A heat exchanger, also known as a heat exchanger, is a device that transfers part of the heat of a hot fluid to a cold fluid, which can transfer heat between fluids with different temperatures. Heat exchanger is a common equipment in chemical, petroleum, power, food and many other industrial sectors, and is one of the main equipment to improve energy utilization. In chemical production, heat exchanger can be used as heater, cooler, condenser, Evaporators and reboilers, etc., play an important role in production.
1 Introduction to heat exchangers
What is a heat exchanger
A heat exchanger is a A device that transfers heat to a cold fluid, also known as a heat exchanger. Its main function is to ensure the specific temperature required by the process for the medium.
It can transfer heat between two or more fluids at different temperatures, so that the heat is transferred from the fluid with a higher temperature to the fluid with a lower temperature, so that the fluid temperature reaches the process specification. To meet the needs of process conditions, the heat exchanger is also one of the main equipment to improve energy utilization.
The role of heat exchangers
Heat exchangers are widely used, such as radiator fins for heating in daily life, condensers in steam turbine installations And oil coolers on aerospace rockets, etc., are heat exchangers. In petroleum, chemical, light industry, pharmaceutical, energy and other industrial production, heat exchangers are often used to heat low-temperature fluids or cool high-temperature fluids, vaporize liquids into vapors or condense vapors into liquids. A heat exchanger can be either a unit equipment, such as a heater, a cooler and a condenser, or a component of a certain process equipment, such as a heat exchanger in an ammonia synthesis tower. Heat exchanger is an important unit equipment in chemical production. According to statistics, the tonnage of heat exchanger accounts for about 20% of the entire process equipment, and some even as high as 30%. Its importance can be imagined.
2 The principle of heat exchanger
The working principle of heat exchanger is: the relative flow direction of the fluid in the heat exchanger generally has two kinds of co-current and counter-current. In co-current flow, the temperature difference between the two fluids at the inlet is the largest and gradually decreases along the heat transfer surface. In countercurrent flow, the temperature difference distribution between the two fluids along the heat transfer surface is relatively uniform.
Under the condition of constant inlet and outlet temperatures of cold and hot fluids, when both fluids have no phase change, the average temperature difference in countercurrent flow is the largest and the smallest in cocurrent flow. Under the condition of completing the same amount of heat transfer, the use of countercurrent can increase the average temperature difference and reduce the heat transfer area of the heat exchanger; if the heat transfer area remains unchanged, the consumption of heating or cooling fluid can be reduced when countercurrent is used. The former can save equipment costs, while the latter can save operating costs, so countercurrent heat transfer should be used as much as possible in design or production.
The principle of the heat exchanger is actually to transfer the heat from one side of the object to the other side through heat conduction. However, according to the different types of heat exchangers, the specific working principles and usage methods are also different. For example, the air heat exchanger is mainly used for air heating in the drying system, and it is the main equipment in the hot air device.
3 The meaning of heat exchanger model
1. Front-end tube box form
A: flat cover tube box;
B: sealed Head tube box;
C: tube box for detachable tube bundle and tube sheet integrated;
N: fixed tube sheet tube box integrated with tube sheet;
D: Special high-pressure pipe box;
2. Shell form
E: One-way shell;
F: With longitudinal
G: split flow;
H: double split flow;
I: U-tube heat exchanger;
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J: Split flow without baffle (or condenser shell);
K: Axe reboiler;
O: External diversion;
3. Back-end structure type
L: fixed tube sheet structure similar to A;
M: fixed tube sheet structure similar to B;
N: Fixed tube sheet structure similar to C;
P: Stuffing box floating head;
S: Hook and loop floating head;
T : Withdrawable floating head;
U: U-shaped tubular type;
W: With casing stuffing box floating head.
4 Types of heat exchangers
Heat exchangers are classified by heat transfer principle
1. Surface heat exchanger
The surface heat exchanger is that two fluids with different temperatures flow in the space separated by the wall, and the heat conduction through the wall and the convection of the fluid on the wall surface , heat exchange between the two fluids. Surface heat exchangers include shell and tube, casing and other types of heat exchangers.
