Application of electronic control design of soil heat pump unit in radiant panel system

0 Introduction With the continuous improvement of social living standards, people have put forward higher requirements for their working and living environment. Comfort, health, energy saving and pollution-free have become the goals pursued by the new generation of air conditioning systems. Following this trend, the company successfully developed a soil heat pump unit for radiant panel systems.
1 Introduction to the soil heat pump unit used in radiant panel systems The soil heat pump unit used in radiant panel systems has its own unique features in function and control. First, a new fan system with heat pump function was added to the unit. The fresh air fan not only delivers fresh air with suitable temperature to users all year round, but also bears the dehumidification load of indoor air to prevent condensation on the radiant panels. In winter, a fresh air preheating function can be added. Furthermore, the control reflects the people-oriented concept: safety, convenience and flexibility. For the system structure, see the working principle diagram of the soil heat pump unit (Figure 1).
The electronic control system of the soil heat pump unit is relatively complex. The main unit and the fresh air fan are two independent refrigeration systems. They are closely linked through the key parameter of indoor dew point temperature and work in coordination to create a comfortable and healthy environment for users. Living space.
2 Design of electronic control system of soil heat pump unit 2.1 Overview of electronic control spring design attention and roll forming method of soil heat pump unit
The soil heat pump unit has three working modes: cooling, heating and ventilation.
Cooling mode: Start the unit, the fresh air fan starts, and works in the dehumidification mode. At this time, the air supply fan runs at high speed, the fresh air valve is closed, the indoor return air bypass valve is opened, the exhaust fan does not work, and the indoor air is circulated for dehumidification, gradually reducing the indoor air temperature. Air dew point temperature. After 30 minutes, the indoor dew point temperature is detected. If the dew point temperature drops to the set temperature point tdew1, a start-up signal is sent to the host, and the host starts cooling and delivers chilled water to the radiant panel. The set value of the host water supply temperature will be dynamically adjusted as the indoor dew point temperature changes. When the dew point temperature drops to the set temperature point tdew2, the fresh air valve opens, the exhaust fan starts, and the air blower runs at low speed to deliver fresh air to users.
Heating mode: The fresh air fan and the main unit are started at the same time. The fresh air valve opens and the blower runs at low speed to deliver fresh air to the room. In the heating mode, the fresh air can be set up with a preheating function, and a humidification device is set up on the air supply side to detect the relative humidity in the room. When it is detected that the indoor air is dry and does not meet the design requirements, the supply air will be humidified. The host compressor starts and delivers appropriate hot water to the radiant panels.
Ventilation mode: The fresh air valve is opened, the indoor return air bypass valve is closed, and the supply fan and exhaust fan run at high speed to deliver fresh air to users.
2.2 Key points of electronic control design of soil heat pump unit
2.2.1 Dew point temperature collection: Dew point temperature is a key parameter of the soil heat pump unit system. In the design, two parameters, indoor relative humidity and dry bulb temperature, are used. A table lookup program is set up in the control mainboard microcontroller to indirectly detect the related parameters. The dew point temperature of the corresponding air-conditioned area. The working status of the fresh air fan and the start and stop of the host are controlled according to the value of the indoor dew point temperature, and the parameters tdew1, tdew2, Δt1 and Δt2 are required to be adjustable. See Figure 2
Figure 1 Working principle diagram of the unit 1. Fresh air valve 2. Return air bypass valve 3. Heat recovery device 4. Exhaust fan 5. Air supply fan 6. Fin heat exchanger 7. Four-way reversing valve 8. Compressor 9. Gas Liquid separator 10, outdoor heat exchanger 11, two-way filter 12, two-way thermal expansion valve 13, expansion tank 14, indoor water pump 15, outdoor water pump

Figure 2 Schematic diagram of dew point temperature setting
2.2.2 Cooling hanging plate water supply temperature setting: In order to improve the operating efficiency of the host and avoid frequent startup of the host, the set value of the water supply temperature of the host (delivered to the radiant panel) is required to be dynamic within a certain range with changes in the indoor dew point temperature. adjustment. As the dew point temperature drops, the water supply temperature set value of the host computer also drops, increasing the cooling capacity of the radiant panel.
