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We want to recover the energy from high temperature flue gas from our four process heaters. The heaters main design condition is shown below. Please advise.
The design information of these heaters
Heater-1, Vertical Cylindrical
Design Duty 4.48 Gcal/hr
Fuel type – Fuel Gas only
Flue gas from convection section 359 oC, mass velocity 1.959 kg/sec m2
Stack OD 1.2m
Heater-2/3, Vertical Cylindrical
Design Duty 6.91 Gcal/hr, each
Fuel type – Fuel Gas only
Flue gas from convection section 389 oC, mass velocity 2.315 kg/sec m2
Stack OD 1.378m
Heater-4, Vertical Cylindrical
Design Duty 6.57 Gcal/hr
Fuel type – Fuel Gas only
Flue gas from convection section 348 oC, mass velocity 2.482 kg/sec m2
Stack OD 1.4m
 
Answers
13/06/2012 A: S Banik, Centre for High Technology, sbdr@rediffmail.com
What is the composition of your fuel gas? Does it contain H2S? How much moisture it contains? There are novel methods of moisture and energy recovery depending on above parameters.Siemens has a process to to recover moisture using liquid desicant. Flue gas energy can be utilised for desalination also.
10/05/2010 A: Virendra Kapoor, Petroleum Refining Consultants, vkkapoor9@yahoo.com
Try Ashutosh Garg, Furnace Improvements, Sugarland, TX
16/04/2010 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
I agree. Consider air preheat first. If a convection section is added, check the calculated draft. If not enough draft, consider FD or ID fan. Be sure that any new convection section has a relief valve in the process side system if it is possible to block in the new coil.
16/04/2010 A: Raghavendra Sangam, Libra Techcon Ltd., rsangam0504@yahoo.co.in
Based on the flue gas temperatures you have indicated, it appears there is/are no air-preheater or any such recovery exchanger.
(1) Find out the fluegas composition- average- and determine the acid dew point
(2) You can include a heat exchanger in the combined fluegas duct to recover the remaining heat so that final exit temperature of the fluegas is well above the acid dew point
(3) Evaluate the operating pattern of these heaters- continuous or on-off. Identify the optimum design case
(4) Evaluate space availability and feasibility of introducing additional heat exchangers for heat recovery
(5) If air-pre-heater presently does not exist, check the pressure requirements post-the preheater introduction and implications on blower. If you want to pre-heat the combustion air, consider two types of exchangers with the second one having glass or ceramic tubes
(6) Alternatively check any other stream- such as boiler feed water- is available nearby for heat pick-up.
(7) Check pressure balnce and necessary controls required for combining the flue gas streams and the overall system
(8) Eavluate the benefits of energy saved versus the new investment you are making.
(9) Consult your in-house process design engineer.