaged this mode of operation when supplying the boiler but the plant engineer may be required to check this option several years after the boiler has been in operation.

• Plant engineers can see how steam generator and HRSG characteristics vary with

load; the effect of supplementary firing and savings in fuel input compared to steam generators may be evaluated. Thus, one may maximize steam generation with a minimum fuel input.

• Complete design and off-design performance calculations of fire tube boilers are

explained. An example illustrates how boiler performance improves when an economizer is added. Plant engineers often think of adding an economizer to an existing boiler to improve efficiency and can carry out this exercise with minimal help from any boiler vendor or completely on their own.

• Calculations for tube sheet temperature in a fire tube boiler with and without tube

sheet refractory are illustrated. Often tube sheet refractory falls off during operation, leading to overheating of tube sheet, and plant engineers can now investigate how high the tube sheet temperature can go without refractory.

• Design and off-design performance of tubular air heaters are illustrated with a

few examples. Low-temperature corrosion is likely at low loads and plant engineers can check this.

• Significance of pinch and approach points is explained with several examples,

including calculation of gas-steam temperature profiles and steam generation in single or multiple pressure HRSGs. Simulation of performance of complex HRSGs may be evaluated and optimized before approaching the HRSG supplier for a quotation.

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• Examples compare results for a HRSG obtained from the simulation process with

that obtained using actual tube geometry details to check accuracy of simulation techniques.

• Using a simulation method, examples illustrate if a single-pressure HRSG can be a

better choice than a dual-pressure HRSG under certain circumstances. The effect of the ratio of HP (high pressure) to LP (low pressure) steam flow and steam pressure is illustrated. This helps one to arrive at the least-expensive HRSG configuration instead of simply accepting a design from a HRSG vendor.

• An example illustrates how a plant may optimize the configuration of a multi- module HRSG to maximize energy recovery using the simulation method. (HRSG suppliers may not have the time to study these options.)

• A simulation example illustrates how by splitting superheaters so that a portion

is located upstream and another downstream of the burner a flat steam temperature may be obtained in both unfired and fired modes and over a wide load range without the need for a desuperheater!

• An example shows how the condensation duty may be estimated when flue gas

is cooled beyond its water dew point. Examples also show how plain tube and finned tube condensing economizers may be sized.

• An example shows how a coil located inside the steam drum may be sized for a

given duty. An example design of a steam desuperheater coil located inside the steam drum is given.

• Effects of sudden changes in process steam demand or feed water cut-off on drum

level and steam pressure are illustrated with examples. Examples also show how one may estimate the time to heat up an evaporator or a superheater using flue gases.

Several more examples on basic and applied heat transfer calculations related to boiler, HRSG design, and performance are provided.

Articles on boilers published by the author can be downloaded from the website http:// vganapathy.tripod.com/boilers.html. The author may be contacted at v_ganapathy@ yahoo.com.

A Few Typical Solved Problems