CCB 2092 Unit Operation Lab 1Jan 2014Group Project Plate Heat
Exchanger Group:1
Group Members:1. ALI ABDULLAH SALEHAL-YAFEAI(17036)
2.AMIRA NURAIN BINTI RAMDAN(18624)
3.AZLAN BIN RAMALI (18645)
4. CHEN SWEE KEAT(18604)
Lab Instructor :Dr. Maziyar Sabet
Date of experiment:6th April 2014
INTRODUCTIONA plate heat exchanger is a type of heat exchanger
that uses metal plates to transfer heat between two fluids. This
has a major advantage over a conventional heat exchanger in that
the fluids are exposed to a much larger surface area because the
fluids spread out over the plates. This facilitates the transfer of
heat, and greatly increases the speed of the temperature change.
Plate heat exchangers are now common and very small brazed versions
are used in the hot-water sections of millions of combination
boilers. The high heat transfer efficiency for such a small
physical size has increased the domestic hot water (DHW) flowrate
of combination boilers. The small plate heat exchanger has made a
great impact in domestic heating and hot-water. Larger commercial
versions use gaskets between the plates, smaller version tend to be
brazed.The plate heat exchanger (PHE) is a specialized design well
suited to transferring heat between medium- and low-pressure
fluids. Welded, semi-welded and brazed heat exchangers are used for
heat exchange between high-pressure fluids or where a more compact
product is required. In place of a pipe passing through a chamber,
there are instead two alternating chambers, usually thin in depth,
separated at their largest surface by a corrugated metal plate. The
plates used in a plate and frame heat exchanger are obtained by one
piece pressing of metal plates. Stainless steel is a commonly used
metal for the plates because of its ability to withstand high
temperatures, its strength, and its corrosion resistance. The
plates are often spaced by rubber sealing gaskets which are
cemented into a section around the edge of the plates. The plates
are pressed to form troughs at right angles to the direction of
flow of the liquid which runs through the channels in the heat
exchanger. These troughs are arranged so that they interlink with
the other plates which forms the channel with gaps of 1.31.5 mm
between the plates.
Figure 1: Schematic conceptual diagram of a plate heat
exchanger
The plates produce an extremely large surface area, which allows
for the fastest possible transfer. Making each chamber thin ensures
that the majority of the volume of the liquid contacts the plate,
again aiding exchange. The troughs also create and maintain a
turbulent flow in the liquid to maximize heat transfer in the
exchanger. A high degree of turbulence can be obtained at low flow
rates and high heat transfer coefficient can then be achieved.A
plate heat exchanger consists of a series of thin, corrugated
plates which are mentioned above. These plates are gasketed, welded
or brazed together depending on the application of the heat
exchanger. The plates are compressed together in a rigid frame to
form an arrangement of parallel flow channels with alternating hot
and cold fluids.
Figure 2: An individual plate for a heat exchangerAs compared to
shell and tube heat exchangers, the temperature approach in a plate
heat exchangers may be as low as 1 C whereas shell and tube heat
exchangers require an approach of 5 C or more. For the same amount
of heat exchanged, the size of the plate heat exchanger is smaller,
because of the large heat transfer area afforded by the plates (the
large area through which heat can travel). Increase and reduction
of the heat transfer area is simple in a plate heat-exchanger,
through the addition or removal of plates from the stack.All plate
heat exchangers look similar on the outside. The difference lies on
the inside, in the details of the plate design and the sealing
technologies used. Hence, when evaluating a plate heat exchanger,
it is very important not only to explore the details of the product
being supplied, but also to analyze the level of research and
development carried out by the manufacturer and the
post-commissioning service and spare parts availability.A very
important aspect to take into account when evaluating a heat
exchanger are the forms of corrugation within the heat exchanger
itself. There are two types of corrugations; intermating and
chevron corrugations. In general, greater heat transfer enhancement
is produced from chevrons for a given increase in pressure drop and
is more commonly used than intermating corrugations.
