Electric water heater program development and design

According to national standards, electric water heaters can be divided into storage type electric water heaters and instant electric water heaters. Instant electric water heaters are mostly used in public places because of their high power consumption. Storage type electric water heaters are divided into closed type, water tank type, open outlet type and open type. Household electric water heaters generally have two types of storage-type electric water heaters: closed type and open outlet type, and the former is the main type.

1. Technical introduction of electric water heater scheme

1.1 Liner technology

The core technology of electric water heater manufacturing is the liner technology, which is subdivided into liner materials and anti-corrosion technology and liner processing technology. At present, the electric water heaters on the domestic market are divided into two camps according to the different inner tank technology, the stainless steel inner tank represented by Haier and the enamel inner tank represented by Ariston. The advantages of the stainless steel liner are that the material is better and the weight is lighter, but the disadvantage is that it has low pressure resistance and is not resistant to impact, especially the weld seam is easy to leak. However, the production process of stainless steel liner is simple and the equipment investment is small, so it is adopted by most domestic manufacturers. Some foreign brand manufacturers such as Ariston use enamel liners made of imported thickened special steel plates, which have excellent anti-corrosion performance and can withstand internal pressures up to more than ten kilograms. They have long service life and are extremely safe, but they produce enamel liners. The equipment investment is huge, the process is complicated, and the technology content is high. With the intensification of market competition, enamel liner technology is bound to become the dominant technology for electric water heaters.

1.2 Safety performance, insulation performance and thermal efficiency

The main technology corresponding to safety performance is hydroelectric separation technology. This is mainly reflected in the manufacturing technology of the heating tube. At present, brand-name electric water heaters in the domestic market generally use gold hook heating tubes. The main materials of the existing heating pipes are generally red copper pipes and special stainless steel pipes, with the latter mainly being used. The isolation means generally uses magnesium oxide powder as the hydroelectric isolator. As an auxiliary to water and electricity isolation technology and a market measure, some manufacturers have installed leakage protection devices and water-outlet and power-off devices on electric water heaters.

Another technology related to safety performance is thermostat manufacturing technology. A good thermostat can prevent excessive water temperature and avoid scalding accidents. The thermostats used in electric water heaters can be divided into two categories: mechanical temperature control and electronic temperature control, and mechanical temperature control is the dominant one. At present, domestic famous brand electric water heaters all adopt imported thermostats.

Corresponding to the thermal insulation performance is the selection of thermal insulation materials and processing technology for electric water heaters. The choice of materials is determined to a certain extent by manufacturing technology, and there are three main categories: polyurethane foam, polypropylene foam and glass fiber cotton. Among them, polyurethane foam has the highest density and the best thermal insulation performance.

The thermal efficiency of electric water heaters is a comprehensive index. Thermal efficiency is closely related to the quality of the heater and the quality of the insulation layer. The denser and thicker the insulation material, the better the insulation performance, and the better the insulation performance, the higher the thermal efficiency. At the same time, the higher the efficiency of the heating tube, the higher the thermal efficiency of the whole machine.

Second, the purchase suggestion of electric water heater

2.1 Choice of instant heating and water storage

There are two types of household electric water heaters: instant heating type and water storage type. Compared with gas water heaters, electric water heaters have the advantages of safety, hygiene, and convenience. They can provide hot water at any time and the water temperature is easy to adjust. Instant water heaters are small in size, do not need to be preheated, but have high power, usually above 4-6kw, and working current up to 20A. Due to domestic wiring restrictions, it is difficult to widely use. Furthermore, the instant electric water heater is heated during the flow of water, also known as the flow water heater, so it is easily affected by the water pressure during use, and the water temperature changes. The storage water heater is heated by volumetric water storage, can automatically keep warm and keep constant temperature. It can also provide hot water during power failure. It can also be used as a domestic hot water supply center for warm and hot water in many places in the family, but it is larger in size and needs Warm up.

2.2 The choice of closed and open

Storage type electric water heaters are divided into two types: closed type and open type. The inner pot of the open-type electric water heater has no pressure resistance requirements, no heat preservation, and the structure is relatively simple, and cannot supply water at multiple places at the same time; the inner pot of the closed electric water heater can withstand pressure, and the structural design is relatively safe and reasonable. It has a constant temperature and heat preservation function. Supply water to multiple places at the same time. At present, most users choose the closed type.

