1.1 solid state heater construction
The distance between the heated solid and the PTC heater should be within 30mm when the material with high thermal conductivity (such as aluminum, copper, graphite) conducts heat the distance between the heated solid and the PTC heater should be within 10mm when the material with higher thermal conductivity (such as steel, stainless steel, titanium, heat conducting ceramics, etc. The distance between the heated solid and the PTC heater should be within 3mm in the case of heat conduction of the materials with poor thermal conductivity, such as plastics, non-porous rubber, insulating paper, Mica Sheet, etc. Do not use thermal insulation materials (such as fiber paper, rubber, Foam Board, etc.) to conduct heat. With better thermal conductivity material, the heated solid will get a more stable temperature when the distance between the heated solid and the PTC heater is smaller.
The area of heat dissipation structure and the heat transfer area ratio of PTC ceramic body, preferably not more than 3 times. The large proportion of the area of the heat dissipation structure will make the temperature of the heated solid unstable.
1.2 surface temperature
The temperature tolerances of PTC ceramic heating plates at 25 °C and 5 °C are shown in Table 1. However, if the PTC ceramic heating plate is installed in the cooling structure, the temperature accuracy will become worse and the tolerance will increase. When the heat transfer is poor in the heater and the area of the heat transfer structure is larger than that of the PTC ceramic body, the temperature precision will decrease.
Fig. 2 is the PTC heating / Cooling Equilibrium Diagram. The resistance value of PTC heater increases sharply with the increase of its temperature. So, when the temperature rises, the heating power will drop sharply; when the Heating Power and the cooling power reach the balance, the temperature will not rise, the power will not change. However, when the heat dissipation conditions (such as ambient temperature, blowing, soaking, etc.) change, PTC power and temperature readjusted to a new balance.
The factors affecting the surface temperature of PTC are shown in Table 2.
There are many factors that affect the surface temperature of PTC heater, so the precision of surface constant temperature cannot be very high. PTC Heater and temperature controller can be used for high precision temperature control. When the external temperature controller fails, the heating surface temperature of the PTC heater will not exceed the set constant temperature and has the function of over-temperature protection.
1.3 heating rate
The influencing factors of temperature-rising speed of PTC heater are shown in Table 3.
Electric Mosquito repellent, massager, heater, electric iron, electric iron, humidifier, hair curler, hair straightener, glue machine, Electric Incense Burner, hot melt glue gun, electric iron, melting wax, electronic component insulation, circuit board drive moisture and so on.
The PTC heater is used as solid constant temperature heating, the suitable heating temperature is 0 ~ 300 °C, and the heating temperature is 50 ~ 300°C at normal temperature. The suitable ambient temperature is-40 ~ 250 °C. PTC with too high or too low surface temperature is difficult to manufacture and does not even have PTC characteristics.
Operating Voltage 3.7 v ~ 420V, both AC and DC. When the working voltage is low, the resistance of PTC at room temperature is very small, the rate of change of resistance-temperature is also small, and the precision of constant temperature is poor.
Considering the safety and reliability, PTC heater is safer, has higher insulation voltage, less leakage current and more stable than traditional electric heating wire. Some applications using traditional electric heating wire cannot pass the safety certification, and the use of PTC heater can pass the safety certification, so PTC can be used in higher safety requirements. However, not all PTC, all PTC, security will meet the requirements, so choose good quality PTC. PTC is used for solid heating, the main safety and reliability issues: First, PTC Ceramic Body burn-out, resulting in short circuit, Burn Insulation Layer; Second, insulation layer breakdown leakage, so that the shell lives.
From the cost consideration, small area of constant temperature heating, using PTC heater can save temperature control and overtemperature protection, PTC volume can be small, installation is also relatively simple. For constant temperature heating with large heating area, the temperature is more even when PTC heater is used. The Life of PTC constant temperature heater is several times longer than that of traditional electric heating wire heater. Using PTC also saves life cost and maintenance cost.
2. Heating of liquids (including water, diesel oil, hydraulic oil, etc.).
2.1 liquid heater construction
Fig. 3 is a structural diagram of the immersion liquid heater.
Figure 5 shows the placement of the heater in the container. Vertical heating pipe installation is relatively simple, but the liquid upper and lower layer of the temperature is not uniform. The heating temperature of the horizontal heating pipe is more uniform, but when it is installed, the container needs to be punched, and the installation hole needs to be sealed. The heating temperature of the curved heating pipe is more uniform and does not need to be punched, but the cost is higher. When the bottom heater of the container is installed at the bottom, the heating temperature is relatively uniform.
