1 Introduction
This is a new type of air-cooled combined unit, a PLC-based air-heat pump rooftop air conditioning unit, integrating air supply, cooling, heating, humidification, air purification, and electrical control. It features large cooling capacity, simplified cooling circuit, high reliability, and compact structure. The unit can be installed on the roof, saving valuable space; the air handling unit can also be installed in a machine room, and the air supply duct connection is simple. The PLC control system ensures precise temperature and humidity control. This article summarizes the control characteristics of this type of air conditioning product using an example of an air conditioning system designed and installed by a specific organization. Overview of Air Conditioning System Control Requirements:
(1) Temperature control range and sensitivity: 10±2℃ in summer, 8±2℃ in winter;
(2) Humidity control range and sensitivity: 55±10% in summer, 40±10% in winter;
(3) Variable air volume operation, and a backup fan section is configured to automatically start operation in case of fan failure;
(4) The unit has a cooling capacity of 200kw and a heating capacity (heat pump type) of 124kw. The energy control is divided into the following levels: 0%, 25%, 50%, 75%, 100%.
2 System Design
2.1 Hardware Architecture
(1) Each circuit is equipped with an air switch, an AC contactor, and a thermal relay;
(2) The blower is equipped with a frequency converter, with an adjustable frequency range of 30~50Hz;
(3) The control system adopts Siemens S7-200CN programmable controller and the human-machine interface adopts Siemens touch screen with full Chinese display. The specific configuration is as follows: one CPU226, one EM222, one EM235, three EM231RTD, one EM232, one KTP178 touch screen, one fresh air sensor, one indoor temperature and humidity sensor, four defrost probes, and one dew point temperature sensor.
2.2 Controlled Object
The controlled objects of the unit include: blower motor (one in operation and one on standby), blower frequency converter, preheating electric heater, electric heater, humidifier, air valve, scroll compressor, condenser fan, four-way reversing valve, etc. The control flow of the new air-heat pump rooftop air conditioner based on PLC is shown in Figure 1.
3 System Principle Design
3.1 Summer Control
The unit operates in cooling mode and is controlled according to dew point temperature. The dew point temperature is set by the user on the touchscreen. When the evaporator dew point temperature is greater than the set value △t1℃, compressor 1 starts first; when it is greater than the set value △t2, compressor 2 starts. After compressor 1 runs for a period of time, the measured value of the indoor temperature and humidity sensor is compared with the set values of indoor temperature and humidity. If the temperature is higher than △t3, compressor 3 starts; if it is higher than △t4, compressor 4 starts. At the same time, if the indoor humidity is greater than the set value, electric heating is activated (electric heating is also activated if the indoor temperature is lower than the set value). If the fresh air temperature is lower than the set value, compressors 1 and 2 stop; at the same time, if the humidity is lower than the set value, compressors 3 and 4 also stop.
3.2 Winter Control
During winter operation of the heat pump unit, compressor 1 starts when the fresh air temperature is below the default value t1℃, compressor 2 starts when it's below t2℃, compressor 3 starts when the fresh air temperature is below the default value t3℃ and the indoor temperature is above t4℃, and compressor 4 starts when the fresh air temperature is below the default value t5℃ and the indoor temperature is above t6℃. Preheating is activated if the fresh air temperature is below t7℃; electric heating starts in groups if the indoor temperature is below each set value t8℃; electric heating stops sequentially when the temperature exceeds the set value t9℃; both preheating and electric heating stop when the fresh air temperature rises to t10℃; and electric heating stops if the compressors start. After the heat pump has been running for a period of time, each compressor stops sequentially when the indoor temperature exceeds each set value t11℃. In addition, indoor temperature and humidity can be controlled and adjusted by the heat recovery device and humidifier based on measured values.
3.3 Defrosting Control
When the unit operates as a heat pump in winter, especially when the ambient temperature is close to or below 0°C, frost will form on the unit. Interlocking control is used during defrosting to prevent excessive temperature fluctuations; that is, when system 1 enters defrosting, system 2 cannot simultaneously enter defrosting, and when system 3 enters defrosting, system 4 cannot simultaneously enter defrosting. Necessary measures are taken during defrosting.
3.4 Humidifier Control
After the blower starts, the humidifier will operate proportionally when the indoor relative humidity is lower than the set value. If an electrode humidifier is selected, it is equipped with a humidification control board that can automatically control the water inlet and outlet. For user convenience, manual drainage and timed drainage control for the humidifier are also added to the PLC .
3.5 Blower control
When the main supply fan fails and stops, it can automatically switch to the standby supply fan, closing the corresponding air valve of the main supply fan and activating the corresponding air valve of the standby supply fan. While the standby supply fan is operating, the main supply fan should be repaired. The supply fan's frequency is changed via a frequency converter, thereby altering its speed and ultimately achieving variable air volume operation and energy savings.
3.6 Inverter anti-interference treatment
Inverters are prone to generating interference, exacerbating temperature and humidity fluctuations. In severe cases, this can cause controller malfunctions and disrupt the normal logic control of the air conditioner. Therefore, eliminating inverter interference must be considered from both hardware and software perspectives. During wiring, power lines and control lines should be separated, the inverter must be reliably grounded, and the PT100 RTD should use a bridge connection. On the software side, filtering measures should be implemented for all analog input interfaces. These measures can largely eliminate interference from the inverter.
3.7 Human-Computer Interface
Featuring a fully Chinese touchscreen display, it is easy to operate and displays a wealth of information. It can monitor temperature and humidity parameters in real time, set indoor temperature and humidity values, and has timer on/off functions. It also displays real-time and historical faults. For security, multiple levels of password protection are implemented for some important parameters.
3.8 Safety Protection
The unit is mainly equipped with the following protections: blower frequency converter fault, blower air pressure fault, electric heating high temperature protection, user external fault interlock, power phase sequence protection, compressor high and low pressure protection, differential pressure alarm, and humidification fault.
4. Conclusion
The new air-cooled heat pump rooftop air conditioner has been operating reliably for over a year at a certain company, meeting the user's temperature and humidity control requirements. User feedback has been positive, and the unit has not experienced any malfunctions to date. The biggest feature of the new air-cooled heat pump rooftop air conditioner is its ability to provide fresh, comfortable air to the controlled environment.
