Regenerative Feed Water Heating System

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Text of Regenerative Feed Water Heating System

RANKINE CYCLE

PRESENTATION ON REGENERATIVE FEED WATER HEATING SYSTEMPresented By:- ASHRANT DASS

DefinitionsMany of the impracticalities of Carnot cycle can be eliminated by super heating the steam in the boiler and condensing it completely in the condenser the cycle that results is called as Rankine Cycle.Working fluid is alternately vaporized and condensed as it recirculates in a closed cycle.Water is typically used as the working fluid because of its low cost and relatively large value of enthalpy of vaporization.The first practical installation was on a 3 MW set in a British power station at Blaydon Burn near New castle-upon-Tyne in 1916.

RANKINE CYCLE

The condensation process is allowed to proceed to completion betweenstate points 4 1 provides a saturated liquid at 1

The water at state point 1 can be conveniently pumped to the boiler pressure at state point 2But the water is not at the saturation temperature corresponding to the boiler pressureHeat must be added to change the water at 2 to saturated water at aWhen heat is added at non-constant temperature (between 2 a), the cycle efficiency will decrease

We know the efficiency is proportional to

The question ishow do we increase efficiency TL and/or TH .Effects of Boiler and Condenser Pressure

INCREASED TH BY ADDING SUPERHEATThe average temperature at which heat is supplied in the boiler can be increased by superheating the steam dry saturated steam from the boiler is passed through a second bank of smaller bore tubes within the boiler until the steam reaches the required temperature

The value of TH, the mean temperature at which heat is added, increases, while TL remains constant.Therefore the efficiency increases. the quality of the turbine exhaust increases, hopefully where x > 0.85

The wetness at the exhaust of the turbine should not be greater that 10% -because this can result in physical erosion of the turbine bladesTo improve the exhaust steam conditions, the steam can be reheated with the expansion carried out in two stepsRANKINE CYCLE WITH REHEAT

The temperature of the steam entering the turbine is limited by metallurgical constraints Newer materials, such as ceramic blades can handle temperatures up to 750 C.

Carnot cycle has efficiency: = 1 TL/TH add QH at as high a TH as possible reject QL at as low a TL as possible The Rankine cycle can be used with a Feed water Heater to heat the high pressure sub-cooled water at the pump exit to the saturation temperature most of the heat addition (QH) is done at high temperatureRankine Cycle with Regeneration

The heating process by means of extraction steam is referred to as being regenerative. The feedwater heaters are an integral portion of the power plant thermodynamic cycle. Normally, there are multiple stages of feedwater heating. Each stage corresponds to a turbine extraction point. These extraction points occur at various stages of the expansion of steam through the turbines. The presence of the heaters in the cycle enhances the thermal efficiency of the powerplant; the greater the number of extraction stages, the lower the amount of thermal energy required to generate a given amount of electrical energy. RELATION TO POWER PLANT CYCLE

The regeneration process in steam power plants is accomplished by extracting (or bleeding) steam from turbine at various stages and feed that steam in heat exchanger where the feedwater is heated. These heat exchangers are called regenerator or feedwater heater (FWH). FWH also help removing the air that leaks in at the condenser (deaerating the feedwater).

Rankine Cycle with Regeneration

Open (DirectContact) Feedwater Heaters-An open FWH is basically a mixing chamber where the steam extracted from the turbine mixes with the feedwater exiting the pump. Ideally, the mixture leaves the heater as a saturated liquid at the heater pressure.Closed Feedwater Heaters-In closed FWH, heat is transferred from the extracted steam to the feedwater without any mixing taking place. Thus; two streams can be at different pressures, since they dont mix.

There are two types of FWHs

14A steam power plant with one open and three closed feedwater heaters.The closed feedwater heaters are more complex because of the internal tubing network, and thus they are more expensive. Heat transfer in closed feedwater heaters is less effective since the two streams are not allowed to be in direct contact. However, closed feedwater heaters do not require a separate pump for each heater since the extracted steam and the feedwater can be at different pressures. Open feedwater heaters are simple and inexpensive and have good heat transfer characteristics. For each heater, however, a pump is required to handle the feedwater.

Most steam power plants use a combination of open and closed feedwater heaters.

