1 INTRODUCTION TO HEAT EXCHANGER – A DETAILED STUDY
HEAT EXCHANGER – A DETAILED STUDY: Heat Exchanger is a piece of equipment built for efficient heat transfer (between two or more fluids, between a solid surface and a fluid, or between solid particulates and fluid at a different temperature). The fluid may be in direct contact or separated by a solid wall so that they don’t get mixed. In exchangers, usually, there are no external heat or work interactions. They are used in both cooling and heating purposes.
1.1 Working Principle
In the Heat exchanger, heat flows from hot stream to the cold one. So the hot fluid coming out of the exchanger is a little colder, and the cold stream gets a little hotter at exit point without mixing. That is why heat exchangers are widely used for heat up or cooling down a fluid stream in a processing plant.
2.1 Based on Transfer Processes
Based on the heat transfer process, heat exchangers are classified as indirect contact and direct contact.
2.1.1 Indirect contact
In an indirect-contact heat exchanger, the fluid streams remain separate, and the heat transfers continuously through an impervious wall. Thus, there is no direct contact between thermally interacting fluids. This heat exchanger also referred to as a surface heat exchanger, can be further classified into direct-transfer type, storage type, and fluidized-bed exchangers.
2.1.2 Direct contact
In a direct-contact exchanger, two immiscible fluids under different temperatures come into direct contact, exchange heat, and are then separated. Typical applications of a direct-contact exchanger involve a mass transfer in addition to heat transfer, such as in evaporative cooling and rectification; means this exchanger involves phase change of working fluid, which enhances the heat transfer rate, this can be further classified into Immiscible Fluid Exchangers. Gas-Liquid Exchangers, Liquid-Vapor Exchangers. Compared to indirect-contact, direct-contact heat exchangers have the following advantages:
Very high heat transfer rates are achievable
Exchanger construction is cheap
Due to the absence of a heat transfer surface, the fouling problem is less
2.2 Based on Construction
Heat exchanger are classified according to their construction features as below
2.2.1 Tubular Heat Exchangers
Tubular exchangers are widely used, and they are manufactured in many sizes, flow arrangements, and types. They can accommodate a wide range of operating pressures and temperatures. They are used for gas-to-liquid and gas-to-gas heat transfer applications mostly when the operating temperature or pressure is very high, or fouling is a significant problem. They are further classified as below:
Double pipe exchanger
Shell and tube exchanger
Coiled Pipe exchanger
2.2.2 Plate Heat Exchanger
Plate Heat Exchangers consist of two rectangular end members who hold together several rectangular plates with holes on the corner for the fluids to pass through. The plates are arranged alternatively so that hot fluid flows in 1, 3 and 5 numbered plate while cold fluid flows in 2, 4 & 6 numbered plates. This arrangement lets the hot and cold fluid exchange heat while not mixing.
Each of the plates is separated by a gasket which seals the plates and arranges the flow of fluids between the plates. The plates are either smooth or have some form of a groove. This type of exchangers is generally used for low pressure & low-temperature application. The plate type heat exchanger requires less installation and maintenance space than shell and tube type of equivalent surface. This type of exchanger is widely used in the food industry because it can quickly be taken apart to clean.
Fig 1: Plate Heat Exchanger
2.3 Based on Flow Arrangements
Heat exchangers are classified according to the flow arrangement. The simplest heat exchanger is when the hot and cold fluids move in the same or opposite directions. This heat exchanger consists of two concentric pipes of different diameters.
2.3.1 Co-current Flow
In the Parallel/Co-current flow arrangement, the hot and cold fluids enter at the same end, flow in the same direction, and leave at the same end. The streams flow parallel to each other and in the same direction. This is less efficient than countercurrent flow but does provide more uniform wall temperatures.
2.3.2 Counter-current Flow
In the countercurrent flow arrangement, the fluids enter at opposite ends, flow in opposite directions, and leave at opposite ends. The two fluids flow parallel to each other but in opposite directions. This type of flow arrangement allows the largest change in both fluids’ temperature and therefore is most efficient. More heat is transferred in a counterflow arrangement than in a parallel flow heat exchanger.
Fig 2: Flow arrangement and profile
Here both fluids flow parallel perpendicular to each other. In this type, efficiency lies between countercurrent flow and co-current flow exchangers. In these units, the streams flow at right angles to each other.
2.4 Based on Flow Passes
2.4.1 Single-pass Exchangers
The heat exchanger is considered as a single-pass unit if both fluids make one pass in the exchanger. Here the fluid flows through the exchanger along its length once.
