Calculation method of plate exchange
Currently more popular way is homogeneous temperature difference method and NTU method to log. When the computer is not popular, the plate heat exchanger is a more complicated argument. Most of the heat exchanger manufacturers use the approximate calculation of the parameters and the method of estimating the total heat transfer coefficient. At present, more and more manufacturers use computer calculation, so that the plate heat exchanger process becomes fast, convenient, correct. The following is a brief description of the calculation method of plate heat exchanger without phase change. The method is based on the heat transfer and pressure drop criterion.
The following five parameters are necessary in the selection and calculation of plate heat exchanger
Total heat transfer (unit: kW.)
The inlet and outlet temperature of the first and two sides
Allowable pressure drop on one side and two side
Maximum mission temperature
Maximum working pressure
The specific heat capacity and the temperature difference between the inlet and outlet. Total heat transfer can be calculated.
temperature
T1= hot side inlet temperature
T2= hot side outlet temperature
T1= cold side inlet temperature
T2= cold side exit temperature
thermal load
The heat exchanger is well preserved, and the heat flux equation reflects the mutual relation of the two fluid in the process of heat transfer. In the case of no heat loss, the relationship between the heat flux and the heat flux is stable:
Heat flow from a hot fluid) = heat flow absorbed by a cold fluid
The expression of the heat transfer process with and without phase change is different. Thermal balance time.
Phase change heat transfer process
Type
WQ---- heat flow rate of cold fluid receiving or thermal fluid.
Kg/mh, mc----- hot, cold fluid moral flow.
KJ/kgKCph, Cpc------ heat and cold fluid specific heat capacity at constant pressure.
KT1, t1------ hot and cold fluid inlet temperature.
KT2, t2------ hot and cold fluid inlet temperature.
For example, the heat transfer process on the other side of the side of the condensation of the two sides of the logistics phase change.
Phase change heat transfer process
Phase change on one side
The two party logistics in the heat exchange process. Such as steam condensation or liquid boiling, one side of the logistics phase change. The heat flow equations for:
Type
J/kgr, R1, r2-------- phase change heat.
Kg/D, D1, D2-------- phase change logistics quantity.
Fouling factor
Shell and tube type heat exchanger compared. The fouling coefficient of the same medium is smaller than that of plate heat exchanger. The water flow in the plate heat exchanger is in a state of high turbulence. Calculate the amount of 10% can be retained.
Should be in accordance with the above paragraph format to perform the addition and calculation. Calculation of heat flux in the phase transition of supercooled or superheated flow.
Logarithmic mean temperature difference LMTD
In the medium and in the flow and countercurrent logarithmic mean temperature under the condition of the calculation way is not some extraordinary circumstances, with homogeneous temperature instead of logarithmic mean temperature difference arithmetic. The logarithmic mean temperature difference is directly related to the difficulty of heat transfer of heat exchanger, and the logarithmic mean temperature difference can not be calculated under some special circumstances. With the uniform temperature on behalf of logarithmic mean temperature difference arithmetic.
The following four physical qualities of the medium affect the heat transfer
Density, viscosity, specific heat capacity, thermal conductivity
Overall heat transfer coefficient
The total heat transfer coefficient of M2 is a parameter to measure the heat transfer resistance of heat exchanger. The heat transfer resistance is mainly composed of heat transfer plate material and thickness, dirt and fluid itself. Unit: W/m2 C orkcal/h..
pressure drop
Pressure drop directly affects the size of plate heat exchanger. You can perhaps add or heat exchanger on, if there is a larger allowable pressure drop. But it will lose the power of pump and increase the operating cost. Under the condition of water and heat exchange, the pressure drop is generally acceptable in 20-100KPa
Calculation method: heat load can be used to express
Q=mcpdt
Q=kALMTD
Q= heat load (kW)
M= moral flow rate (kg/)
Cp= specific heat (kJ/kg C)
Temperature difference between inlet and outlet of dt= medium (c)
The total heat transfer coefficient k= (W/m2 C)
A= heat transfer area (M2)
LMTD= logarithmic mean temperature difference
The total heat transfer coefficient is calculated by the following formula:
Among:
The total heat transfer coefficient k= (W/m2 C)
Heat transfer coefficient of alpha 1= (W/m2 C)
Heat transfer coefficient of alpha 2= (W/m2 C)
Delta = heat transfer plate thickness (M
Thermal conductivity lambda = plate (W/m C)
R1R2 is the coefficient of dirt on both sides (M2 /W)
Alpha 1 alpha may be obtained by the Nusair criterion.
