Chapter_1 Condenser Performance

Chapter 1 Condenser Performance

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Condenser Performance In Power Plants.

Table of content

1. INTRODUCTION................................................................................................................3

2. DEARATION.......................................................................................................................5

3. HOT WELL.........................................................................................................................5

4. CLEANLINESS FACTOR..................................................................................................9

5. CONDENSER WATER CIRCULATION SYSTEMS......................................................11

6. THE OPERATING-POINT DIAGRAM FOR THE PUMPING SYSTEM......................12

7. THE ENERGY-GRADIENT CURVES FOR THE CIRCULATING-WATER SYSTEM13

8. ANALYZING THE PUMP OPERATING POINTS.........................................................14

9. CHOOSING THE TYPE OF INTAKE STRUCTURE AND TRASH RACK TO USE.. 15

Table of Figures

Figure 1.1 Equipment arrangement, schematic...........................................................................2

Figure 1.2 One-pass rectangular surface condenser...................................................................5

Figure 1.3 Average inlet temperature of circulating water, 9 F, United States............................7

Figure 1.4.a- One throughcirculating water system discharges warm water from the condenser

directly to river or sea . fig 1.4.b - Recirculating water system reuses water after it passws

through cooling tower stationary Screen.................................................................................10

Figure 1.5 Operating Point Diagram shows the correct operating range of the circulating water

Pump..........................................................................................................................................13

Figure 1.6 Energy Gradiant Diagram shows the actual system values and valuable in system

design and operation..................................................................................................................15

Figure 1.7 Intake Structure has trash rack , traveling screen , pumps, and crane for dependable

operation of the circulating – water System .............................................................................16

List of Table

Table 1.1 Steam Flow to Condensers*........................................................................................7

Table 1.2 Normal Condenser Pressures and Circulating-Water Temperatures..........................8

Table 1.3 Pressure-Temperature Conversion Table for Water...................................................9

Table 1.4 Typical Small Condenser Proportions......................................................................10

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1. INTRODUCTION

The functions of the condenser are to:

(a) Provide the lowest economic heat rejection temperature for the steam cycle.

(b) Convert the exhaust steam to water for re-use in the cycle.

The associated cooling plant maintains a supply of cooling medium to extract the necessary heat for

these functions. The field cycle which attempted to pressurize exhaust steam without first

condensing it was not an economic proposition. The condensing plant is, therefore, still an

integral part of the cycle and is likely to remain so unless a method is adopted of utilising the 'low.

grade' heat that is rejected to waste. District heating and sea water distillation plant are possible

substitutes for the condenser.

It follows that the main problem in condenser design is to achieve conditions which will ensure

that only the unusable heat in the steam at the turbine exhaust is rejected. Any deviation from

these conditions will cause a decline in efficiency and a very small departure from the optimum

back-pressure, if continuous, add thousands of pounds to the annual cost of running a machine.

Modern condensers are generally of the surface type operating under vacuum. There are limits to

the vacuum which can be economically employed, since the volume of each pound of steam

increases rapidly with decrease in pressure. For a given size of L.P. cylinder, the velocity of the

steam leaving the last row of blades can be such that the loss due to kinetic energy in the exhaust

steam ('leaving loss') outweighs the gain from improved vacuum. Within the limits of 'leaving loss'

the vacuum should be as high as possible, but in practice it is dictated by the size of the

condenser, cooling plant, cooling towers, pumps, culverts, screens, etc. A point would be reached

when the efficiency advantage of better vacuum would be outweighed by the increased cost of the

cooling plant. This is an example of the 'law of diminishing returns'. In addition to its main

functions the condenser is a suitable point at which to reintroduce into the feed water line, drains

from the turbine feed heating and other auxiliary plant and also any boiler feed water make up

which may be required. If used in conjunction with a 'once through' boiler the condenser must be

capable of accepting steam direct' from the steam raising unit. On start-up this steam may be of a

low quality that is unsuitable for use in the turbine. When steam is passed direct to the

condenser it is referred to as 'steam dumping'.

We will cover some points like a description of the construction of condensers and a discussion

of the thermal design and various design considerations. Air extraction equipment is necessary

to remove the air and incondensable gases from the steam space in the condenser and the

considerations governing its design are deal with. The possibility and effect of leakages of air of 17


and cooling water into the steam space are also discussed. The objectives of condenser design

can be summarized as: to condense the exhaust steam while providing

(a)The lowest economic heat sink temperature.

