1. System Structure and Basic Configuration
Control Layer Architecture and PLC Selection
The heat exchange station automation control system typically consists of field sensors, PLC controllers, actuators, human-machine interfaces (HMI), and control cabinets. At the control layer, the Siemens S7-200 SMART PLC serves as the core, offering a compact and cost-effective solution that meets the automation needs of heat exchange stations.
The S7-200 SMART provides a range of CPU models (20, 40, and 60 I/O points), categorized into standard and economical types to suit different I/O scale requirements. For small heat exchange stations (with 5–50 I/O points), the SR20/SR40 standard CPUs (with built-in relay outputs) or ST40 (with transistor outputs) can be selected, with up to four expansion modules available for additional I/O or functions. For stations with higher I/O demands (approaching 50 points), the SR60/ST60 60-point CPUs can be used with analog expansion modules. For simpler stations with fewer I/O needs, the CR40 economical CPU is a lower-cost option but does not support expansion.
When analog input/output is needed, the EM analog expansion modules can be added. The selection principle is to fully meet current requirements while leaving room for expansion.
Typical I/O Point Configuration
Heat exchange stations monitor and control multiple process parameters, including:
Analog Inputs (AI): To monitor primary and secondary network temperatures (supply/return), network pressure (pressure or differential pressure sensors), tank liquid levels, and flow rates. Signals are typically 4-20mA, collected by the PLC for real-time processing. A typical medium-sized station might use 4–6 AI points for temperature and pressure regulation.
Digital Inputs (DI): For monitoring switch signals and equipment status, such as pump run/fault feedback, valve position limit switches, overpressure/underpressure switches, and low liquid level alarms. These ensure system safety interlocks.
Digital Outputs (DO): For controlling devices such as circulation and make-up pumps (start/stop), solenoid valves, electric valves, and alarm buzzers. Example: DO points for starting/stopping pumps, triggering safety relief valves, and alarm indicators.
Analog Outputs (AO): For continuous control of actuators like electric control valves (modulating 4-20mA) and frequency converters for pumps. At least one AO is needed to control the primary side valve, and 1-2 AOs for variable-frequency pumps on the secondary side.
With this I/O setup, the PLC can autonomously monitor and adjust key station parameters, ensuring temperature, pressure, and flow stability in both primary and secondary networks. For instance, if the secondary side supply temperature deviates, the PLC adjusts the primary side valve to maintain desired conditions. For pressure regulation, it adjusts pump speeds to keep the network within the desired pressure range, ensuring reliable, energy-efficient heating.
Integration with HMI Touchscreen
For on-site monitoring and parameter setting, HMIs such as Weinview or Kunlun Tongtai screens are commonly used. The S7-200 SMART can communicate with HMIs via:
RS485 Serial Communication: The onboard RS485 supports Modbus RTU or PPI protocols, allowing HMIs to retrieve data from PLC registers.
Ethernet Communication: Some standard CPUs have built-in Ethernet ports for TCP/IP communication, suitable for higher-speed data exchange and remote/local monitoring integration.
Signal Board Expansion: Optional SB boards like SB CM01 can add RS485/RS232 ports, supporting multi-channel communication with HMIs, VFDs, or SCADA systems.
Through proper configuration in the HMI software (e.g., MCGS or Weinview), seamless data exchange and visualization of temperature, pressure, and flow can be achieved. Operators can easily view real-time data, adjust parameters, and receive alarm notifications on the screen.
Control Cabinet Layout and Wiring Standards
The design of the PLC control cabinet must balance compactness and safety. Typical layout includes:
Equipment Layout: PLCs and I/O modules are mounted on upper DIN rails, with low-voltage terminals nearby, while high-power devices like contactors, circuit breakers, and VFDs are isolated to reduce EMI.
Wiring Standards: Separate routing for DI/DO, analog signals, and power cables, color-coded for clarity. Shielded cables are grounded at a single point to enhance noise immunity.
Grounding and Protection: Unified grounding ensures electrical safety and signal integrity. RC snubbers or diodes protect against inductive load surges.
Power Supply: 24V DC for PLCs and sensors, with independent AC power for motors and heaters, equipped with proper circuit protections. Optionally, UPS can ensure continued operation during short outages.
A properly designed and wired control cabinet provides the foundation for a stable, efficient automation system, with local/automatic modes and remote adjustability.
2. Stability and Cost Analysis
Operational Stability and Anti-Interference Capabilities
The S7-200 SMART, as an industrial-grade PLC, is recognized for its reliability even in environments with pumps, motors, and VFDs that generate electrical noise. Its robust design, including power filtering, EMC certification, opto-isolated I/O, and triple isolation for analog modules, ensures stability.
Rated for 0-60°C operating temperature and IP20 protection, it handles heat, humidity, dust, and vibration typical in heating stations. Built-in diagnostics and watchdog timers prevent crashes and enable safe recovery from anomalies, reducing the risk of supply disruptions.
Comparison with Domestic PLCs
Though domestic PLCs (e.g., Xinjie, Inovance, Fatek) have improved in recent years, the S7-200 SMART stands out in:
Reliability: Proven stability in harsh conditions, robust execution speed (0.15 μs per bit operation), and efficient PID and communication handling.
After-Sales Service: Siemens offers comprehensive technical support, training, and spare parts availability nationwide.
Ease of Programming: MicroWIN SMART software is intuitive, with structured logic, Chinese support, and comprehensive function blocks, making it engineer-friendly.
While domestic PLCs are cheaper and sometimes easier for newcomers, Siemens’ mature ecosystem, support, and reliability are favored for critical applications like heating stations.
Cost Composition Overview
A typical cost structure includes:
PLC Hardware: S7-200 SMART CPU and expansion modules (several thousand RMB for a 40-point CPU). Domestic alternatives are 50%-70% cheaper but may trade off reliability.
HMI Costs: 7–10 inch touchscreens, from a few thousand RMB (domestic brands) to slightly higher for Siemens’ own SMART LINE panels.
Accessories and Installation: Cabinet components, breakers, terminals, and cables, plus engineering design, assembly, and programming fees. Labour costs can rival hardware costs.
While Siemens PLC solutions may cost ~20% more overall, the reduced downtime and maintenance expenses provide better lifecycle value, especially in public service sectors like heating.
Cost-Effectiveness and Application Recommendations
S7-200 SMART strikes a balance between performance and cost, ideal for 5-50 I/O point heat exchange stations. It eliminates the need for complex distributed systems and handles multiple control loops efficiently.
Best for: Single-unit stations with 1 heat exchanger, 1-2 circulation pumps, and a make-up pump. The PLC’s max 60 I/O (expandable to ~188) covers most monitoring needs.
Advantages: Built-in Ethernet enables easy integration with SCADA or cloud platforms for multi-station supervision.
When to consider domestic PLCs: For highly cost-sensitive projects or where local support for domestic brands is readily available. However, quality and reliability remain critical for systems involving safety interlocks and continuous public services.
Conclusion
The Siemens S7-200 SMART PLC offers a comprehensive, reliable, and cost-effective solution for small to medium-sized heat exchange stations. It ensures safe, stable, and energy-efficient operations, supporting both local and remote management. Heating companies and system integrators can confidently adopt this solution for modernizing heating infrastructure with minimal maintenance and maximum reliability.
For more information, please contact Henan Rentai Electric:
Phone: 17638563962 / 0371-56520104
Email: info@hnrentai.com