The Materials of the metal shell in contact with the heated liquid can be selected: al-si alloy, al-zn alloy, al-mg alloy, red copper, brass, iron (galvanized, etc.), 304 stainless steel, 316 stainless steel, titanium, cu-ni alloy, etc. According to the use, the nature of the liquid, liquid corrosion of metal and other factors determines the selection of metal shell materials. For the heating of oil, low-cost Aluminum Alloy Shell can be used; for the heating of washing water, aluminum alloy, copper, brass shell can also be used; for the heating of edible water, 304 stainless steel, 316 stainless steel. When the water quality is poor, or sea water, or for the heating of electroplating water solution, titanium shell can be used; corrosive liquid, copper-nickel alloy can be used. In addition, the metal shell surface attached to a layer of fluoroplastic, in the temperature tolerance, can resist most of the liquid corrosion, but the heating power will be significantly reduced.
Liquid heater at bottom (or side) of container, heater does not contact liquid directly, liquid will not corrode heater shell. But this kind of heater has the advantages of large volume, low power and high cost.
2.2 heating power
The relation between the heating power and the heating time of the PTC heating tube is quite different from that of the traditional electric heating wire heating tube. At the beginning of heating, PTC heating tube power from small to large, to reach the maximum (maximum impact power), after a gradual decline, finally stable (boiling water stable power). The steady power of dry-burning is much less than that of boiling water (about 8% of boiling water).
The stable power is related to the liquid temperature. The higher the liquid temperature is, the smaller the stable power is. According to the actual conditions of use, a standard liquid temperature, when the heating tube heating, liquid temperature from low to high, liquid temperature up to the standard temperature, the recorded power is the standard temperature of the stable power.
The stable power depends on the type of liquid. When the liquid is water, the power is bigger, because the specific heat capacity of water is bigger, the fluidity is better, easy to form the convection, heating other liquid stable power is generally smaller than water.
The higher the dry burning surface temperature of PTC heating pipe is, the greater the stable power is. However, the increase of surface temperature will lead to the increase of power attenuation, PTC heating piece breakdown opportunities, viscose, wire, insulation layer and other performance degradation, reliability.
When the surface temperature of the PTC heating tube is the same as the liquid temperature, the heating power is basically 0, that is, no more heating. Therefore, the PTC heating tube's dry burning surface temperature will always be higher than the liquid temperature above 20 °C, otherwise it cannot be heated. The smaller the difference between the surface temperature of PTC heating tube and the liquid temperature, the smaller the power density, the larger the volume and the higher the cost.
The maximum impact power of the heating pipe is large, the stable power is not necessarily large. If the maximum impact power is too large, the normal operation of the power supply circuit and the switch will be affected, but if the maximum impact power is too small, the stable power will not meet the requirements.
2.3 power attenuation
PTC power attenuation is too large due to the quality difference among PTC manufacturers. Shenzhen LC Electronics' PTC liquid heating pipe, a year of uninterrupted power attenuation is generally around 10%, completely in the user's acceptable range. In some PTC manufacturing plants, the power PTC is about 30% a month, and the power attenuation of continuous working for a year is between 40% and 50%, which is practically unusable. So, when buying a PTC, be sure to do a power attenuation test.
The power attenuation is positively correlated with the surface temperature of PTC ceramic heater. The higher the surface temperature, the greater the power attenuation. That is to say, reducing the surface temperature of the PTC heater can reduce the power attenuation, but the size of the heating tube will increase, and the cost will also increase.
There are two main reasons for the power attenuation, one is the increase of the resistance of PTC ceramic heater at room temperature / The decrease of the impulse current, the other is the loosening of the heat transfer structure which makes the heat transfer worse.
Storage type water heater, namely heat type water heater, Electric Kettle, Hot Water Dispenser, Hot Water Dispenser, hot water for washing vegetables, washing dishes, washing face, bathing, soaking feet, foot bath, auxiliary heating of Solar Energy and heat pump water heater, electric heating of photovoltaic solar water heating, HOT WATER CLEANING OF EXHAUST HOOD, cleaning agent, hot water bed, plating tank, diesel fuel tank, filter heating, hydraulic oil, lubricating oil, cooking oil heating, sauna, beauty, disinfection, humidification to produce steam, medicated fumigation, aquarium, aquarium pool insulation, water pipes, faucets, pipes, tubing anti-freezing, three-way catalysis, heating of urea solution, water heating, heating oil ting type, water heat sink, etc. .
