05 March 2020
Miller Food Service supplies ambient, chilled and frozen food to hospitality and leisure venues throughout Yorkshire. Responding to increased demand, it was decided to expand LT storage with the addition of a new freezer plant. Refrigeration specialist Personal Refrigeration designed and installed the new system, prioritising low energy consumption and running costs.
Instead of using standard technologies with only basic control options and intensive energy consumption, Personal Refrigeration developed a bespoke solution which reduced environmental impact and increased the site’s energy efficiency.
Energy efficiency and optimal control of plant equipment and defrosting formed the primary objectives. The aim was to reduce energy consumption and operating costs whilst providing advanced, environmentally-friendly solutions.
Standard technology has high energy consumption and only provides basic control. To avoid those pitfalls, custom technologies were adopted to provide variable speed compressor systems and advanced defrosting functionality. Inverter driven compressor PLC algorithms were developed to provide a custom isothermal, low-pressure hot gas defrosting system.
A dual three-compressor LT freezer pack system with inverter-driven lead compressor serves multi-evaporators in the LT area and a small HT area. Evaporators were selected and split over two packs for contingency in case of single point failure. Compressor loading matched real time demand condition to integrate the advanced tech designed in the solution. All electronic EEV, EPR and hot gas modulation valves were selected for efficient operation.
Environmental considerations included a multi-point refrigerant leak detection system and independent plant shutdown on leak condition. In addition, high-efficiency evaporator and condenser coil design featuring reduced refrigerant volume and EC fans were chosen to further decrease the environmental impact.
System design & benefits
All bespoke control algorithms were written in-house for the entire project utilising Resource Data Management’s (RDM) TDB software. Strategies include monitoring and control of compressor loading, including inverter-driven lead compressors with floating condensing and evaporating temperatures. The algorithm for monitoring and controlling LT defrost consists of isothermal low-pressure hot gas defrost with the ability to automatically change over to electric defrost if required or in case of hot gas failure. The option to drive an electronic hot gas regulation valve is also included.
The custom isothermal, low-pressure hot gas defrosting delivered exciting results as the advanced hot gas system overcame negative features associated with standard systems. Thermal shock from high-pressure changeover and the risk of liquid flood back were avoided because of the low-pressure operation of the defrosting phase. Modulation of the hot gas keeps hot gas pressure in the evaporator below saturation point for sensible heat to be used to achieve ice removal.
The residual heat algorithm for defrost heaters was optimised with LT electric defrost. Heaters can now be pulsed on or off as needed which reduces energy costs and helps prevent unnecessary heat overspill from each evaporator and ice formation on the ceiling. HT area cooling EEV and cooling cycle EEV are controlled via twin stepper controller with EEV driver. HT area EPR is also controlled via twin stepper controller with EPR driver with auto-float and auto-setpoint adjustment capabilities for varying loads included in the algorithm. If the load in the HT storage area increases, the evaporating temperature will be lowered automatically to control the additional load.
Depending on predicted coil frost algorithm, either “off cycle” or electric option will automatically be selected for HT defrosting. High-efficiency condensers and evaporators with reduced refrigerant volumes and EC condenser fans are also controlled by a bespoke PLC algorithm.
In addition, RDM’s front-end control and monitoring system, DMTouch, monitors energy usage. The device also predicts running costs based on consumption statistics to facilitate energy management. To help keep the system running smoothly, DMTouch provides an overview of all assets and alerts staff to potential problems.
System running costs can be proven, due to energy and site energy cost monitoring software installed on the DMTouch. Energy savings have been conservatively estimated to be 25-30%, based on the assumptions of compressor running times, with reduced system loadings and defrost costs.
Site Manager Gareth Wilson said: "The system installed here has been very reliable and has created a comfortable “frost free” and therefore safe environment for our staff.
"The control and automation side of the plant also means that it pretty much looks after itself.
Our due diligence is also looked after by the monitoring system provided. Electricity bills are also noticeably lower than we would have expected.”