Principle and function of PTC heater

PTC heater
PTC heater, also known as PTC heating element, is composed of PTC ceramic heating element and aluminum tube. This type of PTC heater has the advantages of low thermal resistance and high heat transfer efficiency, making it an automatic constant temperature and energy-saving electric heater. The prominent feature lies in its safety performance. Under any application, there will be no surface "redness" phenomenon such as electric heating tube heaters, which can cause safety hazards such as burns and fires.

It is composed of galvanized outer pressure plate, stainless steel corrugated spring plate, galvanized inner pressure plate, single-layer aluminum heat sink, PTC heat sink, double-layer aluminum heat sink, nickel plated copper electrode terminal, and PPS high-temperature plastic electrode sheath. Due to the use of U-shaped corrugated heat dissipation fins, this product has improved its heat dissipation rate, combined with the advantages of adhesive and mechanical, and fully considering various thermal and electrical phenomena of PTC heating elements during operation. Its strong bonding force, excellent thermal conductivity and heat dissipation performance, high efficiency, and safety and reliability. This type of PTC heater has the advantages of low thermal resistance and high heat transfer efficiency, making it an automatic constant temperature and energy-saving electric heater. One of its prominent features lies in its safety performance, that is, when the fan malfunctions and stops running, the PTC heater's power will automatically drop sharply due to insufficient heat dissipation. At this time, the surface temperature of the heater is maintained at around Curie temperature (usually around 250 ℃), so as not to produce the phenomenon of "redness" on the surface of electric heating tube heaters
​
Principle of PTC heater
The PTC thermistor with constant temperature heating has the characteristic of constant temperature heating. Its principle is that after the PTC thermistor is charged, it self heats up and causes the resistance value to enter the jump zone. The surface temperature of the PTC thermistor with constant temperature heating will remain constant, which is only related to the Curie temperature and applied voltage of the PTC thermistor, and is basically independent of the ambient temperature. PTC heater is a heating device designed using the constant temperature heating characteristics of PTC thermistors. In small and medium power heating applications, PTC heaters have advantages that traditional heating elements cannot match, such as constant temperature heating, no open flame, high heat conversion rate, minimal influence from power supply voltage, and long natural life. Their application in electric heating appliances is increasingly favored by research and development engineers. Thermostatic heating PTC thermistors can be made into various external structures and specifications, commonly including circular, rectangular, elongated, circular, and honeycomb porous shapes. Combining the above PTC heating elements and metal components can form various forms of high-power PTC heaters.

PTC heaters are divided by conduction mode
(1) PTC ceramic heaters primarily based on heat conduction. Its characteristic is to transfer the heat emitted by the PTC heating element to the heated object through multi-layer heat transfer structures such as electrode plates (conductive and heat transfer), insulation layers (insulation and heat transfer), thermal storage plates (some also have thermal adhesive attached), etc. installed on the surface of the PTC heating element.

(2) Various PTC ceramic hot air heaters that use the generated hot air for convective heat transfer. Its characteristic is high output power and the ability to automatically adjust the blowing temperature and output heat.

(3) Infrared radiation heater. Its characteristics actually utilize the heat rapidly emitted from the surface of PTC components or thermal conductive plates to directly or indirectly excite the far-infrared coating or far-infrared material in contact with its surface to emit infrared radiation, forming a PTC ceramic infrared radiation heater.

PTC heaters are classified according to their structural characteristics
(1) Ordinary practical PTC ceramic heater. These types of appliances mainly include electric mosquito repellent, hand warmer, dryer, electric heating plate, electric iron, electric soldering iron, electric adhesive, curling perm, etc. Its characteristic is that it has low power, but high thermal efficiency and is very practical.

(2) Automatic constant temperature PTC heater. This type of equipment mainly includes: small crystal device constant temperature bath, constant temperature incubator, electronic thermos, incubator, insulated cup, insulated plate, insulated cabinet, insulated table, etc. Its characteristics include automatic insulation, simple structure, good constant temperature characteristics, high thermal efficiency, and a wide temperature range for use in the environment.

