Views:1 Author:Site Editor Publish Time: 2020-04-16 Origin:Site
Indirect heating must have two conditions to perform thermal energy displacement. It can be seen from the heat transfer formula:
1. There must be a temperature difference in conduction, that is, △ T ≠ 0. It cannot be isothermally exchanged. Generally, △ T≥20 ℃ is required, otherwise the smaller the temperature difference, the larger the heat exchange area.
2. K value. The K value of a metal heat transfer system is constant. If 0.1 mm thick scale is formed on the metal surface, the K value will be reduced by several times, and the heat exchange surface will be reduced by several times. This is the reason for using soft water in heating and air conditioning systems. Since steam and water are two separate systems, the pressure will not affect each other.
Steam heat exchange should adopt two-stage heat exchange: the first stage is steam-water heat exchange (using latent heat); the second stage is water-water heat exchange (using sensible heat). In saturated steam, because the latent heat is 6-10 times greater than the sensible heat, due to the cost of the project, steam-water primary heat exchange is generally used.
Indirect heat exchange equipment can be divided into tube heat exchangers, spiral plate heat exchangers, bellows heat exchangers, floating coil heat exchangers, mechanical heat exchange units, vortex hot film heat exchangers.
1. Tube heat exchanger
Adopting laminar heat transfer, the thermal efficiency of primary heat exchange does not exceed 80%, the condensate temperature is high, exceeds 100 ℃, easy to vaporize, when the steam pressure is lower than 0.2MPa, it is easy to produce the impact noise of steam and water, and has the function of storing hot water , The water temperature is hot and cold. The portion is heavy and easy to scale. Due to the need for a certain distance of pumping for maintenance, and the large area and high price, the product is basically eliminated.
2. Spiral plate heat exchanger
Using laminar heat transfer, there are two different materials: one is carbon steel and the other is stainless steel. The thermal efficiency does not exceed 80% and cannot be repaired for one-time use. The tube-and-tube type covers a relatively small area, is easy to scale, and has a low cost. The temperature of the condensate water exceeds 100 ° C. When it is easy to vaporize and the steam pressure is less than 0.2 MPa, the condensate and steam generate soda impact noise. It is not widely used because of its low price.
3. Bellows heat exchanger
Adopting vibration and laminar mixed heat transfer, the thermal efficiency of the first-level heat exchange does not exceed 80%, it occupies a small area, is easy to scale, the condensation temperature exceeds 100 ° C, and is easy to vaporize. When the steam pressure is less than 0.2 MPa, water and steam produce impact noise Because of its small size, it was a popular product in the early 1990s.
4. Floating coil heat exchanger
Using vibration heat transfer, the technology is basically developed from the introduction of American technology. It is the use of steam kinetic energy, the heat exchange surface generates vibration to destroy the water film, and the laminar flow becomes stable flow heat transfer. It is an advanced method in the world and can use the temperature difference to shed hard scales, but it is not effective for micro-pore soft scales. Depending on the manufacturing process, the thermal efficiency is about 80-90%. Steam below 0.2MPa is easy to produce soda impact noise. The storage type has a large temperature difference between upper and lower, and there are still water areas. And the floor space is small, the price is slightly higher, and it is used in many places. All domestic manufacturers have not tested it. The steam pressure and water pressure are appropriate. The vibration frequency is too large. The water molecules do not contact the heat exchange surface and cannot transfer heat. If it is too small, it will not damage the water film and affect the heat transfer coefficient.
5. Mechanical heat exchange unit
It is a two-stage heat exchange: the first stage turns steam into high-temperature water by spraying method; the second stage uses a pump with high flow rate (3M / S) to produce high-speed friction in the plate heat exchanger and then produce a surge heat exchanger. This heat exchanger unit has the highest thermal efficiency in the world (97.8%), condensate is below 100 ℃, steam below 0.2MPa is normally used, but due to the large footprint, high cost (more than 5 times higher than floating coil), consumption Electricity, large maintenance, expensive price, can not be universally used.
6. Vortex hot film heat exchanger
The vortex heat film heat exchanger uses the latest vortex heat film heat transfer technology to increase the heat transfer effect by changing the fluid motion state. When the medium passes through the surface of the vortex tube, the surface of the tube is strongly washed, thereby improving the heat exchange efficiency. Up to 10000W / m2 ℃.
This structure realizes the functions of corrosion resistance, high temperature resistance, high pressure resistance, and anti-fouling. The fluid channels of other types of heat exchangers are in the form of fixed-direction flow, forming a flow around the surface of the heat exchange tube, and the convective heat transfer coefficient is reduced.
The biggest feature of vortex hot film heat exchanger is the unity of economy and safety. Due to the consideration of the flow relationship between the heat exchange tubes, the heat exchange tubes and the shell, the turbulent flow is no longer forced by the baffle to force the turbulence, but the alternating vortex flow is naturally induced between the heat exchange tubes, and Under the premise of ensuring that the heat exchange tubes do not rub against each other, the proper tremor power should be maintained. The heat exchange tubes have good rigidity and flexible configuration, and will not collide with each other, which not only overcomes the problem of damage caused by the collision between the floating coil heat exchangers, but also avoids the problem of easy scaling of ordinary shell and tube heat exchangers.