History of multiplex parallel compressor systems
In the past individual or single compressor systems were used for most medium and low temperature applications, however with continuous refrigeration technology development, multiple or parallel refrigeration systems were introduced into the industry. Parallel Compressor systems are central refrigeration units utilising 3 to 6 parallel commonly piped compressors, single common oil separator, an electrical control panel, and a liquid receiver tank mounted on a remote base frame.
The ‘multiplex’ (multiple parallel compressor rack) system may be designed for either indoor or outdoor use. The overall selection and design of the multiplex is based on the requirements and limitations of each individual commercial refrigeration specifications. The most important determining factor for the multiplex specification is the proper selection of the system components for the particular application.
Applications in the commercial refrigeration industry are mainly focussed on the cold chain used to refrigerate perishable consumables, produce, and products that are required to be stored at different temperature ranges, these include supermarkets, abattoirs, agricultural sector, cold storage facilities, and freezer warehouses.
Design philosophy and engineering methodology for the new Metraclark multiplex range
Metraclark’s engineering department investigated the base design of the previous Tsunami multiple parallel semi-hermetic compressor rack range that assisted the creation of the design philosophy for the new Metraclark multiplex range.
The design philosophy was set to create a standardised modular design, that maintains simplicity, reduces manufacturing time and manufacturing complexity, increases ease of installation and maintenance, and increases overall efficiency of the refrigeration system. Furthermore, the design philosophy of the new multiplex frame was set to create a complete in-board design of all components fitted to a multiple parallel compressor rack.
By creating an in-boards design it becomes possible to build a single frame for both indoor and outdoor application with the addition of cladding. To start the process of designing a new multiplex frame, an investigation into thermodynamic first order principle design sizing and selection of all of the components was performed to ensure proper sizing of refrigeration line components (individual and common check valves, individual and common discharge lines, oil separator, and individual and common suction lines) used in manufacturing a parallel compressor rack for a Freon commercial refrigeration system. This directly ties into the design process of a homogenised multiplex design to reduce manufacturing time and costs.
Design summary of the new Metraclark multiplex range
The overall design of the multiplex frame is directed by the layout between each individual compressor and its connection in parallel, thus the individual discharge and suction lines are the pivot points to create a modular homogenised design. Therefore, the frame has been moulded around the Copeland semi-hermetic compressors and a fixed conceptual 3D volume. To be able to accommodate all Copeland D3, 4M, and 6M compressors the 3D volume needs to be based on the largest Copeland semi-hermetic compressor, i.e. the 6MU-40X.
The design project aimed to develop a computer-based 3D model of all iterations of the new multiplex on offer at Metraclark, including 2D manufacturing and assembly drawings of the entire mechanical and electrical multiplex design. The development of the design model required thermodynamic first order calculations to determine the correct size selection of individual discharge and suction line components for the 6MU-40X first, and then the rest of the semi-hermetic compressor range allocated to multiple compressor applications. The individual discharge components include: a vibration eliminator, and a non-return check valve. The individual suction line components include: an isolating ball valve, a replaceable filter drier, and a vibration eliminator. The isolating ball valve facilitates individual compressor shut-off for maintenance purposes, and the non-return check valve protects the head of the compressor during non-running times. After the completion of the individual compressor sub-assembly, the design of the frame becomes indicative.
The new semi-hermetic multiplex utilises a centralised bent profile member that serves as:
• Mounting point for the suction lines.
• Bracket for the pressure switches, gauges, and transducers.
• Electrical kopex connection and wire trunking.
• Oil return header mount.
The centralised main profile shapes the overall frame configuration, and with small alteration it can facilitate both possibilities of standardised and bespoke multiplex designs, resulting in the range of 3 to 6 Copeland semi-hermetic compressors in single and dual suction circuit applications. Furthermore, the new multiplex incorporates a high oil return efficiency helical oil separator coupled with a low pressure oil management system. The low pressure oil management system makes use of full column gravitational flow from reservoir, through the common 5/8” oil return header, to each individual compressor’s oil level management system (mechanical or electronic OLM).
The design of the electrical enclosure and electrical switchgear layout flowed from the modular compressor and frame design. The new electrical enclosure now has a full left to right switchgear access philosophy, which gives the possibility for the switchgear to be divided into modular individual compressor compartments. Further improvements to the electrical wiring schematic was to designate a specific letter and number combination to the control wires of each compressor and the main control circuit.
The final aspect of the structural frame design required first order stress and strain calculation verification together with FEA validation to ensure that the stresses exerted on to the multiplex frame during manufacturing, transport, installation, and final standard steady state operation does not cause any mechanical or electrical failure.
New Metraclark multiplex name and model reference
The new Copeland semi-hermetic multiplex represents the flagship of Freon commercial refrigeration products that Metraclark offers in the market. Therefore, with a new design identity, a new name and new method of distinguishing between different multiplexes is required. The name for the new Copeland semi-hermetic multiplex is a naturally occurring black volcanic glass, which is produced when felsic lava cools rapidly, known as Obsidian. Together with the new name of the multiplex comes new short code model reference numbers to distinguishing between different multiplexes.
Illustrated in the figure on the left, is the breakdown of the model reference number.
Obsidian Metraclark multiplex 3D model rendering and first installation
The new Obsidian Copeland semi-hermetic multiplex was modelled using the computer aided design software SolidWorks, and rendered on SolidWorks Visualise.
The illustration below shows a rendering of the MPMT-4TL060X-ZN multiplex that would be installed by DTS at Checkers Riverside Mall in Nelspruit, Mpumalanga. The multiplexes for Checkers Riverside Mall have been successfully installed and operating to the end-user’s satisfaction.