This is the second blog in a three-part series about identifying issues with feedwater heaters. The series discusses why feedwater heaters are important to efficient plant operations, how health of feedwater heaters can be determined, and common level control and feedwater heater problems that occur during operation of the heaters. View the first installment here.

There are two types of feedwater heaters in power plants:  open and closed.  In an open-type feedwater heater such as the deaerator heater, steam and water come into direct contact.  All of the rest of the feedwater heaters are normally closed-type shell-and-tube heat exchangers.  Closed feedwater heater types may be categorized as:

• Straight Condensing
• Condensing with drain cooler (sub-cools liquid after condensing from steam)
• Condensing with drain cooler and desuperheater (removes remaining superheat before condensing, then sub-cools after condensing)

There are three power plant performance indicators relative to heater level and heat transfer performance that should be monitored.  These are:

• Feedwater Temperature Rise
• Terminal Temperature Difference (TTD)
• Drain Cooler Approach (DCA)

Feedwater Temperature Rise

Feedwater Temperature Rise is the difference between the feedwater outlet temperature and the feedwater inlet temperature:

Feedwater Temperature Rise = T_{FW Out} – T_{FW In}

A properly performing heater should meet its design specifications, provided the level controls are working properly.

Terminal Temperature Difference (TTD)

Terminal Temperature Difference, or TTD, provides an excellent indication of the performance of a feedwater heater.  As shown in Figure 1, the TTD is calculated by subtracting the tube-side feedwater outlet temperature from the feedwater heater shell-side steam saturation temperature (at the operating shell pressure):

TTD = T_{Sat Shell} – T_{FW Out}

Figure 1 – TTD Illustrated in a Straight Condensing Feedwater Heater

The temperature of the feedwater leaving the heater may be found on a local temperature gauge and/or on the power plant’s DCS.  The saturation temperature of the steam entering the heater must be calculated by using the shell pressure indication on the local gauge and/or the DCS and using a steam table to look up the saturation temperature for that pressure.  Typical ranges for TTD are as follows:

• Feedwater heater with a desuperheating zone: -3°F to 0°F
• Feedwater heater without a desuperheating zone: 5 to 10°F
• Deaerator: 0°F

An increase in TTD indicates reduced heat transfer.  TTD increases can be caused by several internal issues such as tube sheet bypass, fouling, etc.  A high drain level begins to cover tubes in the condensing section and will also increase TTD.

Drain Cooler Approach (DCA)

For those power plants that have feedwater heaters with a drain cooler section, a second parameter called Drain Cooler Approach (or DCA) can be used to measure feedwater heater performance.  A drain cooler is usually extra tube surface area in the FWH that is baffled off from the rest of the tube bundle so that all of the condensate draining from the FWH flows through the drain cooler.  This allows the condensate draining from the FWH to be used to heat the incoming feedwater.  The condensate is subcooled (cooled to less than saturation temperature) in the drain cooler.  A measure of the effectiveness of the drain cooler is the DCA.  As shown in Figure 2, DCA is calculated by subtracting the temperature of the feedwater entering the heater from the shell condensate drain temperature:

DCA = T_Drain – T_{FW In}

Figure 2 – Feedwater Heater with Drain Cooler

High DCA is the problem to look for.  The DCA is only hurt when the level in the heater shell falls so far that the tubes in the drain cooler are uncovered (there is usually a “snorkel” inlet).  Flashing can then occur that can literally blow up the baffles that separate drain cooler tube area from the rest of the tube bundle.  This will cause the DCS to go up and could also cause tube leaks.

Nothing bad happens to the drain cooler on high level.  If the level in the heater were so high that it covered other tubes, the DCA would probably go down.

In summary:

• DCA is an indicator of level issues in a power plant:
  • An increasing DCA temperature difference indicates the level is decreasing.
  • A decreasing DCA temperature indicates a rise in level.
• TTD is an indicator of heat transfer issues in a power plant:
  • An increase in TTD indicates reduced heat transfer or a water box partition plate leak.

Additional Articles in this series:

Part 1 of Series: Why Do Plants Require Feedwater Heaters?

Part 3 of Series: Level Control and Feedwater Heater Problems