CN115111931A - Absorber for alumina waste heat and heat absorption and discharge control method - Google Patents

Abstract

The invention provides an absorber for alumina waste heat and a heat absorption and discharge control method, wherein a stepped heat storage main body is arranged on the absorber, a contact heat collection end which is in contact with a target heat source and consists of a plurality of tubular heat conductors extends into the stepped heat storage main body, a frequency modulation induced draft fan is connected with the other end of the stepped heat storage main body, a plurality of heat storage pipes filled with five sections of heat storage materials with different phase-temperature temperatures are arranged in the stepped heat storage main body, and temperature detectors are respectively arranged on the heat storage pipes of a certain heat storage material with different phase-temperature temperatures, the contact heat collection end and the inlet of the frequency modulation induced draft fan. This absorber utilizes the heat absorption flow with the heat absorption of the back aluminium oxide ingot of coming out of the stove to the notch cuttype heat-retaining main part through the temperature that different temperature detectors detected, releases the heat of notch cuttype heat-retaining main part storage again and recycles in the heat exchanger, has reduced the waste of heat energy, has practiced thrift the energy, has also ensured that operating personnel can the safety in production to reduce the emergence of accident.

Description

Absorber for alumina waste heat and heat absorption and discharge control method

Technical Field

The invention relates to an absorber, in particular to an absorber for alumina waste heat, and also relates to a heat absorption and discharge control method of the absorber.

Background

In the alumina smelting process, the alumina ingot needs to be cooled after being discharged from the furnace to carry out the next step of work. The natural cooling and heat dissipation of the aluminum oxide ingot discharged from the furnace are slow, the production efficiency of enterprises is greatly influenced in the cooling process, the aluminum oxide ingot is cooled and simultaneously energy is wasted to a great extent, and the high-temperature aluminum oxide ingot also has a serious influence on the production safety of operators.

Disclosure of Invention

Aiming at the situation of heat loss of the existing aluminum oxide ingot after the aluminum oxide ingot is taken out of the furnace, the invention aims to provide an absorber capable of absorbing heat released by the aluminum oxide ingot after the aluminum oxide ingot is taken out of the furnace.

In order to solve the technical problems, the technical scheme adopted by the invention is as follows: an absorber for alumina waste heat comprises a fixed frame, wherein a stepped heat storage main body is arranged on the fixed frame, one end of the stepped heat storage main body is connected with a plurality of contact heat collection ends which extend into the stepped heat storage main body and are composed of tubular heat conductors, and the other end of each contact heat collection end is in contact with a target heat source; the other end of the stepped heat storage main body is connected with a frequency modulation induced draft fan through a pipeline; a plurality of heat storage pipes filled with heat storage materials are arranged in the stepped heat storage main body along the direction from the contact heat collection end to the frequency modulation draught fan, the heat storage materials are composed of heat storage materials with different temperature changes in five sections, and the phase change temperature of the heat storage materials is sequentially filled in the heat storage pipes from the contact heat collection end to the frequency modulation draught fan end from high to low; a first temperature detector is connected to the contact heat collection end, a second temperature detector, a third temperature detector, a fourth temperature detector, a fifth temperature detector and a sixth temperature detector which are connected with heat storage materials with different temperature of five sections are sequentially arranged on the stepped heat storage main body from the contact heat collection end to the end of the frequency modulation draught fan, and a seventh temperature detector is arranged on a pipeline between the stepped heat storage main body and the frequency modulation draught fan; the electric control device on the fixing frame is in control connection with each temperature detector and the frequency modulation induced draft fan.

The stepped heat storage main body is a tank body, a plurality of heat storage pipes are arranged in the tank body along the direction from the contact heat collection end to the frequency modulation draught fan, and a heat insulation layer is arranged on the outer layer of the tank body.

And fire valves are respectively arranged on the connecting end of the stepped heat storage main body and the contact heat collection end and on the pipeline between the stepped heat storage main body and the frequency modulation induced draft fan, and the electric control device is connected with the control valve.