AIMThroughout this group project, our objectives are to :-
Understand how a plate heat exchanger works basically Compare the
advantages of using a plate heat exchanger with other types of heat
exchanger such as shell and tube heat exchanger and double pipe
heat exchanger Able to give example of applications of plate heat
exchanger in modern industries Study about the design of a plate
heat exchanger and methods for calculating the area of heat
transfer by using the NTU and LMTD methods Suggest ways to improve
the performance of a plate heat exchanger
ADVANTAGESAs an alternative to the conventional method of heat
transfer, the plate heat exchanger has many advantages over the
other types of heat exchangers. The key features are as follows:-
CompactnessThe units in a plate heat exchanger occupy less floor
space and floor loading by having a large surface area that is
formed from a small volume. This in turn produces a high overall
heat transfer coefficient due to the heat transfer associated with
the narrow passages and corrugated surfaces.
FlexibilityChanges can be made to heat exchanger performance by
utilizing a wide range of fluids and conditions that can be
modified to adapt to the various design specifications. These
specifications can be matched with different plate corrugations.
Low Fabrication CostsWelded plates are relatively more expensive
than pressed plates. Plate heat exchangers are made from pressed
plates, which allow greater resistance to corrosion and chemical
reactions. Ease of CleaningThe heat exchanger can be easily
dismantled for inspection and cleaning (especially in food
processing) and the plates are also easily replaceable as they can
be removed and replaced individually.
Temperature ControlThe plate heat exchanger can operate with
relatively small temperature differences. This is an advantage when
high temperatures must be avoided. Local overheating and
possibility of stagnant zones can also be reduced by the form of
the flow passage. ExpandableA significant benefit of the plate heat
exchanger is that it is expandable, allowing anincrease in heat
transfer capability.As your heat transfer requirements change, you
cansimply addplates instead of buying an entire new frame unit,
saving time and money. High EfficiencyThe pressed plate patterns
and narrow gaps allow for very high turbulence at relatively low
fluid velocity. Combined with counter directional flow will results
in very high heat transfer coefficients. Close Approach
TemperatureThe same features that give the plate heat exchanger its
high efficiency also makes it possible to reach a close approach
temperatures which is particularly important in heat recovery and
regeneration applications. Approach temperatures of 1F are
possible. Multiple Duties In a Single UnitThe plate heat exchanger
can be built in sections, separated with simple divider plates or
more complicated divider frames with additional connections. This
makes it possible to heat, regenerate, and cool a fluid in one heat
exchanger or heat or cool multiple fluids with the same cooling or
heating source. Avoid Cross ContaminationEach medium is
individually gasketed and as the space between the gaskets is
vented to the atmosphere, cross contamination of fluids is
eliminated. Less FoulingVery high turbulence is achieved as a
result of the pattern of the plates, the many contact points, and
the narrow gap between the plates. This combined with the smooth
plate surface reduces fouling considerably compared to other types
of heat exchangers. Lower CostHigh heat transfer coefficients mean
less heat transfer area and smaller heat exchangers, and sometimes
even less heat exchanger. This and less space requirements will
reduced flow rates and smaller pumps means.
APPLICATIONSPlate heat exchangers offer the highest efficiency
mechanism for heat transfer available in industry today. Many
industries prefer to use plate heat exchangers compared to other
types of heat exchangers because of its wide variety of
applications. Below are some of the examples of applications of a
plate heat exchanger in industries :- Food and BeveragesMilk or
cream pasteurization, syrup pasteurization, juice pasteurization,
milk reception, nectar pasteurization, cultured milk treatment (ice
cream or cheese treatment), sugar dissolving and pasteurization of
concentrates. Petroleum/Chemical ProcessingBrine cooling,
heating/cooling of corrosive fluids, sea water coolers, crude oil
cooling/interchanging, lean/rich fluid interchanger & cooler,
isobutene condenser & reactor interchanger, crude oil heat
treatment, acid gas condenser and treated/untreated crude
interchanger. Hydrocarbon ProcessingSodium hydroxide cooling,
propylene condensers & coolers, naphtha preheating, methanol
preheating, formalin cooling and liquid product cooling.
PolymersNylon salt cooling,glycol cooling, solvent heating, polyol
product cooling, pellet, refrigerated matter cooling,
heating/cooling isocyanate, heating/cooling of naoh,
heating/cooling viscose & acids and heating of spin bath
solution. PharmaceuticalsProduct heating/cooling, heating jacketed
reactor, cooling water systems, condensers & interchangers and
hot water systems. Energy and PowerAuxiliary cooling circuit
isolation, co-generation applications, geothermal applications,
lubrication oil cooling, diesel engine cooling and heat recovery.
