How extraction steam heats feedwater
Extraction steam enters the hot-side passages and condenses as it releases latent heat. Feedwater flows through separate cold-side passages and absorbs that heat. The resulting condensate must drain from the steam side, while non-condensable gases need a suitable venting arrangement. The steam pressure sets its saturation temperature and therefore helps determine the temperature available for feedwater heating.
Why integrate a feedwater heater?
Preheating raises the temperature of water entering later stages of the cycle. This redistributes heat within the plant and can improve regenerative-cycle performance when the extraction arrangement is properly selected. Extraction steam is not a free energy source: the effect on turbine work, boiler duty and plant efficiency must be evaluated together.
Why use a block heat exchanger?
A welded plate pack offers a compact way to provide the required heat-transfer surface. This can be useful when plant modifications leave limited floor space. A duty-specific flow arrangement balances steam distribution, condensation and water-side pressure drop. Designs that permit access through removable panels also support inspection and cleaning of the accessible surfaces.
Part-load operation, drainage and control
Sizing should cover start-up, normal load and turndown. Changing steam conditions can affect the pressure available to discharge condensate. Poor drainage can flood heat-transfer area and destabilize the heating duty. Review steam control, condensate return pressure, venting and the need for assisted drainage as part of the system design.
Selection data for a feedwater-heating duty
Provide water flow, inlet and target outlet temperatures, steam pressure and temperature, available steam flow, backpressure in the condensate return and permissible pressure drops. Include water chemistry and cyclic operating conditions. The actual operating pressure on both sides governs selection; the term low-pressure feedwater heater refers to its role in the cycle and does not replace a mechanical design check.
Frequently Asked Questions
How is a feedwater heater different from a flue-gas economizer?
This application transfers heat from condensing steam to water. A flue-gas economizer recovers heat from boiler exhaust gas; its heat source and equipment arrangement are different.
Can a block exchanger replace an existing shell-and-tube heater?
It may be suitable after checking duty, pressure drop, steam distribution, drainage, controls, materials and installation interfaces. Replacement is not determined by thermal area alone.
Does the power plant electrical rating size the exchanger?
No. Exchanger sizing requires the actual steam and water duty. The plant electrical output is not the heater’s thermal load.