(3) Hot air PTC heater. This type of hot air PTC heater mainly includes small warm air heaters, hair dryers, warm room heaters, dryers, wardrobes, dryers, industrial drying equipment, etc. Its characteristics are high output hot air power, fast heating, safety, and the ability to automatically adjust wind temperature and power consumption.

PTC plays a role in air conditioning
The PTC corrugated heater for air conditioning is a type of self-control temperature heater.

1. The PTC thermistor with constant temperature heating has the characteristic of constant temperature heating. Its principle is that after the PTC thermistor is charged, it self heats up and causes the resistance value to enter the jump zone. The surface temperature of the PTC thermistor with constant temperature heating will remain constant, which is only related to the Curie temperature and applied voltage of the PTC thermistor, and is basically independent of the ambient temperature.

PTC heating element is a heating element designed using the constant temperature heating characteristics of PTC thermistors. In small and medium power heating applications, PTC heating elements have advantages that traditional heating elements cannot match, such as constant temperature heating, no open flame, high heat conversion rate, minimal influence from power supply voltage, and long natural life. Their application in electric heating appliances is increasingly favored by research and development engineers.

3. Thermostatic heating PTC thermistors can be made into various external structures and specifications, commonly including circular, rectangular, elongated, circular, and honeycomb porous shapes. Combining the above PTC heating elements with metal components can form various forms of high-power PTC heating elements.

Application scenarios and precautions for PTC heaters
1. Solid constant temperature heating.

1.1 Solid heater structure.

In the case of materials with high thermal conductivity (such as aluminum, copper, graphite blocks, etc.) conducting heat, the distance S between the heated solid and the PTC heater should be within 30mm; When conducting heat in materials with slightly higher thermal conductivity (such as steel, stainless steel, titanium, thermal ceramics, etc.), the distance between the heated solid and the PTC heater should be within 10mm; When conducting heat in materials with poor thermal conductivity (such as plastic, non porous rubber, insulating paper, mica sheets, etc.), the distance between the heated solid and the PTC heater should be within 3mm; Do not use insulation materials (such as fiber paper, rubber, foam board, etc.) to conduct heat. If a material with good thermal conductivity is used and the distance between the heated solid and the PTC heater is small, the heated solid will achieve a relatively stable temperature.

The ratio of the area of the heat dissipation structure to the heat transfer area of the PTC ceramic body should not exceed three times. The proportion of heat dissipation structure area is too large, which can cause the temperature obtained by the heated solid to be unstable.

1.2 Surface temperature.

The temperature tolerance of PTC ceramic heating element at an ambient temperature of 25 ℃± 5 ℃ is shown in Table 1. But if the PTC ceramic heating element is installed in the heat dissipation structure, the temperature accuracy will deteriorate and the tolerance will increase. When the internal heat transfer of the heater is poor, and the area of the heat dissipation structure is relatively large compared to the heat transfer area of the PTC ceramic body, the temperature accuracy will decrease.

There are many factors that affect the surface temperature of PTC heaters, so the accuracy of surface constant temperature cannot be very high. In situations where high temperature control accuracy is required, a PTC heater and an external temperature controller can be used. When the external temperature controller malfunctions, the heating surface temperature of the PTC heater will not exceed the set constant temperature and has an over temperature protection function.

1.3 Application scenarios.

Electric mosquito repellent, massager, heater, electric soldering iron, electric iron, humidifier, curler, straightener, glue applicator, electric fragrance applicator, hot melt glue gun, electric iron, wax melting applicator, electronic component insulation, circuit board moisture removal, etc.

Using a PTC heater as a solid constant temperature heating, the suitable heating temperature is 0-300 ℃, and the heating temperature at room temperature is 50-300 ℃. The suitable ambient temperature is -40~250 ℃. PTC with a surface temperature that is too high or too low is difficult to manufacture, and even lacks PTC characteristics.

Working voltage: 3.7V~420V, both AC and DC. When the working voltage is low, the room temperature resistance of PTC is very small, and the resistance temperature change rate is also relatively small, resulting in poorer accuracy of constant temperature; In addition, under low voltage, the heating power is small, the heating rate is slow, and it cannot even rise to a higher temperature.

