How to use the indispensable heating equipment in the laboratory?

Heating is one of the essential tasks in every experiment. There are various types of heating equipment in the laboratory, such as electric furnaces, electric heating plates, heating sleeves, etc. However, can you use so many heating equipment? Today, Xiaoxi will focus on introducing the structure, performance, usage methods, and precautions of each device to everyone. If you don't pick it up soon, please do it.

electric furnace

Electric furnaces, like gas lamps, are commonly used heating equipment in laboratories. An electric furnace mainly generates heat by passing current through a resistance wire (commonly known as nickel chromium alloy wire), which is commonly referred to as an electric furnace wire.

There is another type of electric furnace that can adjust different heat values, commonly known as a "universal electric furnace". Its appearance is shown in Figure 1. The furnace plate is located above, and there is a single pole multi position switch installed under the furnace plate. There are several contact points on the switch, and an additional resistance is installed between each two contact points. The additional resistance is covered with multiple ceramic tubes to avoid short circuits or electric leakage injuries caused by mutual contact and contact with the furnace shell. By rotating the sliding metal plate to change the size of the additional resistance connected in series with the furnace wire, the current intensity passing through the furnace wire can be adjusted to achieve the purpose of regulating the heat generation of the electric furnace. The circuit of the universal electric furnace is shown in Figure 2.

 

 

Figure 1 Universal Electric Furnace

When metal sliding plate 2 is at power-off point 3, the circuit is not connected and the electric furnace is in the closed position. When the metal sheet turns to contact point 4, all additional resistors are connected in series with the furnace wire. At this time, the total resistance is the highest and the current passing through is the lowest, so the furnace wire releases the least heat. When the metal sheet turns to contact point 5, the additional resistance decreases by half, the current intensity is moderate, and the heat released by the electric furnace is also moderate. When the metal sheet turns to contact point 8, the additional resistance is zero, the current intensity reaches its maximum, and the heat released by the electric furnace is also maximum.

If there is no universal electric furnace in the laboratory, a normal electric furnace can also be connected to an autotransformer with a power equivalent to or greater than it. Adjusting the output voltage allows for arbitrary changes in current intensity and heating capacity of the electric furnace, making it more convenient than a universal electric furnace.

 

Figure 2 Schematic diagram of the universal electric furnace circuit

1- Temperature control knob; 2- Metal sliding plate; 3- Power outage point; 4-terminal block (also the contact point of the metal sliding piece); 5- Mid point terminal block; 6- Additional resistance; 7- Electric furnace wire; 8- Contact points

Electric heating plate and electric heating sleeve

An electric heating plate is actually a closed electric furnace, usually rectangular or circular in shape, with adjustable temperature. The plate can accommodate multiple heating objects simultaneously, and there is no open flame. Electric heating plate is a conventional digestion equipment recognized and accepted by domestic laboratories in the past 10 years. The electric heating plate has good temperature control, high stability, and strong safety, which can greatly help experimental personnel solve some of the problems faced by electric furnace digestion. However, electric heating plates also have some drawbacks: high energy consumption, low thermal energy utilization, small effective heating area, limited sample processing, and weak uniformity of experimental results.

Electric heating sleeve is a specialized equipment for heating round bottomed flasks for distillation. The shell is made into a hemispherical shape, and the interior is composed of electric heating wires, insulation materials, and insulation materials. A suitable electric heating sleeve is selected according to the size of the flask, and an automatic voltage regulator is often connected to adjust the required temperature.

Precautions for using electric furnaces, electric heating plates, and electric heating sleeves

(1) The power supply voltage should match the voltage specified by the electric furnace, electric heating plate, and electric heating sleeve itself.

(2) When the heating container is made of glass or metal products, asbestos mesh should be placed on the electric furnace to prevent uneven heating from causing the glassware to break and the metal container to touch the electric furnace wire, resulting in short circuits and electric shock accidents.

(3) The continuous working time of electric furnaces, electric heating plates, and electric heating sleeves should not be too long to avoid affecting their service life.

