In precision laboratories, data centers, museums, pharmaceutical production facilities, and other environments with extremely 

stringent requirements, constant temperature and humidity machines serve as tireless guardians, silently maintaining stable 

temperature and humidity levels within spaces. How exactly do they achieve this? This article will provide a clear and accessible 

breakdown of the four core cycles and their collaborative working principles.

I. Overall Workflow: Sense-Judge-Execute

A constant temperature and humidity machine is not a simple aggregation of single functions but a highly integrated 

intelligent system. Its operation follows a classic closed-loop control logic:

• Sense: Temperature and humidity sensors distributed throughout the unit and the controlled area act as the equipment's

  “senses,” continuously collecting environmental data.

• Judge: The central controller (typically a microcomputer or PLC) serves as the equipment's “brain.” It compares and

  calculates the real-time data from sensors against user-set target values.

• Execution: Based on the calculations, the “brain” issues precise commands to corresponding functional modules

  (such as compressors, heaters, water pumps, etc.).

• Feedback and Adjustment: Post-execution environmental changes are captured again by sensors, forming feedback.

  The controller then makes a new round of adjustments, repeating this cycle to achieve dynamic equilibrium.

II. Breakdown of the Four Core Cycle Principles

1. Refrigeration Cycle: The “Heat Porter”

This forms the foundation for cooling and (accompanying) dehumidification. Its core operates on the vapor compression 

refrigeration principle:

• Key Components: Compressor, condenser, throttling device (e.g., expansion valve), evaporator.

• Process: Low-temperature, low-pressure liquid refrigerant absorbs heat from the air flowing through the evaporator,

  boiling and vaporizing to significantly lower air temperature. Simultaneously, excess water vapor in the air condenses

  onto the evaporator fins, achieving dehumidification. The heat-absorbed gaseous refrigerant is compressed by the

  compressor into a high-temperature, high-pressure gas. This gas enters the condenser, where it releases heat to the

  surroundings (via air or water), condensing back into a liquid. After passing through the throttling device to reduce

  pressure and temperature, it returns to the evaporator to begin the next cycle.

2. Heating Cycle: The Energy “Supplementer”

When heating is required, the unit activates its heating function. Two common methods exist:

• Electric Heating: Directly activates built-in electric heating elements, functioning like a large “hair dryer” to heat

  airflow directly. This method offers rapid response and high control precision.

• Heat Pump Heating: Switches the four-way valve to reverse the refrigeration cycle. The original evaporator now

  functions as a condenser, releasing heat indoors, while the former condenser acts as an evaporator, absorbing heat

  from outdoors. This method achieves higher energy efficiency but is significantly affected by outdoor temperatures.

3. Dehumidification Cycle: The “Condenser” for Moisture

Dehumidification primarily occurs alongside the cooling process (as described above). However, under specific conditions 

(e.g., low temperatures and high humidity), independent or enhanced dehumidification is required:

• Conventional Dehumidification: Naturally achieved during refrigeration as described above.

• Reheat Dehumidification: When temperature targets are met but humidity remains excessively high, the unit simultaneously

  operates the refrigeration cycle (dehumidification) and heating cycle (to compensate for temperature drop caused by

  refrigeration). This ensures efficient dehumidification while maintaining constant temperature, demonstrating precise

  coordination among functional modules.

4. Humidification Cycle: The “Supplier” of Moisture

When the environment becomes excessively dry, the equipment must add moisture to the air. Mainstream technologies include:

• Electrode/Electric Heating Humidification: Water is heated to boiling point via electrodes or heating elements, producing

  pure steam that mixes with the supply airflow. This method offers rapid humidification and precise control.

• Wet Membrane/Ultrasonic Humidification: Water circulates to saturate a wet membrane for evaporation, or ultrasonic

  high-frequency oscillation generates fine water mist absorbed by the air. Relatively high energy efficiency, but stringent

  water quality requirements.

III. Precision Coordination: Achieving Dual Stability

The challenge of constant temperature and humidity lies not in activating individual functions, but in harmonizing potentially 

conflicting operations while rapidly responding to changes. This relies entirely on the controller's intelligent algorithms and 

flexible module coordination.

Scenario 1: High Temperature & High Humidity

The controller detects elevated temperature and humidity. It simultaneously activates cooling and dehumidification, potentially 

adjusting fan speed to rapidly reach the setpoint.

Scenario 2: Low Temperature & Low Humidity

The controller detects low temperature and humidity. It activates heating and humidification, precisely calculating their output 

ratio to prevent overheating from affecting humidity or excessive humidification causing discomfort.

Scenario 3: Temperature Reached, Humidity Excessive

This presents a classic challenge. The controller activates “reheat dehumidification” mode: air first passes over the 

low-temperature evaporator coil to cool and remove moisture, then flows through the heater (or condenser) to precisely 

restore it to the set temperature, ultimately delivering air that is comfortably warm with lower humidity.

Scenario 4: Humidity at Target, Temperature Too Low

The controller initiates heating while closely monitoring conditions. Since heating may cause relative humidity to drop, the 

humidification system intervenes to compensate if humidity falls below the setpoint.

Throughout this process, sensors provide data feedback multiple times per second. The controller performs millisecond-level 

calculations and judgments, continuously adjusting the compressor frequency, valve opening, heating power, and humidification 

volume. This achieves exceptional control precision exceeding ±1°C and ±5%RH.

Conclusion

The constant temperature and humidity unit is a precision environmental control system integrating thermodynamics, fluid 

mechanics, automatic control, and sensor technology. Its intelligence lies not in breakthroughs in any single technology, but 

in seamlessly integrating four seemingly fundamental—and even conflicting—physical processes: cooling, heating, dehumidification, 

and humidification. Through precise sensing and intelligent control, these functions coalesce into a stable, efficient, and rapidly 

responsive whole. It is precisely this collaborative capability that enables it to safeguard our critically important constant environment 

within a compact footprint.