The heat absorption and discharge control method of the absorber of the invention comprises the following steps:

A. heat absorption process

Step 1, after the integrated device is installed in place, enabling the contact heat collection end to be in contact with alumina, opening fire valves at two ends of the stepped heat storage main body, and simultaneously starting a frequency modulation induced draft fan which runs at the lowest frequency;

step 2, when the value of the first temperature detector is above 600 ℃, increasing the frequency of a fan of the frequency-modulated induced draft fan according to the frequency of 5HZ per minute until the temperature of the seventh temperature detector is 80-100 ℃;

step 3, when the value of the first temperature detector reaches 500-600 ℃, whether the temperature of the second temperature detector reaches more than 500 ℃ is examined, when the temperature reaches more than 500 ℃, whether the temperature of the third temperature detector, the fourth temperature detector, the fifth temperature detector and the sixth temperature detector reaches corresponding 400 ℃, 300 ℃, 200 ℃ and 100 ℃ is detected, and if the temperature reaches the corresponding 400 ℃, 300 ℃, 200 ℃ and 100 ℃, the heat storage is judged to be finished; if the frequency does not reach the preset frequency, operating the frequency-modulated induced draft fan by using 30 percent of frequency until all sections meet the requirements;

step 4, when the value of the first temperature detector reaches 400-500 ℃, whether the temperature of the third temperature detector reaches more than 400 ℃ is examined, when the temperature reaches more than 400 ℃, whether the temperature of the fourth temperature detector, the fifth temperature detector and the sixth temperature detector reaches corresponding 300 ℃, 200 ℃ and 100 ℃ is detected, and if the temperature reaches the corresponding 300 ℃, 200 ℃ and 100 ℃, half of the heat storage is judged to be completed; if the frequency does not reach the preset frequency, operating a frequency modulation induced draft fan to a detection working section of a fourth temperature detector, a fifth temperature detector and a sixth temperature detector by using the frequency of 30 percent to meet the requirement; when the detection sections of the third temperature detector, the fourth temperature detector, the fifth temperature detector and the sixth temperature detector all meet the requirements, whether the temperature of the second temperature detector reaches more than 500 ℃ is examined, if not, a new alumina block is prompted to be replaced for absorption until the stored energy reaches the standard;

step 5, when the value of the first temperature detector reaches 300-400 ℃, whether the temperature of the fourth temperature detector reaches more than 300 ℃ is considered, when the temperature reaches more than 300 ℃, whether the temperature of the fifth temperature detector and the sixth temperature detector reaches corresponding 200 ℃ and 100 ℃ is detected, and if the temperature of the fifth temperature detector and the sixth temperature detector reaches corresponding 100 ℃, half of the heat storage is judged to be completed; if the frequency does not reach the preset frequency, operating a frequency modulation induced draft fan to a fifth temperature detector and a sixth temperature detector by using 30 percent of frequency to meet the requirements; when the detection sections of the fourth temperature detector, the fifth temperature detector and the sixth temperature detector all meet the requirements, whether the temperatures of the second temperature detector and the third temperature detector reach 500 ℃ and above 400 ℃ is examined, if the temperatures of the second temperature detector and the third temperature detector do not reach, a new alumina block is prompted to be replaced for absorption until the stored energy reaches the standard;

step 6, when the value of the first temperature detector reaches 200-300 ℃, investigating whether the temperature of the fifth temperature detector reaches more than 200 ℃, when the temperature reaches more than 200 ℃, detecting whether the sixth temperature detector reaches more than 100 ℃, and if so, judging that the heat storage is half completed; if the frequency does not reach the preset frequency, operating a frequency modulation induced draft fan to a detection section of a sixth temperature detector by using the frequency of 30 percent to meet the requirement; when the detection sections of the fifth temperature detector and the sixth temperature detector meet the requirements, whether the temperatures of the second temperature detector, the third temperature detector and the fourth temperature detector reach more than 500 ℃, 400 ℃ and 300 ℃ is examined, if the temperatures of the second temperature detector, the third temperature detector and the fourth temperature detector do not reach the requirements, the replacement of an alumina block is prompted for absorption until the stored energy reaches the standard;

step 7, when the value of the first temperature detector reaches 100-200 ℃, prompting to replace the alumina block;

B. exothermic process

Step 1, vertically installing and inverting a stepped heat storage main body, connecting an outlet of a frequency modulation induced draft fan with a heat exchanger, and controlling a fire damper valve at a contact heat collection end to be opened by 30%; starting a frequency modulation induced draft fan to guide hot air to enter a heat exchanger to exchange heat with water in the heat exchanger;

step 2, detecting whether the water in the heat exchanger reaches the required water temperature or steam temperature, and keeping the opening degree of a valve when the water in the heat exchanger reaches the required water temperature or steam temperature; when the temperature exceeds the preset temperature, the frequency of the frequency-modulated induced draft fan is adjusted to be low so as to reach the required water temperature or steam temperature; and when the temperature of the water is not reached, the frequency of the frequency-modulated induced draft fan is modulated to be higher so as to reach the required water temperature or steam temperature.

