Working Principles of Nine Heat Exchangers

Working Principles of Nine Heat Exchangers

Heat exchanger is an energy-saving device that transfers heat between two or more fluids at different temperatures. For large-area heating, the existence of heat exchanger is essential.
According to the heat transfer mode of heat exchangers, heat exchangers can be divided into three categories:

▲shell-and-tube heat exchanger

According to the compensation measures adopted, shell-and-tube heat exchangers can be divided into four types: fixed tube-plate heat exchanger, floating head heat exchanger, U-tube heat exchanger and stuffing box heat exchanger.

Second, the fixed tube-plate heat exchanger

Fixed tube-plate heat exchanger is a kind of shell-and-tube heat exchanger. The tubesheet at both ends of the fixed tubesheet heat exchanger is connected to the shell by welding and mainly consists of shell, tubesheet, tube bundle, top cover (head) and other components.

▲Fixed tube-plate heat exchanger

The advantages of fixed tube-plate heat exchanger are:

◆ simple structure;

◆ Within the same shell diameter, the number of pipes is the largest and the bypass is the smallest.

◆ Each heat exchange tube can be replaced and the inside of the tube is easy to clean.

The disadvantages of fixed tube-plate heat exchanger are:

◆ Mechanical cleaning is not allowed on the shell side;

◆ When the temperature difference between the heat exchange tube and the shell is large (greater than 50℃), temperature difference stress will be generated. The solution is to install expansion joints on the shell, so the shell side pressure cannot be too high limited by the strength of the expansion joints.

◆ It is only suitable for working occasions where the fluid is clean and not easy to scale, and the temperature difference between the two fluids is not large or the temperature difference is large but the shell side pressure is not high.

Three, floating head heat exchanger

Floating head heat exchanger is a kind of shell-and-tube heat exchanger. It has one end of the tube plate which is not connected to the shell and can float freely along the axial direction. It is also called floating head. The floating head consists of a floating tube plate, a hook ring and a floating head end cover, which are detachably connected, and the tube bundle can be extracted from the shell.

▲movable tube sheet heat exchanger

The advantages of floating head heat exchanger are:

◆ If there is a temperature difference between the heat exchange tube and the shell, that is, when the shell or heat exchange tube expands, there will be no temperature difference stress;

◆ The tube bundle can be pulled out of the shell to facilitate cleaning inside and between tubes.

The disadvantage of floating head heat exchanger is:

◆ Complex structure, large amount of materials and high cost;

◆ If the seal between the floating head cover and the floating tube plate is lax, internal leakage will occur, resulting in the mixing of the two media.

Four, U-tube heat exchanger

U-tube heat exchanger is a kind of shell-and-tube heat exchanger, which consists of tube sheet, shell, tube bundle and other parts. Each tube of the U-tube heat exchanger is bent into a U-shape, the inlet and outlet are respectively installed on two sides of the same tube plate, and the head is divided into two chambers by a partition plate, so that each tube can freely expand and contract independently of other tubes and shells.

▲U-tube heat exchanger

The advantages of U-tube heat exchanger are:

◆ The tube bundle can float freely without considering temperature difference stress, and can be used in large temperature difference situations.

◆ It has only one tubesheet, few flanges, few leakage points and simple structure.

◆U-tube heat exchanger is reliable in operation and low in cost.

The disadvantages of U-tube heat exchanger are:

◆ Cleaning inside the pipe is difficult. Because the tubes need to have a certain bending radius, the utilization rate of the tubesheet is relatively low.

◆ The distance between tubes in the innermost layer of the tube bundle is large, and the shell side is easy to short circuit. When the flow velocity in the pipe is too high, the U-shaped bent pipe section will be seriously eroded, affecting its service life.

◆ If the inner tube is damaged, it cannot be replaced, thus the rejection rate is high.

Five, spray heat exchanger

Sprinkler heat exchanger is a type of tubular heat exchanger, which fixes heat exchange tubes in rows on a steel frame, hot fluid flows in the tubes, and cooling water is uniformly sprayed from the upper spray device, so it is also called spray cooler. The outside of the spray heat exchanger is a layer of liquid film with higher turbulent degree, and the heat transfer coefficient outside the tube is much higher than that of immersion type.

In addition, the spray heat exchanger is mostly placed in the place where air circulates, and the evaporation of cooling water also takes away some heat, which can reduce the temperature of cooling water and increase the driving force of heat transfer. Compared with immersion heat exchanger, the heat transfer effect of spray heat exchanger is greatly improved.