(4) The groove of the electric furnace should be kept clean regularly, and the burnt and burnt materials should be removed in a timely manner (power must be cut off during removal) to maintain good conductivity of the furnace wire. Prevent liquid splashing inside the electric furnace and heating sleeve from causing leakage or affecting their service life.

Digestive furnace

The digestion furnace adopts a well type electric heating method, which enables the sample to achieve better thermal effect when heated in the well type electric heating furnace, shortens the digestion and digestion time of the sample, and thus improves the detection speed of organic matter content determination such as protein. The heating element (module) adopts infrared quartz tube, which is resistant to strong acid and alkali, anti explosion, long service life, and meets CE standards. The characteristic is that the digestive tract has a large heating area, small temperature difference, good consistency in sample digestion, high thermal efficiency, and is conducive to sample digestion. The temperature control adopts a digital temperature controller, with accurate temperature control and fast heating. At the same time, harmful gases such as SO2 that overflow from the digestion tube are discharged into the sewer through the sewage collection pipe on the digestion furnace through a filtration pump, effectively suppressing the escape of harmful gases.

 

Schematic diagram of digestion furnace

Digestion steps of samples

Weigh 0.3-1g of the sample that has been crushed and passed through 40-60 mesh/inch, and place it undamaged into a washed and dried digestion tube. Add water, catalyst, and 10mL of sulfuric acid.

1. Place the digestive tubes into the holes of each digestion rack, then place them on the digester and place the sewage pipes with sealed rings on them.

2. Open the suction tee to allow water (tap water) to enter, so that the suction tee is in the suction state.

3. Connect the power again, turn on the respective control switches, rotate the potentiometers, and adjust the indicated voltage to 220V to quickly digest.

4. In the initial stage of digestion, attention should be paid to observation to prevent the sample from splashing due to rapid heating (relief method: it can be turned off for 5 minutes when digestion reaches splashing and then turned on again to continue heating).

5. After digestion, the digestive tube, sewage pipe, and the entire trailer are moved to the cooling rack for cooling. Attention: During the cooling process, the sewage pipe must be kept in an aspirated state (do not put the digestion pipe into water for cooling).

6. Prevent exhaust gas from escaping.

7. After the instrument is used, fill in the usage and maintenance records.

Maintenance and upkeep of digestion furnace

1. The heating temperature of the digestion furnace should not exceed 500 ℃ to prevent damage to the instrument.

2. When abnormal phenomena occur during the use of the digestion furnace, the power should be turned off in a timely manner to check the cause of the malfunction.

3. Without the presence of a professional maintenance engineer, it is not allowed to dismantle the digestion furnace without authorization.

4. The maintenance and upkeep of the digestion furnace requires a record to be filled out.

Muffle furnace

Also known as a high-temperature electric furnace, it is commonly used in weighing analysis, such as burning precipitates, and measuring ash content.

The muffle furnace with a thermal wire structure can operate at a maximum temperature of 950 ℃ and can be used up to 1000 ℃ in a short period of time. The heating element of the silicon carbon rod muffle furnace is the silicon carbon rod inside the furnace, with a maximum operating temperature of 1350 ℃ and a commonly used working temperature of 1300 ℃. The muffle furnace can be divided into two types based on usage requirements: fixed temperature heating and programmed temperature heating.

The furnace of the muffle furnace is made of high-temperature resistant and non expanding and non cracking silicon oxide composite. There is an empty groove between the inner and outer walls of the furnace, and furnace wires are strung in the empty groove. There are furnace wires all around the furnace, as shown in the figure. So, after being powered on, the entire furnace is uniformly heated around and generates high temperature.

 

Furnace shape

Silicon carbon rod muffle furnace, with heating element silicon carbon rods (usually equipped with platinum platinum rhodium thermocouples) distributed on both sides of the furnace. Resistance wire muffle furnaces are generally equipped with nickel chromium nickel aluminum thermocouples.

