Calculation of plate heat exchanger
Read the number：(2971) Time：2014/3/9 17:52:54
Calculation of plate heat exchanger
Plate heat exchanger calculation is a more complex process, the more popular method is logarithmic mean temperature difference method and the NTU method. When the computer is not universal, various manufacturers mostly used to estimate the approximate calculation parameters and flow - overall heat transfer coefficient curve estimation methods. At present, more and more manufacturers using computer calculations, so that the process of calculating the plate heat exchanger becomes fast, convenient and accurate. The following brief description of the general calculation method of the plate heat exchanger no phase change, this method is based on the calculation method of heat transfer and pressure drop correlations based guidelines.
The following five parameters calculated in the selection of the plate heat exchanger is required:
• The total heat transfer (Unit: kW).
• One inlet and outlet temperature side, the secondary side
• primary, secondary-side pressure drop allows the
• The maximum operating temperature
• Maximum working pressure
If the flow of heat transfer medium is known, the difference between imports and exports of specific heat capacity and temperature, total heat transfer can be calculated.
Temperature
T1 = hot side inlet temperature
T2 = hot side outlet temperature
cold side inlet temperature t1 =
t2 = cold side outlet temperature
Heat load
Heat flow balance equation reflects two fluids in the heat exchanger temperature changes during the relationship, under the heat insulation is good, no heat losses, the steady-state heat transfer process, the heat flow accounting relationship:
(Releasing heat thermal fluid flow) = (absorption of thermal cooling fluid flow)
During the heat balance, for there is no phase change heat transfer process of its expression and differentiated.
(1) no phase change heat transfer process
Where
Q ---- cold fluid or hot fluid discharge absorbing heat flow, W;
mh, mc ----- heat, the cooling fluid mass flow rate, kg / s;
Cph, Cpc ------ hot and cold fluid than the heat capacity at constant pressure, kJ / (kg • K);
T1, t1 ------ hot and cold fluid inlet temperature, K;
T2, t2 ------ hot and cold fluid outlet temperature, K.
(2) there is a phase change heat transfer process
Two logistics in the heat transfer process in which one side of the stream undergoes a phase change, such as steam condensate or liquid boiling, the heat flow balance equation is:
There are side phase change
Heat transfer process on both sides of the logistics phase change occurred, as the other side of the side of the boiling condensation
Where
r, r1, r2 -------- logistics phase change heat, J / kg;
D, D1, D2 -------- stream with variable volume, kg / s.
For hot or cold stream flow accountancy thermal phase transition, the above method should be applied and computing segments.
Logarithmic mean temperature difference (LMTD)
Logarithmic mean temperature difference is the driving force of heat transfer, and the size of the logarithmic mean temperature difference is directly related to the degree of difficulty of heat transfer. Impossible to calculate in some special cases the logarithmic mean temperature difference, this time with the arithmetic mean temperature difference instead of the logarithmic mean temperature difference, media and countercurrent flow conditions and in case of logarithmic mean temperature difference is calculated differently. In some special cases, instead of using the arithmetic mean temperature difference of average temperature difference.
Countercurrent time:
And stream:
Hot Long (F)
And the temperature difference between the hot side of the long and logarithmic mean temperature difference associated with it. F = dt / LMTD
Physical properties of the following four medium heat affected
Density, viscosity, specific heat capacity, thermal conductivity
Overall heat transfer coefficient
Overall heat transfer coefficient is a parameter used to measure the heat transfer resistance. Heat transfer resistance is dominated by the heat transfer material and the plate thickness, dirt and other factors that constitute the fluid itself. Unit: W/m2 ℃ or kcal / h, m2 ℃.
Pressure drop
Pressure drop directly affects the size of the plate heat exchanger, if a larger permissible pressure drop, the heat exchanger may reduce the cost, but at the cost of the pump power and increase operating costs. Under normal circumstances, in the water of the heat exchanger, the pressure drop allowed in general is 20-100KPa acceptable solution.
Fouling factor
And compared to shell and tube heat exchanger flow of water in the plate heat exchanger is in a high state of turbulence, with a medium relative to the plate heat exchanger fouling factor much smaller. Water can not be determined in the case of fouling factor in the calculation can retain 10% of the amount of wealth.
Calculation Method
Thermal load represented by the following formula:
Q = m • cp • dt
Q = k • A • LMTD
Q = heat load (kW)
m = mass flow rate (kg / s)
cp = specific heat (kJ / kg ℃)
dt = medium inlet and outlet temperature difference (℃)
k = overall heat transfer coefficient (W/m2 ℃)
A = heat transfer area (m2)
LMTD = logarithmic mean temperature difference
The overall heat transfer coefficient calculated by the following formula:
Where:
k = overall heat transfer coefficient (W/m2 ℃)
α1 = a measure of the heat transfer coefficient (W/m2 ℃)
α2 = a measure of the heat transfer coefficient (W/m2 ℃)
δ = thickness of the heat transfer plate (m) of
λ = thermal conductivity of the sheet (W / m ℃)
R1, R2, respectively, on both sides of the fouling factor (m2 ℃ / W)
α1, α2 can be obtained by Nusselt criterion style.