2.3 The choice of inner tank

The inner tank of the storage type electric water heater is used to store water. It must have the characteristics of heat preservation, pressure resistance, no rust, no scale, and no water seepage. Otherwise, it is difficult to repair once damaged, so the life of the electric water heater depends on the internal The material and manufacturing process of the tank, and when users buy an electric water heater, choosing a good tank is the key. At present, there are mainly the following types of inner tanks of electric water heaters on the market: Galvanized sheet inner tank: The inner tank of this kind of zinc protective layer is thin, easy to form, easy to process, but it is easy to rust after long-term use, which affects the service life. So generally not used. Stainless steel inner tank: Stainless steel is better than galvanized sheet in material, and it is not easy to rust; many water heater inner tanks use this material, but the disadvantage is that it is difficult to find weld defects of stainless steel inner tank, which is easy to cause hidden dangers. After long-term use , Chromium in stainless steel will be corroded by chloride ions in tap water and cause weld leakage. Hot-dip galvanized anti-rust resin liner: When the liner is processed, the zinc alloy protective layer formed by the hot-dip galvanizing process has a thickness of 0.7 mm or more, which can prevent rust contact and electrolysis, but the quality and life of the liner It depends on the craftsmanship and quality of zinc alloy and anti-rust resin. Enamel liner: The surface enamel of this liner is non-metallic material, which is neither rust nor scale; the liner itself is made of thick steel plate, which has strong resistance to pressure, certain impact resistance, and good anti-corrosion performance. Therefore, the reliability of the enamel liner is relatively better.

2.4 Selection of outer body shape

In the early days, the inner tank of electric water heaters had various shapes, but from a geometrical point of view, the design of the round tank shape was the most uniform and able to withstand high pressure. After more than 20 years of development, the inner tanks of some well-known brands of water heaters on the market are designed in the shape of a round tank.

2.5 Selection of capacity and power

Generally, 30-40 liters are more suitable for a family of 3, and 60-80 liters for 4-5 people; the heating power of the storage-type electric water heater is generally 1-1.5 KW, so the electric meter (electric load capacity) in the home is required Preferably greater than 10A. Now some electric water heaters have been designed for dual-power heating, which provides users with options. At the same time, it also provides convenience for large-capacity water heaters due to long-term insulation needs.

3. Structure composition of instant water heater

The structure of the water heater mainly includes three parts: heating cylinder, circuit board, casing and accessory parts.

3.1 Heating cylinder

The heating cylinder is the core component of the water heater and the place for heat exchange. Multiple groups of heating tubes are installed in the barrel, and electrodes are drawn from the top of the barrel. The water inlet channel is equipped with a water flow sensor (the gear is a water flow switch), and temperature sensors are installed on the water inlet and outlet channels. The heating cylinder has a diameter of 5cm and a length of 24.5cm, and four U-shaped heating tubes are squeezed out. The actual volume is less than 300ml. Therefore, the water flow can be heated up quickly to achieve "hot and ready to use".

The temperature control switch is installed on the cylinder body, which is used for over-temperature power-off and the action temperature is 98°C. To ensure effective contact between the bottom of the temperature control switch and the heating cylinder, the contact surface is coated with thermal grease (silicone grease) to reduce thermal resistance. There is a coin-sized circular bimetallic concave-convex piece inside the temperature control switch. The concave-convex spacing is about 1 mm. The convex surface is close to the metal bottom surface of the switch. When the temperature is overheated, the bimetallic piece is concave-convex flipped, and the center point can produce 2mm stroke , Push the transfer rod to push the conductive beam of the switch away from the contact, thereby cutting off the circuit. Manual reset is required after the action.

Over-temperature can occur when the cylinder is full of water, lack of water or even no water. Heat is transferred to the cylinder body through water, steam or the top of the cylinder body, and the temperature of the cylinder body rises fastest, so the temperature control switch is also installed In the upper part. Based on this analysis, this type of water heater should be installed vertically.

3.2 Control circuit board

There are two circuit boards, one is the main circuit board, which is directly fixed on the casing, and the other is the display and operation circuit board, which is fixed on the front panel, and the two are connected by a multi-strand cable. After consulting the data, comparing the actual products, ignoring some circuit details, the logical block diagram of the circuit can be sorted out as follows.

