The Condensate Feedwater System Part 2 Closed feedwater

The Condensate. Feedwater System Part 2 Closed feedwater heater Open feedwater heater Boiler makeup and treatment Assoc. Prof. Sommai Priprem, Ph. D. Department of Mechanical Engineering Khon Kaen University ��. �������

6 -5 Closed Feedwater Heater • • • To increase plant efficiency 2 -types of feedwater heater 1. Closed type: surface or shell-and-tube type. 2. open or direct-contact or deareating type Large Powerplant may has 5 -7 closed-FWH + 1 open-FWH Closed type FWH acting like Condenser but subject to high pressure • classify: Low pressure LP FWH and High pressure HP FWH. • If bled steam id superheat FWH has 3 -sections: Desuperheating zone, Condensing zone, and Subcooling (or Drain-cooling) zone. ��. ������� 2

T Subcooling zone Condensing zone Desuperheating zone Steam d e e F te wa r Path Length ��. ������� Temperature-Path length diagram for a 3 -zone feedwater heater 3

Tubes and Materials Closed type FWH are subjected to higher pressure than a condenser, therefore their tubings have to be designed to withstand such a pressure. ��. ������� 4

6 -6 Closed Feedwater Heater Calculations Heat Transfer ��. ������� 5

Tf = Ts – 0. 8 ΔTm (6 -13) where Tf = average film temperature, o. F. Figure 6 -15 Overal heat-transfer coefficient for condensing zone only as a function of average film temperature as defined by eqn. (6 -13) ��. ������� 6

Pressure drop • Usually large because of flow friction in long small-diameter tubes. • Calculation of Pdrop for design of Feedwater pump ��. ������� 7

6 -7 Open Feedwater Heaters • • Also called direct-contact and deaerating (DA) heater. Bled steam from turbine is directly mixed with feedwater. There are 3 -type 1. Spray type deaerators 2. Tray-type deaerators 3. Combination spray-tray deaerator ��. ������� 15

6 -8 Boiler Makeup and Treatment • • Circulating water loss ~ 0. 5 -1. 5% mainly due to: leakage, deaeration, blowdown Makeup water need to be well treated to prevent: – deposition of suspened solids and scale on boiler surface – silica deposition and corrosion damage to turbine blades and condensate-feedwater system. ��. ������� 17

A water makeup system: • Pretreating raw water • Demineralizing system • Condensate-polishing system (for once through boiler, for drum type boiler using ‘Blowndow’ Raw water impurity includes: suspended solid, turbidity, organics, hardness (calcium and magnesium), alkalinity (bicarbonate, carbonates, hydrate), dissolved iron(sodium, sulfate, chloride), silica, dissoved gases (O 2, CO 2) ��. ������� 18

Pretreatment well water filtration Surface water: Clarification softening Filtration Hardness = HCO 3 - of Ca or Mg Softening by lime, Ca(OH)2 + Ca(HCO 3)2 2 Ca. CO 3 + 2 H 2 O 2 Ca(OH)2 + Mg(HCO 3)2 2 Ca. CO 2 +Mg(OH)2 + 2 H 2 O 2 Ca(OH)2 + Mg. SO 4 Mg(OH)2 + Ca. SO 4 Add soda ash, Na 2 CO 3+ Ca. SO 4 Ca. CO 3 + Na 2 SO 4 ��. ������� 19

Demineralizing: remove dissolved solids by iron exchange. using Rasin : Catiron rasin (+) attached (-) polymer --- cation Aniron rasin (–) attached (+) polymer --- anion ��. ������� 20