(b)Minimum under cooling of the condensate.

(c)Minimum pressures drop on the steam side.

(d)Effective removal of incondensable in the steam, and air from leakages.

(e)Effective deaeration of condensate.

(f) Effective prevention of water leakages.

All these objectives to be qualified by the following practical considerations:

(a) Economy of overall size, i.e. surface.

(b) Economy of space usage for a given size while still allowing good steam flow, small

Pressure drops, etc.

(c) Economy of cooling water pumping power.

(d) Ease of maintenance and construction.

Figure 1.1 Equipment arrangements, schematic.

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An economical turbine back pressure is from 1.0 to 3.5 in Hg abs. The factors involved in

establishing this pressure are involved and will not be discussed here. An equipment diagram of a

closed power plant cycle is shown in Fig. 1.1, for a condenser to desecrate the condensate, it must

remove oxygen and other no condensable gases to an acceptable level compatible with material

selection and/or chemical treatment of the feed water (condensate). Depending on materials and

treatment, the dissolved O2 level must normally be kept below 0.005 cm3/L for turbine units

operating with high-pressure and - temperature steam.

2. DEARATION

Deaeration in a condenser is accomplished by applying Henry’s law, which states that the

concentration of the dissolved gas in a solution is directly proportional to the partial pressure of

that gas in the free space above the condensate level in the hot well, with the exception of those

gases (e.g., CO2 + NH3) which unite chemically with the solvent. In a condenser droplets of

condensate are continually scrubbed with steam, liberating the O2 and permitting it to flow to the

low-pressure air-removal section, where it is discharged to the atmosphere by the air removal

equipment.

To remove the last traces of O2 from the condensate, an ammonia compound such as hydrazine is

normally added. Free ammonia is liberated in the cycle and is either removed with the non

condensable as a gas or is condensed and retained in the condensate, depending on the detailed

design of the condenser air-removal section. If the ammonia is concentrated as a liquid, it can be

very corrosive to certain copper-base materials.

3. HOT WELL

To make sure that inlet water to condensate pump liquid we must care about NPSH ( Net Positive

Suction Head) , so saturation should be Sub cooled by minimum 2 9 to 3 9 C to avoid Cavitations at

suction line for Condensate.

As per self Explentry Supose the Condenser base Diamention ( L=20 mt , Width = 10 mt ) , and

Condensate Volume 5 m³ , so the Water Level inside shell side will be = 5/200 = 2.5 Cm Not

Chapter 1 Condensers Performance

enought NPSH ( Net Positive Suction Head) for Condensate Pump. So , HotWell is required to

avoid condensate water Head for suction pump.

Size of Hot Well = 120 sec of Steam Flow.

Ex: plant produce steam 600 ton / hour, at condensation we have 600 m³/hour water flow =10

m³/min, =20m³/120 sec=20 m³/2min

So the Volume of Hot Well = 20 m³.

Ratio Between Cooling and Condensed water

m˙steam x Δh = m˙Cw x Cp ΔT cw

Δh =hi – ho

ho : condensated water enthalpy Cp T (≈ 4.18* 30 9 C)= 125 kJ/kg

hi ≈2125 kJ/kg

So if we have 1000 ton steam /hour ,approximately we need 50 000 ton cooling water / hour , this

as per raising cooling temperature 10 9 C only (vary from 7 9 C at summer to 12 9 C at winter), so

now if we thinking to reduce cooling temperature to reduce condensate temperature and reduce P

sat to increase the efficiency at that time we lose too much power for control Cooling temperature

.

Cooling water Pump, we prefer to use three pumps, each working capacity 50 % (2Duty + 1 Stand

by), this will be economical at part load.

The effect of increasing Velocity inside tube

Ex : if we increase V to 2 V , where V : Velocity in side tubes.

If V 1 = 1.5 m/sec it will be V2 = 3 m/sec.

So h (coefficient of heat transfer ) will increase from h 1 = 1 to h 2 = 1.4 kJ/kg

But Δh (pressure Drop) = Fn(V²)

Δh 1= V² Δh 2 = 4 V²

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We can notice that pressure Drop Increasing by 3 times if i duplicate the cooling speed , so we

want to reach to economical speed inside condenser for operation.

Figure 1.2 One-pass rectangular surface condenser.