The periphery of the silicon carbon rod muffle furnace is covered with refractory bricks, refractory soil, asbestos board, etc. to reduce heat loss. The shell is wrapped with a framework and iron sheet with angle iron, and the furnace door is made of refractory bricks. A small hole is opened in the middle and a transparent mica sheet is embedded to observe the temperature rise inside the furnace. When the furnace is dark red, it is around 600 ℃; When reaching deep peach red, it is around 800 ℃; When it is light peach red, it is around 1000 ℃.

Instructions for using the muffle furnace

The temperature control inside the furnace is currently commonly achieved through temperature controllers. The temperature controller mainly consists of a millivolt meter and a relay, connected to a matching thermocouple for temperature control, and its wiring is shown in the figure.

The thermocouple is installed in a ceramic tube and extends into the furnace through the small hole in the middle of the rear of the muffle furnace. Thermocouples generate different potentials with different furnace temperatures, and the magnitude of the potential is directly displayed on the controller header using temperature values. When the pointer indicating temperature (upper pointer) slowly rises and meets the pre adjusted control temperature pointer (lower pointer), the relay immediately acts to cut off the circuit and stop heating. When the temperature drops and the upper and lower pointers separate, the relay reconnects the circuit and the electric furnace continues to heat up. By repeating these actions repeatedly, automatic temperature control can be achieved. Generally, before burning, move the temperature control pointer to the predetermined temperature position and calculate the burning time from the time it reaches the predetermined temperature.

 

Temperature controller wiring diagram

The working principle of a thermocouple is to connect two wires of different metals to form a closed circuit. One contact is heated while the other contact is in a non heated (cold point) state. Due to the different electron concentrations and velocities in different metals, electron diffusion occurs. In the closed circuit, current is generated, resulting in a temperature difference electromotive force. The circuit formed by connecting these two different metals is called a thermocouple. Connect a millivolt meter to both ends of the thermocouple to measure the magnitude of the temperature difference electromotive force. The larger the temperature difference between the cold and hot spots, the greater the millivolts. The scale on the millivolt meter is divided into corresponding temperature values according to the characteristics of the thermocouple used, and the temperature value can be directly read.

Precautions for using muffle furnace

1. The muffle furnace must be placed on a stable cement platform. Insert the thermocouple rod into the furnace through the small hole behind the muffle furnace, and connect the dedicated wire of the thermocouple to the terminal of the temperature controller. Be careful not to connect the positive and negative terminals incorrectly to avoid damage to the temperature pointer due to reverse direction.

2. Identify the required power supply voltage for the electric furnace, configure plugs, sockets, and fuses with appropriate power, and connect the ground wire to avoid danger. A thick rubber cloth should be laid on the ground in front of the furnace for safer operation.

3. After the burning is complete, the electric switch should be pulled down first to cut off the power supply. But the furnace door should not be opened immediately to avoid sudden cold cracking of the furnace. Generally, a small slit can be opened first to allow it to cool down faster, and finally, a long handled crucible clamp can be used to remove the burned object.

4. When using the muffle furnace, it is important to take frequent care to prevent accidents such as electric furnace wire breakage caused by self-control failure. Do not use the muffle furnace when there is no one on duty at night.

5. Keep the furnace clean and avoid stacking flammable and explosive materials around the furnace.

6. When the muffle furnace is not in use, the power should be cut off and the furnace door should be closed to prevent the refractory material from being corroded by moisture.

Electric constant temperature water (oil) bath

An electric constant temperature water (oil) bath pot is often used for evaporation and constant temperature heating, and has single pores and multiple pores. Commonly used for evaporation and constant temperature heating, there are single pore and porous types.

Electric constant temperature water bath pots generally adopt a sink type structure. Divided into inner and outer layers, the inner layer is made of aluminum or stainless steel plates for the inner liner. The bottom of the gallbladder is equipped with an electric heating tube and a bracket. An electric heating tube is a copper tube that contains electric furnace wires and is wrapped in insulating material, with wires connected to the temperature controller. The outer shell is made of thin steel plate, and insulation materials such as asbestos are filled between the outer shell and the inner liner. All electrical components of the temperature controller are installed in the electrical box on the right side of the water bath, and the temperature sensing tube carried by the controller is inserted into the inner container. Surface display and operation panel of the electrical box. There is a drain valve on the lower left side of the water bath pot.