2, Regenerative heat exchanger
Regenerative heat exchanger is a heat storage body formed by solid matter , transfer the heat from the high temperature fluid to the low temperature fluid, the heat medium first reaches a certain temperature by heating the solid material, and then the cold medium is heated through the solid material to achieve the purpose of heat transfer. Regenerative heat exchangers include rotary, valve switching, etc.
3Fluid-connected indirect heat exchanger
The fluid-connected indirect heat exchanger is the The type heat exchanger is a heat exchanger connected by a heat carrier circulating in it. The heat carrier circulates between the high temperature fluid heat exchanger and the low temperature fluid, receives heat in the high temperature fluid, and releases heat to the low temperature in the low temperature fluid heat exchanger. fluid.
4Direct Contact Heat Exchanger
Direct Contact Heat Exchanger is the exchange of two fluids in direct contact Hot equipment, for example, cold water towers, gas condensers, etc.
Heat Exchangers by Purpose
1, Heaters
The heater heats the fluid to the necessary temperature without changing the phase of the heated fluid.
2, Preheater
The preheater preheats the fluid and provides standard process parameters for process operation.
3, Superheater
A superheater is used to heat a fluid (process gas or steam) to a superheated state.
4, Evaporator
The evaporator is used to heat the fluid to a temperature above the boiling point to evaporate the fluid, generally with phase change.
Classified by the structure of heat exchangers
Can be divided into: floating head heat exchangers, fixed tube sheet heat exchangers, U-shaped tube sheet heat exchangers Heaters, plate heat exchangers, etc.
5 Heat Exchanger Selection
There are many types of heat exchangers, and each type has a specific application range. A heat exchanger with good performance in a certain occasion may change its heat transfer effect and performance greatly if it is changed to another occasion.
Therefore, it is very important to select the correct type of heat exchanger for the specific situation. There are many factors that need to be considered in the selection of heat exchangers, mainly including:
1. Heat load and flow rate;
2. The nature of the fluid;
3. Temperature, pressure and allowable pressure drop range;
4. Requirements for cleaning and maintenance;
4. Equipment structure, material, size, weight;
6. Price, safety of use and service life;
In addition to considering the above factors in the selection of heat exchangers, structural strength, material sources, processing conditions, sealing properties should also be considered. , safety, etc. All these are often mutually restricting and influencing each other, and are solved through the optimization of the design.
For different process conditions and operating conditions, we sometimes use special types of heat exchangers or special heat exchange tubes to reduce costs. Therefore, the requirements of process conditions and mechanical design should be comprehensively considered, and the appropriate type of heat exchanger should be correctly selected to effectively reduce the energy consumption of the process.
For engineers and technicians, when designing heat exchangers, they should pay enough attention to the reasonable selection of types, economical operation and cost reduction, and when necessary, they must be calculated technically. Analysis of economic indicators, comparison of investment and operating costs, so that the design can achieve the best design under the specific conditions.
6 Heat Exchanger Design
Basic Requirements for Heat Exchanger Design
With the development of economy, various types and Various types of heat exchangers are developing rapidly, and heat exchangers with new structures and new materials are constantly emerging. In order to meet the needs of development, China has established standards for certain types of heat exchangers and formed series. A perfect heat exchanger should meet the following basic requirements when designing or selecting:
1. Reasonably realize the specified process conditions;
2. The structure is safe and reliable;
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3. Easy to manufacture, install, operate and maintain;
4. Economically reasonable.
Design standard of plate heat exchanger
1. High efficiency and energy saving: its heat transfer coefficient is 3000～4500kcal/m2·°C·h, which is more efficient than tube The thermal efficiency of the shell heat exchanger is 3 to 5 times higher.
2. Compact structure: the plates of the plate heat exchanger are closely arranged. Compared with other types of heat exchangers, the plate heat exchanger occupies less floor space and space. The plate heat exchanger is only 1/5 of the shell and tube heat exchanger.
3. Easy to clean and easy to disassemble: the plate heat exchanger is clamped by clamping bolts, so it is easy to disassemble and clean, and can be opened and cleaned at any time. High, not easy to scale.
4. Long service life: The plate heat exchanger is made of stainless steel or titanium alloy plates, which is resistant to various corrosive media.
5. Strong adaptability: The plate heat exchanger plate is an independent component, and the process can be increased or decreased at will according to the requirements, in various forms; it can be applied to various different process requirements.