▲Spray heat exchanger

The advantages of spray heat exchanger are:

◆ simple structure and low cost;

◆ It can reduce the temperature of cooling water and increase the driving force of heat transfer.

◆ Capable of withstanding high pressure;

◆ Easy maintenance and cleaning, low water quality requirements.

The disadvantage of spray heat exchanger is:

◆ uneven cooling water spraying will affect the heat transfer effect;

◆ Can only be installed outdoors.

Six, double tube heat exchanger

Double-tube heat exchanger is a kind of tube heat exchanger. It is a concentric tube formed by connecting two standard tubes with different sizes. The outer tube is called shell side and the inner tube side. Two different media can flow reversely (or in the same direction) in the shell side and the tube side to achieve the effect of heat exchange. Bushing heat exchanger usually consists of shell (including inner shell and outer shell), U-shaped elbow, stuffing box, etc.

▲Double tube heat exchanger

The advantages of double-tube heat exchanger are:

◆ The structure is simple and can withstand high pressure.

◆ The heat transfer area can be increased or decreased as required, and the application is convenient.

The disadvantages of double-tube heat exchanger are:

Mixer tube indirect head, easy to leak;

◆ Large floor area and large metal consumption per unit heat transfer surface.

Seven, jacketed heat exchanger

Jacket heat exchanger is a kind of plate heat exchanger, which is made by installing jacket on the outer wall of the container and has simple structure. However, its heating surface is limited by the container wall and its heat transfer coefficient is not high. In order to improve the heat transfer coefficient and make the liquid in the kettle heated evenly, a stirrer can be installed in the kettle. When cooling water or non-phase change heating agent is introduced into the jacket, spiral baffles or other measures to increase turbulence can also be arranged in the jacket to improve the heat transfer coefficient on one side of the jacket. In order to supplement the shortage of heat transfer surface, snake pipes can also be installed inside the kettle. Jacketed heat exchangers are widely used for heating and cooling in the reaction process.

▲Jacketed heat exchanger

The advantages of jacketed heat exchanger are:

◆ simple structure;

◆ Processing is convenient.

The disadvantages of jacketed heat exchangers are:

◆ Small heat transfer area and low heat transfer efficiency.

Eight, spiral plate heat exchanger

Helical plate heat exchanger is a kind of plate heat exchanger, which is formed by rolling two parallel metal plates, and two concentric spiral channels are formed in it. A partition plate is arranged at the center of the heat exchanger to separate the spiral channels, and a spacing column is welded between the two plates to maintain the channel spacing.

▲spiral plate heat exchanger

The advantages of spiral plate heat exchanger are:

◆ High heat transfer coefficient;

◆ Not easy to scale and block;

◆ Can utilize heat source with lower temperature;

◆ Compact structure.

The disadvantage of spiral plate heat exchanger is:

◆ Operating pressure and temperature should not be too high;

◆ It is not easy to overhaul.

Nine, heat pipe heat exchanger

The heat pipe is formed by filling a certain amount of certain working liquid into a sealed metal pipe for pumping out non-condensable gas. The working liquid absorbs heat at the hot end and boils and vaporizes. The generated steam flows to the cold end to condense and release latent heat. The condensate returns to the hot end and boils and vaporizes again. As the cycle repeats, heat is continuously transferred from the hot end to the cold end.

▲Heat pipe heat exchanger

The advantages of the heat pipe heat exchanger are:

◆ simple structure, long service life and reliable operation;

◆ It has extremely high thermal conductivity and good isothermal property.

◆ The heat transfer area on the hot and cold sides can be changed at will, heat transfer can be carried out at a long distance, and temperature can be controlled.

The disadvantages are:

◆ Poor oxidation resistance and high temperature resistance.

Matters needing attention in the use of heat exchangers:

◆ Keep the pipe network clean. No matter before or after the work is completed, the pipe network must be cleaned to avoid blockage of the heat exchanger. In addition, attention should be paid to the cleaning of the dirt remover and filter so that the whole heat exchange work can be completed smoothly.

◆ Strictly control demineralized water. Before heating medium treatment, check the water quality problems in the system and softening tank, and inject it after the water quality is qualified.

◆ Check the new system. For the new system, it cannot be used alternately with the heat exchanger immediately, but the heat exchanger can only be incorporated into the system after the operation inspection is completed in order to make the new system run within the specified time period. The purpose of this is to prevent impurities in the pipe network from damaging the internal equipment of the heat exchanger.