The absorber for the alumina waste heat and the heat absorption and discharge control method designed by the technical scheme can absorb the heat released by the alumina ingot discharged from the furnace, and use the absorbed heat for other purposes, so that the heat released by the alumina ingot is utilized, the energy waste is reduced, and the energy is saved. Through the absorption of the heat released by the aluminum oxide ingot after discharging, the serious influence of the high-temperature aluminum oxide ingot on the production safety of operators is avoided, the operators can safely produce the aluminum oxide ingot, and the accidents are reduced. The invention has simple structure and simple heat absorption and release method, and realizes better utilization of the released heat of the aluminum oxide ingot after discharging.

Drawings

FIG. 1 shows a schematic diagram of the structure of an absorber of the present invention;

FIG. 2 is a schematic view of the internal structure of the stepped heat storage body of the present invention;

fig. 3 shows a distribution diagram of a temperature detector provided on the stepped heat storage body according to the present invention.

Detailed Description

The absorber for alumina waste heat and the heat absorption and release control method of the present invention will be specifically described with reference to the accompanying drawings.

The invention relates to an absorber for alumina waste heat, which comprises a fixed frame, wherein a step-shaped heat storage main body 3 is arranged on the fixed frame, one end of the step-shaped heat storage main body 3 is connected with a contact heat collection end 1, the contact heat collection end 1 consists of a plurality of tubular heat conductors extending into the step-shaped heat storage main body 3, and the other end of the contact heat collection end 1 is in contact with a target heat source (high-temperature alumina ingot after being discharged from a furnace). And a frequency modulation induced draft fan 5 is connected with the other end of the stepped heat storage main body 3 through a pipeline. The connection end of the stepped heat storage main body 3 and the contact heat collection end 1 is provided with a fire damper 2, a fire damper 4 is also arranged on a pipeline between the stepped heat storage main body 3 and the frequency modulation induced draft fan 5, and the fire damper 2 and the fire damper 4 are controlled by an electric control device arranged on the fixing frame.

The step type heat storage main body 3 is a tank body, a plurality of heat storage pipes 7 are arranged in the tank body, the heat storage pipes 7 are arranged along the direction from the contact heat collection end 1 to the frequency modulation induced draft fan 5, heat storage materials are filled in the heat storage pipes 7, the heat storage materials are composed of five sections of heat storage materials with different temperature changes, and the phase change temperatures of the heat storage materials are sequentially filled in the heat storage pipes 7 from the contact heat collection end 1 to the frequency modulation induced draft fan 5 in the order from high to low. The temperature of the aluminum oxide ingot is conservatively estimated to be more than 800 ℃ when the temperature of the aluminum oxide ingot is reduced, so that the step section of the step type heat storage main body 3 is designed to be 5 sections, and the heat can be exchanged and stored better. The first temperature detector 8 is connected to the contact heat collecting end 1, and the first temperature detector 8 is used for detecting the temperature inside the contact heat collecting end 1, namely detecting the temperature of the alumina ingot. A second temperature detector 9, a third temperature detector 10, a fourth temperature detector 11, a fifth temperature detector 12 and a sixth temperature detector 13 are sequentially arranged on the heat storage pipe 7 from the contact heat collection end 1 to the frequency modulation induced draft fan 5 end, and the second temperature detector 9, the third temperature detector 10, the fourth temperature detector 11, the fifth temperature detector 12 and the sixth temperature detector 13 respectively detect the temperatures of the heat storage materials with different temperature change in the five sections in the heat storage pipe 7. And a seventh temperature detector 14 is arranged on a pipeline between the stepped heat storage main body 3 and the frequency modulation induced draft fan 5, and the seventh temperature detector 14 is used for detecting the temperature of the inlet end of the frequency modulation induced draft fan 5 so as to ensure that the frequency modulation induced draft fan 5 works within the working temperature range. The first temperature detector 8, the second temperature detector 9, the third temperature detector 10, the fourth temperature detector 11, the fifth temperature detector 12, the sixth temperature detector 13, the seventh temperature detector 14 and the frequency modulation induced draft fan 5 are controlled by an electric control device. The outer layer of the tank body is also provided with a heat-insulating layer 6 for insulating the tank body.

