A centralized system, typically referred to as a “parallel rack,” consists of one or more racks housed in a mechanical room located in the back of a supermarket sized for the store’s entire refrigeration needs. Centrally located on the roof are usually large air-cooled condensers sized to match the requirements of the large rack systems. The racks provide the refrigeration to display cases and walk-in coolers throughout the store.
Various compressors can be used in the centralized systems including reciprocating, scrolls, open-drive and screw compressors. Refrigerant options typically used today are HFCs-404A, -507A, and -407A. There are a variety of other options available to storeowners and designers when it comes to centralized systems. With multiple components, manufacturers, optional features, and even custom configurations, it’s quite difficult to define a standard centralized system.
Distributed systems, on the other hand, are one of the newer commercially adopted refrigeration technologies. A distributed system consists of several miniature parallel compressor racks distributed throughout the store, located next to, or in close proximity to, their display case or walk-in cooler refrigeration loads. A typical traditional grocery store will have five to six units. Distributed systems come in a variety of sizes and options to meet the needs of large and small applications and their more standard configuration allows engineers, architects, and store planners a more consistent product to design.
Most distributed systems use quiet, low-vibration scroll compressors. The most common distributed systems consist of four to six compressors arranged either in a stacked configuration in a vertical cabinet or in-line in a horizontal configuration in a longer low-profile cabinet (Figure 1, page 18).
For air-cooled distributed systems, the remote condensers are typically located directly above the refrigeration unit on the store roof. With a water-cooled system, the heat exchanger/condenser is located inside the refrigeration system’s cabinet along with the compressors.
Distributed systems are typically used with a loop piping system. A loop piping layout uses fewer main liquid and suction lines that loop around the store with branch liquid and suction lines serving individual case line-ups (Figure 2, page 18).
Today, distributed systems utilize non-ozone depleting refrigerants, such as R-404A and R-407A. Distributed secondary systems are also available for medium-temperature display cases and walk-in coolers using a glycol mixture to remove the heat from the refrigerated display fixtures.
With many redundant components, centralized refrigeration systems provide reliable operation. Factory leak and electrical testing are generally completed to ensure that a system meets performance standards prior to its installation.
Similar to centralized systems, the multiplexed scroll compressors used in distributed systems provide efficient operation and back-up protection as well. With individual units responsible for specific case loads, a distributed system reduces the risk of shutting down large sections of the store’s refrigerated merchandise in the event of a major failure, remodel, or service situation. For example, a remodel in the dairy section of a store using distributed refrigeration technology will not affect the operation of the meat section allowing continued operation and no disruption to product sales. The smaller distributed systems can be easy to service and maintain. With controls, mechanical equipment and case electrical located in one cabinet, troubleshooting is simplified.
Refrigerant Charge Requirements
Centralized commercial refrigeration systems found in supermarkets have long piping lines connecting the display cases and walk-in coolers to the backroom rack system. These systems require several thousand pounds of refrigerant to charge and operate correctly.
Distributed systems reduce piping by 50 percent to 75 percent over centralized systems and require 60 percent to 80 percent less refrigerant to operate efficiently.
A recent Environmental Protection Agency GreenChill study found that the average leak rate for a supermarket is 15 percent to 30 percent annually. Retail stores can spend thousands of dollars checking and repairing these leaks.
With a centralized refrigeration system, the risk of refrigerant leaks can be relatively high. The systems run refrigerant through several thousand feet of copper tubing. When a leak does occur, these multiple long insulated lines can make it difficult to find the breach. In addition, the centralized systems use hundreds of brazed joints. Each joint is a potential leak point.
With shorter piping runs and fewer brazed joints, distributed systems provide less opportunity for piping failure, reducing leaks by as much as 50 percent to 75 percent.
While centralized refrigeration systems are fairly energy efficient, several features of the distributed systems can offer an advantage over the centralized technologies. With shorter piping runs, distributed technologies have less refrigerant in the system. They can also consume less energy due to more suction groups, less pressure drop in refrigerant lines, and better load matching capabilities from multiple compressors and multiple temperature groupings. The high-efficiency scroll compressors used with the distributed systems are one of the most energy-efficient options available today.
Installation + Maintenance
Installation of a centralized refrigeration system can be complicated. It takes more labor and materials, such as copper tubing for the long piping lines. A centralized system also requires a machine room or mezzanine location. If the equipment is placed on the roof, additional support steel is required.
Distributed systems take less time and material for installation. In addition, these systems do not require a centralized machine room for the equipment.
For servicing, centralized systems are located in an open, accessible area. Many food store owners and employees are comfortable with the centralized system technology and technicians are familiar with servicing the equipment.
Distributed systems are easy to service and maintain for any person familiar with parallel systems. With refrigeration cabinets located throughout the store, the technician can move around the store to the individual units to service them. The units might, at times, be located in tight spaces and require the service technician to crouch to service them. With proper store planning, this service concern can be eliminated.
Centralized systems will generally have lower first costs than distributed systems, but properly applied distributed systems can be equal to or even less expensive than a centralized system in addition to having a lower cost of ownership over the life of the equipment.
With less installation time required and less copper needed for piping, distributed systems can reduce installation costs by 17 percent over a centralized system. Better floating suctions and condensing pressure of the smaller, efficient compressors used with a distributed system can save retailers up to 15 percent in energy costs. Shorter piping runs, smaller refrigerant lines, and fewer leaks result in refrigerant savings. The energy efficiency, reduced refrigerant charge, and lower leak rates of the distributed systems also reduce the store’s carbon footprint.
Distributed systems are a cost-effective refrigeration option that helps answer today’s pending regulations. The systems also offer a solution to many customers participating in the EPA GreenChill program, a cooperative alliance with the supermarket industry. GreenChill works with supermarkets to reduce refrigerant emissions and decrease their impact on the ozone layer and climate change, and provides information and assistance to help them adopt alternate refrigeration technologies, strategies, and practices. Distributed systems can help stores meet the EPA GreenChill objectives and achieve GreenChill certification.
The refrigeration system that’s best depends on the application and an individual store’s needs. Centralized systems are reliable, flexible, and have a lower first cost. However, when comparing energy efficiency, refrigerant loads, leak risks, ease of installation, and total cost of ownership, distributed systems offer advantages over the centralized systems.
Publication date: 09/05/2011