heat exchangers biofuels

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Selecting the correct type of heat exchanger maximises efficiency and enables future
modifications to be made.

Still going strong!

he vast majority of products

produced globally today require
either heating or cooling at some
point in their manufacturing
process. Ethanol production is no
exception. In an effort to keep production
costs at a minimum, it is critical to
understand and utilise the proper, most
efficient form of heat transfer available
for the application at hand. When this
is accomplished, excessive costs
for heat transfer equipment, utilities,
replacement parts, lost production,
and maintenance can be eliminated.
In the North American market place,
the British Thermal Unit (BTU) is used as
the standard unit for measuring energy.
One (1) BTU is the amount of energy
required to heat one pound of water by
1F. This equates to 1055.06 joules or
251.99 calories. Heat is a form of energy,
and the laws of thermal dynamics tell
us that the amount of heat put into or
taken away from a product (such as
ethanol) requires the same amount
of energy on the adjacent side (with
the exception of minimal losses). The
efficiency level at which a heat exchanger
transfers heat can be calculated and
expressed by its overall heat transfer
coefficient or U-Value. The square feet
of heat transfer surface required to
perform a specific duty is dependent
upon the heat exchangers U-Value.
Overall heat transfer coefficient U-Value
U=1
1/ho+X/K+1/hi
Where:
hi = inside fluid film heat transfer
coefficient
ho = outside fluid film heat transfer
coefficient
X = thickness of the plate material
K = thermal conductivity of the plate
material
Required heat transfer area A
A=

Q
U X T

Where:
A = Heat Transfer Surface Area (sq. ft.)

biofuels international

Q = BTUs
U = Overall heat transfer coefficient
T = LMTD (log mean temperature
difference)
Selecting the right type of heat
exchanger
There are many different types of heat
exchangers on the market today. Each
has its own unique set of advantages and
disadvantages that must be considered to
ensure a proper selection for the specific
application at hand. With the exception
of some evaporation applications,
gasketed plate heat exchangers are
used predominately in the production
of ethanol as they provide the highest
efficiency ratings of all other choices.
They are also easily modified for future
capacity expansion, easily opened for
cleaning and inspection, and take up a
fraction of the floor space required by
conventional shell and tube designs.
While efficiency is a major consideration
in the type of heat exchanger to select,
first and foremost is to consider the heat
exchanger as a material handling piece
of equipment. This is critical because
in the ethanol production process not
all process fluids are clear in nature. In
fact, throughout much of the process
the fluid contains particulates which can
plug some types of heat exchangers.
Fortunately today, plate type heat
exchangers can
provide wide
channels to pass
particulates, yet
facilitate high
turbulence leading
to higher U-Values
than conventional
shell and tube
designs. They are
also capable of
design pressure
and temperature
ratings suitable for
the requirements
of the ethanol
process.
Standard gap
plate designs

A standard gap plate heat exchanger is

the most efficient type of heat exchanger
available today for fluids that are clear
in nature, and carry no particulates or
fibres. It consists of a series of thin gauge
metal plates (typically made of stainless
steel) embossed with a wave pattern
to induce turbulence. The plate design
incorporates a rubber gasket (Nitrile,
EPDM material) around the perimeter
which provides a sealing surface between
plates. When stacked to create a plate
pack and compressed, they produce
narrow flow channels. Typical plate
pressing depths are 4mm or less. At
proper compression, these designs
will have numerous metal-to-metal
contact points that allow the efficient
thin gauge plates to handle design
pressures up to approximately 450 PSI.
Plate designs
A wide gap plate heat exchanger is the
most efficient type of heat exchanger
available today for fluids that carry
solids or particulates. These plate heat
exchangers typically are similar to the
standard gap plates herringbone or
washboard design but have wider flow
channels with pressing depths up to
12mm, and fewer metal-to-metal contact
points. Wide gap plates are available
today with design pressure ratings of
up to approximately 180 PSIG, and

