If hot air temperature heated with hot air heaters is T[ºC], air flow rate is F[L/min.], power consumed by air heater is P[w]:

- T ≈ 50P/F [ºC] --- Formula to determine the hot air temperature Considering hot air temperature T = 50 x (Heater power consumption P) ÷ (Air flow rate F) Thermal efficiency ratio E (0.7 to 0.95), the following equation is obtained. T ≈ 50EP/F
- P ≈ 0. 02FT [w] [ºC] --- Formula to determine required the power Considering heater consumption power P = 0.02× (Air flow rate F) x (Hot air temperature T) Thermal efficiency ratio E (0.7 to 0.95), the following equation is obtained. P = 0.02P/(EF)

When the flow rate F has been selected, the heater power consumption P has to be set to obtain the target hot air temperature T. If P needs to be larger than the rated power of the available hot air heater, a different hot air heater model with a large output has to be selected. If the required power P is smaller than the rated power of the selected hot-air heater, it can be managed by lowering the supply voltage. The method of controlling the voltage will be described in a separate section. Thermal efficiency has to be considered when the air temperature is high and the flow rate is low. In models with metal cases, the thermal efficiency may decrease to ~60% and to about ~50% in glass tube exposed type.

When the influence of thermal efficiency cannot be ignored, the power of the heater to be selected can be calculated by dividing the necessary power P obtained by the equation of P = 0.02 FT with the thermal efficiency (division).

For example, if P is 1000 W and the thermal efficiency is expected to be 70% (0.7), the heater power to be selected will be 1000 ÷ 0.7 = 1429 W. In practice, heaters with even higher power than this value is selected considering the margin. The above hot air temperature T is actually the temperature rise value. That is, T = (T-out)- (T-in) Since T is a temperature rise value, usually T can be calculated as the hot air outlet temperature without any problems. (T-in) generally ranges from 32ºF (0ºC) to 86ºF (30ºC), T-out is generally as high as 1472ºF (800ºC), and the error will be negligible in most cases even if calculated assuming (T - in) is zero. However, when the hot air temperature is low or the air inlet temperature is high, the air inlet temperature (T-in) has to be taken into account.

**Capacity of hot air heater (air heater)** - products are available in the range of 30 W ~ 70,000 W. Also, BCE Inc. can manufacture any custom specifications provided such specifications are feasible. The service life of the air heater can vary significantly depending on the usage method, and assigning a specific value and offering warranty is not possible.

Generally, the maximum temperature of the heating element in use state is measured with a non-contact thermometer such as a pyrometer, and the life of heating element is estimated from the Temp Graph using the measured temperature. This graph presents considerably fairly modest service life values for safety of the manufacturers. Actually, the service life is longer. Also, since the heating element temperature is approximately 572ºF (300ºC) higher than the hot air temperature, heating element temperature can also be estimated from the hot air temperature. This temperature difference varies depending on the flow rate and is a very rough estimation.

From this, if hot air temperature is 1472ºF (800ºC), the life is approximately 1000 hours and if the hot air temperature is 1292ºF (700ºC) or less, the lifetime value approaches infinity and this indicates that service life need not be considered. However, this is a generalized statement and results will be significantly different depending on individual conditions. Service life is also significantly affected by the method of voltage control and by impurities (water, oil, metal powder) contained in air, shock, and vibration.

**Formula for calculating the necessary power is P = 0.02 FT**

The formula for the required power P W is obtained with flow rate as F, unit as L/min, and air temperature rise value dt as T K. Furthermore, the unit of power (watt: W) is equal to J/s. When the flow rate F L/min is corrected every second, 1/60F L/s Heat capacity of air is 1.01 J/g/K, Density of air is 1.2 g/L, F L/min. = 1/60 F L/s F L/min. = 1/60F L/s x 1.2 g/L = 0.02 F g/s, Power P W (J/s)=1.01 J/g/K x 0.02F g/s) x t K = 0.0202 x F x T

In other words, the equation Power P =0.0202FT W is obtained. 0.0202 is taken as 0.02 for practical use.

**Required power P = 0.02FT W**

This is a theoretical formula and thermal efficiency is not considered. In case of hot air heaters, thermal efficiency is 0.9 ~ 0.7, and the value obtained by dividing P by this number is the actual required electric power. T is the temperature rise value. This means T = (hot air outlet temperature - air inlet temperature).

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