PLC Solution Library · Cold storage simulator
Keep a cold room in its band, door, defrost and power cut included
A cold room is a thermostat, an evaporator that ices up, a door that people leave open and a record that must prove the cold chain. In this simulator, one Raspberry PLC 21 runs the compressor, the fans and the defrost, watches the door and the alarm button inside, and keeps the record.
Choose what is stored, run a day, leave the door open, break the defrost or cut the power. Watch the air, the product and every alert.
Choose your scenario
-
AirProductAlarm limitsDoor openDefrost
Reset stands for the operator at the cabinet: it silences the buzzer, acknowledges the alerts and lets the compressor start again once its fault input is clear.
Your design choices
These are choices you make in your own cold room program. They are explained below. A change applies at once, also in the middle of a scenario.
What is in the room
| Product | Thermostat | Alarm |
|---|---|---|
| Vaccines | 4 to 6 °C | under 2, over 8 °C |
| Fresh fish | 0.5 to 1.5 °C | under -1, over 2 °C |
| Frozen food | -23 to -20 °C | over -18 °C |
Example values for this simulator. The bands follow common rules: 2 to 8 °C for many vaccines and medicines, close to melting ice for fresh fish (Regulation (EC) 853/2004), -18 °C or colder for quick-frozen food. Use the conditions of your product and your local rules.
Alerts and events
| time | event | detail | sent to |
|---|
One row per alert or event. The CSV has one row every 15 minutes with the air, product and coil temperatures, the door, the compressor, the defrost and the alarm. A minute with the PLC off has no row: the record shows the gap, it does not invent values.
How does a PLC control a cold room?
With a thermostat on the air, a defrost cycle for the evaporator and a set of alarms that protect the product and the people. Every minute, the PLC runs these steps:
- Thermostat. Over the upper set point of the air, the PLC starts the compressor. At the lower set point it stops it. The gap between the two is the hysteresis, and it keeps the compressor from short cycles.
- Fans. The evaporator fans move the cold air through the room. They stop when the door opens, so they do not blow the cold out, and start again if the door stays open past its alarm time. They also stop during the defrost.
- Defrost. Every 6 hours the PLC stops the compressor and heats the coil to melt the ice. It ends when the coil reaches 8 °C, or after 30 minutes at most.
- Alarms. The PLC compares the product probe with the band of the product, watches the door, the compressor fault and the alarm button inside the room, and sends each alert by 4G.
- Record. Every 15 minutes the PLC writes the temperatures and the states with the time of its real-time clock, as a temperature recorder does.
Run the A normal day scenario and compare the air with the product in the trend.
Why alarm on the product probe and not on the air?
The air of a cold room jumps every time the door opens and every time the coil defrosts. The product, with its mass, barely moves. An alarm on the air fires at each delivery, and after a few days nobody looks at it.
A probe in a bottle of glycol among the product follows the product. The PLC raises the alarm when it stays out of the band for 30 minutes. A short opening does not fire it. A door left open, a stopped compressor or a coil full of ice do.
Set Temperature alarm on to The air probe and Alarm delay to At once, run the A normal day scenario and count the alerts.
Run the Door left open scenario: the door alarm sounds after 3 minutes, long before the product leaves its band.
Why does an evaporator need a defrost?
The coil of the evaporator is colder than 0 °C, so the moisture of the air freezes on it. Each door opening brings in more moist air. The ice blocks the air between the fins, and the evaporator gives less and less cold. The compressor then runs longer for the same room.
An electric defrost stops the compressor and the fans and heats the coil. Ending on the coil temperature stops the heater as soon as the ice is gone. A fixed time either wastes heat in the room or leaves ice on the coil. With a coil probe, the PLC also sees a defrost that ends on its maximum time with the coil still cold: a sign of a heater fault, which it reports.
Run the Defrost heater fails scenario and watch the ice on the coil and the running time of the compressor.
What happens with a power cut or a person inside?
With a power cut, the refrigeration stops and the room warms up slowly. A closed room with its product holds the cold for a while, so the first job is to know about it. A PLC on mains only goes off with everything else: no alert and a hole in the record. A 24 V DC UPS with a battery keeps the PLC and its 4G board alive. The PLC reads a mains contact, sends the alert at once and keeps recording. When the power comes back, the outputs of the Raspberry PLC keep their last state until Linux has started, as its datasheet says. Keep the anti-short-cycle timer of the condensing unit, and do not rely only on the program for the restart delay.
The UPS built into the Raspberry PLC 21 is there for a safe shutdown of Linux, to protect its files. It is not meant to keep the PLC running through a long cut. For that, use an external DC UPS sized for the PLC and the 4G board.
A person can get locked in a cold room. Safety rules for refrigerated rooms, such as EN 378, ask for a way to open the door from inside and to raise an alarm. Here the alarm button inside the room starts the beacon and the buzzer and sends an alert at once, day or night, with no delay. This alarm must also work in a power cut. Feed the button, the beacon and the buzzer from the DC UPS, or fit a self-contained cold room alarm with its own battery and let the PLC read its contact.
Run the Power cut and alarm button scenario with the UPS, then with mains only, and compare the record.
Design choices you make
1. The probe for the alarm
The air probe is the right input for the thermostat: it reacts fast. The product probe is the right input for the alarm: it says what the product feels. Keep both, and record both.
2. The alarm delay
A delay filters the normal events of the room. It must stay short enough for the people on call to act before the product is lost. The door alarm is separate and much faster.
3. How the defrost ends
On the coil temperature, with a maximum time as a backstop. A fixed time is the simple choice when there is no coil probe, at the cost of energy and ice.
4. The supply of the PLC
An external DC UPS turns a power cut into an alert and a complete record. Without it, the PLC can only report the cut when the power comes back.