6. No liquid leakage: The sealing groove of the plate heat exchanger is provided with a liquid leakage channel, and various media will not be connected with each other. Even if there is leakage, the medium is always discharged outward.
7 Heat Exchanger Installation
1. The basis for installing the heat exchanger must be satisfied so that the heat exchanger does not sink, or the pipeline transmits excessive deformation to the heat exchanger. Take over.
The foundation is generally divided into two types: one is a brick saddle foundation, the heat exchanger is directly placed on the saddle foundation without a saddle support, and the heat exchanger and the foundation are not fixed. , which can move freely as thermal expansion requires. The other is a concrete foundation, and the heat exchanger is firmly connected to the foundation by an anchor bolt through a saddle support.
2. Before installing the heat exchanger, the basic quality inspection and acceptance work should be strictly carried out. The main items are as follows:
Foundation surface overview; foundation elevation, plane position, shape and Whether the main dimensions and the reserved holes meet the actual requirements; whether the position of the anchor bolts is correct, whether the threads are in good condition, whether the nuts and washers are complete; whether the foundation surface on which the pads are placed is flat, etc.
3. After the foundation acceptance is completed, put a horn on the foundation before installing the brazed plate heat exchanger. The surface of the foundation where the horn is placed must be leveled so that the two can be in good contact . The thickness of the horn can be adjusted so that the heat exchanger can reach the designed level. After the horn is placed, the stability of the heat exchanger on the foundation can be increased, and its weight can be evenly transferred to the foundation through the horn. The horns can be divided into flat horns, inclined horns and open horns. Among them, the inclined horns must be used in pairs. There should be pads on both sides of the anchor bolts, and the installation of the pads should not hinder the thermal expansion of the heat exchanger.
4. After the heat exchanger is in place, use a leveler to level the heat exchanger, so that each pipe can be connected to the pipeline without stress. After leveling, the inclined pad iron can be welded firmly with the base, but must not be welded with the flat pad iron or slide plate below. When two or more overlapping heat exchangers are installed, the upper heat exchanger should be installed after the alignment of the lower heat exchanger is completed and the anchor bolts are fully fixed. Before installation, the pumpable tube bundle heat exchanger should be checked by core pulling, cleaned, and the sealing surface and baffle should be protected when the tube bundle is pumped. When moving and lifting the tube bundle, the tube bundle should be placed on a dedicated support structure to avoid damage to the heat exchange tubes.
5. According to the form of the heat exchanger, there should be enough space at both ends of the tube plate heat exchanger to meet the needs of condition (operation) cleaning and maintenance.
The fixed head end of the floating head heat exchanger should have enough space to be able to pull out the tube bundle from the shell, and the outer head end must also leave more than one meter for the installation and removal of the outer head cover and the floating head cover.
6. Sufficient space should be reserved at both ends of the fixed tube-sheet heat exchanger so that the tubes can be pulled out and replaced, and the inside of the tubes should be cleaned mechanically. The tubes can be brushed at both ends, and the fixed head cover of the U-shaped tube heat exchanger should leave enough space to extract the tube bundle, and also leave enough space at the opposite end to be able to remove the shell.
7. The heat exchanger shall not operate under the conditions specified on the nameplate. The temperature and pressure drop of the medium on the tube and shell side should be monitored frequently, and the leakage and scaling of the heat exchange tubes should be analyzed.
8 Heat exchanger cleaning
1. Descale cleaning
Determine the cleaning cycle time and the amount of chemicals according to the amount of scale, and add the configured descaler cleaning to the circulating water of the cleaning tank in proportion 2. Clean with clean water
After connecting the cleaning equipment and the boiler, clean it with clean water for 10 minutes, and check the system status and whether there is any leakage , while cleaning the rust.
3. Stripping and anti-corrosion cleaning
Add surface stripping agent and slow-release agent to the circulating water of the cleaning tank according to the proportion, and circulate for 20 minutes to make the dirt and the parts cleaned. Separation, and anti-corrosion treatment on the surface of objects without scale.
4. Passivation coating treatment
Add passivation coating agent to carry out passivation coating treatment for boiler cleaning system to prevent corrosion of pipelines and components and new rust formation.