From the perspective of safety and reliability, PTC heaters generally have higher safety than traditional electric heating wires, higher insulation voltage resistance, lower leakage current, and more stability. In some applications, traditional electric heating wires cannot pass safety certification, while PTC heaters can pass safety certification. Therefore, PTC can be used in situations with high safety requirements. But not all manufacturers of PTC will meet the safety requirements, so it is necessary to choose PTC manufacturers with excellent quality for supply. PTC is used for solid heating, and the main safety and reliability issues are: firstly, the PTC ceramic body breaks down and burns out, causing a short circuit and burning out the insulation layer; The second is that the insulation layer breaks through and leaks electricity, causing the shell to become charged.

From a cost perspective, using a PTC heater for small area constant temperature heating can eliminate the need for temperature control and over temperature protection. The volume of PTC can be very small and installation is relatively simple. Constant temperature heating with a large heating area, using a PTC heater for more uniform heating temperature. The lifespan of PTC constant temperature heaters is several times longer than that of traditional electric wire heaters, and using PTC also saves lifespan and maintenance costs.

The materials for the metal shell in contact with the heated liquid can be selected: aluminum silicon alloy, aluminum zinc alloy, aluminum magnesium alloy, copper, brass, iron (galvanized, etc.), 304 stainless steel, 316 stainless steel, titanium, copper nickel alloy, etc. The selection of the metal shell material is determined based on factors such as the purpose, properties of the liquid, and the corrosiveness of the liquid to the metal. Used for heating oil, a low-cost aluminum alloy shell can be used; Used for heating washing water, aluminum alloy, copper, or brass shells can also be used; Heating of edible water can be done using 304 stainless steel or 316 stainless steel. When the water quality is relatively poor, or it is seawater or used for heating electroplating solutions, a titanium shell can be used; A highly corrosive liquid can be made of copper nickel alloy. In addition, a layer of fluoroplastic is attached to the surface of the metal shell, which can resist the corrosion of the vast majority of liquids under temperature resistance, but the heating power will significantly decrease.

The liquid heater at the bottom (or side) of the container does not come into direct contact with the liquid, and the liquid will not corrode the outer shell of the heater. However, this type of heater has a larger volume, lower power, and higher cost.

2.2 Heating power.

The relationship between heating power and heating time of PTC heating tubes is significantly different from traditional electric wire heating tubes. In the initial heating stage, the power of the PTC heating tube increases from small to large, reaches its maximum value (maximum impact power), gradually decreases, and finally stabilizes (stable power for boiling water). The stable power of dry burning is much smaller than that of boiling water (the dry burning power is about 8% of boiling water).

The stable power is related to the liquid temperature, and the higher the liquid temperature, the smaller the stable power. According to the actual usage conditions, a standard liquid temperature can be specified. When the heating tube is heated, the liquid temperature increases from low to high. When the liquid temperature reaches the standard temperature, the recorded power is the stable power at the standard liquid temperature.

The stable power is related to the type of liquid. When the liquid is water, the power is relatively high because water has a larger specific heat capacity, better fluidity, and is prone to convection; When heating other liquids, the stable power is generally lower than that of water.

The higher the surface temperature of the PTC heating tube during dry burning, the greater the stable power. However, increasing the surface temperature will cause an increase in power attenuation, an increase in the chance of PTC heating element breakdown, a decrease in the performance of adhesives, wires, insulation layers, etc., and a decrease in reliability.

When the surface temperature of the PTC heating tube during dry burning is the same as the liquid temperature, the heating power is basically 0, which means that it is no longer heated. So the surface temperature of the PTC heating tube during dry burning is always higher than the liquid temperature by more than 20 ℃, otherwise it cannot be heated. The smaller the difference between the surface temperature and liquid temperature during dry burning of PTC heating tubes, the smaller the power density of the heating tubes, the larger the volume required, and the higher the cost.