1 STEAM INLET CONNECTION 9 RETURN WATER BOX 17WATER BOX PASS PARTITION

2 EXTENSION NECK 10 SHELL 18 SPRING SUPPORTS

3 TRANSITION PIECE 11 HOTWELL 19 SUPPORT FEET

4 VENT OUTLET CONNECTION 12 TUBESHEETS 20 SOLE PLATES

5 CONDENSATE OUTLET CONNECTION 13 TUBE SUPPORT PLATES 21 ANTI-VORTEX BAFFLE

6 CIRCULATING WATER INLET OR OUTLET 14 ACCESS OR INSPECTION OPENINGS 22 WATER BOX COVER PLATE

7 Tubes 15 SHELL EXPANSION JOINT 23 WATER BOX DIVISION

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PLATE

8 INLET-OUTLET WATER BOX 16 EXHAUST NECK EXPANSION JOINT

In sizing a condenser, the steam flow and heat rejected to the condenser are obtained from the

turbine heat balance. Table 1.1 gives representative steam flows; heat rejected to the condenser is

approximately 950 Btu/lb of steam for non reheat turbines and 975 Btu/lb for reheat machines .

local water temperature should be used, when known. The number of passes is usually dictated by

the plant arrangement, with total length of water travel and tube diameter dictated by economic

considerations. Normally, small-diameter tubes, single-pass condensers are used where water is

plentiful, and larger-diameter tubes, two-pass condensers when water is scarce. The vacuum, or

back pressure, is determined by economic evaluation, but Table 1.2 gives normal recommended

values for average water temperatures. Table 1.3 is a pressure-temperature conversion table.

Table 1.1 Steam Flow to Condensers*

Table 1.2 Normal Condenser Pressures and Circulating-Water Temperatures

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4. CLEANLINESS FACTOR

A cleanliness factor is applied to the heat-transfer rate of new, clean tubes to allow for gradual

decrease by fouling. To clean the inside surfaces of the tubes, it is necessary to force brushes or

plugs through them individually by the use of water or air jets. To do this, the affected portion of

the condenser must be shut down. As an alternative, recirculating sponge rubber ball or brush

systems are available that can be used during normal operation.

To remove scale formations, sponge rubber balls with abrasive coatings are effective, but their

continued use will shorten tube life.

Major factor in condenser performance is tube cleanliness. Tubes can be plugged with leaves,

marine life, and other debris deposited on the face of the tube sheets. By valving, various

arrangements for back washing, or flushing, are effected to remove these materials.

A standardized cleanliness factor of 85 percent is frequently used, but this can often be misleading

and even erroneous. The fouling is attributable to (1) sedimentation, (2) scaling, (3) steam-side

deposits, (4) corrosion, and (5) biological growth. The fouling is correctly determined by use; in

some cases the cleanliness factor may be 90 percent, whereas in other cases it may never rise

above 75 percent.

Velocities normally used are: for clean water, 7.0 ft/s; for very clean water with cooling towers,

8.0 ft/s; and for seawater, with entrained sand, as low as 6.0 ft/s to minimize erosion. Prevalent

velocities are 6.5 ft/s with aluminum-brass tubes, 7.0 ft/s with admiralty metal, and 8 + ft/s with

stainless steel or titanium. Water-temperature rise is about 10 9 F for single-pass condensers and 15 9

F for two-pass condensers, with a minimum 5 9 terminal temperature difference (TTD).

Approximate general rules for condensers serving turbines rated up to 100 MW. The surface area,

ft2, is equal to the steam flow, lb/h, divided by 10 for single-pass condensers and by 7.5 for two-

pass condensers.

Circulating-water quantity, gal/min, is equal to the area, ft2, for a two pass condenser and is twice

the area for a single-pass condenser. Empty weight of an installed condenser is 5 to 6 lb/ft2 of

surface.

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Table 1.3 Pressure-Temperature Conversion Table for Water

Table 1.1 Typical Small Condenser Proportions

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5. CONDENSER WATER CIRCULATION SYSTEMS

CHOOSING THE TYPE OF CIRCULATING-WATER SYSTEM TO USE

There are two basic types of circulating-water systems used in steam power plants today , the

Once-through systems, Fig.1.4 a, and the Recirculating-water system, Fig.1.4 b. Each has

advantages and disadvantages.

Figure 1.4 a- One throughcirculating water system discharges warm water from the condenser directly to river or sea . Figure 1.4.b - Recirculating water system reuses water after it passws through cooling tower

stationary Screen

In the Once-through system, the condenser circulating water is drawn from a nearby river or sea,

pumped by circulating-water pumps at the intake structure through a pipeline to the condenser.

