Modernising and upgrading a control system

A new customer came to us with a problem he hoped we could help with: he wanted to control the supply and temperature of CO2 into several large greenhouses. Controlling the temperature of CO2 (the exhaust from the gas engines and boilers) is vital, it can’t go above 60 degrees centigrade: too hot and the PVC ducting will melt.

His current panel system consisted of relays, contactors and Star Delta motor starters – a system which was perfectly fine but not meeting his needs, because it wasn’t possible to control the fan speed, which eats into consumption and, therefore, profits.


After discussing what the customer wanted, our team designed and built a new electrical panel system. We replaced the Star Delta starters with inverters, and the old relays, contactors, timers, switches and lamps with a PLC and touch screen display (HMI).

The whole system is now controlled by a PLC programme, which our engineer designed to control the valves, read temperatures via sensors, start fans and monitor fault mechanisms – for example if a valve does not close within a set timescale.

6. interior of the inverter drive cabinet

Before: interior of the inverter drive cabinet

The HMI screen allows the operator to see and set the temperature of the CO2 going through the system, which the PLC regulates by mixing normal air with CO2.

5. interior of the new CO2 control panel which houses the PLC, HMI relays and power supply unit

Work in progress: interior of the new CO2 control panel which will house the PLC, HMI relays and power supply unit

One of the main advantages of updating to a PLC operated system is that the customer can add extra features without adding and wiring extra components, for example a fault detection mechanism. Fault-finding becomes easier with a PLC driven system, as mechanical timers and relays can cause intermittent faults.

4. HMI showing the main page

HMI showing the main page with CO2 valve not closed
* Warning lamps indicate valves that aren’t closed when the fan is off
* CO2 failure warning lamps indicate that an inverter has failed,
or the over-temperature thermostat is active
* Switches put each fan into manual or automatic mode, or off
As our customer’s business continues to expand, incorporating extra features can be easily, efficiently and cost-effectively managed.