How does a heat exchanger achieve convection heat transfer?
Plate heat exchangers mainly use convection between two cold and hot media to achieve heat exchange, and liquid-liquid exchange is one of the commonly used methods of heat exchangers.
Convection heat transfer is one of the most common and fundamental methods of heat transfer. During the heat transfer process, the liquid medium is always in contact with the heat exchanger wall. Therefore, heat transfer is achieved by the continuous countercurrent flow of liquids. The heat is then exchanged through the temperature difference between the heat exchanger wall and the fluids. This is what we are talking about today: convection heat transfer.
Plate heat exchangers achieve efficient convection heat exchange between two fluids with different temperatures (usually cold fluid and hot fluid) in an isolated state through special plate structure design, forced fluid conduction and efficient heat transfer path.Its core principle can be broken down into three key links: structural design → fluid flow → heat transfer. The specific implementation process is as follows:
The heat transfer capacity of a plate heat exchanger depends primarily on the special design of the heat exchange plates. These structures directly determine the flow pattern and heat transfer area of the fluid, and are the basis of convective heat transfer:
The essence of convective heat transfer is the combination of "macro-fluid flow + molecular micro-heat transfer". Plate heat exchangers use external power (pumps, fans) to force fluid flow, driving the heat transfer process in two steps:
Driven by external pumps, the cold and hot fluids enter their respective independent flow channels:
The cold fluid enters another set of flow channels from the "cold fluid inlet," also flowing in a turbulent pattern, exchanging heat with the plates.
Due to the extremely small gaps between the flow channels (typically 2-5 mm), the fluid is "squeezed" during flow, further enhancing the turbulent flow and preventing localized fluid stagnation that could reduce heat transfer efficiency.
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The core of convective heat transfer is "heat transfer from hot fluid to cold fluid". The plate acts as an isolation and heat transfer medium, playing a key role in heat transfer. It is completed in three steps:
First: Thermal fluid → Plate (Convection heat transfer)
When the hot fluid flows turbulently, the high-temperature molecules collide violently with the surface of the plate, transferring heat to the plate through "convection" (at this time, the temperature of the side of the plate closest to the hot fluid increases).
Second time: inside the plate (heat conduction)
The plates are made of metal (with high thermal conductivity, such as stainless steel (about 16W/(m・K) and titanium alloy (about 17W/(m・K))). Heat is rapidly transferred from the high-temperature side (hot fluid side) to the low-temperature side (cold fluid side) within the plates through "molecular thermal motion."
Third time: Plate → Cold fluid (convection heat transfer):
The low-temperature side of the plate contacts the cold fluid, and through the collision of molecules of the cold fluid in the turbulent flow, the heat is transferred to the cold fluid again through "convection" (at this time the temperature of the cold fluid increases and the temperature of the hot fluid decreases).
In addition to the core principles, the following design details of the plate heat exchanger also provide guarantees for convective heat transfer: Detachable structure: maintains cleanliness.
Typically, because the two media used are different, their flow dynamics within the equipment are also different, which can lead to significant differences in convective heat transfer.Convective heat transfer is generally divided into two situations. One is natural convection heat transfer, which is the flow heat transfer generated by the different temperatures and densities of two media through the wall. The second is forced convection heat transfer, which is the flow heat transfer generated by external forced forces (such as pumps, fans and other equipment).In the case of forced convection, the flow rate of the liquid itself will be higher than the flow rate in the natural state, and the efficiency of convective heat transfer will also be high. For example, the heat transfer coefficient of air in natural flow is only 5~25W/(m2.℃), but when forced flow is performed, the heat transfer coefficient of air increases to 10~100W(m2.℃).
There are many factors that affect the heat transfer efficiency of the medium, such as the physical properties of the fluid medium itself: density, specific heat capacity, thermal conductivity, etc., as well as the design of the heat exchange equipment itself: the size of the heat exchange plate, the shape of the plate, etc., and the flow method of the medium in the equipment, all of which will affect the actual efficiency of convective heat transfer.