Stable heating power can range from 10W to 10KW. However, the more PTC ceramic heating plates are used for high-power heating pipes, which will increase the chance of PTC ceramic heating plates breakdown, the larger the insulation area and the higher the chance of insulation failure. So, it is best to be divided into several heating tubes, each of the heating tubes has less power, this can reduce failure and waste. Operating Voltage Range 12V ~ 420V, both AC and DC. When the working voltage is low and the heating power is large, the impulse current and the stable current will be very large, the power supply is difficult to bear, so when high power is needed, it is best not to use low voltage.
The advantage of electric heating wire electric heating pipe is lower cost. The existing problems: First, after using for a period of time, insulation voltage, leakage current, insulation resistance exceeded the standard; second, cannot dry burning, if the temperature protection device failure, then the electric heating pipe will be dry burning, or even burn out the surrounding materials, a fire; Three is, after the electric heating wire works for a long time, it may oxidize and burn out, which may affect its life and even cause electric leakage; four is, when the water quality is bad and the concentration of cation (calcium, magnesium, etc.) is relatively high, the surface of the electric heating pipe is easy to appear scale, and the surface heat transfer becomes worse That caused the internal heating wire to burn out.
PTC liquid heating pipe can overcome the shortcomings of electric heating pipe, insulation voltage resistance, leakage current, insulation resistance excellent and stable, resistant to dry burning, long service life, scale will not burn. However, if the quality of PTC manufacturing is poor, there may also be problems: First, PTC ceramic body burn down, resulting in a short circuit, burning insulation layer; second, insulation layer breakdown leakage, so that the shell live; third, heating power with the increase in the use of time and decline; Fourth, the impact current is too large, resulting in power supply lines and switch failure. PTC heating pipe users should carefully choose PTC heating plate to ensure the quality of reliability.
3. Air Heating
3.1 Air Heater construction
Air Heating structure, according to whether the radiator charged division, divided into charged and insulation type.
Fig. 7 is a charged air heater. The radiator is charged. The electrode layer of the PTC ceramic heater is in direct contact with the aluminum radiator, and the aluminum radiator acts as an electrode outlet. The electric type air heater is easy to obtain the bigger power, the cost is also lower, but because the radiator is electrified, easy to have the security hidden danger.
Fig. 8 is an insulated air heater. The radiator and PTC ceramic heater are insulated. The radiator is not electrically charged. Insulated air heaters are safer, but have lower power and higher cost.
According to the structure of the radiator, there are corrugated structure, insert structure, aluminum extrusion structure, Compressed air heating and so on. The corrugated structure of the air heater is more flexible, length, width, easier to adjust, composed of various sizes, various power, to adapt to various fan size, duct size, and heating power requirements. Corrugated Structure Heater radiator and PTC heating plate (or heating pipe), is through adhesive bonding, in high temperature for a long-time use, or long-term drying without drying, adhesive may fall off. The length of insert structure and aluminum extrusion structure can be adjusted properly, but the width is difficult to adjust. Insert structure gives the impression that the structure is compact, strong, and relatively small wind resistance.
According to whether you need to blow to differentiate, divide into again blow wind type and natural convection type air heater. The blower type air heater is more powerful, but requires additional fans, which generate noise. Convection type air heater does not need fans, no noise, especially suitable for the bedroom heating, heating under the bed, but the heating power is small.
3.1 heating power
The change of heating power with heating time is shown in Fig. 9. At the beginning of power, the initial power is relatively small (the initial power is usually 1 / 3 to 1 / 5 of the maximum impact power), then the heating power gradually increases to the maximum impact power, then the power decreases and finally reaches the stable power. In the case of heat dissipation too fast, the heater is always at the initial power, cannot reach the maximum power, so the stable heating power is very small.
Maximum impact power c * Using Voltage / Ambient Resistance
The value of the maximum impact power depends on the resistance at room temperature, the surface temperature, the grain size in the ceramic body and the applied voltage of the PTC ceramic heater. The higher the surface temperature is, the higher the C is, and the higher the room temperature resistance is, the higher the C is. If a heater has several PTC ceramic heating plates with different resistances, the maximum impulse current of the heater to be energized at cold state (room temperature) will be smaller, but at hot state (about 40 °c lower than the surface temperature) it will be energized the maximum impulse current will be greater. The maximum impact power of the air heater, the stable power will be slightly larger. If the maximum impact power is too large, it will affect the normal operation of the whole power supply circuit and the switch, but if the maximum impact power is too small, it will cause the heating power to fail to meet the requirements, and even the heating power will be smaller when the blowing speed is higher.
