Condensate -THERMAL POWER PLANTS

1. Condensate
ASHISH KUMAR JAIN

2. Steam utilisation in various
sections in pulp & paper mill
• Digesters - preheater
• Bleaching - extraction stage
• Paper machine
– Press section - hot showers
– Dryers
• Evaporators
• Deareators

3. Condensate
• Condensate is simply water, formed in the
distribution system because of
unavoidable heat losses.
• It also forms in the heating equipment
because of the heat transfer from the
steam to the substance heated.
• All of the condensate must be removed
from the distribution system and the heat
transfer equipment.

4. Effects of condensate
• Condensate in the heat transfer unit
takes up space and,in effect, reduces
the physical size, and therefore the
capacity of the equipment.
• Condensate must be removed promptly
so the heat transfer unit (heating coil,
radiator, heat exchanger, etc.) can be
filled with steam.

5. • The distribution system needs to be drained as well.
• Condensate that is permitted to collect in pipes is
blown into waves by steam passing over the
condensate.
• Steam moves very fast in mains and supply lines.
The waves will block steam flow at some point in a
line. Condensation occurs after that point, causing a
pressure differential that accelerates the slug of
water, turning the slug into a battering ram when it
encounters an ell, valve, orifice plate, etc., that can
damage fittings of valves. This water hammer is not
only annoying, but actually can be dangerous.

6. AIR & CO2 entrapment
• Air is always present in a system on
start-up, and is in boiler feed water.
CO2 is released from dissolved
carbonates in boiler feed water.
• Air and CO2 must also be removed.

7. Effect of Air
• Air is a wonderful insulator, however, it
is undesirable in a steam heating
system.
• As steam condenses, air “plates out” on
heat exchanger surfaces. One-half of
one percent by volume of air in steam
reduces heat transfer efficiency by 50
percent!

8. Effect of CO2
• CO2 can dissolve in condensate to form
carbonic acid that will eat pipes and
equipment.
• 02 in air causes pitting and speeds
corrosion.

9. • Condensate, air, and CO2 should be
removed as quickly and completely as
possible by a steam trap, and not allow
steam to be wasted.
• A specific application may require a less
efficient trap, but most of the time a trap
that can accomplish all the
requirements can be used.

10. • Condensate carries sensible heat with it,
which can be used to heat process water or
• the flash steam can be recovered for re-use.
• Cost of condensate -

11. Evaporators
• Clean condensate
– taken back as feed water to deareator
• Dirty condensate
– contaminated with black liquor droplets

12. PAPER DRYING
Low pressure steam used to evaporate water in dryers.
Drying can be divided into four phases

13. OBJECTIVES OF PAPER DRYING
 Must efficiently evaporate water with least amount of equipment
 Sheet quality should be maintained. No cross direction variation
 Minimum energy usage. Ideal figure is 2960KJ/kg water evaporated
 Must be designed to support overall M/C efficiency

14. BASIC DRYING THEORY
The transfer of heat from steam into the paper web while on drying cylinder
Evaporation of water from the paper web into air during draw between the
cylinders.
Heat Transfer
Q = UA (Ts - Tp )
where Q = Rate of heat flow from steam to paper (KJ/h)
U = Overall heat transfer coefficient
A = Dryer surface area in contact with paper web (m2)
Ts = Steam temperature (C )
Tp= Paper Temperature (C )

15. LIMITATION OF STEAM TEMPERATURE AND PRESSURE
 Practical limitation of high pressure used
 Cannot use high pressure in wet end dryers
 High pressure causes fiber on outer surface of sheet to stick to dryer
surface
 Poor runnability
General rule is that " higher the printing requirements, lower the wet end
steam pressure and more gradually increase the steam pressure"
With fixed number of dryers only two variables left that can be directly
controlled
Saturated steam Temperature (Ts)
Over all heat transfer coefficient (U)

16. Over all Heat transfer coefficient
Measure of resistance to heat transfer
 Condensate layer thickness in drying cylinders
 Dryer shell material
 Scale or fiber built-up on outer surface of dryers
 Thin air film between sheet and cylinder
 Sheet properties like water content, thickness, surface roughness, porosity etc.
 Non- condensable in dryers

17. EVAPORATION
EVAP = KA( Ps - Pa)
Where EVAP = water evaporated from the sheet (kg water/h)
K = Mass transfer coefficient
A = Evaporation area determined by length of draw (m2)
Ps = Vapor pressure of water in sheets (Pa)
Pa = Partial pressure of water vapor in air surrounding
the sheet
Ps is directly related to the temperature of sheet
Pa is directly related to the absolute humidity of air.

18. EFFECT OF DRYING VARIABLES ON THE DRYING
EFFICIENCY
Variable % Influence
1. Condensate Removal 30%
2. Furnish grade and sheet 25%
Properties
3. Dryer fabric design, Permeabilitty 20%
& Tension
4. Pocket Ventilation 15%
5. Hood and dryer air system 5%
6. Others 5%

19. DRYER ENERGY CONSUMPTION
Dryer section uses steam for sheet heating, evaporation, air
heating, non- condensable bleed and venting.
Typical Dryer Energy Consumption
Consumer Heat consumption
(KJ/kg of water evaporated)
1. Sheet Heating 200
2. Evaporation 2300
3. Air Heating 420
4. Non condensable bleed 45
5. Venting 0

20. STEAM AND CONDENSATE SYSTEM
Condensate layer effect on drying

21. DRYER SYPHONS
Used to remove the condensate from the dryers
Rotary syphon Stationary syphon

22. Main difference between a rotary and stationary syphon is the
amount of differential pressure needed to overcome the centrifugal
force in a rotary syphon
Extra differential is needed to overcome the centrifugal force.
Differential pressure requirement is 40 kPa to 95kPa
Stationary Syphon
Better than rotary syphon
Does not have to overcome the centrifugal force
Differential pressure & blow through steam requirement is low
Differential pressure requirement is 35 - 40 kPa
Most suitated for high speed machines
Mechanical maintenance is more

23. DRYER BARS
 Meant for machine speeds over 900m/min
 Series of flat bars running lengthwise, mounted on the dryer
surface
 Increase turbulence in condensate layers increasing heat transfer
rate
Dryer bar Improvement Potential
Machine speed (m/min) Drying capacity improvement
150 -365 0
365 - 460 0 - 5
460 - 610 3 - 10
610 - 760 5 -12
760 - 915 10 - 15
915+ 12 - 20

24. CASCADE SYSTEM