Exiting the condenser, the water returns to the river or sea.

Advantages of a Once-through system include:

1- Simple piping arrangement.

2- Lower cost where the piping runs are short.

3- Simplicity of operation—the cooling water enters, then leaves the system.

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Disadvantages of Once-through systems include:

1- Possibility of thermal pollution—i.e., temperature increase of the river or sea into which

the warm cooling water is discharged.

2- Loss of cooling capacity in the event of river or sea level decrease during droughts.

3- Trash accumulation at the inlet, reducing water flow, during periods of river or sea

pollution by external sources.

In Recirculating systems use small amounts of water from the river or sea, once the system has

been charged with water. Condenser circulating water is reused in this system after passing

through one or more cooling towers. Thus, the only water taken from the river or sea is that

needed for makeup of evaporation and splash losses in the cooling tower, the only water

discharged to the river or sea is the cooling-tower blow down.

Advantages of the recirculating-water system include:-

1- Low water usage from the river or sea.

2- Little or no thermal pollution of the supply water source because the cooling-tower blow

down is minimal.

3- Remote chance of the need for service reductions during drought seasons.

Disadvantages of recirculating systems include:-

1- Possible higher cost of the cooling tower(s) compared to the discharge piping in the once-

through system.

2- Greater operating complexity of the cooling tower(s), their fans, motors, pumps, etc.

3- Increased maintenance requirements of the cooling towers and their auxiliaries.

The final choice of the type of cooling system to use is based on an economic study which factors

in the reliability of the system along with its cost. For the purposes of this procedure, we will

assume that a Once-through system with an intake length of 4500 ft (1372 m) and a discharge

length of 4800 ft (1463 m) is chosen. The supply water level (a river in this case) can vary

between +5 ft (1.5m) and +45 ft (13. 7 m).

6. THE OPERATING-POINT DIAGRAM FOR THE PUMPING SYSTEM

The maximum cooling-water flow rate required, based on full-load steam flow through the

turbine-generator, is 314,000 gpm (19,813 L/ s). Intermediate flow rates of 283,000 gpm (17,857

L/ s) and 206,000 gpm (12,999 L/ s) for partial loads are also required.

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To provide for safe 24-hour, 7-day-per-week operation of a circulating-water system, plant

designers choose a minimum of two water pumps. As further safety step, a third pump is usually

also chosen. That will be done for this plant.

Obtaining the pump characteristic curve from the pump manufacturer, we plot the operating-point

diagram, Fig.1.5 , for one-pump, two-pumps , and three-pump operation against the system

characteristic curve for river (weir) levels of +5 ft (1.5 m) and +45 ft (13.7 m). We also plot on the

operating-point diagram the seal-well

weir curve. The operating-point diagram is a valuable tool for both plant designers and operators

because it shows the correct operating range of the circulating-water pumps. Proper use of the

diagram can extend pump reliability and operating life.

7. THE ENERGY-GRADIENT CURVES FOR THE CIRCULATING-WATER SYSTEM

Using the head and flow data already calculated and assembled, plot the energy gradient curve,

Fig.1.6 , for several heads and flow rates. The energy-gradient curve, like the operating-point

diagram, is valuable to both design engineers and plant operators. Practical experience with a

number of actual circulating-water installations shows that early, and excessive, circulating-pump

wear can be traced to the absence of an operating-point diagram and an energy-gradient curve, or

to the lack of use of both these important plots by plant operating personnel.

In the once-through circulating-water system being considered here, the total conduit (pipe) length

is 4500 +4800 = 9300 ft (2835 m), or 1.76 mi (2.9 km).

This conduit length is not unusual; some plants may have double this length of run. Such lengths,

however, are much longer than those met in routine interior plant design where 100 ft (30.5 m) are

the norm for ‘‘long’’ pipe runs. Because of the extremely long piping runs that might be met in

circulating-water system design, the engineer must exercise extreme caution during system design

checking and double-checking all design assumptions and calculations.

