Heat exchangers are vital in various processes where heating, cooling, condensing, evaporating or heat recovery must be carefully controlled. For a process system engineer, it is crucial to appreciate how different types of equipment achieve the transfer of thermal energy, and in what applications each design is more effective. Applications are evaluated on many bases including temperatures, pressures, flow rates, fluid characteristics, heat duty, space available and the need for maintenance. Familiarization with the major types of Heat Exchangers can aid in decision making when designing or optimizing process systems.
1. Shell & Tube Heat Exchanger
Shell and tube configurations are one of the oldest options for industrial heat transfer applications. Shell and tube configurations use a bundle of tubes housed in a shell. One fluid is pumped through the tubes and another fluid is pumped around the outside of the tubes.
Key Features
- Applicable for high temperature applications
- Can accommodate relatively high operating pressures
- Available in a range of sizes and setups.
- Suitable for a range of industrial fluids
- For heating or cooling.
This unit finds use in chemical processing power, petroleum, and other industrial plants.
2. Plate Heat Exchanger
A plate heat exchanger employs a set of thin metal plates to define alternating channels of flow. The plates are semi permeable and allow heat to transfer between the two separated fluids.
Why Engineers Consider Them
- A concise structure.
- Large heat transfer surface area
- Effective Thermal performance
- Easy enough access to maintain where applicable
- Practical if the available space for fitting is tight
Consideration of fluid compatibility, pressure, temperature and fouling conditions.
3. Brazed Plate Heat Exchanger
Brazed plate units: a Brazed plate unit consists of a stack of plates that are brazed together. The unit is therefore relatively compact and has a large surface area for heat transfer.
Common Considerations
Engineers generally evaluate:
- Operating temperature:
- Operating pressure
- Fluid formulation
- The heat duty required:
- Fouling potential
- Installation prerequisites
Due to their compact design, these may also have application in situations where both space and effective heat transfer are critical factors.
4. Air-Cooled Heat Exchanger
Air-cooled designs transfer heat from a process fluid to the air. The air is forced over finned tubes by a fan.
Where They Can Be Useful
Air-cooled equipment may be considered when:
- Cooling water is scarce
- The use of water should be curtailed
- A completely independent cooling water system is not wanted
- The needed cooling could be achieved from the atmosphere.
The designer should consider the ambient conditions, airflow and fan capability and noise and the space to be occupied.
5. Plate Fin Heat Exchanger
The stacked plate fin type of heat exchanger consists of alternating plates and fins so as to produce a large surface area in a small volume. Such a heat exchanger has good thermal performance at relatively small overall size.
Important Selection Factors
- Type of fluid
- Temperature range
- Pressures needed
- Heat transfer rate required
- Material compatibility
- Cleanliness of process fluids
The construction is beneficial when high heat transfer rates and construction space are limited.
6. Double-Pipe Heat Exchanger
A double pipe heat exchanger contains one pipe within another, with one fluid flowing through the inner pipe and the other flowing through the space between the two pipes.
Advantages
- Easy to build.
- The simple operating mechanism
- Suitable for low heat loads
- Relatively simple to learn and to administer
- Adaptable for some arrangements of process
For large heat duties there can be too much piping required. In this case other configurations are suitable.
7. Spiral Heat Exchanger
The spiral heat exchanger employs metal plates which are arranged in a spiral configuration. This forms two distinct passages for the fluids to flow through.
Key Benefits
- Tight-packed order
- effective utilization of heat transfer surface
- Helpful in some liquid uses
- The possibility exists to use for certain fouling duties
- Ideal for use in heat recovery
However, engineers will still have to consider fluid properties and cleaning needs.
8. Finned Tube Heat Exchanger
A finned tube heat exchanger uses extended surfaces, or fins, which are arranged around tubes, to maximize the heat transfer surface area. Such a system is especially effective for transferring heat from a fluid which passes through the tubes, to the surrounding air.
Factors to Evaluate
- Fin material
- The construction material of the tube
- Airflow
- Temperature Difference
- Fin spacing
- Conditions of fouling
Commonly associated with air heating and cooling applications where an increased surface area may enhance heat transfer
9. Condenser
A condenser is the equipment that extracts the heat from vapor and transforms it into liquid form. It is also widely used in industrial applications such as power production, chemical processing, refrigeration, and in process plants.
Condenser Selection Depends On
- Vapour characteristics
- Condensing temperature
- Operating pressure
- Cooling agent
- The capacity needed.
- Compatibility of materials
Efficient operation of a condenser is required as variations in the condensing environment can influence the performance of the overall process.
10. Evaporator
Evaporator: an apparatus that introduces heat into a liquid such that it vaporizes in part or in full. They are commonly used in processes for concentration, drying, refrigerating and chemical production.
Important Design Considerations
Engineers may need to assess:
- The rate of evaporation
- working pressure
- The parameters that influence the values are: Fluid characteristics.
- Thermal sensitivity
- Fouling and scaling
- Cleaning regime
There are various configurations of evaporators and the design should be matched to the particular process.
How to Select the Right Heat Exchanger
Knowing the main types is only the first step. Engineers must match the equipment against the actual needs of the process.
Key Factors to Consider
- Heat Transfer duty
- Water temperature
- Working pressure
- Flow Rate
- Fluid characteristics
- Potential corrosion.
- Fouling tendency
- Space available for mounting.
- Maintenance needs
- The expected life time
Initial Cost Is Not the Only Factor
Purchase price is only one aspect and perhaps the least significant. The cost of operation must include energy consumption, upkeep, cleaning, spare parts, and non-operating hours/expected life of equipment.
A design with the lowest initial cost may not achieve the lowest operating cost over its lifetime. When evaluating options, performance, as well as life cycle, needs to be taken into account by engineers.
Final Thoughts
Different types of heat transfer designs have their own pros and cons. Shell and tube units can be applicable for heavy industrial services, plate units are known for compact construction. Air cooled and finned tube units are ideal for use with air as a coolant. Plate fin designs are an alternative and also the spiral designs.
A working knowledge of how each type of device is designed, how it functions, and its advantages and disadvantages, will provide a good initial point for selecting the equipment. Selection should always be made on the basis of the process conditions, and not necessarily by virtue of familiarity of design.
The engineer could compare equipment from heat exchanger suppliers for his industrial needs based on his heat duty, operating parameters, fluid properties and installation configuration.