A heating tube with a high maximum impact power may not necessarily have a high stable power. If the maximum impact power is too large, it will affect the normal operation of the entire power supply circuit and switch; But if the maximum impulse power is too small, it will cause the stable power to not meet the requirements.

2.3 Power attenuation.

Due to differences in quality among PTC manufacturers, users feel that the power attenuation of PTC is too significant. The PTC liquid heating tube from Dongguan Tiancheng has a power attenuation of about 10% for continuous uninterrupted operation for one year, which is completely within the acceptable range for users. However, some PTC manufacturers have PTCs that have a monthly power output of around 30% and a continuous power attenuation of 40-50% for a year, which is actually unusable. So when purchasing PTC heating tubes, it is necessary to conduct a power attenuation test.

The power attenuation is positively correlated with the surface temperature of PTC ceramic heating elements, and the higher the surface temperature, the greater the power attenuation. That is to say, reducing the surface temperature of the PTC heating element can reduce power attenuation, but the volume of the heating tube will increase, and the cost will also increase.

There are two main reasons for power attenuation: firstly, the increase in room temperature resistance of PTC ceramic heating elements/the decrease in impulse current; The second is the loosening of the heat transfer structure, which leads to a decrease in heat transfer.

2.4 Application scenarios.

Water storage water heater, namely hot water heater, electric kettle, hot drinking water machine, hot water for washing vegetables, dishes, washing face, bathing, soaking feet, and foot bath, auxiliary heating of solar and heat pump water heaters, electric heating of photovoltaic solar water heaters, hot water cleaning of range hood and cleaning agent, heating of hot water bed, electroplating tank, diesel car oil tank, filter, heating of hydraulic oil, lubricating oil, edible oil, steam generation for sauna, beauty, disinfection, and humidification, fumigation of medicinal solution, insulation of aquarium and fish tank, antifreeze of water pipes, faucets, pipes, and oil pipes, three-way catalysis, heating of urea solution, water heating of floor heating, oil heating, oil heating, oil heating, oil type Heating with hydrothermal fins, etc.

The stable heating power can range from 10W to 10KW. However, for high-power heating tubes, a larger number of PTC ceramic heating plates are used, which increases the chance of PTC ceramic heating plate breakdown; In addition, the insulation area is also relatively large, which increases the chance of insulation failure. So it's best to divide it into several heating tubes, each with a lower power, which can reduce failure and waste. The working voltage range is 12V~420V, both AC and DC can be used. When the working voltage is low and the heating power is high, both the impulse current and stable current will be high, making it difficult for the power supply to withstand. Therefore, when high power is needed, it is best not to use low voltage.

The advantage of electric heating wire electric heating tubes is their lower cost. Existing problems: Firstly, after using it for a period of time, the insulation withstand voltage, leakage current, and insulation resistance exceed the standard; Secondly, dry burning is not allowed. If the temperature protection device malfunctions, the electric heating tube will be burned out due to dry burning, and even the surrounding materials will be burned, leading to a fire; Thirdly, after long-term operation, the electric heating wire may oxidize and burn out, affecting its lifespan and even causing leakage; Fourthly, when the water quality is poor and the concentration of cations (such as calcium and magnesium) is relatively high, scale is prone to appear on the surface of the electric heating tube, and the surface heat transfer deteriorates, leading to the burning of the internal electric heating wire.

PTC liquid heating tubes can overcome the shortcomings of electric wire heating tubes, with excellent and stable insulation voltage resistance, leakage current, and insulation resistance. They are resistant to dry burning, have a long service life, and will not burn out even if there is scale. However, if the quality of PTC manufacturing is poor, problems may also occur: firstly, the PTC ceramic body may break down and burn out, causing a short circuit and burning out the insulation layer; The second is that the insulation layer breaks through and leaks electricity, causing the shell to become charged; The third is that the heating power decays with the increase of usage time; The fourth is that the surge current is too large, causing faults in the power supply line and switch. PTC heating tube users should carefully choose PTC heating plates to ensure reliable quality.

3. Air heating.

3.1 Air heater structure.

The structure of air heating is divided into charged and insulated types based on whether the radiator is charged or not.

