Analysis of Proportional Valves in Use of Gas Fired Boilers

The proportional control originates from the regulation of the temperature of the electric heating furnace. In the ON-OFF and ON-OFF bit control methods, due to insensitivity zone, thermal inertia, and the thermal inertia problem of the temperature probe, the adjustment of the motion difference occurs, that is, the furnace temperature fluctuates. The larger the fluctuation, the smaller the frequency of the fluctuation. On the contrary, the frequency becomes larger to make the amplitude smaller. In order to overcome this contradictory malady, a proportional regulator came into being. Proportional regulation means that the power of the electric furnace is proportional to the deviation of the input. That is, the closer the furnace temperature is to the set value, the lower the power. In the early 1960s, with high-power thyristors, proportional control was widely used. If it is too large, it will cause strong oscillation of the regulating system. In actual use, parameters and debugging must be determined according to the characteristics of the adjusted object (power, thermal inertia, stability, etc.) and the matching suitability of the actuator. Keep the furnace temperature stable at the set value. The PID adjustment due to the existence of thermal inertia, the proportional band can not be too small, otherwise it will cause instability like a bit adjustment, resulting in greater furnace temperature fluctuations. The larger the proportional band, the greater the static difference, so that the furnace temperature after stabilization can never reach the set furnace temperature. To this end, people have added integral adjustment based on the proportional adjustment. This proportional-integral regulator has the function of automatically overcoming the static error. However, the output of the integral action increases gradually with time, and the effect of interference on the measured value cannot be overcome quickly. In particular, furnaces with large thermal inertia are bound to cause great fluctuations in furnace temperature. Therefore, people added a differential adjustment based on the proportional-integral adjustment and became a proportional-integral derivative regulator, ie, a PID regulator.

The proportional band, integration time, and derivative time are three important PID parameters. Since their outputs are in the same feedback circuit, changing any one of them has an impact on the three parameters. The smaller the proportional band is, the stronger the proportional effect is. The smaller the integral time is, the stronger the integral action is; the greater the differential time, the more the differential function is. Strong, but the proportion is too small, the integration time is too small, the differential time is too large, will make the adjustment system has a strong shock, in the actual use must also be based on the characteristics of the object being adjusted (power, thermal inertia, stability, etc.) And the matching of the actuator's suitability to determine the parameters and debugging, in order to make the furnace temperature stable at the set value.

The adjustment and change of electric heating power can be done very accurately and timely, but the adjustment of burning power is not so easy. Even the use of a PID regulation system cannot accurately and timely adjust the size of the flame. Proportional valve is through the water temperature feedback signal, automatically adjust the current, but also through the coil to generate magnetic force, magnetic force to promote the permanent magnet and the ball valve, so that the valve opening and current size corresponding changes. Regardless of several energy conversions, whether the proportional relationship can be linear or not, the relationship between the opening of a single finger valve and the flow rate is not linear. The current-flow characteristic curve given by most proportional valve manufacturers is actually a current-pressure curve. Gas-fired boilers can only be said to be basically linear. So the following questions arise:

(1) With the harsh environmental protection requirements, the boiler combustion is also very strict, a certain gas flow should correspond to a certain amount of air supply and a certain amount of flue gas emissions, otherwise it can not guarantee complete combustion and high thermal efficiency. The amount of smoke exhausted is too large, the amount of air is large, and the thermal efficiency is reduced; exhaust smoke is insufficient, air is less, and combustion is incomplete. The gas flow rate does not change linearly, which means that it is difficult to follow the use of variable frequency fans; so that the exhaust gas is proportional to the gas flow rate.

(2) The technical parameters of the burner heat exchanger are generally based on the rated load and nothing, such as the inlet, throat, protection pipe, nozzle position, etc. on the ejector, both with the amount of primary air and the mixed intensity There are relationships. If it is not burning under the rated load, the proportion of primary air is incorrect, and the combustion state is not good. When entering the proportional valve segment, the firepower is small, the flue gas temperature is low, and the flow rate is slow, the heat release coefficient is greatly reduced, and the heat loss of the heat exchanger itself is also relatively increased. At present, the exhaust fan of most manufacturers is only two-speed transmission, row The amount of smoke is large, and the air coefficient is very high, so that the thermal efficiency of entering the proportional section is greatly reduced. Many manufacturers have explicitly admitted in the specification or the sample that the thermal efficiency of the proportional section is less than 80% or less.

Different uses and conditions of use

The furnace temperature control of large industrial furnaces is close to continuous PID regulation in electrical principle. The air ratio directly tracks the gas flow. The gas flow itself also has a self-checking system, which can comprehensively monitor the combustion status. It can also record the furnace temperature, gas flow, and air flow throughout. Such a large and complex control system has not yet been used in the boiler industry. The precedent. Because the use environment of boilers is different from that of industrial furnaces, the industrial furnaces emphasize the stability and accuracy of the furnace temperature and the proportional control is not only a matter of the proportional valve but the mutual cooperation of the entire system. We hope that with the rapid development of science and technology, in the near future, the real boiler controlled by the proportional valve control system can be put into the market.

The fluctuation of the water temperature in the boiler does not affect the temperature fluctuation of the room. This is determined by the heat preservation of the room and the heat capacity of the room itself. If the temperature difference of the radiator is 15°C and the fluctuation is once every 1h, the room temperature will hardly change. The purpose of using a proportional valve is not this, but to prevent frequent start of the solenoid valve, first to avoid ignition noise, and second, to extend the service life of the solenoid valve.

Through tests, under the normal heating area and rated load, if the hysteresis (setting and recovery start temperature difference) is set to 15°C, the volume-type wall-hung boiler will only start 1 or 2 times per 1 hour, and it will be fully on 24 hours per day. Use up to 5,000 times a year for 4 months. The current standard stipulates that the life of the solenoid valve is 30,000 times, but actually exceeds 50,000 times, that is, normal use for 10 years should be no problem; for the direct-flow type hot water, the valve start ratio is not stored because there is no hot water stored therein. The volumetric type is more frequent, but the use of 8 years is also sufficient.

Bathing temperature fluctuations are very small, volume type setting 1 ~ 2 °C temperature difference, volume type boiler 2 ~ 3min is activated once, DC type is constantly on and off the solenoid valve, if there is no fire regulation, in the summer bathing The maximum flow of hot water, it will also start several times a few times to ten times per minute, a greater impact on the life of the solenoid valve, so the best of dual-flow type wall-hung boiler fire control valve, although in a small fire The state is low in thermal efficiency, but because of the short bath time, there is little impact on energy waste, but the operation is a bit tricky and firepower needs to be adjusted in advance.

If a special control method is used to make the time when the bathing time and the thermal inertia are exactly one-half cycle apart, the number of times the solenoid valve is activated during bathing will be reduced by a factor of two, and the fluctuation of the water temperature will be small. (1) The thermal efficiency of the test report and the nameplate and the instructions are all tested under rated load conditions. In actual use, the thermal efficiency of the proportional section is very low, and the actual thermal efficiency of the finned tube DC water heater is far lower. In volumetric type. (2) Finned-tube DC dual-use boilers should use proportional valves, otherwise their service life will be seriously affected. (3) The volume boiler can store a certain amount of energy, and it can also maintain the output of thermal energy for a period of time after shutdown. It does not frequently start when heating, and there is no need to use a proportional valve.

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