PLC-based rooftop air conditioner with fresh air heat pump

1 Introduction

Air-cooled combined unit is a brand-new air-heat pump rooftop air-conditioning unit based on PLC, which integrates air supply, refrigeration, heating, humidification, air purification, and electrical control. It has the characteristics of large cooling capacity, simplified refrigeration circuit, strong reliability, and compact structure. The unit can be installed on the roof without occupying effective space. The air handling part can also be installed in the machine room, and the air supply duct connection is simple. The use of PLC control system can ensure the accurate temperature and humidity of the unit. This article summarizes the control characteristics of this type of air-conditioning product based on the example of an air-conditioning system designed and installed by a certain unit. Overview of air conditioning system control requirements:

(1) Temperature control range and sensitivity: 10±2℃ in summer and 8±2℃ in winter;

(2) Humidity control range and sensitivity: 55±10% in summer and 40±10% in winter;

(3) Variable air volume operation, and a spare blower section is configured to automatically start operation in the event of a blower 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%, and 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 air blower is equipped with a frequency converter, and the frequency can be adjusted in the range of 30~50Hz;

(3) The control system uses a Siemens S7-200CN programmable controller, and the human-machine interface uses a Siemens touch screen with full Chinese display. The specific configuration is: one CPU226, one EM222, one EM235, three EM231RTDs, one EM232, one touch screen KTP178, one fresh air and one indoor temperature and humidity sensor, four defrost probes, and one dew point temperature sensor.

2.2 Control Objects

The control objects of the unit include: air blower motor (one for use and one for backup), air blower frequency converter, preheating electric heater, electric heater, humidifier, air valve, scroll compressor, condensing fan, four-way reversing valve, etc. The control process of the fresh air heat pump type rooftop air conditioner based on PLC is shown in Figure 1.

Figure 1 Fresh air conditioning system control process

3 System principle design

3.1 Summer control

The unit performs cooling operation and is controlled by the dew point temperature. The dew point temperature is set by the user on the touch screen. When the evaporator dew point temperature is greater than the set value △t1℃, compressor 1 is started first, and when it is greater than the set value △t2, compressor 2 is started. After compressor 1 runs for a period of time, the indoor temperature and humidity sensor measures the value and compares it with the indoor temperature and humidity set value. If the temperature is higher than △t3, compressor 3 is started again. If it is higher than △t4, compressor 4 is started. At the same time, if the indoor humidity is greater than the set value, electric heating is turned on (if the indoor temperature is lower than the set value, electric heating is also turned on). If the fresh air temperature is lower than the set value, compressors 1 and 2 stop. At the same time, when the humidity is lower than the set value, compressors 3 and 4 also stop.

3.2 Winter control

In winter, the unit heat pump runs. When the fresh air temperature is lower than the default value t1℃, compressor 1 starts. When it is lower than the default value t2℃, compressor 2 starts. When the fresh air temperature is lower than the default value t3℃ and the indoor temperature is lower than t4℃ or more, compressor 3 starts. When the fresh air temperature is lower than the default value t5℃ and the indoor temperature is lower than t6℃ or more, compressor 4 starts. If the fresh air temperature is lower than t7℃, preheating is put into operation; if the indoor temperature is lower than the set value t8℃, the electric heating group starts; when it is higher than the set value t9℃, the electric heating stops in sequence; if the fresh air temperature rises to t10℃, both preheating and electric heating stop; if the compressor starts, the electric heating stops. After the heat pump runs for a period of time, when the indoor temperature is higher than the set value t11℃, the compressors stop in sequence. In addition, the indoor temperature and humidity can also be heated and humidified by the heat recovery device and humidifier according to the measured values.

3.3 Defrost control

When the unit is running as a heat pump in winter, especially when the ambient temperature is close to 0℃ or lower than 0℃, the unit will be frosted. Interlocking control is used during defrosting to avoid excessive temperature fluctuations, that is, when system 1 enters defrosting, system 2 cannot enter defrosting at the same time, and when system 3 enters defrosting, system 4 cannot enter defrosting at the same time; necessary electric heating is used during defrosting. When the unit enters the heating condition, low-temperature and low-pressure refrigerant enters the fin heat exchanger, the coil temperature continues to drop, and the suction temperature also drops. When the coil temperature drops to the set value and the defrost cycle has expired, the PLC controls the four-way reversing valve to switch to the defrost mode, and at the same time stops the condensing fan and starts the electric heater; when the evaporator coil temperature rises to the set value, or the defrost execution time reaches the set maximum defrost time, the four-way valve reverses, the defrost ends, the electric heating stops running, and the unit enters the heating condition again, and the cycle continues.

3.4 Humidifier control

After the blower is started, when the indoor relative humidity is lower than the set value, the humidifier is put into use proportionally. If an electrode humidifier is selected, the humidifier is equipped with a humidification control board, which can automatically control the inlet and outlet. For the convenience of users, the PLC also adds manual drainage and timed drainage control of the humidifier.

3.5 Fan control

When the main fan fails and stops, it can automatically switch to the standby fan, close the corresponding air valve of the main fan, and start the corresponding air valve of the standby fan. When the standby fan is working, the main fan should be repaired. The fan changes the frequency through the inverter, thereby changing the speed of the fan, and finally achieves variable air volume operation and energy saving.

3.6 Anti-interference processing of inverter

The inverter is easy to generate interference sources, which aggravates the fluctuation of temperature and humidity. In severe cases, it will cause the controller to malfunction and affect the normal logic control of the air conditioner. Therefore, both hardware and software must consider eliminating frequency conversion interference. When wiring, the power line and the control line are separated, the inverter must be reliably grounded, and the thermal resistor pt100 should use a bridge connection. In terms of software, measures such as filtering should be taken for each analog input interface. This can basically eliminate the interference caused by the frequency converter.

3.7 Human-machine interface

The touch screen display in full Chinese is used, which is easy to operate and has rich display content. It can monitor the temperature and humidity parameters of various places in real time, set the indoor temperature and humidity values, and set the timed start and shutdown functions, and display real-time faults and historical faults. For safety reasons, multi-level password protection is set for some important parameters.

3.8 Safety protection

The unit is mainly set with the following protections: fan frequency conversion fault, fan air pressure fault, electric heating high temperature protection, user external fault interlocking, power supply phase sequence protection, compressor high and low pressure protection, pressure difference alarm, humidification fault.

4 Conclusion

The fresh air heat pump rooftop air conditioner has been operating reliably in a certain unit for more than one year, meeting the user's temperature and humidity control requirements. The user feedback is good, and the unit has not had any bad faults so far. The biggest feature of the fresh air heat pump rooftop air conditioner is that it can provide fresh and comfortable air to the controlled environment. For different seasons, different operating modes are used, which is both efficient and energy-saving. This type of air conditioner will be widely used in metallurgy, chemical industry, electromechanical and other industries and has broad application prospects.