Figure 7 shows a charged air heater, with the radiator charged. The electrode layer of the PTC ceramic heating element directly contacts the aluminum radiator, and the aluminum radiator serves as an electrode outlet. Electric air heaters are easy to obtain higher power and have lower costs; However, due to the fact that the radiator is charged, there are potential safety hazards.

Figure 8 shows an insulated air heater. The radiator and PTC ceramic heating element are insulated, and the radiator is not charged. Insulated air heaters are relatively safe, but they have lower power and higher cost.

According to the structure of the radiator, there are corrugated structures, plug-in structures, aluminum extrusion structures, compressed air pipeline heating, etc. The air heater with a corrugated structure has a flexible structure, and its length and width are easy to adjust, forming various sizes and powers to meet the requirements of various fan sizes, duct sizes, and heating power. The radiator of the corrugated structure heater is bonded to the PTC heating element (or heating tube) through adhesive. After prolonged use at high temperatures or long-term dry burning without wind, the adhesive may come off. The length of the insertion structure and the aluminum extrusion structure can be adjusted appropriately, but the width is difficult to adjust. The impression given by the plug-in structure is that it is compact, sturdy, and has relatively low wind resistance.

According to whether blowing is necessary, it can be divided into blowing type and natural convection type air heaters. The power of the blowing air heater is relatively high, but it requires the addition of a fan, and the fan will produce noise. Convective air heaters do not require fans and are noise free, making them particularly suitable for heating bedrooms and beds, but with lower heating power.

3.2 Heating power.

The variation of heating power with heating time is shown in Figure 9. At the beginning of power on, the initial power is relatively small (usually 1/3 to 1/5 of the maximum impulse power), then the heating power gradually increases to the maximum impulse power, then the power decreases, and finally reaches stable power. In the case of rapid heat dissipation, the heater is always at its initial power and cannot reach its maximum power, so the stable heating power is very small.

Maximum impulse power=C * operating voltage/room temperature resistance

The maximum impact power value depends on the room temperature resistance, surface temperature, grain size inside the ceramic body, and operating voltage of the PTC ceramic heating element. The multiple C is 1-6, and the higher the surface temperature of the PTC ceramic heating element, the greater the C; The higher the room temperature resistance, the greater the C. If there are multiple PTC ceramic heating elements with different resistances in a heater, the maximum impulse current of the heater starting to power on in the cold state (at room temperature) will be smaller; However, starting from the hot state (at a temperature about 40 ℃ lower than the surface temperature), the maximum impulse current will be larger. An air heater with a maximum impact power will have a slightly higher stable power. If the maximum impact power is too large, it will affect the normal operation of the entire power supply circuit and switch; However, if the maximum impact power is too small, it will cause the heating power to not meet the requirements, and even the heating power will decrease as the blowing speed increases.

The stable heating power of PTC air heaters is related to wind speed. Generally speaking, the higher the wind speed, the faster the heat dissipation, and the greater the heating power. The wind speed of a typical fan is 3-5m/s. The power without blowing is about 10% of the power at a wind speed of 5m/s. When necessary, the heating power can be adjusted by adjusting the wind speed. If the impulse power of the heater is close to the stable power at low wind speeds, increasing wind speed cannot increase power; Excessive wind speed can actually reduce power.

The heat dissipation of non insulated PTC air heaters is faster than that of insulated ones, so the heating power is also relatively high. The heat sink is relatively dense, and the increase in wind area will also increase heat dissipation and heat generation power.

The increase in the number of PTC heating plates does not result in a completely proportional increase in stable heating power, and the heating power of each heating plate will decrease.

The relationship between the heating power of PTC air heater and the influencing factors is as follows:

Stable heating power=heat dissipation coefficient * (dry burning temperature - inlet air temperature)

The heating power is related to the inlet air temperature, and the higher the inlet air temperature, the smaller the stable power. When the surface temperature of the PTC ceramic heating element is the same as the inlet air temperature during airless dry firing, the heating power is basically 0, indicating no further heating. So the surface temperature of the PTC ceramic heating element during airless dry firing is always higher than the inlet temperature by more than 20 ℃, otherwise it cannot be heated. The smaller the difference between the surface temperature and the inlet air temperature of the PTC ceramic heating element during airless dry firing, the smaller the heating power density, the larger the volume required, and the higher the cost.