The heat absorption flow of the absorber of the invention is as follows:

step 1, installing the integrated device in place, enabling a contact heat collection end 1 to be in contact with an alumina ingot, opening fire valves 2 and 4 at two ends of a stepped heat storage main body 3, and simultaneously starting a frequency modulation induced draft fan 5, wherein the frequency modulation induced draft fan 5 runs at the lowest frequency;

step 2, when the value of the first temperature detector 8 is above 600 ℃, increasing the fan frequency of the frequency-modulated induced draft fan 5 according to the frequency of 5HZ per minute until the temperature of the seventh temperature detector 14 is 80-100 ℃;

and 3, when the numerical value of the first temperature detector 8 reaches 500-600 ℃, inspecting whether the temperature of the second temperature detector 9 reaches more than 500 ℃, and when the temperature reaches more than 500 ℃, inspecting whether the temperature of the third temperature detector 10, the fourth temperature detector 11, the fifth temperature detector 12 and the sixth temperature detector 13 reaches the corresponding temperature of more than 400 ℃, 300 ℃, 200 ℃ and 100 ℃, and if so, judging that the heat storage is finished. If the frequency does not reach the preset frequency, the frequency-modulated induced draft fan 5 is operated by using 30 percent of frequency until all sections meet the requirements;

and 4, when the value of the first temperature detector 8 reaches 400-500 ℃, checking whether the temperature of the third temperature detector 10 reaches more than 400 ℃, when the temperature reaches more than 400 ℃, detecting whether the fourth temperature detector 11, the fifth temperature detector 12 and the sixth temperature detector 13 reach corresponding 300 ℃, 200 ℃ and 100 ℃, and if so, judging that the heat storage is half completed. If the frequency does not reach the preset frequency, the frequency modulation induced draft fan 5 to the fourth temperature detector 11, the fifth temperature detector 12 and the sixth temperature detector 13 are operated by using the frequency of 30 percent to detect that the workshop section meets the requirement; when the detection sections of the third temperature detector 10, the fourth temperature detector 11, the fifth temperature detector 12 and the sixth temperature detector 13 all meet the requirements, whether the temperature of the second temperature detector reaches more than 500 ℃ is examined, if the temperature of the second temperature detector does not reach the temperature, a new alumina block is prompted to be replaced for absorption until the stored energy reaches the standard;

and 5, when the value of the first temperature detector 8 reaches 300-400 ℃, checking whether the temperature of the fourth temperature detector 11 reaches more than 300 ℃, when the temperature reaches more than 300 ℃, detecting whether the temperature of the fifth temperature detector 12 and the sixth temperature detector 13 reaches more than 200 ℃ and 100 ℃ correspondingly, and if so, judging that the heat storage is half completed. If the frequency of the frequency-modulated induced draft fan 5 to the fifth temperature detector 12 and the sixth temperature detector 13 does not reach the frequency of the frequency-modulated induced draft fan, the frequency of the frequency-modulated induced draft fan 5 to the fifth temperature detector 13 is 30% of the frequency-modulated induced draft fan, and the detection section meets the requirements. When the detection sections of the fourth temperature detector 11, the fifth temperature detector 12 and the sixth temperature detector 13 all meet the requirements, whether the temperatures of the second temperature detector 9 and the third temperature detector 10 reach 500 ℃ and 400 ℃ or not is examined, if the temperatures do not reach, a new alumina block is prompted to be replaced for absorption until the stored energy reaches the standard;