2.2.3 Automatic fan gear shifting: The supply fan and exhaust fan should automatically shift cnc machined uhmw pe plastic impellers and gears according to the different operating modes of the unit. On the premise of meeting the system performance requirements, the fan can operate at low speed to reduce system consumption and noise.
2.2.4 The indoor temperature set value takes precedence over the host water supply temperature set value to control the start and stop of the soil heat pump unit. That is, when the indoor temperature reaches the user’s setting requirements, one or all of the host compressors can be controlled to be put into or out of operation.
2.2.5 The main machine adopts a dual-head or multi-head combination (except for units with very small capacity, cooling capacity ≤10kW), and can perform energy level adjustment.
2.2.6 Auxiliary heating function: Fresh air preheating function in heating mode: In the severe cold season, when the fresh air temperature drops to the set value, the fresh air preheater is started to prevent frost in the heat exchanger channel. The fresh air preheater is made of PTC ceramic material. This type of ceramic heating element has small thermal resistance and high heat exchange efficiency. Its most outstanding advantage lies in its safety performance. That is, when the fan fails and stops, the PTC heater cannot dissipate sufficient heat. The power will drop sharply. At this time, the surface temperature of the heater is maintained at around the Curie temperature (generally around 250°C), and there is no danger of burning.
2.2.7 The system can be configured with remote telephone control to turn on and off the unit remotely and pre-cool (heat) the room in advance.
2.2.8 Various safety assurance measures necessary for system operation: including exhaust pressure protection, suction pressure protection, overload protection, antifreeze protection of the compressor, various temperature protections of the fresh air blower circuit, and the differences between the three operating modes. Interlocking etc.
2.3 Hardware composition (see Figure 3)
Figure 3 Schematic diagram of hardware composition
The PIC16F874 microcontroller is the core component of the soil heat pump unit control motherboard. It is a product of the American Microchip company. It has an 8-channel 10-bit A/D converter, an internal 4K FLASH memory, 256*8 EEPROM memories, five Multi-function port, three timers/WDT, and rich configuration of this microcontroller can fully meet the needs of air conditioning controllers. The control mainboard uses two single-chip microcomputers of the same model. One single-chip microcomputer is dedicated to the collection and display of dew point temperature and the output of dew point temperature and control signals. Another single-chip microcomputer is used to control the fresh air blower and host machine.
The main control board considers the electromagnetic compatibility of the system in terms of circuit design, component selection, printed circuit board design, wiring and manufacturing processes. Its main measures include: grounding, filtering, and isolation. Printed circuit board aspects include: reasonable zoning of circuit boards, distinguishing digital and analog floors, reasonable wiring, and floor laying to reduce interference.
2.3.1 Control panel input and output
Six-channel analog input: four-channel NTC thermistor, two channels are used to detect the supply and return water temperature of the host, and two channels are used to detect the supply and return air temperature of the fresh air system.
Two 4-20mA standard signal inputs are used for temperature and humidity input.
Switch input: including various safety protections of the unit, remote start-up, etc.
Keyboard input: including cnc turning industrial copper aluminum clad carbon fiber machinery parts on and off the unit, operating mode selection, parameter setting, etc.
Relay output: Drive loads include: compressors, pumps, fans, auxiliary heaters, control signals, etc.
2.3.2 Power circuit
The power circuit in this system uses dual independent power supplies to supply digital and analog power supplies respectively. It consists of AC transformer (220V/12V), bridge rectifier circuit, filter capacitor, integrated regulated power supply LM7812CT, MC7805T, which provides +12V and +5V DC voltage for the electronic control board.
2.3.3 Temperature acquisition circuit
PIC16F874 has its own A/D conversion circuit and can directly collect temperature signals, simplifying the peripheral circuit. In this system we use four temperature signals. Two channels are used to collect the supply and return water temperatures of the host. Two channels are used to collect the supply and return air temperatures of the fresh air fan. All four temperatures can be displayed on the unit control panel. In this way, when the unit is debugging and running, the operating status of the system can be directly judged through the supply and return water (air) temperatures.
2.3.4 Relative humidity and dry bulb temperature acquisition circuit
The two 4-20mA temperature and humidity standard signals collected on site are converted into millivolt electrical signals through standard resistors and sent to the microcontroller. The corresponding dew point temperature is found through the software lookup table method and the output is displayed. .
In order to prevent interference when collecting analog data, this solution uses digital filtering technology—average filtering. average filter