For a heat exchange device, heat load refers to the amount of heat exchanged or transferred, that is, the amount of heat exchanged per unit time, in units of KW (KJ/S) or Kcal/h (kilocalories per hour). (Please remember the heat exchange relationship between the two: 1 KW=860 Kcal/h. The heat load in engineering is commonly represented by Q.
existFinned radiatorIn the design, the heat load is usually not directly proposed by the user, but calculated by the designer based on the actual needs of the user and the technical parameters on site. Here are a few examples to illustrate.
Example 1: A heating company wants to install a finned radiator for a 10t/h hot water boiler used for heating, hoping to reduce the exhaust temperature from 220 º C to 120 º C. The smoke flow rate is uncertain, it may be over 20000 cubic meters per hour, and please inform us of the model and flow rate of the induced draft fan.
To determineFinned radiatorThe designer needs to obtain as much information as possible from the user regarding the exhaust gas volume, such as the amount of coal burned, the calorific value of coal, whether the boiler is operating at full capacity, and the type of fan. Finally, based on their own experience, help users determine the design value of smoke exhaust volume: 16000Nm ³/h. Then calculate the heat load of the finned radiator according to the following formula:
Q=Gg×（Tg1 ×Cpg1 － Tg2 ×Cpg2） KW
Here: Gg： Mass flow rate of flue gas, kg/s
Cpg1 Cpg2： Specific heat at the inlet and outlet of flue gas, refer to the physical property table, KJ/(Kg ·º C)
Tg1： Inlet temperature of flue gas, º C
Tg2： Flue gas outlet temperature, º C
In this example, Gg=16000 × 1.295/3600=5.755 kg/s
Cpg1=1.102 KJ/（Kg·ºC） ， Cpg2 = 1.074 KJ/（Kg·ºC）
Tg1=220 º C Tg2=120 º C, 1.295 is the density of flue gas at 0 º C (kg/m3).
Calculated Q=653 KW
It should be remembered that the thermal energy contained in flue gas (or air) at a certain temperature can be calculated by the following formula:
Qg = Gg×（Tg ×Cpg）
Finned radiatorThe calculation of heat load should follow the following principles:
(1) Classify and compare the parameters provided by the user according to the "hot fluid side" and "cold fluid side", such as
Serial Number Hot fluid side Serial Number Cold fluid side
1 Inlet temperature of hot fluid 4 Cold fluid inlet temperature
2 Hot fluid outlet temperature 5 Cold fluid outlet temperature
3 Hot fluid flow rate 6 Cold fluid flow rate
Generally, only 5 out of the above six quantities need to be given, and the fluid with the most comprehensive conditions should be selected for heat load calculation. For example, if all conditions 1, 2, and 3 are given on the hot fluid side, while only two conditions 4 and 5 are given on the cold fluid side, the heat load should be calculated based on the conditions given on the hot fluid side, that is:
Q=[fluid flow rate, (kg/s)] × [inlet enthalpy value (kJ/kg) - outlet enthalpy value (kJ/kg)], the unit obtained is kJ/s or KW.
In calculationFinned radiatorThe import and export enthalpy values can be directly checked in the physical property table for water and water vapor. For flue gas and air, if the user does not provide enthalpy data, the following formula can be used for calculation:
Enthalpy value (kJ/kg)=Specific heat (kJ/kg º C) × Temperature (º C)
That is, i=Cp × T