I. Fluid-related factors (core influence on heat transfer driving force)
Fluid is the carrier of heat transfer in a heat exchanger. Its own characteristics and flow state directly determine the heat exchange efficiency, which are key parameters that need to be confirmed when selecting heat exchangers for export.
1. Fluid velocity: a key variable affecting the heat transfer coefficient
Flow velocity is one of the most significant factors influencing the heat transfer coefficient; the higher the velocity, the greater the degree of fluid turbulence, the thinner the convective heat transfer boundary layer, and consequently, the higher the heat transfer coefficient and efficiency. For instance, within the turbulent flow regime-typically at velocities ranging from approximately 0.1 to 1.0 m/s-plate heat exchangers can achieve high heat transfer coefficients. However, it is crucial to note that increasing flow velocity results in a quadratic increase in pressure drop. Therefore, a balance must be struck between heat transfer efficiency and operational energy consumption; during the selection process, the design should be optimized by taking into account the actual allowable pressure drop requirements.
2. Fluid physical properties: determine the strength of the heat exchange foundation.
Physical properties of fluids-such as thermal conductivity, specific heat capacity, viscosity, and density-directly influence the convective heat transfer coefficient. Specifically, higher thermal conductivity and specific heat capacity result in greater heat transfer capability; for instance, water, with a thermal conductivity of approximately 0.6 W/(m·K) and a specific heat capacity of about 4.2 kJ/(kg·K), ranks among the fluids with superior heat transfer performance among common media. Conversely, higher viscosity leads to increased fluid flow resistance, reduced turbulence, and consequently lower heat transfer efficiency; for example, when antifreeze agents such as ethylene glycol are added to water, the thermal conductivity decreases while the viscosity increases, resulting in a corresponding reduction in the overall heat transfer coefficient.
3. Fluid temperature difference: the core driving force of heat transfer
The temperature difference between hot and cold fluids (especially the logarithmic mean temperature difference) is the driving force for heat transfer. The greater the temperature difference, the faster the heat transfer rate and the better the heat exchanger performance.Provided that process requirements are met, appropriately increasing the inlet temperature of the heat source medium or decreasing the inlet temperature of the cold source medium can increase the temperature difference and improve heat exchange efficiency; conversely, too small a temperature difference will lead to insufficient heat transfer driving force and affect the heat exchange effect. In addition, the outlet temperature difference between the hot and cold media needs to be controlled within a reasonable range to avoid excessive temperature difference leading to increased thermal stress on the equipment.
4. Fluid cleanliness: To avoid a decrease in heat exchange efficiency.
If the fluid contains impurities, oil, scale, etc., a fouling layer will form on the heat exchanger surface. The thermal conductivity of the fouling is much lower than that of the metal, which will create significant additional thermal resistance, resulting in a decrease in the heat transfer coefficient and a decline in heat exchange performance.For example, the wastewater side channel is prone to scale buildup due to the accumulation of impurities. Even a thin layer of scale can significantly affect the heat exchange effect. It is necessary to clean the fluid impurities regularly or optimize the channel design to delay scale formation.
II. Equipment structural factors (determining the heat exchange foundation capacity)
The structural design of a heat exchanger directly affects the fluid flow state and the utilization rate of the heat transfer area. It is the core determining factor of the equipment's performance before it leaves the factory and is also the key to the differentiated competition of foreign trade products.
1. Heat transfer area: the fundamental guarantee of heat exchange capacity
The larger the heat transfer area, the more complete the contact between hot and cold fluids, the higher the total heat transfer, and the stronger the heat exchanger performance. The heat transfer area design varies greatly among different types of heat exchangers. For example, the heat transfer area per unit volume of a plate heat exchanger can reach 250-1000㎡/m³, with a compact structure and high heat transfer area utilization.Shell-and-tube heat exchangers have a smaller heat transfer area per unit volume, making them suitable for low-flow-rate heat exchange scenarios. When selecting a model, the heat transfer area must be precisely matched according to the heat load requirements.
2. Heat transfer surface structure: affects turbulence intensity and heat transfer efficiency
The design of the geometry of the heat exchange surface directly affects the flow state and turbulence intensity of the fluid, and thus the heat transfer efficiency. Taking plate heat exchangers as an example, the corrugation depth, corrugation angle, pitch, and other parameters of the corrugated plates are crucial. Herringbone corrugations are a typical example of efficient heat transfer. The corrugations of adjacent plates are in opposite directions, forming a large number of contact points, which both bear pressure and promote turbulence. The corrugation depth is generally 3-5 mm. The greater the depth, the stronger the turbulence, but the greater the pressure drop.The fin type (straight fins, corrugated fins) and spacing of finned tube heat exchangers also affect the air-side heat transfer area and turbulence. Serrated fins are suitable for high flow velocity scenarios to enhance heat transfer.
3. Flow channel design: determines the rationality of fluid flow.
The shape, size, and arrangement of the flow channels determine the flow path and uniformity of the fluid within the heat exchanger. If the flow channels are too narrow, blockage and excessive pressure drop are likely; if they are too wide, flow velocity will decrease, turbulence will weaken, and heat exchange efficiency will be affected; uneven flow channel arrangement will lead to localized fluid stagnation, resulting in uneven heat exchange. Furthermore, the design of the number of flow channels (tube pass, shell pass) needs to be optimized in conjunction with fluid flow rate and pressure drop requirements. In the design of export products, both heat exchange efficiency and operating energy consumption must be considered.
4. Material selection: Affects heat transfer efficiency and equipment lifespan.
The thermal conductivity and corrosion resistance of heat exchanger materials directly affect heat transfer efficiency and equipment stability. Better thermal conductivity results in faster heat transfer. Materials such as copper, stainless steel, and titanium alloys are commonly used high-efficiency thermal conductive materials. Copper has the best thermal conductivity but poor corrosion resistance; titanium alloys have strong corrosion resistance and are suitable for corrosive media environments, but they are more expensive.The appropriate material should be selected based on the characteristics of the medium (such as whether it contains corrosive components), operating temperature and pressure, to avoid heat exchange attenuation or equipment damage due to unsuitable materials.
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