It is possible to predict the performance of water tube HRSGs in clean gas applications by using a simulation process instead of physically designing the unit. Thus anyone familiar with heat balances such as consultants and those planning cogeneration or combined cycle plants can obtain a good idea of the performance of the HRSG under various modes of operation. This information may be used to arrive at the HRSG configuration and optimize the major parameters for the steam system. Several ‘‘what if’’ scenarios may be looked at. The performance of an existing HRSG may also be evaluated to see if its performance is reasonable, as discussed in Q8.45. Though simulation may be used for any clean gas convective type of HRSG, it is particularly useful in gas turbine applications, because the HRSG designs involving multiple-pressure, multiple-module designs are more complex as discussed in Chapter 1.

Because of the large amount of exhaust gases and the low inlet gas temperature of an HRSG, one cannot arbitrarily assume an exit gas temperature and compute the steam generation. The problem of evaluating steam generation and temperature profiles gets complicated further as it is not often possible to recover a substantial amount of energy from the exhaust gases with steam at a single pressure level. Multiple-pressure steam generation with split modules alone can optimize energy recovery, makes the task of performing energy balance calculations very tedious. Gas and steam temperature profiles and hence energy balance in an HRSG are governed by what are called pinch and approach points (Fig. 2.17) Q8.34 and Q8.37 explain this in greater detail.

Basically, we estimate the term UA, the product of the overall heat transfer coefficient and the surface area, for each heating surface in the design mode and then correct it for the effect of gas flow, temperature, and analysis. Using this corrected UA, one can use the NTU method to evaluate the performance of any exchanger and then the overall performance.

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