Water Cooling System |
Difference Between Cooling Tower and Spray Pond
In this article explained about Terms
in cooling system, Design specification of cooling system and formula
for Cooling System Efficiency. Finally discussed about difference between spray pond and cooling tower
Design specification of cooling system | Cooling Tower | Spray Pond
Condensing and cooling system plays a major role in
sugar industry process house.
Essential Terms in cooling system
·
Evaporation
·
Dry Bulb
·
Wet Bulb
·
Relative Humidity
·
Approach
Evaporation: In cooling process some quantity of water
evaporated from the circulating water into the atmosphere in cooling system
Dry Bulb temperature
(DBT): The DBT of the inlet or ambient air adjacent to the
cooling tower as measured by a dry-bulb thermometer
Wet Bulb Temperature
(WBT) : The lowest temperature to which the water can be cooled
by contact with the air is the temperature indicated by the wet-bulb thermometer. The
WBT readings are obtained by means of a mechanically aspirated psychrometer.
Relative Humidity: The ratio of the mole fraction of water vapour
present in the air to the mole fraction of water vapour present in saturated
air at the same temperature and barometric pressure is called relative humidity
Approach: The
temperature difference between WBT of incoming air and outgoing temperature
(cooling system outlet water) of water is known as “Cooling tower approach“.
Design specification of cooling systems
Water recirculation quantity calculation in
cooling system of sugar process
To estimate cooling system capacity in sugar
plant boiling house from four sources
1. Evaporator
condenser: Quantity of
vapour goes to condenser from evaporator set last body. Its quantity having
generally in the range of 5
to 8% on cane.
2. Pan condensers: Total
vapour load on condensers form all pans having 20 to 25% on cane. In case of
back-end refinery it will increased upto 25
to 30% on cane.
3. Vacuum filter
condenser: Total load on
condenser including vapour and humid air having 0.8 to 1.2% on cane.
For more information please go through the
below link
Rotary
vacuum filter Equipment capacity details in sugar industry
4. Miscellaneous: Cooling
water will be also used for odd duties such as ejector for vacuum
crystallizers, NCG (non-condensible gases) and condensers common air ejector.
Its total water requirement approximately 100% to 150% on cane
The total quantity of water circulating
through the cooling system is about approximately 40 to 50 times the amount of
vapour condensed. Its quantity depended upon the so many parameters like
condenser efficiency, cooling system efficiency and wet bulb temperature.
For example 5000 TCD (210 TCH) plant cooling
system water required
Vapour Load = 8% ( by evaporators) + 25% (By
pans) + 1% (by vacuum filters) = 34% on cane
Total vapour load on condensers = 210 x 34% =
71.40 T/hr
Water required for vapour condensation =
71.40 x 50 = 3570 t/hr
Water required for ejectors = 210 x 150% =
315 t/hr
Total water required in cooling system = 3570
+ 315 = 3885 t/hr ~ 4000 T/hr
Evaporation Rate in cooling system
The heat load to be removed in the cooling
system is determined from the quantity of water and the change in temperature
Q = M x Cp x ( Ti – To)
Here Q = Quantity of heat load to be removed
in evaporated in Kcal/kg
M = Water recirculation flow rate through the
cooling system in T/hr.
Cp = Specific heat of water in Kcal/kg/oC
Ti = Water inlet temperature to cooling
system ( Before cooling)
T0 = Water outlet temperature to cooling
system ( After cooling)
For example M = 4000 t/hr ( recirculation
flow rate),
Cp = 1 Kcal/kg/oC
Ti = 40 oC
T0 = 32 oC
Q = heat load to be removed in
evaporated = 4000 x 1 x 8 = 32000 Kcal/kg
Latent heat of vaporization of water =
2260 KJ/kg = 540 Kca/kg
W = Quantity of water evaporation = 32000/540
= 59 T/hr
The proportion of the circulating water
evaporated is about 2
to to 2.5% in circulation flow rate.
The cooling is directly dependent on the difference between the water
temperature and the wet bulb temperature, both these
temperatures are major factors affecting the size of the cooling system.
The wet bulb temperature is a function of the
local conditions, and varies significantly during the course of the year.
A smaller quantity of water returning at a
higher temperature may have the same heat duty to be removed as a system a
larger quantity of water returning at a lower temperature.
However the higher temperature system will
require a smaller cooling system because the difference between water and wet
bulb temperature is higher.
Thus it is important to ensure that
condensers have a close approach, returning the smallest quantity of water at
the highest temperature possible.
Cooling System Efficiency
The efficiency of cooling system design
of condenser play a very important role in reducing the consumption of water
and power.
The efficiency “
Ti = temperature of the warm water entering
the cooling system
To = temperature of the cold water leaving the cooling system
Twb= wet-bulb temperature.
Types of cooling system
There are two principal types of cooling
system used in sugar factory
1. Spray Pond
2. Cooling Tower
The water is cooled by contact with cooler
ambient air either in conventional cooling
towers or in spray ponds. Most of the cooling occurs by evaporation
and only a small proportion is by sensible heat transfer. Thus a significant
proportion of the condensed vapour is evaporated in the cooling system.
Spray Pond Cooling System
Introduction: The spray pond cooling is the one of the simplest
method of cooling the condenser water. In spray pond warm water is broken up
into a spray by means of nozzles.
The hot water coming out of condenser is
sprayed through the nozzles to expose maximum surface area of water to air for
effective cooling.
Advantage
a) Pumping cost is low
b) Head required is less
c) Lesser wastage of water
d) Low drift losses
e) Least choking
f) Easy to maintain
g) High Durability and long life
Disadvantages
a) Large area is required. Approximately
25-50 times the area of cooling tower
b) Spray losses due to evaporation and
windage losses
c) There is no control over the temperature
of cooled water.
e) Cooling efficiency is low.
f) Cooling effect reduces with reduced wind
velocity.
g) When load on the plant increases it does
not respond to change.
Cooling Tower
Introduction: The cooling
tower are desired when positive control on the temperature of water is
required, the space occupied by the cooling system is considerable factor and
the plant is situated near the load center.
Higher the surface area, more time of
exposure, lower relative humidity, higher difference between WBT of air and
water inlet temperature and cross flow gives effective cooling and reduced the
tower size.
The cooling tower are mainly divided into two
groups as Natural draft or Atmospheric cooling tower and Mechanical draft tower
as per the air flow through the tower.
Advantages
a) Less floor space area required
b) Induced draft design with fan motor
c) More efficient compare to spray ponds.
Disadvantages
a) The carry over losses which are 0.2-0.6 %
of water flow ,causes corrosion of blades forms the fog around tower with
increase in carry over losses make up water required is also increased.
b) The drift droplets have the same chemical
impurities as the circulating water and the deposition of these chemicals on
object causes potential environmental concern.
c) Cooling tower where sewage effluent is
uses for making up is highly conductive for growth of algae.
d) More maintenance.
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