Heat exchange
The
cooling water system in any process plant is vital to keep the process going
efficiently. Just as with electricity, gas or steam, which may be needed to
operate parts of the plant, the cooling system must be reliable and monitored
constantly.
As a plant
operator, you are the first line of defense when problems arise, such as
corrosion, scale, or biological contamination. As a plant operator, you can take
many steps to stop these problems arising at all .... that's what this is all about.
Every
plant, whether for manufacturing, process cooling, or air conditioning has a
specific operating temperature. Correct cooling is essential as overheating
costs money and wastes energy. In some cases it can also be dangerous.
The most
common way of removing excess heat is by water. Water is readily available, it
covers three quarters of the earth's surface.
It is
easily handled and is capable of removing large quantities of heat
cost-effectively. Kilogram for kilogram it can hold more heat than almost any
other liquid.
Heat
exchange
The way we
use water to take away unwanted heat is by the use of a "heat exchanger".
Regardless
of the type of heat exchanger in use at your plant, the cooling process is
basically the same.
In a
shell-and-tube heat exchanger (fig. i), cold water enters the shell from the
pipeline and is passed through a series of tubes.
The fluid
to be cooled is pumped into the shell and circulates over and around the tubes
containing the cold water. The heat naturally transfers through the tubes into
the cooler water and is carried away through the outlet to the pipeline.
The fluid,
now cooler, leaves the exchanger to continue its process. Some systems work the
other way around: the fluid to be cooled is confined in the tubes and the water
flows around the shell.
Sometimes,
also, the water carrying the heat away is to be used again. So it must pass
through a cooling tower - or cooling pond - where it is cooled by evaporation
before being recirculated back to the heat exchanger to begin the process again.
In open
evaporating systems, water is cooled in two ways: firstly by what is called
 "radiated
heat losses" and secondly by "evaporation heat losses". When water is cooled
naturally by heat transfer to the cooler air, we speak of "radiated heat
losses".
On the
other hand, as the warm water comes in contact with the cooler, drier air,
evaporation takes place and some of the water converts into vapor. When a
substance changes from a liquid into a vapor it requires heat-energy. It takes
this heat from the water and thus the remaining water has a lower temperature.
In
a cooling tower, 80 to 90 percent of the cooling is the result of evaporation.
Only 10 to 20 percent is the result of basic radiated heat loss.
Because of
this, cooling ponds are designed with large surface areas. Sprays are sometimes
added to aerate the water and increase the evaporation.
In cooling
towers, the water to be cooled is distributed and cascaded over a "fill"
section. This reduces the bulk water flow into small droplets, increasing the
surface area of water in contact with the air and improving evaporation still
further.
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