These are suitable for a wide range of temperatures, from very low to extremely high.
A shell and tube heat exchanger is necessary when there is an extreme temperature difference between the two fluids between which the heat transfer is occurring.
Shell and tube heat exchangers work with a close approach temperature of up to 5°C, requiring more energy than a plate heat exchanger.
Plate Heat Exchangers
The plate gaskets of plate heat exchangers can limit the temperature and pressure range that can be handled.
Temperatures lower than –50°C or exceeding 160°C are not normally tolerated in plate heat exchangers because they can cause standard gaskets to leak.
Gaskets made from special materials can withstand temperatures up to 400°C, but plates can be welded or brazed together to operate under harsher conditions.
These upgrades would also increase the operational limits and the possibility of working with corrosive fluids because the need for gaskets would be removed.
The drawback of completing these upgrades is that you lose the flexibility and ease of cleaning of the traditional “gasketed” model, and the product becomes more expensive.
However, plate heat exchangers can be designed with a very close approach temperature of 1°C or even lower.
This makes plate heat exchangers more energy efficient than shell and tube heat exchangers.
Because a plate heat exchanger forms narrow channels between its plates, only a low volume of fluid is contained within.
This allows rapid responses to change in process conditions with short lag times, so temperatures are easily controllable.
The shape of the channels (see Diagram 1) also reduces the possibility of dead zones and areas that might overheat.
These factors are always important when high temperatures need to be avoided.
DIAGRAM 1:
Pressure Capabilities
Shell + Tube Heat Exchangers
This style of heat exchanger is suitable for higher pressures compared to a plate heat exchanger.
Shell and tube heat exchangers can handle pressures ranging from a few bar to several hundred bar.
Pressure drops within shell and tube heat exchangers are low due to reduced turbulence compared with plate heat exchangers and their narrow flow channels.
Plate Heat Exchangers
These are typically used in applications with lower pressure requirements, typically ranging from a few bar to around 25 bar.
Because of the corrugated plates and the small flow space between them, friction causes the pressure drop to be high.
This increases pumping costs as higher-pressure pumps are required to achieve the necessary flow.
Pressure drops can be reduced by increasing the number of passages per pass and splitting the flow into more channels.
This diminishes the flow velocity within the channel and therefore the amount of friction.
In turn, this reduces the convective heat transfer coefficient, which decreases the effectiveness of the heat exchanger.
Pressures exceeding 25atm are not normally tolerated in plate heat exchangers because they can cause standard gaskets to leak.
Heat Transfer & Recovery Capabilities
Shell + Tube Heat Exchangers
The type of flow inside the heat exchanger has great impact on the heat transfer capabilities of a heat exchanger.
The more turbulent the flow, the better the heat transfer.
Shell and tube heat exchangers have turbulent flow within the cylindrical shell, but laminar flow within the tubes.
This means the transfer of heat is less effective than that within a plate heat exchanger.
To transfer heat effectively with a shell and tube heat exchanger, many tubes should be used to increase the thermal contact between the substance inside the tubes and the substance in the cylindrical shell.
The more tubes used increases the heat exchanger’s heat transfer capabilities, but also increases the purchase price and maintenance time for the piece of equipment.
Plate Heat Exchangers
The formation of turbulent flow is enhanced by the corrugations of the plates and the small hydraulic diameter of plate heat exchangers.
Plate heat exchangers are often considered the most efficient due to turbulent flow on both sides of each plate.
This allows high rates of heat transfer for the fluids passing through.
IMAGE 1:
A shell + tube heat exchanger.
Ease of Cleaning
Shell + Tube Heat Exchangers
Cleaning this type of heat exchanger might prove difficult due to their complex internal structure and tube bundles.
Cleaning should be done using high-pressure water blasting, wire brushes, scrapers, or chemical cleaners.
The tubes of this kind of heat exchanger need to be cleared out regularly to maintain the unit’s efficiency.
The required frequency of this cleaning may differ depending on the type of liquids being used.
Plate Heat Exchangers
Gasketed plate heat exchangers can be separated, therefore it is extremely easy to clean and inspect all parts that are exposed to the fluid.
Note that brazed plate heat exchangers will not have this easy-clean function.
Methods used to clean these heat exchangers include chemical cleaning, mechanical brushing, or ultrasonic cleaning, depending on fouling type and severity.
Size Compactness
Shell + Tube Heat Exchangers
Shell and tube heat exchangers are bulkier and require more space in a plantroom than plate heat exchangers.
When the internals of the unit need to be inspected, it takes more space to open and adjust the tubes.
These are typically installed where space is not a major constraint.
Plate Heat Exchangers
Plate heat exchangers only need a small amount of space in your plantroom in relation to their effectiveness.
For the same area of heat transfer, a plate heat exchanger can take up 80% less floor space than a shell and tube heat exchanger.
IMAGE 2:
A gasketed plate heat exchanger.
Fluid Suitability
Shell + Tube Heat Exchangers
These can handle a wide range of fluids, including ones that are corrosive and viscous.
Tube material is selected based on the fluid’s corrosiveness, and the tube geometry can be tailored for fluids with higher viscosities.
Plate Heat Exchangers
In particular cases, plate heat exchangers can be used in condensation or evaporation operations but are not recommended for gases and vapours because of pressure limitations and limited space within the flow channels.
Plate heat exchangers will struggle to process fluids that are highly viscous or contain fibrous material.
This is because of the high associated pressure drop and flow distribution problems within plate heat exchangers.
Compatibility between the fluid and gasket material should also be considered, as highly flammable or toxic fluids must be avoided due to possible leakage.