MarineSeawater isolation exchanger, central cooling, jacket fresh
water cooling, lube oil cooling, camshaft lube and oil cooling.
DESIGN
Other than chemical industries, plate heat exchangers are widely
used in automobile, aerospace, and cryogenic due to high
effectiveness, compactness (high surface area density), low weight
and affordable cost. Heat exchanger design is one of the problem
that are commonly faced by engineers. The fluid inlet temperatures
and flow rates, as well as a desired hot or cold fluid outlet
temperature, are described in the heat exchanger design problem.
Normally, design problem is faced when the heat exchanger is
custom-built for a specific application. Unlike heat exchanger
performance calculation, heat exchanger design problem is
responsible in specifying a specific heat exchanger type, and
determining the size or area of the heat exchanger that are
essential to achieve the desired outlet temperature. Meanwhile,
performance calculation analyzed the existing heat exchanger in
order to determine the heat transfer rate and fluid outlet
temperatures, for prescribed flow rates and inlet temperatures.The
plate component of plate heat exchanger are described in the Figure
below :Figure 3: Plate heat exchanger components
Plate heat exchanger is also called a plate and frame heat
exchanger. A plate heat exchanger is a form of compact heat
exchanger consisting of layers of plates which are gasketed or
copper, Cu brazed sandwiched between two frames. One frame is
fixed, and the other one is adjustable. The adjustable frame allows
the number of plate to be modified, either increased or decreased,
depends on the usage of the heat exchanger itself. This criteria
made plate heat exchanger as a compact heat exchanger because the
number of plates are changeable, without the need of buying a new
heat exchanger. The plates are commonlymade of aluminum or
stainless steel, due to the reason of low weight and do not rust
easily. While the frames are made of iron.Hence, it can be said
that the compact construction of the plate heat exchangerallows for
an expansion in capacity without difficulty. Meanwhile, on the
plate itself, it has an enhanced surface called fins. There are
various types and sizes of fins, either tubular or plate in shape.
However, the main purpose of fins is to increase the surface area
of the heat exchanger. According to the equation , it is obviously
stated that area, A is directly proportional to heat transfer rate,
Q. Therefore, with the presence of fins, the surface area of heat
exchanger is increased, and the heat transfer will be
increased.Plate heat exchanger is made of a stack of corrugated
fins alternating with nearly equal numbers of flat separators known
as parting sheets, bonded together to form a monolithic block.
Appropriate headers are welded to provide the necessary interface
with the inlet and the exit fluid streams. The schematic view of
such an exchanger is given in Figure below. The corrugations serve
both as secondary heat transfer surface and as mechanical support
against the internal pressure between layers.
Figure 4: Plate fin heat exchanger assembly and details of Side
bars, Plates or Parting Sheets, Fins, Fluid 1, Fluid 2, Cap Sheet
HeaderIn addition, cost is one of the most crucial aspect for the
heat exchanger design. Few factors need to be taken into
consideration to design the heat exchanger, so that the cost
required is affordable. The important factors to minimize the heat
exchanger cost are Pressure drop and Log Mean Temperature
Difference, LMTD. The higher pressure drop the smaller the heat
exchanger and the higherthe temperature difference, the smaller the
heat exchanger will be.For heat exchanger design problem, basically
there are two ways to calculate the area, A. First is by using the
NTU (Number of Transfer Units) method. Few calculations are needed
to find the heat exchanger surface area, as explained below : When
the outlet and inlet temperatures are known, calculate .
Calculate the capacity ratio Cr = Cmin/Cmax Calculate the
overall heat transfer coefficient, U. When and Cr and the flow
arrangement are known, determine NTU by using appropriate -NTU
equations. When NTU is known, calculate the total heat transfer
surface area, A.
For this method, in order to find NTU, different -NTU equation
is used for different flow. However, if , regardless of the flow
pattern, the -NTU equation used is :
This equation shows that the heat exchanger behavior is
independent on flow arrangement. Besides that, Area can also be
calculated by using the LMTD method. The steps are explained below
: Calculate Q and the unknown outlet temperature by using the
equation and
Calculate LMTD and obtain the correction factor (F) if
necessary. Calculate the overall heat transfer coefficient, U using
the equation Determine Area.
