1. Introduction to Plate Heat Exchangers
　　A plate heat exchanger is a new type of high-efficiency heat exchanger formed by stacking a series of metal plates with a certain corrugated shape. Thin rectangular channels are formed between the plates, and heat exchange occurs through the half-plates. Compared with conventional shell-and-tube heat exchangers, under the same flow resistance and pump power consumption, its heat transfer coefficient is much higher, and it has a tendency to replace shell-and-tube heat exchangers within its applicable range.
　　Plate heat exchangers are mainly divided into two categories: frame type (dismountable type) and brazed type. The main types of plates are chevron corrugated plates, horizontal straight corrugated plates, and dimpled plates.
　　1.1 Basic Structure of Plate Heat Exchangers
　　Plate heat exchangers are mainly composed of two parts: the frame and the plates.
　　The plates are made of thin plates of various materials, pressed into various corrugated shapes using different molds, and have corner holes at the four corners for the flow path of the medium. The periphery and corner holes of the plates are sealed with rubber gaskets.
The frame consists of a fixed pressure plate, a movable pressure plate, upper and lower guide rods, and clamping bolts, etc.
The plate heat exchanger is assembled by stacking the plates between the fixed pressure plate and the movable pressure plate and then clamping them with clamping bolts.
1.2 Characteristics of Plate Heat Exchangers (Comparison with Shell-and-Tube Heat Exchangers)
a. High Heat Transfer Coefficient Due to the different corrugated plates being inverted, a complex flow path is formed, causing the fluid to rotate in a three-dimensional flow in the channel between the corrugated plates. This creates turbulence at a lower Reynolds number (generally Re=50~200), resulting in a high heat transfer coefficient, generally considered to be 3~5 times that of shell-and-tube heat exchangers.
b. Large Logarithmic Mean Temperature Difference, Small Terminal Temperature Difference In shell-and-tube heat exchangers, the two fluids flow in the tube side and shell side respectively, and the overall flow is cross-flow. The logarithmic mean temperature difference correction coefficient is small, while plate heat exchangers mostly use parallel flow or counter-flow, and their correction coefficient is usually around 0.95. In addition, the flow of cold and hot fluids in plate heat exchangers is parallel to the heat transfer surface without bypass flow, resulting in a small terminal temperature difference in plate heat exchangers. For water heat exchange, it can be less than 1℃, while shell-and-tube heat exchangers are generally 5℃.
c. Small Footprint Plate heat exchangers have a compact structure, and the heat transfer area per unit volume is 2~5 times that of shell-and-tube heat exchangers. They also do not require a space for removing the tube bundle for maintenance like shell-and-tube heat exchangers. Therefore, to achieve the same heat exchange capacity, the footprint of a plate heat exchanger is about 1/5~1/10 that of a shell-and-tube heat exchanger.
d. Easy to Change Heat Transfer Area or Flow Combination By adding or removing a few plates, the heat transfer area can be increased or decreased; changing the plate arrangement or replacing a few plates can achieve the required flow combination and adapt to new heat exchange conditions. The heat transfer area of shell-and-tube heat exchangers is almost impossible to increase.
e. Light Weight The plate thickness of a plate heat exchanger is only 0.4~0.8mm, while the thickness of the heat transfer tubes in a shell-and-tube heat exchanger is 2.0~2.5mm. The shell of a shell-and-tube heat exchanger is much heavier than the frame of a plate heat exchanger. A plate heat exchanger generally weighs only about 1/5 of a shell-and-tube heat exchanger.
f. Low Price Using the same material and the same heat transfer area, the price of a plate heat exchanger is about 40%~60% lower than that of a shell-and-tube heat exchanger.
g. Easy to Manufacture The heat transfer plates of plate heat exchangers are made by stamping, with a high degree of standardization and can be mass-produced, while shell-and-tube heat exchangers are generally made manually.
h. Easy to Clean For frame-type plate heat exchangers, simply loosening the clamping bolts allows the plate bundle to be loosened and the plates removed for mechanical cleaning. This is very convenient for heat exchange processes that require frequent cleaning.