Open FWH simple

inexpensive good heat transfer characteristics (Mixing) bring feedwater to the saturation statemore complex (internal tubing)

more expensive

less effective (no mixing)Closed FWH

A feed water heater is a heat exchanger designed to preheat boiler feed water by means of condensing steam extracted (or bled) from a steam turbine. They are unfired since the heat transfer within the vessel does not occur by means of combustion, but by convection and condensation.The steam extraction process in a closed feedwater is referred to as uncontrolled extraction. The flow rate of steam into a feedwater heater is not limited by the amount of available steam (as opposed to a surface condenser, for instance). The shell side operating pressure in a feedwater heater is determined by the pressure of the steam supplied to it, not by the amount of heat transfer surface.FWH TERMINOLOGY

Most feedwater heaters are of a standard shell-and-tube configuration, although some are of header type (mainly outside of the USA). A few employ straight tubes, although the majority uses U-tubes, which are relatively tolerant to the thermal expansion during operation.CONFIGURATION

Materialcarbon steel;0.5 molybdenum steels;1 chromium steels;2 chromium steels;stainless steels;And Thicknesses:up to 160 mm for plates;up to 500 mm for forged tubesheets.common materials for feedwater heaters

Low Pressure Heater: A heater located between the condensate pump and the boiler feed pump. It normally extracts steam from the low pressure turbine.High Pressure Heater: A heater located after boiler feed pump. The steam source is the high pressure turbine.Intermediate Pressure Heater: (if present). A heater located between the booster pump and the boiler feed pump. the steam is extracted from an intermediate pressure turbine.PRESSURE CLASSIFICATION

Horizontal: Most heaters are of this configuration. These are the most stable in regard to level control, although they occupy more floor space. Vertical, : Although these conserve floor space, the amount of control area available for liquid level fluctuation is less. Disassembly is by shell removal. Installation and removal may be more difficult than for horizontal heaters.ORIENTATION

Zones are separate areas within the shell in a feedwater heater.Condensing Zone: All feedwaters have this zone. All of the steam is condensed in this area, and any remaining non condensable gases must be removed. A large percentage of the energy added by the heater occurs here.Subcooling Zone: (Optional) The condensed steam enters this zone at the saturation temperature and is cooled by convective heat transfer from the incoming feedwater.Desuperheating Zone: (Optional) The incoming steam enters this zone, giving up most of its superheat to the feedwater exiting from the heater.ZONES

COMPONENTS

The removal of dissolved gases from boiler feedwater is an essential process in a steam system. The presence of dissolved oxygen in feedwater causes rapid localized corrosion in boiler tubes.it is more economical and thermally efficient to remove these gases mechanically. This mechanical process is known as deaeration and will increase the life of a steam system dramatically.

The Principle of Deaerator

Spray type Deaerator

Material selectionVessel

Steam charging device

Flow baffles

Spray baffle

Sprayer

Carbon steel

Carbon steel (principal) Stainless steel (erosion, high water content)Carbon steelCarbon steelStainless steel

The Principle of DeaeratorGas solubility in a solution decreases as the temperature of the solution rises and approaches saturation temperature.Spraying feedwater in thin films increases the surface area of the liquid in contact with the steam, which, in turn, provides more rapid oxygen removal and lower gas concentrations. This process reduces the solubility of all dissolved gases and removes it from the feedwater. The liberated gases are then vented from the deaerator.

Another widely used oxygen scavenger ishydrazine(N2H4)

FEED WATER HEATER PERFORMANCEDepends upon two parameters:-

1. Terminal temperature difference (TTD)

it is the difference of saturation temperature of extracted steam and feed water temperature leaving the heater.

2. The final temperature of feed water leaving the heater.

CondenserBlock Diagram of A Large Steam Turbine

Reheat SteamHP

Main SteamSteam for Reheating

IP

LPLP

CFWH 6CFWH 5OFWH 4CFWH 2CFWH 1CFWH 3

Air Blanketing / Improper venting

Water side contamination of the tubes

steam side contamination of tubesFACTORS AFFECTING FEED HEATER PERFORMANCE

Air get into LP heater while in service below atmospheric steam pressure.In HP heater air can get in while the un