 Heat
Exchangers
If you work with a chemical, electronic or mechanical system where
heat is transferred from one place to another, a heat exchanger
is the best way to optimize that process. Our suppliers include
SGL Acotec who specialize in graphite heat
exchangers, AIC for shell and tube, as
well as shell and coil heat exchangers, and Alfa Laval. As exclusive Ontario representatives of Alfa Laval plate
heat exchangers, we provide solutions for solving Alfa Laval
heat transfer problems.
Heat Exchanger Suppliers
alfa Laval | sgl
| aic
A heat exchanger is a device for transferring heat from one medium
to another. It does this in several ways, each of which has its
own advantages and drawbacks. In each and every case, the transfer
of heat takes place even though the two mediums never mix with one
another. In a typical heat exchanger, one medium, i.e., water, enters
the heat exchanger at a certain place at a certain temperature.
The other medium, also water, enters the heat exchanger from a different
location at a different temperature than the first. Regardless of
what function the heat exchanger fulfills, in order to transfer
heat, the fluids involved must be at different temperatures and
they must come into thermal contact. And in accordance with the
second law of thermodynamics, heat cannot of itself pass from a
colder body to a hotter body. It will always move from the hotter
to the cooler medium.
Kinds of Heat Exchangers
Whether you want to heat a fluid using a hotter fluid, reduce the
temperature of a fluid using a cooler fluid, heat pool water using
solar heat, condense a gaseous fluid using a cooler fluid or boil
water while condensing a hotter gaseous fluid, the right heat exchanger
will help you to do this more efficiently. To pick the right heat
exchanger for your application, you will need to know what kind
of heat exchanger is needed. Heat exchangers are classified according
to their flow pattern and their construction.
Construction: heat exchangers are most often classified
by their construction. Two of the most popular types of heat exchangers
are the shell and tube and the plate heat exchanger.
Shell and Tube Heat Exchangers
Consisting of stainless steel tubes inside a carbon steel shell,
shell and tube heat exchangers are ideal for applications with extremely
high flow rates, temperatures and pressure loads. They are also
come highly recommended for handling contaminated liquids with particles
that would normally block the channels of a plate heat exchanger.
Variations of the shell and tube include U-tube, straight, shell
and coil, spiral tube heat exchangers, and finned tube designs.
Plate Heat Exchangers
Using corrugated plates mounted and fastened together within a frame,
plate heat exchangers are ideal for applications where temperature
and pressure demands are not that high and there is very little
particle debris or build-up in the fluid. There are several variations
of the plate heat exchanger including gasketed plate and frame,
brazed heat exchangers, welded plate, and semi-welded plate heat
exchangers which are differentiated in terms of how the plates are
fastened together, i.e., with welds, gaskets or by vacuum brazing.
Variations of the plate heat exchanger include brazed, gasketed,
semi-welded and welded plate exchangers.
*A new form of heat exchanger includes the shell and plate heat
exchanger which consists of a shell that contains plates instead
of tubes.
Flow Pattern: There are three typical flow arrangements
to consider: parallel, counter and cross flow. Parallel and counter
flow patterns are both in line flow patterns: two fluid streams
flow in the same direction or in the opposite directions.
Parallel
In this heat exchanger, two fluids enter the exchanger at the same
end, and travel in parallel to one another in the same direction
to the other side. In this case, the two fluids enter the unit from
the same end with a large temperature difference. As the fluids
transfer heat from the hotter fluid to the cooler one, their temperatures
start to approach one another.
Counter
In this heat exchanger, two fluids enter the unit from opposite
directions and travel against one another. Because the cooler
fluid exits the counter flow heat exchanger at the end where the
hot fluid enters the heat exchanger, the cooler fluid will approach
the inlet temperature of the hot fluid. Counter flow heat exchangers
are the most efficient of the three types. In contrast to the parallel
flow heat exchanger, the counter flow heat exchanger can have the
hottest cold fluid temperature greater than the coldest-hot fluid
temperature.
Cross
In this heat exchangers, the fluids travel perpendicular to one
another through the exchanger, i.e., one fluid flows through tubes
and the second fluid passes around the tubes at 90° angle. These
units are usually found in applications where one of the fluids
changes state. An example is a steam system's condenser, where the
steam exiting the turbine enters the condenser shell side, and the
cool water flowing in the tubes absorbs the heat from the steam,
condensing it into water.
Comparison
While the fluids in a shell and tube heat exchanger are separated
by a series of finned tubes, the fluids in a plate heat exchanger
are separated by plates with very large surface area. This plate
type arrangement is more efficient than the shell and tube and is
mostly used when temperature and pressure demands are not that great.
Its only weakness is the gaskets themselves. Advances in gasket
technology, however, have made the plate type increasingly practical.
When it comes to flow arrangements, all three flow patters have
their advantages and disadvantages. But of the three, the counter
flow design is the most efficient when comparing heat transfer rate
per unit surface area.
Advantages and Disadvantages
Fouling - one of the most serious problems that you will
face with heat exchangers is fouling. Whenever you use a
natural source of water as your cooling medium, i.e., lake, a lot
of biological and human debris will enter the heat exchanger and
build layers, decreasing the heat transfer coefficient.
Scale - another possible problem is scale, which is chemical
deposit layers such as calcium carbonate or magnesium carbonate.
Plate heat exchangers need to be taken apart to be cleaned, while
shell and tube heat exchangers can be cleaned using acid, bullet
cleaning etc. In a large scale operation, water treatment such as
purification, addition of chemicals, and testing, can minimize fouling.
Corrosion - is a never-ending problem that is solved to some
degree by the type of material used to make the heat exchanger.
Heat exchangers are typically made from steel, titanium, copper,
bronze, stainless steel, aluminum or cast iron. Those made with
stainless steel are usually the most corrosion resistant. It is
important, however, that the grade of stainless steel be capable
of handling the environment in which the heat exchanger operates.
Defined as a ferrous alloy with a minimum of 10.5% chromium content,
stainless steel has higher resistance to oxidation (rust) and corrosion
in most environments. To avoid corrosion, pitting, stress-corrosion
cracking (SCC), and other failures, some heat exchangers are designed
with fins to provide greater thermal conductivity.
Call us today. Tel: 905.940.0961
or by Email: info@valutechinc.com
|
