RDM Controls Cut Running Costs by 25%

The Project

A new cold store at Sharrocks Fresh Produce facility, near Preston, is being cooled with a high-efficiency refrigeration system equipped with advanced controls from Resource Data Management (RDM). The custom-built cooling and control solution, designed and installed by Leigh-based contractor Personal Refrigeration Ltd, is estimated to be saving the end user 25 per cent on refrigeration running costs.

Steve Bickett, who headed up the project for Personal Refrigeration, said:

“Sharrocks is a major UK supplier of high-quality fruit and veg, and imports produce from across the world. The company was relocating into new premises, and wanted an efficient new cold store to maintain product quality and maximise shelf life.

“With the rising cost of electricity, it was also important that the design minimised energy use in order to reduce running costs and protect the environment while maintaining produce quality.”

Tight control of both temperature and humidity within the 28m by 13m cold room space was therefore essential. After calculating the heat load, based on anticipated flows of produce through the facility, Personal Refrigeration designed the refrigeration system with an overarching control solution based on RDM’s Intuitive TDB controller, with touch screen interface and I/O expansion board.

“The RDM system provided an excellent solution, and delivers energy efficiency, plant resilience and produce quality – all key requirements of our customer. It manages to be simple to use and program, while offering extremely sophisticated, flexible and intelligent control in a single package.” - Steve Bicket, Personal Refrigeration

Maximising Savings

The contractor wanted to maintain a narrow Delta T of around 6K at all times, to maximise energy savings for the client. Key to delivering tight control was the use of control panels, custom-designed by the contractor, incorporating individual inverter drives for the plant’s two Frascold compressors.

The contractor supplied two “tropical” condensing units to accommodate the inverter-driven compressors. Control is managed by a custom-designed software programme, written by the contractor using TDB programmable logic control (plc) software. This controls the inverters, which in turn modulate the frequency of power to the compressors, enabling control of pressure within approximately 0.2 Barg (+/- 0.5^oC) of evaporating set point.

This in turn also helps maintain high humidity in the storage area, to ensure quality is maintained. Humidity in the store is maintained above 85 per cent most of the time, with more than 90 per cent achieved during times of reduced throughput and product turnover.

Evaporating temperature is set to -3^oC, with the inverter drives ensuring this is maintained through modulation via the PLC algorithm. The RDM system also monitors and controls floating head pressures, to ensure real-time changes in load are matched by the cooling plant.

“This gives incredibly fine control, and enables cooling capacity to be precisely matched to changes in heat load,” said Steve Bickett. “You can’t achieve anything like this with a conventional control solution. We estimate this approach will save around 25 per cent on energy use compared with a traditional system.”

This has now been verified from data from energy monitors, referenced against the standard compressor operating frequency of 50Hz at the design condition.

To ensure resilience, in the event of a failure or trip of either inverter the RDM system is programmed to step in and enable the compressors to start and stop under conventional “part wind start”, bypassing the tripped inverter.

It also monitors compressor run-time and automatically balances usage, ensuring wear and tear on both pieces of plant equipment is equal – extending the working life of the refrigeration system.

“After a few months of operation, the run times for the twin systems were within 0.2 of an hour – which equates to just 12 minutes. This is remarkable run-time matching, and as well as extending working life means that servicing and maintenance schedules on both systems remain in step,” said Mr Bickett.

Temp and humidity readout, the RDM control system was programmed to maintain even temperatures throughout the cold room space with the use of an anti-stratification PLC algorithm. This controls evaporator fans to ensure effective mixing of air in both horizontal and vertical planes, important for both efficiency and the maintenance of product quality.

Electronic expansion valves help efficiency further and assist in making maximum use of the floating head pressure design. When ambient temperatures fall, it means lower condensing pressures can be used, reducing electrical loads in cooler weather.

The RDM system gives the end user and contractor access to real-time and historical data on the cold store via a web browser on a PC, tablet or smartphone, and offers almost unlimited options to expand the system at a later date.

“Steve and the Personal Refrigeration team provided us with a flexible, accessible and efficient solution with professional support throughout the project. Their ‘can-do’ approach and specialist knowledge, particularly in relation to refrigeration and intelligent controls, have been invaluable.” - John Eastham, Sales Manager at Sharrocks