Double pipe heat exchangers are the simplest exchangers used in industries. On one hand, these heat exchangers have low cost for both design and maintenance, making them a good choice for small industries. But on the other hand, low efficiency of these exchangers beside high space occupied for such exchangers in large scales, has led modern industries to use more efficient heat exchanger like shell and tube or other modern heat exchangers. But yet, since double pipe heat exchangers are simple, they are used to teach heat exchanger design basic to students and as the basic rules for modern and normal heat exchangers are the same, students can understand the design techniques much easier.
The aim of heat exchanger design is to calculate several variables to set several other variables to their desired values. For example, in most double pipe heat exchanger designs, operational conditions are known such as input and/or output temperatures of cold and hot flows, flow rate of these two fluids, physical properties of fluids including density, heat capacity, viscosity, thermal conductivity, etc. Beside these properties, there are other limiting factors such as pressure drop through the heat exchanger, length of the heat exchanger, standard pipe sizes available, etc. Having all these variables or some of them, usually it is desired to calculate and report pipe sizes used, length of pipes, number of passes, pressure drop through the exchanger, etc.
Double pipe heat exchanger includes two concentric pipes. The inner pipe is usually called the tube the outer one is called annulus. These two pipes are produced in standard sizes. There are tables like IPS or NPS tables in which the standard pipe size data are represented. In most cases there are only several pipe sizes available or there are limitations on choice of the pipe size. So, considering the operational conditions, it is possible to determine the suitable pipe size for design.
Generally, in heat exchanger design, the first step is the determination of the physical properties of the cold and hot fluids. For double pipe heat exchanger design – or any other heat exchanger – density, heat capacity, thermal conductivity and viscosity are the most basic fluid properties needed. Since these properties are all temperature dependent, and temperature of each flow changes through the exchanger, an average temperature must be determined at which the properties are read. This average temperature can be linear or logarithmic mean temperature. After determination of the physical properties it’s time to calculate other parameter such as Reynolds number, prandtl number, etc.
As was mentioned before, there are two main flow arrangements, concurrent (parallel) and countercurrent flow. In concurrent flow, both cold and hot fluids will enter the heat exchanger at the same head. Therefore, while the hot fluid temperature decrease, the cold fluid temperature increase and as a result the temperature difference between two flows reduces through the exchanger. This will lead to gradual reduction of thermal driving force along the exchanger and will reduce the heat exchanger efficiency and heat transfer rate.
In countercurrent flow arrangement, one of the fluids will enter the heat exchanger at one head and the cold flow enters at the other end. In this flow arrangement as shown in the following figure, the temperature difference between two flow i.e. the driving force, will remain almost constant along the heat exchanger.
In the next articles , there will be more information of double pipe heat exchanger and the design.