1-Calculation of power
The following calculation is mainly based on the thick film heating element in instant hot water dispenser.
M: Mass of water P: density of water V: volume of water C: specific heat of water Q: flow rate s: time AT: Temperature rise of water T: Heating time P: power of heating element
7: thermal efficiency
The formula is as follows
M - pr (1)
V - Q * (2)
The heat absorbed by water from the initial temperature to the final temperature is:
W1 - CMAT
The amount of electricity lost is zero
W2 - P
In the absence of surface loss of the heating element
W1 and W2
С MA Т - P
Cpl. * tAT P > PCpAT
The water in this design is water,C4.2*10
(KG) * K, p = 1.0 * 10 KG/m '
You get the unit
P-4.20AT where Q is in ml/s. If the surface loss of the heating element is taken into account and the thermal efficiency is calculated into N, then P-4.20AT/n is obtained
If the thermal efficiency of the thick film heater is 95%, the inlet water temperature is 25℃, and the outlet water temperature is 100℃, then p-331.60, that is to say, a 2000W heater needs to boil water, and the maximum flow rate of water can only be 6mL /S, that is, 0.361/min. If the flow rate is too high, the water will not boil
The calculation method of other heating methods can also be calculated according to the energy balance W-CMAT, which is not derived here
2- Calculation of thick film printing circuit:
RH is the resistance at the operating temperature,RC is the resistance at the initial temperature; TH is the operating temperature,TC is the initial temperature, and U is the operating voltage. 1p is the current density, printing P is the power density, for stainless steel thick film heating element power density generally below 60W/cm current density below 3A/mm is appropriate; Wire width W, wire length L, wire area S.
The formula is as follows:
P = U/RH
= > RH = U/P
TCR= ((RH -RC) x 106) / (RC x (TH - TC)) =>RC-RH/(1+TCR (TH-TC)*109) =/P(1+TCR(TH-TC)*10) =>RC-U/P(1+TCR (TH-TC)*10")
According to the dielectric size, select the resistance size matching with it, and find the square resistance IiSq, resistance temperature coefficient TCK, current density 1p, power density Pp through the Datasheet of the resistance size, calculate the initial resistance value Rc, and the square resistance Rsy corresponding to the size can be obtained
L - RCW/Rs4
Then calculate the power density according to Pp=PA/L"W to see if it meets our design requirements. The power PA is divided into initial power and steadystate power, and the power density needs to be calculated separately according to the mature resistance and the cold resistance
43 Graphic design
Resistance graphic design, the resistance layer bend arc shape, do not use 90 degree bending, because 90 degree bending may form a hot spot, heat concentrated in the bending, long-term use is easy to lead to open failure, can also be in the bending of the conductor paste to avoid the above problems. At the same time, try to ensure the uniformity of heat distribution of the whole heating body.
The width of the wire is mainly obtained according to the property of the paste and the current density when the wire is working normally, usually based on the calculated value, the design has a reduction factor of more than 1.5 times.
The function of the resistance end is to complete the connection between the resistance and the wire. The width of the lead band at the end is generally no less than the width of the guide band, and the lap connection with the resistance is no less than 50% of the guide band.
The outlet point is the bridge connecting the power supply, which needs to be selected reasonably according to the connection mode. The connection point of welding and collision mode is to ensure that the Lead H line is connected slowly as the basic lead.