The NEW GEA Model NW350 voted

highest in its class for design
pressure rating

september/october 2014 57

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biofuels heat exchangers

are used for the heating and cooling of
beer mash and fermentation coolers.
A free flow plate heat exchanger is the
most efficient type of heat exchanger
available today for fluids that carry
large solids, particulates and/or fibres.
These plate heat exchangers have wider,
consistent flow channels with pressing
depths up to 10mm, and even fewer
metal-to-metal contact points than wide
gap plates. Free flow plates are available
today with design pressure ratings up
to approximately 150 PSIG, and are
particularly well suited for heating and
cooling applications containing a high
percentage of fibre content, or long
fibres that can become lodged at the
contact points of standard and wide gap
plate designs. If fibres are entrained in a
carrier fluid that has a low viscosity, it is
important to design the heat exchanger
with higher velocities over the plate
surface to avoid matting or settling.
The free flow plate design would be a
good plate design to consider for a fibrous
cellulosic ethanol production process.
Considerations
In a standard heat transfer application,
the process fluid is heated or cooled
using a secondary media like water or
steam. Regenerative heating and cooling
allows producers to benefit from the
energy already put into their product to
pre-heat an incoming stream. These heat
exchangers are typically referred to as
interchangers. The use of interchangers
is an excellent way to reduce the overall
BTU/hr requirements of the process.
Application engineers must design the
heat exchanger with adequate velocity
across the plate surface to ensure
turbulent flow and uniform usage of the
entire plate surface. Velocities across a
standard gap plate will typically range
from 1-3 feet per second for most nonviscous applications. Applications sized
in a wide gap or free flow plate may
require plate velocities in the 3-5 feet
per second range in order to provide
turbulent flow and keep particulates and
fibres in suspension. Generally speaking,
high velocity designs will provide the
user with a more thermally efficient heat
exchanger and longer run times between
cleaning; however, as velocity increases,
pressure loss will also increase requiring
larger pumps and higher pumping costs.
In addition to the velocity and
associated pressure losses across the
plate surface, the velocity through the
port of the plates themselves must
also be considered. Excessive port

Erosion as a result of excessive port velocity

velocity can cause erosion of the thin

gauge plate material and lead to costly,
premature plate pack replacement.
Keeping port velocities less than 16 feet
per second should minimise this risk.
The application engineer must select a
heat exchanger model that has a large
enough port diameter to accommodate
the prescribed flow capacity, providing
reasonable port velocity, yet still
maintain adequate plate velocity.

Most recently, a customer in the

US Midwest corn-belt has seen the
advantage and replaced an existing
fermenter cooler. The new NW350 was
able to reduce velocity in the port area
from in excess of 25 feet per second
to less than 11 feet per second. Due
to the longer thermal length of the new
plate, the new fermenter cooler now has
30% fewer heat transfer plates reducing
maintenance costs even further.

Case study: Plate erosion due to

excessive port velocity

The future

Many ethanol producers are running

today at higher capacities than the
original plant nameplate design in an
effort to maximise profit dollars. Also,
plant expansions have been increasing in
the last few years, and rather than replace
older heat exchangers with borderline
port diameters, plants have simply added
additional heat transfer area (more plates).
While this scenario will save money
upfront, it is expensive to continuously
replace plate packs that are destroyed
by excessive flow velocity. Depending on
the size and quantity of plates in these
heat exchangers, replacing a plate pack
could cost $50,000 (37,500) or more.
Engineers at GEA PHE Systems have
designed Free Flow and Wide Gap Plates
with large port diameters (up to 14)
that are well suited for plants with a 100
million gallon per year capacity. The latest
addition to this family of plates is the
NW350 which sports 14 ports and the
highest design and test pressure rating
in the industry. It also utilises a heavy
duty gasket design for extended life.

As energy costs increase, and cost

savings takes a higher priority,
heat exchanger manufacturers
will continue to research and
develop more efficient designs.
Future improvements will likely come in
the area of enhanced embossing pattern
designs to increase turbulence. One
would speculate that narrower channels
will also become more attractive for those
fluids that are clear in nature. In addition
to more efficient materials and plate
designs, newer, more thermally efficient
heat transfer fluids will also be developed.
One thing is for certain: there will
always be a need to heat and cool liquids
in the most cost-effective manner. The
plate and frame heat exchanger has been
this method for the past 80 years. l

For more information:

This article was written by Steven Rooks, aftermarket
manager for the North American market, GEA PHE
Systems. Visit: www.gea-heatexchangers.com

58 september/october 2014 biofuels international