First analyze the heating tube circuit, the heating tube is connected and disconnected using a relay to achieve strong and weak electrical isolation. The SCR device in the second path is also directly connected in series in the heating tube circuit, so electrical isolation is achieved through an optocoupler device. The optocoupler sends the control signal of the single-chip microcomputer to the SCR through the secondary conversion of "electricity-optical-electricity". The second is the microcontroller control circuit. In order to keep the factory confidential, the model identification of the single-chip microcomputer has been erased, but it does not affect the analysis of the circuit. The input signal of the one-chip computer includes the inlet water flow rate, the inlet water temperature, and the outlet water temperature, which are provided by the corresponding sensor circuit. The output control signal of the single-chip microcomputer includes two relays and one thyristor. The single-chip microcomputer uses multiple I/Os to connect the display screen, buzzer, E2 PROM memory, operation buttons and other circuits. In addition, there are power circuits, leakage protection circuits, and zero-crossing detection circuits.

Fourth, the heating tube circuit analysis of the water heater

4.1 The heating circuit control mode of the gear machine

The gear machine is equipped with four heating tubes, the power of which is two 1. 7kW and two 2.3 kW, respectively controlled by four relays, which can combine a total of 8 power combinations from 1. 7 to 8 kW, corresponding to 8 gears. To suit different temperature needs.

4.2 Heating circuit control mode of thermostat

The thermostat also has four heating tubes, each 2kW, divided into 2 groups of wiring, each group of 4kW, the first group is controlled by a relay to be responsible for "coarse adjustment", and the second group is controlled by a relay and a triac in series for power "fine adjustment" ". Thyristor is a kind of high-power semiconductor device, which can control the current by changing the conduction angle and realize any power distribution between 0-4kW. The combination of the two groups can achieve any power output of 0-8kW.

4.3 SCR control principle

To understand the working principle of SCR control, you can understand more design details of the circuit. Triac is a kind of multi-layer semiconductor device, which is equivalent to a special switch. It is characterized by “trigger on, hold after on, and turn off after power off”. T1 and T2 electrodes are connected in series in the heating tube circuit to trigger the signal It is output by the single-chip microcomputer and loaded to the gate G after being isolated by an optocoupler.

After the thyristor is triggered and turned on, even if the trigger signal is removed, the device can maintain the on state until it is turned off and wait for the next trigger. Power-off utilizes the "zero-crossing" of the AC power source 50Hz sine wave. In a half cycle (angle π, duration 10ms), t1 and t3 are the first and last "zero-crossing" moments, and t2 is a certain load driving signal At the moment, then the angle from t2 to t3 is the conduction angle. As long as the conduction angle is changed (0-π), the conduction current can be controlled to achieve any power distribution between 0-100%. The one-chip computer samples the zero-crossing signal at t1, starts timing, and sends out a trigger signal when the delay reaches t2. The conduction angle and its calculation period need to be determined based on complex automatic control theory. The zero-crossing signal comes from the secondary voltage of the power transformer. After rectification and shaping, it is sent to the single-chip microcomputer as a synchronous control signal.

4.4 Over-temperature protection circuit

There are two groups of over-temperature protection circuits: the first group uses a temperature control switch to prevent abnormal high temperatures. The second group of over-temperature protection is "high water temperature protection". The protection value is generally set to 60°C and controlled by the single-chip software. , Once the temperature is over, cut off the heating tube circuit, give a display and sound prompt, when the temperature returns to below the set value, it will automatically resume normal constant temperature control. During the test, when the water temperature was greater than 60°C, it was observed that the single-chip microcomputer cut off the two relays and turned off the thyristor trigger signal, and the buzzer produced three short 90ms long alarm sounds.

The thermostat uses a variety of sensors such as temperature sensors, flow sensors, current transformers, and temperature control switches. The temperature control switch is both a sensor and a controller. Current transformers are used for leakage protection. Here introduce the principle and technology of temperature sensor and flow sensor.

NTC thermistor is used for water temperature sensor, NTC is negative temperature coefficient. This type of sensor uses metal oxides such as manganese, cobalt, nickel and copper as the main materials and is manufactured by ceramic technology. When the temperature is low, these oxide materials have a small number of carriers (electrons and holes), so their resistance value is high. As the temperature increases, the number of carriers increases and the resistance value decreases. The resistance value of the temperature sensor needs to be converted into a voltage after the single-chip AD conversion, and then converted into a temperature value before it can be used for calculation, control, and display.

