Crude heater is twin cabin type which is operated at 367degC for heating crude from preheat-2 to crude distillation column. There are total 8 passes, 4 passes in each cabin.Pass 1-4 in one cabin and rest 5-8 in another cabin.From past 8 months, To minimize the deviation in all passes coil outlet temperature,when flow is increased in pass 5 & 8,its coil outlet temperature increased rather than decreasing and pass 6 & 7 coil outlet temperature reduced when its flow reduced.Also, this effect is observed in skin temperature.
However, it is confirmed by checking temperature of radiation outlet with temperature gun and ensured the deviation.
When flow in passes 5-8 is made equal, deviation in its coil outlet temperature reduced for the same heater box temperature.
However no such issue is found in rest passes i.e. pass 1-4. What can be the possible reason?