When the outlet temperature of a heater cannot reach the required high temperature, to increase the outlet temperature, a second PTC air heater needs to be placed along the wind direction. At this point, the first heater near the fan has a higher power output; The inlet temperature of the second heater away from the fan is higher, so the power will be much lower, and the stable power is often less than 50% of the first heater.

The higher the surface temperature of PTC ceramic heating element during airless dry firing, the greater the heating power. However, increasing the surface temperature of the PTC ceramic heating element results in an increase in heating power attenuation over time, an increase in the chance of PTC heating element breakdown, a decrease in properties such as adhesive, wire, and insulation layer, loosening of the aluminum shell, and a decrease in heater reliability.

3.3 Power attenuation.

The main factors affecting the power attenuation of PTC air heaters are: increased resistance of PTC ceramic heating elements, loose adhesive, and loose aluminum radiators. Some PTC manufacturers produce PTCs with a monthly power output of over 30%, so it is very important to choose high-quality PTCs. When purchasing PTC air heaters, it is necessary to test power attenuation.

Power attenuation is positively correlated with the surface temperature of PTC ceramic heating element. The higher the surface temperature, the more the resistance of PTC heating element increases, the easier the adhesive is to loosen, the easier the aluminum radiator is to loosen, and the greater the power attenuation. If a PTC heating element with a surface temperature above 250 ℃ is used as an air heater, and it is powered on without wind, the adhesive will gradually decrease in strength and loosen after 200 hours. That is to say, reducing the surface temperature of the PTC heating element can reduce power attenuation, but the volume of the air heater will increase, and the cost will also increase.

​
3.4 Application scenarios.

Heating or drying of clothes, shoes, hands, desiccants, tea, and agricultural products using hot air, heating or drying of wardrobes, bookcases, shoe cabinets, rooms, bathrooms, factories, shopping malls, warehouses, electrical control cabinets, high-speed train carriages, electrical equipment, and storage, tunnel type ovens, compressed air heating, and battery discharge loads.

The stable heating power can range from 100W to 10KW. If the power of a single heater is smaller, it can reduce failure and improve the qualification rate. If a larger power is required, multiple heaters can be used. The working voltage is 12V~420V, both AC and DC can be used. When the working voltage is low, both the impulse current and stable current will be large, so it is best not to use low voltage when high power is required.

The advantages of electric hot wire hair dryer are lower cost, higher outlet temperature, and smaller volume. Existing problems: Firstly, after using it for a period of time, the insulation withstand voltage, leakage current, and insulation resistance exceed the standard; Secondly, power cannot be turned on without wind. If the fan stops running, the air duct is blocked, and the temperature protection device malfunctions, the heater will burn out due to overheating without wind, and even damage surrounding materials, leading to a fire; Thirdly, after long-term operation, the electric heating wire may oxidize and burn out, affecting its lifespan and even causing leakage; Fourthly, when combustible substances adhere to the heater, it may cause combustion.

PTC air heaters can overcome the shortcomings of electric wire heaters, with excellent and stable insulation voltage resistance, leakage current, and insulation resistance. They can be powered on without wind, automatically protected, will not ignite or burn, and have a long service life. However, if the manufacturing quality of the PTC heater is poor, problems may also occur: firstly, the PTC ceramic body breaks down and burns out, causing a short circuit and burning out the insulation layer; Secondly, the heating power decays with increasing usage time and cannot reach the expected service life. Some PTC heater manufacturers use low-grade materials in order to reduce production costs, and the manufacturing process cannot be strictly controlled, without life testing tests. Not all PTC manufacturers can guarantee the quality of their PTC heaters. The control of power attenuation, power accuracy, impulse current, voltage resistance, insulation performance, and reliability of use varies among PTC manufacturers. Therefore, PTC manufacturers should be carefully selected to ensure reliable quality.