Application Flexibility
Shell + Tube Heat Exchangers
The cooling capacity of a shell and tube heat exchanger cannot be increased, but a gasketed plate heat exchanger’s can be.
The size of a shell and tube heat exchanger is fixed, and the equipment requires replacement when a bigger capacity is needed.
Plate Heat Exchangers
Plate heat exchangers are simple to dissemble, meaning they can adapt easily to new requirements through the addition or subtraction of plates, or rearranging the number of times the fluid passes through the channels.
The variety of plate designs available also contributes to the flexibility of those units.
Note: brazed plate heat exchangers and shell and tube heat exchangers do not have this function.
IMAGE 3:
Brazed plate heat exchangers.
Likelihood of Fouling
Shell + Tube Heat Exchangers
Shell and tube heat exchangers generally have a lower fouling likelihood when clean water is the fluid used.
Clean water, which doesn’t contain solid particles or impurities, is less likely to cause fouling in heat exchangers.
The larger tube diameters in shell and tube heat exchangers allow for better fluid flow and less fouling accumulation.
However, it’s worth noting that fouling can still occur over time if the water contains dissolved minerals or if biofilm growth takes place within the heat exchanger.
Plate Heat Exchangers
Plate heat exchangers are also suitable for clean water applications.
The narrow passages between the plates create high flow velocities and turbulence, which help minimise fouling.
When clean water flows through these passages, the chances of fouling are generally lower compared to situations where the water contains solids or impurities.
It’s important to monitor the water quality and periodically inspect the plates for any signs of fouling, especially in environments where water quality may vary or where biofouling can occur.
You can catch fouling early by watching for an increase in pressure drops or the lowering of equipment performance.
Likelihood of Leakage
Shell + Tube Heat Exchangers
These have a lower likelihood of leakage due to their robust construction and design.
The tube-to-tube sheet joints are typically welded or mechanically expanded, providing a reliable seal against leakage.
Shell-side and tube-side fluid paths are separated, minimizing the risk of cross-contamination.
To inspect the system for leaks, cracks or corrosion, dye can be added to one of the fluids. This will help you spot leaks between the shell and the tubes.
Alternatively, pressurising the shell side of the system will push air through tubes where cracks or holes are present, offering another way to check for leakage.
Leaking tubes can be plugged to prevent leakage from affecting the rest of the system.
However, this changes the overall flow rate and decreases the efficiency of the heat exchanger.
Plugging might be a sufficient fix temporarily, but the use of these should be limited.
To replace the tubes altogether, the heat exchanger will require an extended down time.
Tubes can be double-walled to reduce the likelihood of leakage within the heat exchanger, although this will raise the manufacturing cost.
Plate Heat Exchangers
These heat exchangers can be more prone to leakage if the plates are not properly sealed or maintained.
Friction between the plates of a heat exchanger can cause wear and the formation of small holes that are difficult to locate.
As a precaution against this, we advise that you pressurise the process fluid so there is less risk of contamination in the event of leakage from a plate.
The rubber gaskets are also prone to leakage as they age, and because these are between every plate, the likelihood of leakage increases dramatically as your plate heat exchanger grows older.
However, these heat exchangers have vents near the gaskets (see Diagram 2) to prevent fluids from mixing in the case of gasket failure.
DIAGRAM 2:
The way this protection is designed also allows the easy identification of leaks, because the fluid can be easily seen flowing out of the heat exchanger.
While leakage from the gaskets is common, the flexible design of plate heat exchangers allows easy dissembling and replacement.
Initial Cost
Shell + Tube Heat Exchangers
Shell and tube heat exchangers usually have higher initial costs than plate heat exchangers due to their complex construction, larger size and tube customisation options.
This cost can vary significantly depending on factors such as construction materials, pressure ratings, and the number of tubes.
However, the decision of which style is right for your application should not be based on price alone.
Plate Heat Exchangers
These are often more cost-effective than the above option, especially for smaller applications.
They have a compact design, require less material and have simpler manufacturing processes.
Plates are pressed or glued together rather than welded. This means the production of these plates is inexpensive.
However, special materials can be used for the plates to make them more resistant to corrosion or chemical properties, which might raise the price of plate manufacture.
Operating Cost
Shell + Tube Heat Exchangers
Shell and tube heat exchangers usually have higher pressure drops due to construction design and a higher likelihood of fouling due to the laminar flow within the tubes.
Both of these factors affect the efficiency of the system’s pumps, resulting in higher pumping operational costs.
Plate Heat Exchangers
These heat exchangers can be designed with a very low pressure drop and a low likelihood of fouling due to turbulent flow within the plates, resulting in better efficiency and therefore a lower ongoing pumping cost.
Maintenance Cost
Shell + Tube Heat Exchangers
The time it takes to perform routine maintenance on a shell and tube heat exchanger is far longer than that for a plate heat exchanger.
This is because of the larger model’s complex construction involving tube bundles and the cleaning, removing and replacing of these.
The laminar flow within the tubes often causes a high level of scaling formation, which increases the likelihood of fouling.
Because of this, the descaling process must be performed periodically.
This could cost you a lot of downtime and extra labour for the maintenance period.
Plate Heat Exchangers
While maintenance time is far quicker with a plate heat exchanger, the ongoing cost of gasket replacement also needs to be considered.
Even when gaskets do not fail, gasket replacement is part of the regular service process.
However, the intervals between servicing can be as long as 5 years from the date of operation.
However, these parts are not expensive to replace compared with the cost of replacing/repairing the tubes of a shell and tube heat exchanger.
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