2.4.2 Multipass Exchangers
When the heat exchanger design results in either an extreme length, significantly low fluid velocities, or low effectiveness, a multi-pass heat exchanger are used, a fluid is considered to have made one pass if it flows through a section of the heat exchanger through its full length. After flowing through one full length, if the flow direction is reversed and fluid flows through an equal or different-sized section, it is considered to have made a second pass. Increasing the number of passes increases overall effectiveness, but it also increases the pressure drop on the multi-pass side.
3 TYPES OF INDUSTRIAL HEAT EXCHANGERS
3.1 Double Pipe Exchanger
This exchanger usually consists of two concentric pipes; The inner pipe acts as the conductive barrier, where one fluid flows in the inner pipe, and the other fluid flows in the annulus between pipes. They are also referred to as hairpin, jacketed pipe, jacketed U-tube exchangers. Flow in a double-pipe heat exchanger can be co-current or countercurrent. Double-pipe exchangers are generally used for small-capacity applications because they are cheap for both design and maintenance. They are simple, but they have low efficiency with the large space occupied in large scales. Therefore more efficient heat exchangers like shells and tubes are used by many industries.
Fig 3: Double Pipe Heat Exchanger
3.2 Shell and Tube Exchangers
Shell and Tube Heat Exchangers are some of the most popular types of exchangers due to its flexibility. In this type, two fluids with different temperatures, one of which flows through the tubes (the tube side fluid) and the other flows outside the tubes but inside the shell (the shell side fluid). Heat is transferred from one fluid to another through the tube walls, either from the tube side to the shell side or vice versa. The fluids can be either liquids or gases. This system handles fluids at different pressures, higher pressure fluid is typically directed through tubes, and lower pressure fluid is circulated through a shell side. The major components of this exchanger are tubes (or tube bundle), shell, frontend head, rear-end head, baffles, and tube sheets.
Fig 4: Shell and Tube Heat Exchanger
3.3 Spiral Heat Exchanger
Spiral heat exchangers are generally used in chemical plants and are of circular construction. The spiral heat exchanger is made by rolling two long metal plates around a central core to form concentric spiral flow passage. The hot fluid flow in one channel and just next to it the cold fluid. This side by side flow is responsible for heat exchange between two fluids without mixing. The heat transfer rate associated with a spiral tube is higher than that for a straight tube. Thermal expansion is no problem, but cleaning is almost impossible. Similar to the plate exchanger, the spiral exchanger is compact and requires less installation and maintenance space.
Fig 5: Spiral Heat Exchanger
4 SELECTION PARAMETERS FOR HEAT EXCHANGER:
4.1 Material of Construction
MOC for the heat exchanger depends on the types of fluids or vapours handled. It should have a definite corrosion rate. The material should exhibit strength to withstand operating temperature and pressure.
4.2 Operating pressure & temperature
The pressure is one of the parameters which determines the thickness of components. Higher is the pressure; higher is the thickness of pressure retaining components. Design temperature indicates whether the material can withstand operating conditions.
4.3 Flow rates
Flow rates determine the flow area. For obtaining a higher flow area, flow rates should be kept quite high.
4.4 Flow arrangements
Exchanger effectiveness is the main factor affected by flow arrangements, whether it is co-current or countercurrent.
The heat exchanger’s thermal effectiveness is the ratio of the heat transfer rate to the maximum possible heat transfer rate.
4.6 Pressure drop
It is an important parameter in heat exchanger design. High-pressure drop results in high pumping power, which ultimately increases the cost. On the other hand, the pressure drop also creates turbulence, which increases the heat transfer coefficient.
The fouling of heat exchangers may be defined as the deposition of unwanted material on heat transfer surfaces, impacting its performance. Fouling increases the resistance to fluid flow and lowers the overall heat transfer coefficient of heat exchangers. Fouling also increases the pressure drop across heat exchangers.
The cost of a heat exchanger includes the initial price of the equipment and the installation, operational, and maintenance costs of the device lifespan. At the same time, it is necessary to choose a heat exchanger, which effectively fulfils the applications’ requirements.
Heat exchanges are used in oil refining processes like oil cooling, steam generation, preheating etc.
The heat generated from the boiler can be recovered using heat exchangers
Tubular Exchangers are used in the industrial paint system to maintain temperature.
Plate type exchangers are mainly used in food & chemical processing due to its ease of cleaning.
Condensers and evaporators are used in distillation processes, refrigeration and power plants.
Compact heat exchangers are used in aircraft & automobile industries.