Fired Heater Pass Balancing

However, pass temperature balancing can be very lucrative in situations where there is an economic incentive to operate at the highest possible combined heater outlet temperature. If limited by the metallurgy of the passes, then raising all the pass outlet temperatures to the limit would, in this example, allow the combined temperature to be increased by 10 °C. 

B alancing of heaters in coking service should be approached with care. A coked pass will be less efficient in transferring heat from the firebox to the fluid in the pass. Its pass outlet temperature will then be lower. If the objective of pass balancing it to equalise pass temperatures then it will reduce the flow through the pass in order to increase the temperature. This will increase the residence time and accelerate coking. Left unchecked this will shorten run length. 

If the heater is a hydraulic constraint then we can use historical data to quantify the relationship between pass flow and valve position. By comparing average valve position to the maximum permitted we can then estimate the overall increase in capacity utilisation. 

In the case of a coking constraint (and skin temperature constraint if caused by coking) we can perform a calculation similar to that for efficiency savings. Coking is a chemical reaction governed by the Arrhenius Equation which states that rate of reaction (k) is governed by the activation energy (E), the universal gas constant (/ ) and the absolute temperature (T). 

Or we might exploit it by decoking less often. However our calculation assumes that the period that each pass operates above average temperature is the same as the time it spends below. If one pass is consistently the hottest then it will limit run length. Balancing would then increase run length by 24 %, i.e. 100/(100-19.5). 

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Fired heater pass balancing is often installed to equalise pass temperatures in order to improve efficiency. Chapter 10 shows that the fuel saving is negligible and that, in some cases, the balancing may accelerate coking. However there may be much larger benefits available from the potential to debottleneck the heater. 

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