Working principle of energy saving high temperature muffle furnace:
Based on the law of conservation of energy conversion by electrical energy into heat energy when intelligent high temperature muffle furnace to work normally, applied voltage and current on the heating element (resistance), heating elements in the studio to produce a large amount of heat, and heat convection and thermal radiation, the work of indoor heat preservation material absorption, send out a few of the heat and isolated from most of the heat, so that the work of indoor temperature rise gradually, to reach the predetermined temperature of heated material.
Installation and use of energy-saving high temperature muffle furnace:
1. After opening the package, check whether the muffle furnace is intact and the accessories are complete. General muffle furnace does not require special installation, just flat on the indoor flat ground or shelf. The controller should avoid vibration and should not be placed too close to the electric furnace to prevent internal components from working normally due to overheating.
2. A thermocouple is inserted into the furnace 20-50mm, and the space between the hole and thermocouple is filled with asbestos rope. Connect the thermocouple to the control, preferably with a compensating conductor (or an insulated steel core), pay attention to the positive and negative terminals, do not connect them in reverse.
3. The power switch shall be installed at the power line to control the total power supply. In order to ensure safe operation, the electric furnace and controller must be reliably grounded.
4. Before use, adjust the thermometer indicator to zero. When using the compensation wire and the cold end compensator, adjust the mechanical zero to the reference temperature point of the cold end compensator.
5. After checking the wiring and confirming it is correct, cover the controller shell. Adjust the setting pointer of the temperature indicator to the required operating temperature, and then switch on the power. When the power switch is turned on, the green light on the temperature indicator instrument will be on, the relay will start to work, the electric furnace will be energized, and the ammeter will have current display. With the increase of the temperature in the electric furnace, the temperature indicator indicator pointer also rises gradually, which indicates that the system works normally. The temperature rise of the electric furnace and the fixed temperature are indicated by the traffic light of the temperature indicator respectively. The green light means the temperature rise and the red light means the fixed temperature.
Calibration method of high temperature muffle furnace:
The working system of high temperature muffle furnace consists of three parts: resistance furnace body (electric wire embedded in the furnace chamber), temperature control thermocouple and temperature control display instrument. The evenly distributed resistance wire in the furnace body heats up after electrification, and the supporting instrument can automatically adjust the heating power or duty cycle. The furnace body is made of thermal insulation materials, and zui can finally control the furnace temperature at a certain set temperature.
1. Traditional verification methods
For the high temperature muffle furnace calibration, the temperature control thermocouple and the temperature control display instrument were removed and sent for inspection respectively, and then put into use after the inspection meets the requirements. This detection method has the following shortcomings:
(1) the insertion depth of temperature control thermocouple should not be too shallow, about 8 to 10 times the diameter of thermocouple protection tube, so as to ensure the accuracy of temperature measurement; But also not too deep, so as not to interfere with the furnace workpiece. Therefore, the insertion position of the instrument controlled thermocouple is generally at the edge of the effective heating area or beyond, which cannot accurately reflect the temperature of the effective heating area where the workpiece is located.
(2) the thermocouples used in some resistance furnaces do not meet the standard specifications and the length is very short, which does not meet the requirements of the verification regulations, so it is impossible to conduct verification in the thermocouple verification furnace.
(3) the production department needs to stop the furnace every time the customer removes it for inspection, which will bring inconvenience to the customer.
2. Overall system calibration method
This method is to take the resistance furnace body, temperature control thermocouple and temperature control display instrument as a whole, and conduct overall on-site calibration. According to the needs of daily calibration, this calibration method is suitable for zui high temperature resistance furnace with high working temperature of 1600℃.
(1) calibration items
The calibration items of high temperature resistance furnace mainly include temperature deviation and temperature uniformity.
(2) composition of calibration device
Standard devices and supporting equipment are standard thermocouples, standard display instruments (dc voltage measuring instruments), digital thermometers, compensation wires and change-over switches.
(3) calibration steps
It is best to measure the muffle furnace without charge (no load) in the furnace. If there is charge, it should be made clear to the customer that the standard must be qualified.
Standard thermocouple installation
Under the premise of power failure, the standard thermocouple is inserted through the measuring port, and the measuring end of the thermocouple is placed in the central sample analysis position to reflect the real temperature of the working area in the furnace well. Connect the reference end of the thermocouple with the input end of the standard display instrument, and place the digital thermometer next to the reference end to obtain the compensation temperature of the reference end. Close the measuring port with asbestos to prevent heat leakage.
