In the plate heat exchanger, only the outer shell of the heat transfer plate is exposed to the atmosphere, so the heat loss is negligible, and no heat preservation measures are required. However, the heat loss of the shell and tube heat exchanger is large, which requires an insulating layer.
Characteristics of plate heat exchanger
1. Smaller capacity is 10% ~ 20% of the shell and tube heat exchanger.
2. The pressure loss per unit length is large. Because the gap between the heat transfer surfaces is small and there are irregularities on the heat transfer surface, the pressure loss is greater than that of the conventional smooth tube.
3. Not easy to scale Because the interior is fully turbulent, it is not easy to scale, and the scale factor is only 1/3 ~ 1/10 of the shell and tube heat exchanger.
4. The working pressure should not be too high, the medium temperature should not be too high, there may be leakage. The plate heat exchanger is sealed with a gasket, the working pressure should not generally exceed 2.5MPa, the medium temperature should be below 250 ℃, otherwise it may leak.
5. Easy to block Because the channel between the plates is very narrow, generally only 2 ~ 5mm, when the heat exchange medium contains large particles or fibrous materials, it is easy to block the channel between the plates.
Application of plate heat exchanger
a. Refrigeration: used as a condenser and evaporator.
b. HVAC Brazed Plate Heat Exchangers : intermediate heat exchangers used in conjunction with boilers, intermediate heat exchangers in 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, cooling of steelmaking process, etc.
e. Machinery industry: various quenching fluid cooling, reducer lubricating oil cooling, etc.
f. Power industry: high-voltage transformer oil cooling, generator bearing oil cooling, etc.
g. Paper industry: heat recovery from bleaching process, heating of washing liquid, etc.
h. Textile industry: cooling of aqueous solution of viscose silk alkali, cooling of boiling nitrocellulose, etc.
i. Food industry: fruit juice sterilization and cooling, animal and vegetable oil heating and cooling, etc.
j. Oil and fat technology: soap-based atmospheric drying, heating or cooling various process liquids.
k. Centralized heating: heating the waste heat area of a thermal power plant, heating bath water.
l. Others: petroleum, medicine, ships, seawater desalination, geothermal utilization.
Problems that should be paid attention to in the selection of plate heat exchanger
Plate type or corrugated type should be determined according to the actual needs of heat exchange occasions. For the case where the flow rate is large and the pressure drop is small, the plate type with low resistance should be selected, otherwise, the plate type with high resistance should be used. According to the fluid pressure and temperature, determine whether to choose the detachable or brazed type. When determining the plate type, it is not advisable to select plates with too small veneer area, so as to avoid too many plates, small flow velocity between plates, and low heat transfer coefficient. This problem should be paid more attention to for larger heat exchangers.
Choice of process and flow path
In general, several flow channels are connected in parallel or in series to form different combinations of cold and hot medium channels. The flow combination form should be calculated based on heat exchange and fluid resistance, and determined under the requirements of process conditions. Try to make the convection heat transfer coefficients in the cold and hot water flow channels equal or close, so as to obtain excellent heat transfer effect. Because the convective heat transfer coefficients on both sides of the heat transfer surface are equal or close to each other, the heat transfer coefficient obtains a larger value. Although the flow rate between the plates of the plate heat exchanger is not equal, in the calculation of heat transfer and fluid resistance, the average flow rate is still used for calculation. Since the U-shaped single-process nozzles are fixed on the pressure plate, it is easy to disassemble and assemble.
Pressure drop check
The design and selection of plate heat exchangers generally have certain requirements for pressure drop, so they should be checked. If the check pressure drop exceeds the allowable pressure drop, the design selection calculation shall be performed again until the process requirements are met