Exploring Temperature Reduction Factor (TRF) in a small-scale heat recovery steam generator under shutdown conditions

Abstract

Investigating the minimization of HRSG temperature drop during its temporary shutdown is crucial for the purpose of its prompt restart. To achieve this goal, the relative Temperature Reduction Factor (TRF) was introduced and explored at each hour of the shutdown process. Therefore, in this paper, a numerical simulation of the HRSG shutdown process, extending to 118800 s, was performed under two distinct scenarios: the first incorporating a stack damper, and the second excluding it. By regulating the stack damper, the exhaust gas flow, and consequently the heat transfer rate and HRSG cool-down, can be controlled, thereby preventing the occurrence of undesirable thermal stresses. This paper investigates the effect of the stack damper on the critical TRF index, employed as a decision-making and comparative criterion, in order to determine the extent of its contribution to reducing the restart time. The results indicated that the maximum increase in TRF after 33 h of HRSG shutdown, under Scenario 2, is approximately 10 % (55 K) higher than that of Scenario 1. Therefore, under Scenario 2, the restart time increases by approximately 30 min (25 %). Additionally, the results indicate that the use of the Damper Open strategy is permissible if the restart occurs before the 20-h critical time. However, should the system restart process extend to 33 h, the use of the Damper Closed strategy is recommended.