The position of the test point
The location of the test point should be distributed in resistance furnace work indoor three plane, scale, middle and lower three layers, the middle by studio parallel to the bottom of the working face geometric center, test points and studio wall distance of not less than 1/10 of each side in the duct, this distance can be increased, such as resistance furnace with sample or sample car, the lower test point can be put in the sample or samples of 10 mm above the car. For the resistance furnace with a small box (less than 0.2m3) used in the laboratory, the position of the test point can be adjusted appropriately. It is usually placed in the central position of the resistance furnace, or at the measuring end of the standard thermocouple and the thermocouple of the calibrated resistance furnace. The test layout is shown in figure 2 (a is when the volume is less than 0.9m3, and b is when the volume is greater than 0.9m3).
Selection of calibration point
In general, the calibration temperature point of newly purchased resistance furnace should choose the lower and upper limit temperature of the range of use, the arbitrary temperature between the upper and lower limits can be selected in use, and the actual temperature points commonly used can be selected according to the user's needs, so as to facilitate the temperature uniformity calibration.
Temperature calibration
The temperature controller of the calibrated resistance furnace is set to the required nominal temperature, usually from low temperature to high temperature, so that the resistance furnace can work normally and start reading after stabilization. The temperature of all test points is recorded once every 2min, and a total of 15 tests are conducted within 30min. When the measured value is corrected, it is the actual temperature. For the resistance furnace with a small volume, only the temperature deviation of the resistance furnace can be calibrated.
Data processing
A. Calculation of temperature deviation
Note: the millivolt value corresponding to the temperature point measured by the standard thermocouple is the sum of the millivolt value shown by the standard display instrument and the millivolt value shown by the digital thermometer. The temperature value measured by the standard thermocouple can be obtained by referring to the thermocouple dividing meter.
B. Calculation of temperature uniformity
The arithmetic mean value of the difference between zui high temperature and zui low temperature measured in each measurement
(5) problems that should be paid attention to in the whole calibration process of the system
- standard thermocouple furnace loading, wiring in the case of the resistance furnace power, check the connection is correct, can open the resistance furnace temperature.
- the measuring end of the standard thermocouple shall be placed near the analysis position of the sample, but shall not touch the sample or the inner wall of the furnace to prevent contamination or damage to the standard thermocouple.
- after the temperature reaches the temperature control point, in order to make the furnace temperature uniform and stable, the temperature should be controlled for 30min before reading. Because the secondary instrument of the resistance furnace is not synchronized with the display of the millivoltmeter, the zui high and zui low values of the millivoltmeter should be read during each reading turn, and the zui high and zui low values displayed by the secondary instrument should be read accordingly.
- measurement of furnace temperature uniformity must be carried out in the case of no load, to reach the thermal stable state. For a resistance furnace with a fan, the fan should work normally during the measurement.
- if the indication error is too large, check whether the furnace is working normally. For example, in order to prevent the stainless steel protection tube of the thermocouple of the control instrument from being short connected with the furnace wire, an insulated porcelain tube is set on the thermocouple of the control instrument. In this way, the insulated porcelain tube should be cut short so as to avoid the actual length of the thermocouple of the control instrument being 8~10 times shorter than that of the thermocouple protection tube. The hole in the thermocouple of the control instrument inserted into the furnace wall should be tightly sealed to prevent heat loss in the nearby area. In practice, some measures can be taken to reduce the calibration value of furnace temperature with large error value.
- after the calibration, in order to avoid damaging the thermocouple or shorten the life of the thermocouple, the standard thermocouple cannot be taken out quickly, nor can the furnace door be opened quickly because rapid cooling and heating will damage the furnace chamber.
Precautions for energy saving high temperature muffle furnace:
1. When muffle furnace is used for the first time or is used again after long-term shutdown, oven drying must be carried out. The oven drying time shall be at room temperature 200 ℃ for four hours. 200 ℃ to 600 ℃ for four hours. When in use, the maximum furnace temperature shall not exceed the rated temperature, so as to avoid burning the electric heating elements. It is forbidden to pour various liquids and soluble metals into the furnace. Muffle furnace is better to work below the maximum temperature of 50 ℃. At this time, the furnace wire has a long service life.
2. Muffle furnace and controller must work in the place with relative humidity no more than 85%, no conductive dust, explosive gas or corrosive gas. When metal materials such as grease need to be heated, a large number of volatile gases will affect and corrode the surface of electric heating elements, so as to destroy and shorten their service life. Therefore, when heating, it is necessary to timely prevent and seal the container or properly open holes to eliminate.
3. Muffle furnace controller shall be limited to the range of ambient temperature 0-40 ℃.
4. According to the technical requirements, regularly check whether the wiring of the electric furnace and controller is good, whether the pointer of the indicator is stuck when it moves, and use the potentiometer to check the increase of the error caused by the magnetic steel, demagnetization, wire expansion, spring fatigue, balance damage, etc. of the instrument.
5. Do not pull out the thermocouple suddenly in case of high temperature, so as to prevent the jacket from cracking.
6. Keep the furnace clean and remove oxides and other things in time.
On 2018-08-08 11:45:46
