Among the many configurations of refrigeration systems available to supermarket customers, distributed refrigeration systems have become an increasingly popular choice. That means you’ll be encountering them in the field more often.
Before you begin servicing these units, it’s important to understand why customers choose them. These typically compact units offer a number of advantages over more traditional approaches, such as small single condensing units or large centralized parallel-rack systems. As typically smaller versions of the rack systems employed in machine houses and motor rooms, distributed systems are similar in their operation to rack systems, but for many customers they offer several distinct advantages.
As pointed out in "Distributed Refrigeration: The Next Wave?" (Contracting Business.com, June 2009, http://bit.ly/distributedrefrig) when compared to rack systems, distributed refrigeration systems are generally better able to match loads and provide improved energy performance with less refrigerant charge. At the same time, they usually offer some simplified installation aspects, such as shorter piping and wiring runs. Another key factor for many customers is that they also potentially require less maintenance.
Distributed systems come in just about as many variations as do centralized systems. The main difference between distributed and traditional systems is that with distributed systems typically two or more individual units, each contained in its own enclosure, will be used for a store instead of one larger centralized system located in a machine house or motor room.
On the face of it, it would seem that the more units there are, the more maintenance that will be required. However, because distributed systems are more closely located to their loads, often on the roof directly overhead or indoors on top of walk-ins (and practically anywhere else in the store that is convenient, such as storage and prep areas), there’s less piping, less wiring, and less refrigerant to deal with for each unit compared to a single, large centralized system.
Consequently, although overall the combined units may actually contain more compressors, the maintenance and repair requirements for these systems can be less demanding for each unit since there are fewer and smaller compressors and coils per unit. In addition, they have smaller piping circuits to contend with when, for instance, finding leaks on direct-expansion (DX) versions of the systems. Also, because the compressors in each unit are closely matched to their loads, compressor cycling is reduced, which can extend the life of the compressors.
Secondary versions of the systems, whether medium-temperature glycol or low-temperature CO2, offer the same advantages, although by their nature leak detection is not as big an issue. Some distributed systems even combine both approaches, allowing customers to reach levels of refrigerant reduction considerably beyond the capabilities of traditional centralized DX systems.
All of these advantages notwithstanding, technicians may find that working on distributed systems is more demanding than working on centralized systems.
The enclosures that the systems are usually contained in can make reaching some components difficult, and it may actually help for a technician to be on the smaller side physically when attempting to work in some of the enclosures in which the systems are contained.
For versions of these systems that are mounted outside on a store’s roof, it’s generally recommended that customers increase the frequency of inspections and preventive maintenances (PMs). It’s also a good idea to check their operation quarterly or at least biannually. Because of the outdoor location of many of these systems, during PM inspections it’s important to check for line abrasions and rub-throughs. Penetrations should also be checked to ensure that they haven’t degraded due to UV exposure.
One other consideration when working on outdoor units is to remember that equipment such as electronic control boards and 208/460V control panels should never be exposed to moisture. Take precautions to keep these components dry when working on them in the rain or snow.
Otherwise, any regular PM that would be performed on a conventional system should also be done on distributed system. Filters on the rack and honeycombs on the cases should be kept clean. Solution concentrations on medium-temperature secondary systems should be checked the same as if they were conventional systems. Compressor operation likewise should be checked for proper loading and unloading, and valves, including thermostatic expansion valves, should be checked for proper operation.
Other items to pay careful attention to during PMs on distributed systems include all electrical contactors and components to ensure that all connections are tight. Over time these connections can become loose due to vibrations. It’s also advisable to leak-check each rack and all systems. On compressor PMs, check the superheat and discharge temperature, and perform an oil test — as should be done with any type of system.
Finally, grit your teeth and dive in. Some technicians simply would rather not have to work on the units that are out in the elements when they could be inside in a dry and comparatively comfortable motor room. But then again, nobody becomes a refrigeration technician for the attractive work conditions!
Bill Katz is the technical writer and course developer, Hill PHOENIX Learning Center, Conyers, GA. He can be reached at 718/324-2231 or by e-mail at [email protected]. Also contributing to this article were Norm Tyche and Rusty Walker, Hill PHOENIX; and Charlie Meisinger, Mike Duffy, and Jim Kirk, AAA Refrigeration Service Inc., with locations in New York City; Plainville, CT; and Montville, NJ.