and 6, when the value of the first temperature detector 8 reaches 200-300 ℃, checking whether the temperature of the fifth temperature detector 12 reaches more than 200 ℃, when the temperature reaches more than 200 ℃, detecting whether the temperature of the sixth temperature detector 13 reaches more than 100 ℃, and if so, judging that the heat storage is half completed. If the frequency of the frequency-modulated induced draft fan 5 to the sixth temperature detector 13 does not reach the frequency of the frequency-modulated induced draft fan, the frequency of the frequency-modulated induced draft fan 5 to the sixth temperature detector 13 is 30% of the frequency-modulated induced draft fan, and the detection section meets the requirements. When the detection sections of the fifth temperature detector 12 and the sixth temperature detector 13 all meet the requirements, whether the temperatures of the second temperature detector 9, the third temperature detector 10 and the fourth temperature detector 11 reach 500 ℃, 400 ℃ and 300 ℃ or not is examined, if the temperatures do not reach the requirements, the replacement of a new alumina block is prompted for absorption until the stored energy reaches the standard;

and 7, when the numerical value of the first temperature detector 8 reaches 100-200 ℃, prompting to replace the alumina block.

The heat release flow path of the absorber of the invention is as follows:

step 1, the step-shaped heat storage main body 3 is vertically installed and inverted, the outlet of the frequency modulation induced draft fan is connected with the heat exchanger, and the valve (high-temperature end valve) of the fire damper 2 is controlled to be opened by 30%. A frequency modulation induced draft fan 5 is started to guide hot air to enter a heat exchanger (not shown) through a pipeline to exchange heat with water in the heat exchanger;

and 2, detecting whether the water in the heat exchanger reaches the required water temperature or steam temperature, and keeping the opening degree of the valve when the water in the heat exchanger reaches the required water temperature or steam temperature. When the temperature exceeds the set temperature, the frequency of the frequency modulation induced draft fan 5 is adjusted to be lower to reach the required water temperature or steam temperature. And when the temperature of the water is not higher than the set temperature, the frequency of the frequency-modulated induced draft fan 5 is increased to reach the required water temperature or steam temperature.

Claims (4)

1. An absorber for alumina waste heat comprises a fixed frame and is characterized in that a stepped heat storage main body is arranged on the fixed frame, one end of the stepped heat storage main body is connected with a plurality of contact heat collection ends which extend into the stepped heat storage main body and are composed of tubular heat conductors, and the other end of each contact heat collection end is in contact with a target heat source; the other end of the stepped heat storage main body is connected with a frequency modulation induced draft fan through a pipeline; a plurality of heat storage pipes filled with heat storage materials are arranged in the stepped heat storage main body along the direction from the contact heat collection end to the frequency modulation draught fan, the heat storage materials are composed of heat storage materials with different temperature changes in five sections, and the phase change temperature of the heat storage materials is sequentially filled in the heat storage pipes from the contact heat collection end to the frequency modulation draught fan end from high to low; a first temperature detector is connected to the contact heat collection end, a second temperature detector, a third temperature detector, a fourth temperature detector, a fifth temperature detector and a sixth temperature detector which are connected with heat storage materials with different temperature of five sections are sequentially arranged on the stepped heat storage main body from the contact heat collection end to the frequency modulation induced draft fan end, and a seventh temperature detector is arranged on a pipeline between the stepped heat storage main body and the frequency modulation induced draft fan; the electric control device on the fixing frame is in control connection with each temperature detector and the frequency modulation induced draft fan.

2. The absorber for the waste heat of alumina as claimed in claim 1, wherein the stepped heat storage body is a tank body, a plurality of heat storage pipes are arranged in the tank body along the direction from the contact heat collection end to the frequency-modulated induced draft fan, and the outer layer of the tank body is provided with a heat insulation layer.

3. The absorber for the waste heat of alumina as claimed in claim 1, wherein fire valves are respectively disposed on the connection end of the stepped heat storage main body and the contact heat collection end and the pipeline between the stepped heat storage main body and the frequency-modulated induced draft fan, and the electric control device is connected with the control valve.

4. A heat absorption and discharge control method for an absorber of alumina waste heat,

the method is characterized in that:

A. heat absorption process

Step 1, after the integrated device is installed in place, enabling the contact heat collection end to be in contact with alumina, opening fire valves at two ends of the stepped heat storage main body, and simultaneously starting a frequency modulation induced draft fan which runs at the lowest frequency;

step 2, when the value of the first temperature detector is above 600 ℃, increasing the frequency of a fan of a frequency modulation induced draft fan according to the frequency of 5HZ per minute until the temperature of the seventh temperature detector is 80-100 ℃;