PERFORMANCETo maximize the performance of a Plate Heat Exchanger
means saving money, especially if the process is built for a long
term project. Here are some ways to improve the performance of a
heat exchanger: Heat transfer area Fluid flow velocity Temperature
gradientThese suggested ways of improvements are based on the
equation for heat transfer rate of a heat exchanger, which is:
Where,Q: Heat transfer rate between the fluidsU: Overall heat
transfer coefficientA: Heat transfer area: Logarithmic mean
temperature difference of the system
Heat Transfer Area
As the equation shown above, the heat transfer area (or contact
area) is directly proportional to the heat transfer rate. If the
heat transfer area increases, heat transfer rate increases as
well.A common way to increase heat transfer area is addingfins to
the surface. It is cheap to put fins to the heat transfer area but
fins also increase fouling, especially in bio-process.
Overall Heat Transfer Coefficient
Overall EfficiencyIn order to predict or design the performance
of a plate heat exchanger, it is essential to determine the heat
lost to the surrounding for the analyzed configuration. A parameter
can be defined to quantify the percentage gains or losses. Such
parameter may be obtained readily by applying the concept of
overall energy balances for hot and cold fluids.
Where, : Heat power emitted from hot fluid. : Heat power
absorbed by cold fluid. : Mass flow rate of hot and cold fluids,
respectively., : Specific heats of hot and cold fluids,
respectively. : Inlet and outlet temperatures of hot fluid,
respectively. : Inlet and outlet temperatures of cold fluid,
respectively.
and should be equal if the plate heat exchanger is well
insulated. In practice, these will not be the same due to heat
gains or losses from/to the environment.
The above formulae were deducted taking into account that hot
fluid is being surrounded by cold fluid. If the average cold fluid
temperature is below the atmospheric temperature, heat will be
gained, resulting > 100%. However, if the average cold fluid
temperature is above the atmospheric temperature, then heat will be
lost to the surroundings thus resulting < 100%.
Temperature Efficiencies
Temperature efficiency of each fluid stream is a very useful
measure of the heat exchanger performance. The temperature change
in each fluid stream is being compared with the maximum temperature
difference between two fluid streams giving a comparison with a
heat exchanger of infinite size.
Figure 5: Co-current and counter-current operation for a Plate
Heat ExchangerWhere,
Plate Film Coefficient
Where,plate film coefficient
Where,mass flow rate per unit cross sectional area =
cross-sectional area for flowlengthfluid specific heat, heat
capacity
Overall heat transfer coefficient UDue to that the temperature
difference between the hot and cold fluid streams varies along the
length of the heat exchanger, it is absolutely essential to derive
an average temperature difference from which heat transfer
calculations can be performed. This average temperature difference
is known as Logarithmic Mean Temperature Difference (LMTD),
Calculation of LMTD in co-current flow:
Calculation of LMTD in counter-current flow:
Hence, we can now define an overall heat transfer coefficient U
as:
Where, : Heat power emitted from hot fluidA: Heat transmission
areaTemperature GradientTemperature gradient is certainly a
important part of heat transfer. It is the driving force for heat
transfer. If we can introduce fluids with greater temperature
difference into the heat exchanger, the heat transfer rate (Q) will
be greater. If we go back to the temperature profiles of the
co-current and counter-current flow as shown in Diagram 1.1, we can
see that the driving force is great for co-current at the beginning
but decreases drastically as it moves along the heat exchanger. The
counter-current flow provides relatively consistent driving force
and therefore performs better than co-current flow.
CONCLUSIONPlate heat exchangers are widely used in warming,
heating, cooling applications food and cosmetics and chemistry. The
plate heat exchanger is widely recognized today as the most
economical and efficient type of heat exchangers on the market, and
that could be noticed through the advantages of the plate heat
exchanger such as compactness, ease of cleaning where its plates
are easy to remove and replaced which make it more expandable as we
can add more plates instead of buying an entire new frame. Those
advantages make it more useful in many fields such as food and
beverages, petroleum/chemical processing, polymers and
pharmaceuticals. Plate fin heat exchangers, because of their
compactness, low weight and high effectiveness are widely used in
aerospace and cryogenic applications. This device is made of a
stack of corrugated fins alternating with nearly equal number of
flat separators known as parting sheets, bonded together to form a
monolithic block. Appropriate headers are welded to provide the
necessary interface with the inlet and the exit streams. While
aluminum is the most commonly used material, stainless steel
construction is employed in high pressure and high temperature
applications.
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