A heat exchanger is a device used to transfer heat between different fluids. In industries, we use heat exchangers for both cooling and heating purposes.
The basic idea of a heat exchanger is that heat always travels from high to low temperature.
However, there are a lot of technical aspects involved in its optimization for a particular process. So it is important to understand the various types of heat exchangers.
In this discussion, we will focus on the most used types of heat exchangers, their working principles, and design aspects.
Following are some of the most used types of heat exchangers. We will discuss each in detail.
- Double Pipe Heat Exchangers
- Plate Heat Exchangers
- Shell and Tube Heat Exchangers
Double Pipe Heat Exchanger
A double pipe heat exchanger consists of two concentric pipes held together. One of the pipes is held inside another pipe that has a larger diameter.
Two different fluids flow inside each pipe.
We can extend the internal pipe by making a u-type shape usually called a hairpin. There may be a number of hairpins as shown in the following figure, depending upon the process requirement.
We use double pipe heat exchangers where the required area of heat transfer is small. It is the simplest type of heat exchanger.
We can further classify it into two types depending upon the direction of fluid flow. The parallel flow and the counter flow. We will discuss these two types of fluid flows in the later section of this blog.
Plate Heat Exchanger
A plate-type heat exchanger uses metal plates to transfer heat between different fluids.
It provides a comparatively large area of heat transfer because the fluid spreads all over the flat plates. The plates are clamped together with the help of clamping bolts. One side of the cover is fixed while the other side is moveable.
The exchanger has two different inlets; one for the hot fluid and the other one for cold fluid. Both the fluids flow in such a way that one plate has hot fluid flowing over it while the other has cold fluid.
The flowing medium thus has a hot-cold, hot-cold flow pattern. Gaskets are provided to direct the flow and also to avoid leakages.
When the plates are tightly pressed together, the smaller (negligible) clearance ensures good thermal contact between the two fluids.
Moreover, the plates are corrugated and thin which provides a large heat transfer area. Corrugations on the plates encourage turbulent flow which enhances the heat transfer rate. They also stiffen the plate structure which enables them to be used for thinner plates.
Plate heat exchangers, generally have the highest efficiency as compared to shell and tube heat exchangers. But we can only use them for low temperature and pressure applications.
Plate Heat exchangers are mostly used in refrigeration systems and for systems using corrosive fluids.
Shell and Tube Heat Exchangers
Shell and tube heat exchangers are one of the most used types of heat exchangers, industrially. The main components include a shell, a tube bundle, tube sheets, and heads.
Shell is the casing of the heat exchanger. The area outside the tube walls is generally called the shell side of the exchanger. One fluid flows inside the tubes while the other flows over the tubes on the shell side.
The shell and tube heat exchangers are used for heavy-duty applications like in refineries, and other process industries.
There are three types of shell and tube heat exchangers. The discussion of these types is beyond the scope of this particular blog.
- Shell and tube heat exchangers are considered “workhorses” in process industries. Because we can use them for multiple purposes, i.e, heating, evaporation, cooling, condensation etc.
- We can use them for a wide range of pressure applications, i.e, from very high to very low (vacuum) pressures.
To get a detailed study of the shell and tube heat exchanger you can read, Shell & Tube Heat Exchangers by Graham Hart Process Technology Limited.
Types of Heat Exchangers in Terms of The Direction of Fluid Flow
There are generally three types of fluid flows in a typical heat exchanger.
In parallel flow, the hot and cold fluids enter at the same end, exchange heat between each other and leave at the same end.
In counterflow, both the fluids enter and exit at different ends. This is the most used type of flow in heat exchangers. The reason for this is that we know that the driving force for heat transfer is the temperature gradient.
We usually measure driving force in terms of LMTD (Log Mean Temperature Difference).
The purpose of measuring it in LMTD is that the driving force changes across the dimensions of a heat exchanger.
It is not possible to calculate it at every instant. So we define the general term LMTD by considering an average driving force across the heat exchanger.
In a parallel flow heat exchanger, the driving force is greater in the beginning when both fluids have a greater temperature difference. And then starts decreasing to the minimum value at the exit as shown below on the left side of the graph.
While in counterflow, the average driving force across the heat exchanger remains the same, making it more useful in the process. The right side of the graph shows almost the same temperature difference across the length of the exchanger, in a counter flow heat exchanger.
Another type of heat flow is crossflow. In a crossflow heat exchanger, both the fluids flow perpendicular to each other.
The driving force or LMTD in a cross-flow is almost similar to that of a counterflow. The crossflow is mostly used when there is a phase change involved in one fluid which is always on the shell side. For example, condensers, reboilers, etc.
Different Types of Heat Exchangers, used for different operations
It is obvious to understand the different types of heat exchangers for particular operations. For example, we can’t use a kettle-type reboiler in a power plant to condense the steam.
Even the name suggests, it is a reboiler that does evaporation. Following are some of the types of heat exchangers depending on the different types of operations, they are used for.
We use a simple Exchanger to recover the heat between two process streams. For example, in some refineries, the crude oil is preheated before sending it to the furnace by the product stream coming from the distillation column.
One fluid in a heater is always hotter while the other one is comparatively cold. The purpose is to heat the process (cold) fluid. The heating medium or the hot fluid must have a greater content of latent heat. For example, steam is used as a heating medium in most process industries.
We use a cooler to cool down the process fluid. The cooling medium is mostly water. The purpose of using water is that it is abundant and has a high specific heat coefficient.
Sometimes, we also use air as a cooling medium, but it requires a large area to use an air cooler. Moreover, its small value of specific heat coefficient makes it less susceptible to be used for heavy-duty operations.
The primary purpose of a condenser is to remove the latent heat of vaporization. We must not confuse it with a cooler that involves the exchange of sensible heat.
A reboiler has its application in the distillation column to supplement the heat requirements. It partially vaporizes the less volatile components of the distillation unit. Examples are, kettle-type reboiler and thermosiphon reboilers.
We use an evaporator, most of the time to concentrate a solution. For example, in the vapour absorption refrigeration cycle, an evaporator helps to concentrate the absorbant (Lithium Bromide Solution). It can also be used as a vaporizer to generate vapours for a particular use.