Retrofitting Air-Cooled Heat Exchangers with Variable Frequency Drive: Challenges and Benefits

Khursaniyah Gas Plant (KGP) had successfully invested in retrofitting one of its air-cooled heat exchanger systems with a variable frequency drive (VFD). This air-cooled heat exchanger system is an integral part in the cogeneration plant operation, and one where its reliability directly affects the cogeneration plant's availability. The original system arrangement consists of a two-double-deck heat exchanger, and three forced draft fin fans operating simultaneously at full speed. A failure or trip to one of these fin fans will automatically trigger a logic known as "Runback," plant power generation and steam production are reduced by 50% and 30% respectively when logic is activated, this protection logic is configured in order to protect the gas turbine hot gas path parts and maintain their temperature within the allowable safe working limits. The new retrofitted VFD allows controlling of the fin fans electric motors rotational speed via controlling both voltage and frequency input to the motors. The primary objective which drove Khursaniyah gas plant to implement this project was to improve the cogeneration plant utilization. This objective was achieved by maintaining the heat exchanger output temperature at an operating range between 420F and 440F, via variable speed fin fans regardless to any changes in the ambient temperature. Several modifications to the existing system components were implemented in order to achieve this objective, some of these modifications include: increasing the fan's blade width, increasing of driver motor's rated torque, and installation of a closed feedback loop control system. The new enhanced system is now capable of operating the cogeneration plant with two fin fans at 100% cogeneration plant operating capacity, while one fin fan can be offline for any planned, or unplanned maintenance activities. An expensive alternative to this retrofitting project was to add additional fin fan to the heat exchanger system. However, adding a new fin fan requires replacement of the double-deck heat exchanger to another unit with larger capacity and additional footprint to accommodate the new extra fan. This retrofitting project required none of the above and was deemed and proven to be the most efficient and practical option to resolve this chronic issue.