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Figure 1.5 Operating Point Diagram shows the correct operating range of the circulating water Pump

8. ANALYZING THE PUMP OPERATING POINTS

Using the operating-point diagram and the energy-gradient curves, plot the intersection of the

system curves for each intake water level vs. The characteristic curves for the number of pumps

operating, Fig.1.6 . Thus, we see that with one pump operating, the circulating-water flow is

120,000 gpm (7572 L/ s) at 48.2 ft (14.7 m) total dynamic head. With a weir level of + 5 ft (1.5

m), and two pumps operating, the flow is 206,000 gpm (12,999 L/ s) at 79 ft (24.1 m) total

dynamic head. When three pumps are used at the +5 ft (1.5 m) level, the flow is 225,000 gpm

(14,198 L/ s) at 79 ft (24.1m) total dynamic head.

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With a weir level of +5 ft (1.5 m), and two pumps operating, the flow is 206,000 gpm (12,999 L/

s) at 79 ft (24.1 m) total dynamic head. When three pumps areused at the 5-ft (1.5 m) level, the

flow is 225,000 gpm (14,198 L/ s) at 79 ft (24.1 m) total dynamic head.

Using the sets of curves mentioned here you can easily get a complete picture of the design and

operating challenges faced in this, and similar, plants. The various aspects of this are discussed

under Related Calculations, below.

9. CHOOSING THE TYPE OF INTAKE STRUCTURE AND TRASH RACK TO USE.

Every intake structure must provide room for the following components: (a) circulating- water or

makeup-water pumps; (b) trash racks; (c) trash-removal screens—either fixed or traveling; (c)

crane for handling pump removal or installation; (d) screen wash pump; (e) access ladders and

platforms.

A typical intake structure having these components is shown in Fig.1.7. This structure will be

chosen for this installation because it meets the requirements of the design.

Trash-rack problems are among the most common in circulating-water systems and often involve

unmanageable weed entanglements, rather than general debris.

The type of trash rack and rack-cleaning facilities used almost exclusively in the United States and

many international plants, is shown in Fig.1.7. Usually, the trash rack is inclined and bars are

spaced at about 3-in (76.2-mm). The trash rake may be mechanical or manual. The two usual rake

designs are the unguided rake, which rides on the trash bars, and the guided rake, which runs in

guides on the two sides of the water channel. If the trash bars are vertical, the guided rake is

almost a necessity to keep the rake on the bars. But neither solves all the problems.

If seaweed or grass loads are particularly severe, alternative trash rakes, such as the catenary or

other moving-belt rakes, should be considered. These are rarely put into original domestic

installations. There are many other alternative types of trash racks and rakes in use throughout the

world that are successful in handling heavy Loads , Log booms, skimmer walls, channel

modifications, and specialized raking equipment can sometimes alleviate raking problems.

Traveling screens follow the trash racks. These usually are of the vertical flowthrough type.

European practice uses alternative screens, such as center-flow, dualflow, and drum screens.

Traveling screens may be one- or two-speed. Most two speed screens operate in the range of 3 to

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12 fpm (0.9 to 3.7 m/min) but speeds as high as 30 fpm (9.1 m/min) have been used. Wear is

much greater at higher speeds.

Figure 1.6 Energy Gradiant Diagram shows the actual system values and valuable in system design and operation

Figure 1.7 Intake Structure has trash rack , traveling screen , pumps, and crane for dependable operation of

the circulating – water System .

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Depending on the type of piping used in the circulating-water system - concrete or steel - some

form of cathodic protection may be needed, in addition to the trash racks and rakes. Cathodic

protection is needed primarily when steel pipe is used for the circulating water system. Concrete

pipe does not, in general, require such protection. Since the piping in once-through systems can be

10 to 12 ft ( 3 to 3.7 m) in diameter, use of the cathodic protection is an important step in

protecting an expensive investment. Cathodic protection methods are discussed Later.

Related Calculations. Designing a condenser circulating-water system can be a complex task

when the water supply is undependable. With a fixed-level supply, the design procedure is

simpler. The above procedure covers the main steps in such designs. Head loss, pipe size, and

other considerations are covered in detail in separate procedures later.

Construction of the operating-point diagram and the energy-gradient chart are important steps in

the system design. Further, these two plots are valuable to operating personnel because they give

the design assumptions for the system. When pressures or flow rates change, the operator will

know that the system requires inspection to pinpoint the cause of the change.

The design procedure given here can be used for other circulating-water applications, such as

those for refrigeration condensers, air-conditioning systems, internal-combustion-engine plants,

etc.

Data given here are the work of R. T. Richards, Burns & Roe Inc., as reported in Power

magazine. SI values

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Chapter_1 Condenser Performance

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