i. Small Heat Loss Only the outer shell of the heat transfer plate of a plate heat exchanger is exposed to the atmosphere, so heat loss can be ignored, and no insulation measures are needed. Shell-and-tube heat exchangers have large heat losses and require insulation layers.
j. Smaller Capacity 10%~20% of shell-and-tube heat exchangers.
k. Large Pressure Drop per Unit Length Due to the small gap between the heat transfer surfaces and the unevenness on the heat transfer surface, the pressure drop is larger than that of traditional smooth tubes.
l. Not Easy to Fouling Due to sufficient internal turbulence, it is not easy to foul, and its fouling coefficient is only 1/3~1/10 that of shell-and-tube heat exchangers.
m. Working Pressure Should Not Be Too High, Medium Temperature Should Not Be Too High, Possible Leakage Plate heat exchangers use gaskets for sealing, and the working pressure should generally not exceed 2.5MPa, and the medium temperature should be below 250℃, otherwise leakage may occur.
n. Easy to Clog Because the channels between the plates are very narrow, generally only 2~5mm, when the heat exchange medium contains larger particles or fibrous substances, it is easy to clog the channels between the plates.
1.4 Application Scenarios of Plate Heat Exchangers
a. Refrigeration: Used as condensers and evaporators.
b. HVAC: Intermediate heat exchangers used with boilers, intermediate heat exchangers for high-rise buildings, etc.
c. Chemical Industry: Soda ash industry, ammonia synthesis, alcohol fermentation, resin synthesis cooling, etc.
d. Metallurgical Industry: Heating or cooling of aluminate mother liquor, steelmaking process cooling, etc.
e. Mechanical Industry: Cooling of various quenching liquids, cooling of reducer lubricating oil, etc.
f. Power Industry: Cooling of high-voltage transformer oil, cooling of generator bearing oil, etc.
g. Papermaking Industry: Heat recovery in bleaching process, heating of pulp washing liquid, etc.
h. Textile industry: cooling viscose alkali solution, cooling boiling nitrocellulose, etc.
i. Food industry: juice sterilization cooling, heating and cooling of animal and vegetable oils, etc.
j. Oil and fat process: atmospheric drying of soap base, heating or cooling various process liquids.
k. Central heating: district heating of waste heat from thermal power plants, heating bath water.
l. Others: petroleum, medicine, ships, seawater desalination, geothermal utilization.
1.5 Issues to note when selecting plate heat exchangers
1.5.1 Plate type selection
The plate type or corrugated type should be determined according to the actual needs of the heat exchange occasion. For situations with large flow rate and small allowable pressure drop, plate types with low resistance should be selected, and vice versa. According to the fluid pressure and temperature, determine whether to choose a detachable or brazed type. When determining the plate type, it is not advisable to choose plates with too small a single plate area, so as to avoid too many plates, too small inter-plate flow velocity, and too low heat transfer coefficient. This issue should be paid more attention to for larger heat exchangers.
1.5.2 Selection of process and flow path
The process refers to a group of parallel flow channels in the plate heat exchanger in the same flow direction of a medium, while the flow channel refers to the medium flow channel formed by two adjacent plates in the plate heat exchanger. In general, several flow channels are connected in parallel or series to form different combinations of cold and hot medium channels.
The process combination form should be determined according to the heat exchange and fluid resistance calculation, under the condition of meeting the process requirements. Try to make the convective heat transfer coefficients in the cold and hot water channels equal or close, so as to obtain the best heat transfer effect. Because when the convective heat transfer coefficients on both sides of the heat transfer surface are equal or close, the heat transfer coefficient obtains a larger value. Although the flow velocities between the plates of the plate heat exchanger are unequal, the average flow velocity is still used for calculation in the heat exchange and fluid resistance calculation. Because the "U"-shaped single-flow piping is fixed on the pressure plate, it is convenient to disassemble.
1.5.3 Pressure drop check
In the design and selection of plate heat exchangers, there are generally certain requirements for pressure drop, so it should be checked. If the checked pressure drop exceeds the allowable pressure drop, the design and selection calculation needs to be re-performed until the process requirements are met.