The internal structure of the water flow sensor, viewed from the top, placed an impeller in the cylindrical internal space. The impeller is covered with a magnetic ring, and then sealed with a top cover. The top cover has a groove, which is just inserted into the Hall device to make the magnetic The distance between the ring and the Hall device is less than 3mm. The water flow drives the impeller to rotate. Using the Hall effect, the element senses the change of the magnetic pole signal and outputs an electrical signal. The Hall effect refers to a phenomenon in which an electromotive force is generated in the direction perpendicular to the current I and the magnetic field B when a current flows through the semiconductor sheet. The device model is W12, which integrates Hall elements and signal processing circuits, including reference voltages, amplifiers, Schmitt triggers, and drive output transistors, which can directly output shaped pulse square waves.

Five, the temperature control circuit of the water heater

According to automatic control theory, typical temperature control generally uses PID analog control, that is, "proportional (P)-integral (I)-differential (D)", where r (t) is the given value and y (t) the actual output value , The deviation e(t)=r(t)-y(t), the deviation value is calculated by PID and linearly combined to form the control quantity u(t) to control the controlled object.

Proportional control is to output the control quantity in proportion to the size of the error, so that the controlled quantity changes in the direction of error reduction. The size of the control quantity depends on the proportional coefficient Kp. Assuming that the set temperature is 40°C and the proportional bandwidth is 20°C, when the outlet water temperature is below 30°C, the controller has full power output and 50°C above zero output. The two will be controlled proportionally. Theoretically, proportional control can achieve a constant temperature effect.

However, proportional control cannot solve the problem of static error (also called static error or steady-state error), so integral control is introduced to memorize and integrate the static error, and the control quantity is adjusted in the direction of reducing the static error. Integral control needs to sample and integrate the previous few errors before generating a control signal, so there is a large lag, so derivative control is introduced. Differentiation reflects the change trend (rate of change) of the error, thereby generating the control quantity in time. The temperature value in the actual system is the discrete value obtained by sampling, and the integral and differential terms need to be discretized.

Another widely used automatic controller is the fuzzy controller, which is very suitable for instant water heaters. Research shows that fuzzy control has a better control effect on uncertain systems such as nonlinear and time-varying. The control process of the fuzzy controller is: after the input signal is processed by the fuzzy interface, the logic stored in the database and the rule library is called by the inference engine, and the data vector is used to solve the simulation equation according to the relevant rules to obtain the fuzzy control value, and then pass the defuzzification interface Converted into control electrical signal. Among them, the membership degree vector value of all fuzzy subsets of each input and output variable is stored in the database, and the rule inventory control rules are stored in the database. These rules are defined by experts and experience. In Yu Hongjie's comparative test, fuzzy control is better than PID control mode in terms of reaching steady-state time, control accuracy and anti-interference ability.

6. Safety technical design of water heater

Safety is the primary consideration for user selection. If the heating tube is broken or perforated, the heating wire in the tube will come into contact with the water, causing the water body to become charged. Instant water heaters take the following measures to ensure safe electricity use.

6.1 Grounding device and leakage protector

The standardized power distribution environment is a barrier for safe use, including grounding systems and leakage protectors. The ground wire in the power cord of the water heater must be reliably connected to the home ground wire, and the leakage protector should be tested for validity by pressing the test button once a month. Instant water heaters have high power and should be equipped with 40A circuit breakers and at least 4mm2 dedicated power lines should be laid.

6.2 Leakage detection circuit

There is a special leakage protection circuit inside the water heater, a transformer is sheathed on the power inlet (phase and neutral, excluding the ground wire), and a coil is wound around the transformer as the primary coil, and the transformer has a secondary coil. More than a thousand turns. If there is leakage, the currents of the phase line and the neutral line are inconsistent, causing magnetic induction, and the secondary induced voltage. After the CS54123 chip circuit is amplified and processed, the single-chip microcomputer controls the water heater to disconnect the relay. In the simulated leakage test, it is observed that the single-chip microcomputer turns off the trigger signals of the 2 relays and the thyristor after 77 milliseconds, and generates an audible alarm signal and a fault code display.

6.3 Anti-electric wall

The anti-electric wall is actually a section of insulated water pipe, which attenuates the leakage current through the resistance of the water body. In order to achieve a sufficient length, the internal design is generally a spiral pipe, which has good reliability in use.