The steady heating power of PTC air heater is related to the wind speed. Generally speaking, the higher the wind speed, the faster the heat dissipation, the higher the heating power. The General Fan's wind speed is 3 ~ 5 M / S. The power without blowing is about 10% of the power at 5M / S. When necessary, the heating power can be adjusted by adjusting the wind speed. If the shock power of the heater is close to the steady power at low wind speed, the power cannot be increased by increasing the wind speed.
NON-INSULATED PTC air heater heat dissipation faster than insulation, so the heating power is also larger. The radiator is denser, the wind area increases, will also increase heat dissipation, heating power will also increase.
With the increase of the number of PTC heating pads, the stable heating power will not increase in direct proportion, and the heating power of each heating pad will decrease.
The relationship between the heating power of PTC air heater and the influencing factors is as follows:
Coefficient of heat dissipation for steady heating power * (dry burning temperature-inlet air temperature)
The heating power is related to the inlet air temperature. The higher the inlet air temperature is, the smaller the stable power is. When the surface temperature of the PTC ceramic heater is the same as the air temperature, the heating power is basically 0, that is, no more heating. Therefore, the surface temperature of PTC ceramic heater without air drying is always higher than 20 °C of air inlet temperature, otherwise it cannot be heated. The smaller the difference between the air-dry surface temperature and the air temperature, the smaller the heating power density, the larger the volume and the higher the cost.
When the outlet temperature of a heater does not reach the desired high temperature, to increase the outlet temperature, a second PTC air heater is placed along the wind direction. At this point, the first heater near the fan is of higher power; the second heater away from the fan has a higher inlet temperature, so the power is much lower and the steady power is often less than 50 per cent of the first heater.
The higher the surface temperature without air drying, the higher the heating power of PTC ceramic heater. However, when the surface temperature of PTC ceramic heater is raised, the heating power decreases with the service time, the breakdown of PTC heater increases, the performance of adhesive, wire, insulation layer and so on decreases, and the reliability of heater decreases.
3.2 power attenuation
PTC Air Heater Power attenuation factors include: PTC ceramic heating plate resistance increased, loose adhesive, loose aluminum radiator and so on. Some PTC manufacturer's PTC, a month of power in more than 30%, so choosing good quality PTC is very important, in the purchase of PTC air heaters, be sure to test power attenuation.
The higher the surface temperature is, the more the resistance of the PTC heater increases, the easier the adhesive is to be loosened, the easier the aluminum radiator is to be loosed, and the greater the power attenuation is. If the surface temperature is above 250 °C PTC heating plate to do air heater, no wind state power, viscose will gradually decline in strength after 200 hours, loose. That is to say, reducing the surface temperature of the PTC heater can reduce the power attenuation, but the volume of the air heater will increase and the cost will increase.
Air Heater, air conditioner, clothes, shoes, hands, desiccant, tea, agricultural products, hot air drying, heating or drying of Wardrobe, bookcase, shoe cabinet, room, bathroom, factory building, shopping mall, warehouse, Electronic Control Cabinet, high-speed railway carriage, electrical equipment, storage, tunnel oven, Compressed air heating, battery discharge load.
Stable heating power can range from 100W to 10KW. If the power of a single heater is smaller, it can reduce failure, improve the qualified rate, need more power can use multiple heaters. Operating Voltage 12V ~ 420V, both AC and DC. When the working voltage is low, the impulse current and stable current will be very large, so when high power is needed, it is best not to use low voltage.
The advantages of the heater are lower cost, higher outlet temperature and smaller volume. The existing problems are: First, after using for a period of time, the insulation withstand voltage, leakage current and Insulation Resistance Exceed The Standard; second, there is no ventilation, if the fan is stopped, the air duct is blocked, the temperature protection device fails, then the heater will burn down due to no wind overtemperature, or even burn the surrounding materials, resulting in a fire; third, after long-term operation of the electric heating wire, it may oxidize and burn down, affecting its life and even causing electric leakage; fourth, when combustible materials are attached to the heater, it could cause a fire.
PTC Air Heater can overcome the shortcomings of electric heater, insulation voltage, leakage current, insulation resistance excellent and stable, no wind ventilation, automatic protection, will not catch fire burning, long service life. However, if the quality of PTC heater manufacturing is poor, there may also be problems: First, PTC Ceramic Body burn-out, resulting in a short circuit, burning insulation layer; second, the heating power with the increase in the use of time and cannot achieve the expected service life. Some PTC heater manufacturers reduce production costs and use low-grade materials, manufacturing process cannot be strictly controlled, there is no life test. The quality of PTC heaters is not guaranteed in all PTC. The control of power attenuation, power accuracy, impulse current, voltage resistance, insulation performance, reliability, etc. Variations from factory-to-factory PTC should be carefully selected to ensure the reliability of quality.