Feed Water Heater – Types , Open and Closed Feed Water Heater

FEED WATER HEATER

Low pressure feed water heaters are used in the condensate system between the condensate pump discharge and boiler feed pumps, and utilize low pressure turbine extraction or auxiliary turbine exhaust steam for heating the condensate.
High pressure feed water heaters are used in the feed water system between the boiler feed pump discharge and the boiler, and utilize high pressure turbine extraction steam for heating the feed water. The condensate or feed water temperature increase for each feed water heater will be in the range of 28 to 56 degrees C with the actual value determined by turbine manufacturer‟s stage location of steam extraction nozzles. Depending on turbine size, some turbines offer alternate number of extraction nozzles with usually a choice of using the highest pressure extraction nozzle. The selection, in this case, of the total number of feed water heaters to use should be based on economic evaluation.

Low Pressure Heater(s)
Use one or more low pressure feed water heaters to raise the temperature of condensate from condensate pump discharge temperature to the de-aerator inlet temperature. The heater drains are cascaded from the higher pressure heater to the next lower pressure heater with the lowest pressure heater draining to the condenser.

High Pressure Heater(s)
Use one or more high pressure feed water heaters to raise the temperature of feed water from de-aerator outlet temperature to the required boiler economizer inlet temperature. The heater drains are cascaded from heater to heater, back to the de-aerator in a fashion similar to the heater drain system for the low pressure heaters.

Advantages
(a) Fuel economy.
(b) Longer life of the boiler.
(c) Increase in steaming capacity.

A feedwater heater is a power plant component used to pre-heat water delivered to a steam generating boiler. Preheating the feedwater reduces the irreversibilities involved in steam generation and therefore improves the thermodynamic efficiency of the system. This reduces plant operating costs and also helps to avoid thermal shock to the boiler metal when the feedwater is introduced back into the steam cycle.

Working Of Feed Water Heater :

It should be noted that the energy used to heat the feedwater is usually derived from steam extracted between the stages of the steam turbine. Therefore, the steam that would be used to perform expansion work in the turbine is not utilized for that purpose. The percentage of the total cycle steam mass flow used for the feedwater heater must be carefully optimized for maximum power plant thermal efficiency since increasing this causes a decrease in turbine power output.

Feedwater heaters can also be open and closed heat exchangers. An open feedwater heater is merely a direct-contact heat exchanger in which extracted steam is allowed to mix with the feedwater. This kind of heater will normally require a feed pump at both the feed inlet and outlet since the pressure in the heater is between the boiler pressure and the condenser pressure. A deaerator is a special case of the open feedwater heater which is specifically designed to remove non-condensable gases from the feedwater.

Closed feedwater heaters are typically shell and tube heat exchangers where the feedwater passes throughout the tubes and is heated by turbine extraction steam. These do not require separate pumps before and after the heater to boost the feedwater to the pressure of the extracted steam as with an open heater. However, the extracted steam must then be throttled to the condenser pressure.
Many power plants incorporate a number of feedwater heaters and may use both open and closed components.

Feedwater heaters are used in both fossil- and nuclear-fueled power plants. Smaller versions have also been installed on steam locomotives, portable engines and stationary engines. An economiser serves a similar purpose to a feedwater heater, but is technically different. Instead of using actual cycle steam for heating, it uses the lowest-temperature flue gas from the furnace to heat the water before it enters the boiler proper. This allows for the heat transfer between the furnace and the feedwater occurring across a smaller average temperature gradient. System efficiency is therefore further increased when viewed with respect to actual energy content of the fuel

Open Feed Water Heater (Deaerator)

An open feedwater heater , also called direct-contact and deaerating (DA) heater, is one that heats the feedwater by directly mixing it with bled steam from the turbine. Usually only one DA is used at Power Plant.

Because the pressure in such a heater can’t exceed the turbine pressure at the point of extraction, a pump (Main Boiler Feed Water Pump) must follow the heater. The confluence of steam and water flows makes possible the efficient removal of non-condensables as well as the heating of the feedwater.

The DA outlet feedwater is at or near saturation. Pumping saturated water results in cavitation because of the pressure drop below saturated pressure, thus causing flashing on the back side of pump vanes. The DA heater is therefore usually positioned in the powerplant steam-generator house high above its pump by perhaps 60 ft. This provides sufficient pump inlet pressure to render the saturated water compressed (or subcooled) and prevents cavitations.

There are three types of DA heaters for industrial and utility use.

1) Spray-Type deaerators .

In this type the feedwater enters the heater through nozzles that spray it into the extraction-steam-filled heater space. The water is heated and scrubbed to release the noncondensables gases. A second agitation of the now-heated feedwater by another steam flow is provided by an internal baffling system.

2) Tray-Type deaerators.

Here the feedwater is directed onto a series of cascading horizontal trays. It falls in sheets or tubes from tray to tray and comes into contact with rising extraction steam admitted from the bottom of the tray system. As scrubbing occurs and noncondensables gases and some steam rise, they come into contact with colder water, resulting in a reduced volume of high concentration of noncondensables to vent into the atmosphere.

3) Combination spray-tray deaerator

In this type, the feedwater is first sprayed into a steam-filled space, then made to cascade down trays. This combination type with horizontal stainless steel trays is currently preferred by utilities.