Pagrindiniai plokštelinių šilumokaičių gedimo veiksniai ir prevencinės priemonės

Pagrindiniai plokštelinių šilumokaičių gedimo veiksniai ir prevencinės priemonės

The plate heat exchanger is a new type of high-efficiency heat exchanger. It has the characteristics of high heat exchange efficiency, small heat loss, and compact and lightweight structure. But its use also fails due to its sealing gasket, blockage, scaling, and corrosion of the medium used. This article analyzes the causes and preventive measures for the failure of the plate condenser.

What is a plate heat exchanger?

The plate heat exchanger is a new high-efficiency heat exchanger composed of a series of metal sheets with a certain corrugated shape stacked together. A thin rectangular channel is formed between various plates for heat exchange through half plates. Plate heat exchangers are ideal for heat exchange between liquid-liquid and liquid vapor. It has the characteristics of high heat exchange efficiency, small heat loss, compact and lightweight structure, small footprint, convenient installation and cleaning, wide application, and long service life. Under the same pressure loss, its Heat transfer coefficient is 3-5 times higher than that of the tubular heat exchanger, covering one-third of the floor area of the tubular heat exchanger, and the heat recovery rate can be as high as 90%. As compact and efficient heat exchange equipment, plate heat exchangers have been widely used in many industrial fields in China and have achieved significant economic benefits. However, the failure of plate heat exchangers also causes certain economic losses to our users, mainly due to sealing failure, scaling blockage, and corrosion, making the heat exchanger unable to function effectively.

1. Analysis of the causes of seal failure and relevant preventive measures

The main reasons for seal failure include pressure, temperature, time, and operating conditions.

1.1 Pressure

The types of plate heat exchangers include detachable and sealed types. For example, when a plate-type detachable heat exchanger is used at rated working pressure, leakage is mainly related to abnormal impact loads in the system, which operators do not detect easily, except for the manufacturing quality of the equipment. The impact causes an instantaneous increase in pressure, which may be 1-3 times higher than the normal pressure, causing the sealing gasket installed in the plate heat exchanger to shift and leading to sealing failure. Moreover, because the heat transfer components of the equipment are made of thin plates formed in one go, with a thickness of 0.5-0.8mm, the sealing rigidity and bearing capacity could be better. Moreover, some heat exchange plates have a longer circumference and relatively poor impact resistance, much worse than other heat exchangers, such as tubular ones.
Corresponding preventive measures should be taken: depending on the operating pressure, increase the design pressure of the equipment to 1.5-2 times the operating pressure; Efforts should be made to avoid impact phenomena in the system during use; In special circumstances, measures should be taken to increase the thickness of the plate.

1.2 Temperature

The rapid temperature change also causes sealing failure. When the temperature changes too quickly, the expansion coefficient of the sealing gasket does not match the elastic deformation and the pre-tightening force of the seal, resulting in a decrease in the pre-tightening force of the seal and causing the equipment to withstand less than the rated working pressure.
The following measures should be taken to address this: the pressure rise and temperature rise should be as slow as possible when operating the equipment; Design a compression preload spring on the clamping bolt to compensate for changes in preload.

1.3 Time

The issue with the service time of plate heat exchangers is that as the service time increases, the material of the sealing gasket will also age. It was causing the sealing and sealing effects of plate heat exchangers.
For this reason, the sealing gasket of suitable material should be selected according to the characteristics of the material, and different sealing gaskets should be used according to the different conditions of use.

1.4 Operating conditions

The different conditions of the process medium can also cause sealing failure. The Supersaturation steam temperature will cause seal failure in a short time. Saturated steam at the same temperature can form a water film on the surface of the gasket, which can protect the gasket.
Choosing the appropriate process medium according to the operating situation can also prevent plate heat exchanger failure.

2. Analysis of Failure cause causes of blocking and scaling and corresponding preventive measures

2.1 Blockage

The flow gap of the plate heat exchanger is relatively small, about 2.5-6mm. Particles and debris with a diameter greater than 1.5-3mm can easily block the channel, causing a sharp change in the pressure drop of the equipment, a decrease in flow rate, a decrease in heat exchange efficiency, and easy failure of the equipment. Preventive measures can be taken by installing a filter or flushing device at the inlet of the medium and regularly cleaning or treating it.

2.2 Scaling

After the plate heat exchanger is used, scaling occurs due to the heat transfer or cooling medium, which can reduce the Heat transfer coefficient of the plate heat exchanger and even block the plate channel in serious cases. The plate-type heat exchanger is designed with many supporting contacts to provide pressure support for the flow of the medium. Its side effect is to cause local stagnation of the fluid and the formation of dirt. Over time and at appropriate temperatures, calcium and magnesium ions in the cooling water precipitate and continuously increase, forming a honeycomb-like scale.
Although blockage causes and scaling differ, the effects are the same. The preventive measures are: plate heat exchangers should not be used in dirty materials or prone to scaling; Do not use unsoftened water as the cooling medium; and there should be relevant temperature requirements to avoid sensitive temperature zones where calcium and magnesium ions precipitate. Moreover, when a new system is implemented, the heat exchanger should be separated from the system and circulated for some time before the heat exchanger is used.

3. Corrosion Failure cause analysis and preventive measures

The corrosion failure types of plate heat exchangers include pitting, Crevice corrosion, stress corrosion cracking, uniform corrosion, and other corrosion failures. Corrosion is a complex chemical phenomenon, such as pitting corrosion of rust or scale on the surface of plates; Crevice corrosion caused by sealing gasket groove bottom or plate sealing; The uniform corrosion of all or most of the metal surfaces in contact with the medium.
The following effective preventive measures should be taken: correct selection of plate materials; Regularly clean dirt to disrupt the conditions and development of corrosion; Select non-chlorine-containing adhesives.

4. Reason analysis and measures for design, manufacturing, and installation

Due to the efficient and energy-saving characteristics of plate heat exchangers are widely used in many fields, and the processed media are diverse. In the production process, the conditions are different and diverse. The selection of gaskets is crucial. If the selection is not appropriate, the material cannot meet the physical properties of the process medium, and the elasticity is poor, resulting in pad detachment, elongation, deformation, aging, and fracture during use.
The residual stress generated during stamping during the manufacturing process of plates can cause stress corrosion when used in corrosive environments, directly causing damage to the plates.
The surface of the plate is uneven, making it difficult to install and align the equipment during installation, use, and maintenance, resulting in uneven compression force, affecting sealing, and causing leakage.
In this regard, it is advisable to install the alignment as much as possible during installation to ensure even compression force to maintain compression and prevent leakage during use.

5. Conclusion

An analysis was conducted on the causes of the failure of plate heat exchangers, and corresponding preventive measures were proposed. During the use process, corresponding maintenance and preventive measures should be formulated for using various plate heat exchangers to make them play their due role.