Making power more efficiently

Improving control of feedwater heaters improves heat transfer and reduces operating cost.


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.

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. 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?

This post was written by Bruce Brandt, PE, DeltaV Technology Leader at MAVERICK Technologies, a leading system integrator providing industrial automation, operational support and control systems engineering services in the manufacturing and process industries. MAVERICK delivers expertise and consulting in a wide variety of areas including industrial automation controls, distributed control systems, manufacturing execution systems, operational strategy, and business process optimization. The company provides a full range of automation and controls services – ranging from PID controller tuning and HMI programming to serving as a main automation contractor. Additionally MAVERICK offers industrial and technical staffing services, placing on-site automation, instrumentation and controls engineers.

No comments
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by...
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
Integrated mobility; Artificial intelligence; Predictive motion control; Sensors and control system inputs; Asset Management; Cybersecurity
Big Data and IIoT value; Monitoring Big Data; Robotics safety standards and programming; Learning about PID
Motor specification guidelines; Understanding multivariable control; Improving a safety instrumented system; 2017 Engineers' Choice Award Winners
This digital report will explore several aspects of how IIoT will transform manufacturing in the coming years.
Motion control advances and solutions can help with machine control, automated control on assembly lines, integration of robotics and automation, and machine safety.
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

Mobility as the means to offshore innovation; Preventing another Deepwater Horizon; ROVs as subsea robots; SCADA and the radio spectrum
Future of oil and gas projects; Reservoir models; The importance of SCADA to oil and gas
Big Data and bigger solutions; Tablet technologies; SCADA developments
Automation Engineer; Wood Group
System Integrator; Cross Integrated Systems Group
Jose S. Vasquez, Jr.
Fire & Life Safety Engineer; Technip USA Inc.
click me