Calculation method for plate heat exchangers

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Below is the calculation method for plate heat exchangers. I am Yang Junfang from Shanghai Erxing Heat Exchange Chemical Equipment Co., Ltd. Our company specializes in the production of detachable plate heat exchangers, fully welded plate heat exchangers, temperature control systems, and various types of units. My contact number is 13297581286, my QQ ID is 673655616, and my email address is yangjfghjyr@163.com. If you have any needs in this area or would like to discuss technical matters, please feel free to contact me. Calculation method for plate heat exchangers: The calculation of plate heat exchangers is a relatively complex process. The currently popular methods are the logarithmic mean temperature difference method and the NTU method. Before computers became widespread, most manufacturers used approximate estimation of calculation parameters and flow velocity-total heat transfer coefficient curve methods. Currently, more and more manufacturers are using computer calculations, which makes the process calculations for plate heat exchangers faster, more convenient, and more accurate. The following briefly describes the general calculation method for plate heat exchangers in the absence of phase change; this is a design calculation method based on the relationships between heat transfer and pressure drop criteria. The following five parameters are essential for the selection and calculation of plate heat exchangers: total heat transfer rate (in kW); inlet and outlet temperatures on the primary and secondary sides; allowable pressure drops on the primary and secondary sides; maximum operating temperature; and maximum operating pressure. If the flow rate of the heat transfer medium, its specific heat capacity, and the temperature difference between the inlet and outlet are known, the total heat transfer rate can be calculated. Temperature T1 = inlet temperature on the hot side; T2 = outlet temperature on the hot side. t1 = inlet temperature on the cold side; t2 = outlet temperature on the cold side. Heat load: The heat flow balance equation reflects the relationship between the temperature changes of the two fluids during heat exchange. In the case of a well-insulated heat exchanger with no heat losses, for a steady-state heat transfer process, the heat flow balance relationship is as follows: (Heat flow released by the hot fluid) = (Heat flow absorbed by the cold fluid). When performing heat balance calculations, the expressions vary depending on whether there is a phase change or not in the heat transfer process. 　　(1) Heat transfer process without phase change 　　　 Where 　　　Q----the heat flow rate absorbed by the cold fluid or released by the hot fluid, W ； 　　　mh, mc-----mass flow rate of hot and cold fluids, kg/s ； 　 　　　Cph, Cpc------Specific isobaric heat capacities of hot and cold fluids, kJ/(kg·K) ； 　　　T1, t1 ------ inlet temperatures of hot and cold fluids, K ； 　　　T2, t2------Exit temperatures of the hot and cold fluids, K. 　　(2) Heat transfer with phase change During the heat exchange process between two fluids, phase change occurs in one of the fluids, such as vapor condensation or liquid boiling. The formula for calculating the heat flow rate is as follows: When phase change occurs on only one side When phase change occurs on both sides, such as in a situation where one side undergoes condensation while the other side undergoes boiling Where, r, r1, r2-------- are the heat of phase change for each fluid, in J/kg ； 　　　　D, D1, D2--------phase change mass flow rate, kg/s. 　　For the heat balance calculation during phase change in subcooled or superheated streams, it should be performed by summing the values in segments using the above method. Logarithmic mean temperature difference (LMTD): The logarithmic mean temperature difference is the driving force behind heat transfer in heat exchangers, and its value directly affects the ease of heat transfer within such exchangers. In some special cases, it is not possible to calculate the LMTD; in such situations, the arithmetic mean temperature difference is used as a substitute. The methods for calculating the LMTD differ depending on whether the fluids flow counterflow or co-currently. In some special cases, the arithmetic mean temperature difference is used in place of the logarithmic mean temperature difference. During counterflow: During co-current flow: Thermal length (F) is related to the temperature difference on one side and the logarithmic mean temperature difference. F = dt/LMTD. Heat transfer is influenced by the physical properties of the following four media: density, viscosity, specific heat capacity, and thermal conductivity. The overall heat transfer coefficient is a parameter used to measure the heat transfer resistance of a heat exchanger. The heat transfer resistance is primarily determined by factors such as the material and thickness of the heat transfer plates, fouling, and the fluid itself. Unit: W/m2 ℃ or kcal/h·m2 ℃. The pressure drop has a direct impact on the size of the plate heat exchanger; a higher allowable pressure drop may reduce the cost of the heat exchanger, but it will increase the power consumption of the pump and thus raise operating costs. Under normal circumstances, in the case of water-to-water heat exchange, a pressure drop of 20–100 KPa is generally considered acceptable. Compared to shell-and-tube heat exchangers, the water flow in plate heat exchangers is in a highly turbulent state; therefore, the fouling coefficient for the same fluid is much lower in plate heat exchangers. When the dirt coefficient of water cannot be determined, a 10% safety margin can be included in the calculations. Calculation method: The heat load can be expressed using the following formula:
Q = m · cp · dt
Q = k · A · LMTD
Where:
Q = Heat load (kW)
m = Mass flow rate (kg/s)
cp = Specific heat capacity (kJ/kg·℃)
dt = Temperature difference between the inlet and outlet of the medium (℃)
k = Overall heat transfer coefficient (W/m²·℃)
A = Heat transfer area (m²)
LMTD = Logarithmic mean temperature difference

The overall heat transfer coefficient is calculated using the following formula:
k = Overall heat transfer coefficient (W/m²·℃)
α1 = Heat transfer coefficient for one side (W/m²·℃)
α2 = Heat transfer coefficient for the other side (W/m²·℃)
δ = Thickness of the heat transfer plates (m)
λ = Thermal conductivity of the plates (W/m·℃)
R1 and R2 are the fouling coefficients on each side respectively (m²·℃/W).
α1 and α2 can be determined using Nusselt’s equation.