Types of Heat Exchangers and Their Selection

What is a Heat exchanger: A heat exchanger is mechanical equipment used to exchange heat between two fluids. Fluids used for heat transfer can be either liquids or gases, and two or more fluids can be used for heat transfer operation. A well-designed heat exchanger can be used for either cooling or heating operations. The heat exchanger in the oil & gas field is referred to as static equipment as the heat exchanger doesn’t contain any rotating components
Heat transfer is an operation in which heat is exchanged between two or more fluids because of temperature differences.

Design Code: Design codes standards that are preferred for heat exchangers are the following:

• ASME BPVC SEC VIII (Construction code for design, manufacturing, inspection, and testing of pressure vessels and covers).
• TEMA (TUBULAR EXCHANGER MANUFACTURERS ASSOCIATION).

Material of Construction: The material of Construction selection for the heat exchanger is based on the heat resistant property, selected material can be one of the following:

• Carbon Steel
• Stainless Steel
• Alloy Steel
• Aluminium
• DSS and others

Application of Heat Exchangers: Heat exchanges are used widely in the following industries:

• Chemical plant or refinery
• Food and beverage industry
• Nuclear and power industry
• Petrochemical platform

Classification of Heat exchanger: generally heat exchangers can be classified on several factors as mentioned below:

• Based on flow configuration
• Pass configuration
• Heat transfer mode
• Function and application
• Construction method
• Fluid phases

Now we will study the detailed classification of heat exchangers:

1. Based on the flow configuration: Flow configuration is the path through which fluid moves in the heat exchanger. Flow configurations in heat exchangers are generally available in four types which are:

• Co-Current Flow: Co-Current flow heat exchangers are also referred to as parallel flow heat exchangers. In Co-Current flow, the fluid stream flows in the same direction or parallel to each other. Co-Current flow provides thermal uniformity through heat exchanger walls and gives lower efficiency.
• Counter current flow: In counter-current flow, heat exchanger fluid streams flow parallel but in opposite directions. Counter current flow gives the highest efficiency.
• Cross flow: Cross-flow heat exchanger in which fluid flowing streams perpendicular (at 90°) to each other. Cross flow heat exchanger gives better efficiency in comparison to the Co-Current flow heat exchanger.
• Hybrid flow: Hybrid flow in which the heat exchanger uses a combination of Co-Current, Counter current, or Cross flow. This type of configuration is used in heat exchangers to resolve the limitation or restrictions.

2. Based on the heat transfer mechanism: heat exchangers are classified into two categories based on the heat transfer mechanism which are direct contact and indirect contact type.

• Direct contact type: In this type of heat exchanger, direct contact means no tube wall is available for fluids, fluids come in direct contact with each other and finally take exit from the respective outlet nozzles. Direct contact types heat exchangers are generally used to attain low-temperature changes.
  Example of Direct contact type heat exchanger is a cooling tower, desuperheater, and scrubber.
• Indirect contact type: In this type of heat exchanger, a separate pipe wall is present for different fluid media (no possibility of mixing fluid with each other).
  Indirect-type heat exchangers are further classified into three categories which are:
  
  a. Direct transfer type: In this type of heat exchanger each fluid contains a separate path with respect to its passes. The temperature difference is deriving force for the heat transfer process. Direct transfer type heat exchangers are also referred to as recuperators. In this type of heat exchanger heat is continuously exchanged from hot to the cold fluid.
  Examples of Direct transfer type heat Exchangers: Shell & tube heat exchangers (STHE), plate type heat exchangers, economizers, etc.
  
  b. Fluidized bed type: Fluidized bed heat exchanger which consists of two compartments for hot and cold fluid including an inlet diverter plate at the inlet. Alumina pallets are used to fill the entire bed. The heat of the hot gases causes the alumina pallets to fall into the bottom chamber, cold air is used to extract the heat from the hot gas. Hot gas is flowing in the upper chamber. For recycling purposes, the recirculating path or arrangements can be incorporated.
  
  A fluidized bed heat exchanger operates on low velocity, due to high velocity the solid particles in the bottom section tend to float in the entire heat exchanger section as the drag force of the fluid becomes much higher than the weight of the fluid.
  
  c. Storage type: In the storage type of heat exchanger containing a fixed bed area, fluids are passed through their respective beds when in operation. Hot & cold fluids are so adjusted, they have to pass through the cold and hot fluid bed following each other. Heat transfer takes place because of this mechanism.
• An example of a Storage type heat exchanger is an air preheater.

3. Based on the passes of the heat exchanger: heat exchanger based on the number of passes can be one of the following:

• Single pass heat exchanger: In single pass type heat exchanger fluid can enter only once throughout the length of the heat exchanger.
• Multi-pass heat exchanger: In the multi-pass type heat exchanger, the fluid movement uses the return bend or U bends which is placed at the end of the tube length. This construction enables the fluid to enter multiple times as there is a number of series utilizing the multi-pass arrangement.

4. Based on the available fluid phases: depending upon the fluid or process media phases, three types of heat exchangers are mentioned below:

• Gas – Liquid: This type of phase availability is in Compressor oil cooling system arrangement is required.
• Liquid-Liquid: This is the most used heat exchanger in industrial applications, shell & tube heat exchanger (STHE) is an example of a liquid-liquid type of heat exchanger.
• Gas-Gas: This type of heat exchanger is mostly used in air preheater operation where with the help of air flue gas or another process media is cooled down, whereas this preheated air can be used for furnaces increases its efficiency.

5. Based on the function and their applications: selection is based on the process application, different types are mentioned below:

• Coolers
• Exchangers
• Condensers
• Reboiler
• Steam generator

6. Based on the heat exchanger construction: Heat exchanger classification based on the construction is described below:

• Plate Type heat exchanger
• Shell & tube heat exchanger
• Finned tube heat exchanger
• Spiral heat exchanger
• Double pipe heat exchanger

Selection of heat exchanger: selection of the heat exchanger is completely based on the design which is done by the process engineer. The different parameters which play important roles in the selection for heat exchange as indicated below:

1. Type of Fluid stream and its properties.
2. Plot plan limitations
3. Design (Thermal) output from the operating condition.
4. Any means of special service.

Reference Links:

https://www.thomasnet.com

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