Views:0 Author:Site Editor Publish Time: 2020-04-26 Origin:Site
The sealing operation of the heat exchanger mainly depends on the comparison between the instantaneous sealing stress of the rubber gasket due to compression and the operating pressure of the heat exchanger. If the sealing stress is greater than the operating pressure, the sealing can be maintained, otherwise, the lagoon occurs. Therefore, the most important thing for heat exchanger sealing is to make the sealing stress as high as possible and keep it as long as possible.
Rubber and metal springs have different elasticities. It is a viscoelastic material, which means that rubber will undergo stress relaxation under long-term deformation, that is, under constant tension or pressure, the sealing stress will increase with time. attenuation. Higher stress relaxation is an important factor limiting the service life of heat exchanger rubber gaskets. There are two types of stress relaxation, one is physical relaxation, which is caused by the rearrangement between polymer molecules and filler particles.This rearrangement gradually approaches equilibrium as the rubber deforms. The number is linear.
The other is chemical relaxation, which is caused by the cracking of chemical bonds in the rubber network. Oxidation and temperature are important factors affecting this relaxation. Therefore, the rate of stress relaxation depends to a large extent on the temperature and the applicable operating temperature range of each rubber gasket. Low temperature nitrile is suitable for the low temperature range, so as the temperature rises, its stress relaxation deteriorates rapidly, while the fluorine rubber suitable for the high temperature range is the opposite. The stress relaxation depends on the composition (formulation) and processing procedure of the rubber gasket, and the The degree (crosslink density) is also very important. The gasket is not well-made, the cross-link density is low, the stress relaxation rate is high, and the gasket life is short. However, although increasing the crosslink density can improve the stress relaxation, it reduces the tear strength of the rubber, and as a result, the rubber gasket breaks under high stress. Sealing stress is a function of temperature. Different rubbers have different temperature dependence. Fluoroelastomer's sealing stress shows a strong dependence on temperature, so cold leakage may occur when the fluoroelastomer plate heat exchanger is installed.
In addition to stress relaxation, another characteristic of rubber gaskets that is often overlooked is that the physical properties are also very strongly dependent on temperature. At high temperatures, the tear strength and tear strength of the rubber gasket will be reduced.Although the reduction range of different types of rubber gaskets is different, if the gasket is compressed and exceeds its ultimate strength, the gasket may be Mechanical damage, such as crushing, has occurred.
The life of the heat exchanger rubber piece can be described as follows. When the gasket is installed on the plate and compressed to a nominal size after assembly, the initial sealing stress is generated, and the stress relaxation of the rubber gasket also begins. During the period of transportation, storage and installation of the heat exchanger, the temperature of heat exchange is lower and the stress relaxation is also more relaxed. After driving, the temperature began to rise and the stress relaxation gradually became serious. After the heat exchanger has been running for a certain period of time, it needs to be stopped for maintenance and cleaning, and the heat exchanger is cooled to the original temperature. On, it can still maintain a seal. After restarting, the sealing stress and stress relaxation start again. After several repetitions, under the condition of cold and heat exchange, the sealing stress of the rubber gasket eventually fell below the minimum sealing stress necessary to maintain the seal.