step 3, when the value of the first temperature detector reaches 500-600 ℃, whether the temperature of the second temperature detector reaches more than 500 ℃ is examined, when the temperature reaches more than 500 ℃, whether the temperature of the third temperature detector, the fourth temperature detector, the fifth temperature detector and the sixth temperature detector reaches corresponding 400 ℃, 300 ℃, 200 ℃ and 100 ℃ is detected, and if the temperature reaches the corresponding 400 ℃, 300 ℃, 200 ℃ and 100 ℃, the heat storage is judged to be finished; if the frequency does not reach the preset frequency, operating the frequency-modulated induced draft fan by using 30 percent of frequency until all sections meet the requirements;

step 4, when the value of the first temperature detector reaches 400-500 ℃, whether the temperature of the third temperature detector reaches more than 400 ℃ is examined, when the temperature reaches more than 400 ℃, whether the temperature of the fourth temperature detector, the fifth temperature detector and the sixth temperature detector reaches corresponding 300 ℃, 200 ℃ and 100 ℃ is detected, and if the temperature reaches the corresponding 300 ℃, 200 ℃ and 100 ℃, half of the heat storage is judged to be completed; if the frequency does not reach the preset frequency, operating a frequency modulation induced draft fan to a fourth temperature detector, a fifth temperature detector and a sixth temperature detector by using 30 percent of frequency to meet the requirements; when the detection sections of the third temperature detector, the fourth temperature detector, the fifth temperature detector and the sixth temperature detector all meet the requirements, whether the temperature of the second temperature detector reaches more than 500 ℃ is examined, if not, a new alumina block is prompted to be replaced for absorption until the stored energy reaches the standard;

step 5, when the value of the first temperature detector reaches 300-400 ℃, whether the temperature of the fourth temperature detector reaches more than 300 ℃ is examined, when the temperature reaches more than 300 ℃, whether the fifth temperature detector and the sixth temperature detector reach more than 200 ℃ and 100 ℃ correspondingly is detected, and if yes, half of the heat storage is judged to be completed; if the frequency does not reach the preset frequency, operating a frequency modulation induced draft fan to a fifth temperature detector and a sixth temperature detector by using 30 percent of frequency to meet the requirements; when the detection sections of the fourth temperature detector, the fifth temperature detector and the sixth temperature detector all meet the requirements, whether the temperatures of the second temperature detector and the third temperature detector reach 500 ℃ and above 400 ℃ is examined, if the temperatures of the second temperature detector and the third temperature detector do not reach, a new alumina block is prompted to be replaced for absorption until the stored energy reaches the standard;

step 6, when the value of the first temperature detector reaches 200-300 ℃, whether the temperature of the fifth temperature detector reaches more than 200 ℃ is examined, when the temperature reaches more than 200 ℃, whether the temperature of the sixth temperature detector reaches more than 100 ℃ is detected, and if the temperature reaches more than 200 ℃, half of the heat storage is judged to be completed; if the frequency does not reach the preset frequency, operating a frequency modulation induced draft fan to a detection section of a sixth temperature detector by using the frequency of 30 percent to meet the requirement;

when the detection sections of the fifth temperature detector and the sixth temperature detector meet the requirements, whether the temperatures of the second temperature detector, the third temperature detector and the fourth temperature detector reach more than 500 ℃, 400 ℃ and 300 ℃ is examined, if the temperatures of the second temperature detector, the third temperature detector and the fourth temperature detector do not reach the requirements, the replacement of an alumina block is prompted for absorption until the stored energy reaches the standard;

step 7, when the value of the first temperature detector reaches 100-200 ℃, prompting to replace the alumina block;

B. exothermic process

Step 1, the stepped heat storage main body is vertically installed and inverted, the outlet of a frequency modulation induced draft fan is connected with a heat exchanger, and a fire damper valve at a contact heat collection end is opened by 30%; starting a frequency modulation induced draft fan to guide hot air to enter a heat exchanger to exchange heat with water in the heat exchanger;

step 2, detecting whether the water in the heat exchanger reaches the required water temperature or steam temperature, and keeping the opening degree of a valve when the water in the heat exchanger reaches the required water temperature or steam temperature; when the temperature exceeds the preset temperature, the frequency of the frequency-modulated induced draft fan is adjusted to be low so as to reach the required water temperature or steam temperature; and when the temperature of the water is not reached, the frequency of the frequency-modulated induced draft fan is modulated to be higher so as to reach the required water temperature or steam temperature.

Images