It is a common practice in the electric utility industry to utilize multiple stages of shell and tube heaters to preheat feedwater going to the boiler. The challenge has always been transferring as much heat as possible from the steam inside the shell to the water in the tubes. The mechanical design of the heaters helps this by providing baffles to prevent steam from exiting the heater rather than condensing. Heaters cascade from the highest shell pressure being the last in the train to the lowest being the first. There is also one heater called a deaerator that is an open heater between the low-pressure and high-pressure closed heaters where steam mixes with feedwater and any entrained air is removed. The steam for heating feedwater comes from extraction stages of the turbine and from the drain of the next higher-pressure heater.
At first blush, this seems to be a mainly mechanical, self-regulating system, but there are control challenges. First, this is a highly interactive system with the pressure in the shell being a function of the extraction stage of the turbine, the rate of condensation, and the rate of what’s coming from the next higher heater’s drain. The feedwater’s temperature increase is dependent on absorbing heat being released by the condensing steam, which is a function of the level in the heater shell. The more tubing surface area exposed to steam, the more heat can be transferred. A flooded heater will transfer very little heat to the feedwater, and an uncovered drain will let steam pass without condensing fully, so controlling feedwater level in the heater shell is critical.
One additional issue is that not all of the steam is coming from the turbine extraction. Some comes from the drain of the next higher heater. As the heater drains, its condensate passes through a control valve where it flashes back to steam on its way down to the next heater. The control valve must not allow pressure in the drain line to drop below the pressure of the turbine extraction feeding the heater. If the drain valve outlet pressure gets too low, the heater being drained will back up. This is an application where valve sizing and performance is critical. Now multiply these complexities by six or seven stages of heating, and you begin to get the magnitude of the control problem.
Control engineers today have some arrows in their quivers that weren’t available when I first started out. For one thing, controllers today have the ability to do their own internal efficiency calculations. For example, a drop in the efficiency of a heater can indicate fouling in the tubes. In older systems, a problem like that would only have showed up after progressing to the point of being a major issue. Some current DCSs have built-in function block libraries covering thermodynamic properties of water and steam, so doing a real-time enthalpy balance around a heater becomes trivial. We can build a multiple-in-multiple-out (MIMO) predictive model running in the controller that can coordinate a group of operating heaters as a set rather than individual units to address interactivity. The bottom line is, with these advances, we can improve on efficiency gains from using heater systems and maintain them better over time.
What other areas of efficiency improvement have you seen?