 

Electric constant temperature water bath pot

The water bath pot is heated by electric heating. The power supply voltage is 220V. The constant temperature range of the water bath is 37-100 ℃ (3 ℃ higher than room temperature), with a temperature difference of ± 1 ℃. The structure of an oil bath and a water bath is similar, except that the heating medium is changed from water to oil. The constant temperature oil bath is widely used in the laboratory and is an essential high-temperature constant temperature equipment. It uses a high-temperature heating tube to heat the heat transfer oil, and then precisely controls the temperature through a precision temperature control instrument. The oil used in the oil bath should be determined based on temperature and experimental requirements.

Glycerin is used for low temperatures, cottonseed oil is used for high temperatures. Generally, the oil commonly used in constant temperature oil baths is oil NO 1. Oil NO 2. Olive oil, cottonseed oil, paraffin oil, sesame oil, engine oil, transformer oil. The flash point of olive oil, cottonseed oil, and sesame oil in these oils is around 300 ℃. Therefore, if high-temperature testing is required, it is recommended to use these types of oils. Otherwise, when the ignition point of the oil is reached, it will be very dangerous.

Usage and precautions

1. Usage of water bath pot

(1) Close the drain valve and inject clean water (preferably pure water) into the water bath to an appropriate depth, generally not exceeding two-thirds of the capacity of the water bath.

(2) Connect the power plug to the socket and install a ground wire in the thick hole of the socket.

(3) Turn on the power switch and turn on the power. Adjust the temperature control button to the set temperature.

(4) When the temperature index of the furnace wire rises to the controlled temperature after heating, the red light goes out, and then the red light continuously goes out, indicating that the constant temperature controller is working.

2. Usage of Oil Bath Pot

(1) When using an oil bath, it is necessary to add oil to the pot first, then connect the power, display the actual measured temperature on the digital temperature control meter, and adjust the knob switch.

(2) At the same time, observe the reading to the desired temperature value. When the set temperature exceeds the oil temperature, the heating indicator light will light up, indicating that the heater has started working.

(3) When the oil temperature reaches the desired temperature, the constant temperature indicator light will turn on and the heating indicator light will turn off.

(4) It should be noted that the oil in the pot should not cause the electric heating tube to leak out of the oil surface, in order to avoid burning the electric heating tube and causing leakage.

3. Precautions for water bath pots

(1) Remember to keep the water level not lower than the electric heating tube, otherwise it will immediately burn out the electric heating tube.

(2) The interior of the control box must not be damp to prevent electrical leakage and damage to the controller.

(3) Always pay attention to whether there is any leakage in the water tank during use.

In addition to ordinary electric constant temperature water bath pots, there are also some precision experimental super constant temperature water bath pots that use electric circulation pumps for stirring and have a good automatic temperature control system, with a constant temperature fluctuation of ± 0.05 ℃.

4. Precautions for oil bath pots

(1) When injecting liquid into the oil bath, the liquid level should be controlled to prevent excessive overflow. In summer, there is a large temperature difference between indoors and outdoors. When the experimental temperature reaches 300 ℃, the liquid level should be controlled at around 80% of the volume.

(2) Prohibit the use of highly flammable and volatile oils, and the oil used should be determined based on temperature and experimental requirements. Use glycerol for low temperatures and cottonseed oil for high temperatures.

(3) Oil bath pots should not be used in areas with poor ventilation, and should be kept away from sources of fire and areas prone to sparks to prevent fires.

(4) It is prohibited to burn without oil, as it may cause electric leakage, fire, and damage the heating tube. It is prohibited to operate with wet hands in areas with excessive moisture, as there is a risk of electric leakage and shock. The power supply must use a grounding plug.