5. Outputs for the refrigeration
The digital outputs of the Raspberry PLC 21 are optoisolated, with 70 mA at most each. Drive the compressor and the defrost heater through contactors, and the fans through a contactor or an interface relay. Keep the safety devices of the refrigeration circuit, such as the high pressure switch, wired to stop the compressor by themselves. The PLC reads their state through the fault input.
The case studies behind this simulator
This simulator brings together our cold chain cases in one room. The values, times and faults here are examples, not data from those projects.
- Vaccine cold storage. A Raspberry PLC outside the freezer reads probes in the tank, raises the alarm at the limit, counts the days of each batch and keeps a database that it sends to the cloud. Read the vaccine cold storage case study
- Fishing vessel holds. PLCs watch the temperature, humidity and air quality of each hold and share them over Modbus with the screens and the alarms of the crew. Read the fishing vessel holds case study
- Refrigerated fleet. One unit on each truck measures the temperature of the frozen load and the position, and sends them over the mobile network. Read the refrigerated fleet case study
To follow battery sensors and a gateway across a warehouse, try the Wireless Sensor Network simulator.
Learn the building blocks
These examples cover the parts of this cold room on Industrial Shields hardware.
- An alarm history in MySQL with Node-RED, for the record and the alarms
- DS18B20 temperature on a Raspberry PLC over 1-Wire, for chilled rooms within its range
- Telegram alerts and remote control, for the alerts to the phone
- Email alerts from a Python monitoring server, for a second channel of alerts
More examples: PLC Solution Library · all simulators.
Raspberry PLC 21: the specs that matter here
| Name | RASPBERRY PLC 21 FLEX |
|---|---|
| Computer | Raspberry Pi 4, 2, 4 or 8 GB of RAM, Debian 64-bit |
| Inputs | 13: 6 analog/digital (0-10 V, 12-bit) and 7 digital optoisolated |
| Outputs | 8: 3 analog/PWM (0-10 V) and 5 digital optoisolated (70 mA max.) |
| Communication | 2 x Ethernet, Wi-Fi, Bluetooth LE, 2 x RS-485, I2C, SPI, USB |
| Built in | DS3231 real-time clock with battery, UPS for a safe shutdown |
| Expansion | One board: 4G (LTE-M/NB-IoT or LTE Cat 4), LoRa, CAN or M-Bus |
| Power and mounting | 12 to 24 Vdc, DIN rail TS35, IP20, 0 to 50 °C |
Example I/O for this cold room
| Signal | PLC I/O | Count |
|---|---|---|
| Air, product and coil probes, Pt1000 with a 0-10 V transmitter | Analog inputs | 3 |
| Door contact, alarm button inside, compressor fault, mains present | Digital inputs, optoisolated | 4 |
| Compressor, evaporator fans, defrost heater | Digital outputs, through contactors | 3 |
| Alarm beacon and buzzer | Digital output, through an interface relay | 1 |
| Alerts and record to the cloud | 4G board, Ethernet | 1 |
This example uses 3 of the 6 analog inputs, 4 of the 7 optoisolated inputs and 4 of the 5 optoisolated outputs. The last output can drive a door frame heater in a freezer room. With 12 bits over a -50 to 50 °C transmitter, one count is about 0.025 °C. Check the rated range of each probe for your band, and your loads and levels against the Raspberry PLC family datasheet. Full specs: Raspberry PLC 21 FLEX.
FAQ
How does a PLC control the temperature of a cold room?
It works as a thermostat on the air probe: it starts the compressor over the upper set point and stops it at the lower one, with a gap that avoids short cycles. It runs the evaporator fans, stops them with the door open and during the defrost, and runs a defrost of the coil every few hours. On top of the control, it raises alarms and keeps a dated record of the temperatures.
Where should the alarm probe of a cold room go?
Among the product, ideally in a bottle of glycol or another product simulant, so it follows the temperature of the product and not every jump of the air. The air probe stays for the thermostat, which needs a fast signal. An alarm on the air fires at each door opening and each defrost, and people stop paying attention to it.
How often does a cold room evaporator need a defrost?
It depends on the room temperature, the humidity and how often the door opens. A schedule of every 6 hours is a common starting point, ended by the coil temperature with a maximum time as a backstop. A room with many openings ices up faster. A defrost that always ends on its maximum time with a cold coil points to a heater or probe fault.
What should a cold room do during a power cut?
Keep the door closed and tell someone. The refrigeration stops, and a closed room with its product holds the cold for a while. To send the alert, the PLC and its 4G board need their own supply, such as a 24 V DC UPS with a battery, and a mains contact to read. Without it, the PLC goes off, and the cut only shows as a gap in the record when the power comes back.
Why does a walk-in cold room need an alarm button inside?
Because a person can get locked in, and the cold is dangerous. Safety rules for refrigerated rooms, such as EN 378, ask for a door that opens from inside and a way to raise an alarm. The PLC treats the button as the most urgent alarm: beacon, buzzer and an alert to the people on call at once, with no delay. It must keep working in a power cut, so feed it from a DC UPS or use a self-contained alarm with its own battery.
Which Industrial Shields PLC fits a cold room?
This simulator uses a Raspberry PLC 21, the model we propose in our vaccine cold storage case. It has 6 analog inputs for probe transmitters, 7 optoisolated inputs for the door, the alarm button and the faults, 5 optoisolated outputs for the compressor, the fans, the defrost and the alarm through contactors, a real-time clock for the record and a slot for a 4G board to send the alerts. It runs Linux, so the same controller can keep the database and the dashboard.
Send me this architecture
Tell us what you keep cold, how many rooms, freezers or vehicles you watch and who must get the alerts. Our team replies with a proposed architecture for your cold chain. Your simulator settings go with the request.