Pass Balancing with Flow/Temperature Controllers
If the flows to the heater passes are single phase, then they can be measured with flow meters. These meters can be used as part of flow control loops that balance the passes.
Figure 600-1 illustrates the principles of a pass-balancing flow/temperature controller arrangement.
The total flow through the furnace is the sum of the individual pass flows. The operator adjusts the set points of the pass flow controllers with the HIC. The pass outlet temperatures are averaged to determine the set points of the pass outlet temperature controllers. Each pass temperature controller trims the flow set point for that pass, raising the flow if the temperature is too high and lowering the flow if the temperature is too low. This tends to raise the low temperature and lower the high temperatures, so that the outlet temperatures converge toward the average pass outlet temperature. There are several constraints that should be considered in the detailed application of these principles to an actual fired heater, including the following:
• Minimum pass flow allowed, which reduces the chance of stalling the flow in a pass if the total flow rate to the heater is reduced
• Minimum and maximum pass valve openings, which keep the controls from driving the pass flow valves beyond a reasonable operating range
• Minimum and maximum ratios of individual pass flow to average pass flow, which keep the controls from driving the heater toward very unbalanced flows (If the pass flows were very far apart, a drastic cut in total flow might stall flow in a pass.)
• How to handle failed transmitters
• How to handle control tuning constants