Its core principle is a closed-loop negative feedback system of "heating - measurement - control". In simple terms, it is to precisely control the power of the heating elements inside the box to counteract the heat dissipation caused by the external environment, thereby maintaining a constant test temperature that is higher than the ambient temperature. The process by which the air valve stabilizes the temperature is a dynamic and continuously adjusting closed loop:
First, set a target temperature. The temperature sensor measures the actual temperature inside the box in real time and transmits the signal to the PID controller.
When the PID controller calculates the error value, it calculates the heating power that needs to be adjusted based on the error value through the PID algorithm. The algorithm will take into account three factors
P (proportion) : How large is the current error? The greater the error, the greater the adjustment range of the heating power.
I (integral) : The accumulation of errors over a certain period of time. It is used to eliminate static errors (for example, if there is always a slight deviation, the integration term will gradually increase the power to completely eliminate it).
D (differential) : The rate of change of the current error. If the temperature is rapidly approaching the target, it will reduce the heating power in advance to prevent "overshoot".
3. The PID controller sends the calculated signal to the power controller of the heating element (such as a solid-state relay SSR), precisely regulating the voltage or current applied to the heating wire, thereby controlling its heat generation.
4. The circulating fan works continuously to ensure that the heat generated by heating is distributed rapidly and evenly. At the same time, it also quickly feeds back the signal changes of the temperature sensor to the controller, making the system response more timely.
The air valve balancer measures air volume, while the density of air varies with temperature. Under the same differential pressure value, the mass flow rate or volume flow rate corresponding to air of different densities is different. Therefore, the temperature must be stabilized at a known fixed value so that the microprocessor inside the instrument can accurately calculate the air volume value under standard conditions based on the measured differential pressure value using the preset formula. If the temperature is unstable, the measurement results will be unreliable.
1.Compression
The low-temperature and low-pressure gaseous refrigerant flows out of the evaporator and is sucked in by the compressor. The compressor does work on this part of the gas (consuming electrical energy) and compresses it violently. When the refrigerant turns into high-temperature and high-pressure superheated vapor, the temperature of the vapor is much higher than the ambient temperature, creating conditions for heat release to the outside.
2. Condensation
The high-temperature and high-pressure refrigerant vapor enters the condenser (usually a finned tube heat exchanger composed of copper tubes and aluminum fins). The fan forces the ambient air to blow over the condenser fins. Subsequently, the refrigerant vapor releases heat to the flowing air in the condenser. Due to cooling, it gradually condenses from a gaseous state into a medium-temperature and high-pressure liquid. At this point, the heat is transferred from the refrigeration system to the outdoor environment.
3. Expansion
The medium-temperature and high-pressure liquid refrigerant flows through a narrow channel through the throttling device, which serves to throttle and reduce pressure, similar to blocking the opening of a water pipe with a finger. When the pressure of the refrigerant drops suddenly, the temperature also drops sharply, turning into a low-temperature and low-pressure gas-liquid two-phase mixture (mist).
4. Evaporation
The low-temperature and low-pressure gas-liquid mixture enters the evaporator, and another fan circulates the air inside the box through the cold evaporator fins. The refrigerant liquid absorbs the heat of the air flowing through the fins in the evaporator, rapidly evaporates and vaporizes, and reverts to a low-temperature and low-pressure gas. Due to the absorption of heat, the temperature of the air flowing through the evaporator drops significantly, thereby achieving the cooling of the test chamber.
Subsequently, this low-temperature and low-pressure gas is drawn into the compressor again, initiating the next cycle. In this way, the cycle repeats itself without end. The refrigeration system continuously "moves" the heat inside the box to the outside and dissipates the heat into the atmosphere through the fan.
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