Considerations in Refrigeration Systems
Refrigerants, Design, Commissioning Come into Play
No sector of refrigeration has been more cutting edge than the supermarket sector. At the most recent Food Marketing Institute Energy & Store Development Conference, papers and presentations focused on these trends. Nearly all trends were actually being used in some supermarkets today and all were expected to gain increasing attention as the sector moves away from high-global warming potential (GWP) refrigerants and toward even more efficient equipment.
This is the first of a two-part series looking at the most recent developments in detail.
Tim Anderson, principal engineer for Hussmann Corp., looked at the type of equipment that is being used with refrigerants that have low or no GWP.
“The goal is to compare and contrast HFC, CO2, glycol, and propane refrigeration systems by explaining from an equipment perspective the strength and weaknesses of each system and how supermarket operators can determine which system is right for their company and culture.”
He said there are three main refrigerant choices: Synthetic HFCs such as -134a and -407A; synthetic blends such as HFO-1234yf and HFO-1234ze; and naturals like R-744 (CO2), R-290 (propane), R-600a (isobutene), and R-717 (ammonia).
“There is no perfect solution,” he said. “Improving one thing always causes something else to get worse. Example: Lowering the environmental impact often means the blend will be flammable.” And, he noted, “The refrigerant choice cannot be separated from the system choice. For example, propane is not compatible with, and A2Ls (slightly flammable refrigerants), are not likely for centralized parallel direct expansion systems.”
He did stress that “the overriding issue” is flammability in noting currently five classifications: Class 1 with no flame propagation consisting of many existing HFCs and blends and CO2; Class 2L with ‘low flammability’ such as HFO-1234yf and R-32; Class 2 as flammable as with R-152a; and Class 3 as highly flammable with examples like hydrocarbons (HCs) such as R-290 and R-600a.
With those refrigerant options have come a range of equipment configurations. Anderson listed those as centralized parallel rack with R-404A; distributed system; secondary CO2 LT and glycol MT; cascade DX CO2 LT and secondary CO2 MT; transcritical CO2 MT and cascade DX CO2 LT; and micro-distributed systems that are water cooled.
Designing a System
With all the options available in refrigerants and systems, the selection process is next. Rob Arthur, an engineer with CTA Architects Engineers, looked at refrigeration system selection. That includes, he said, the in-house engineering team, the equipment manufacturer, and a consulting engineer.
The process involves, he said, “Building time into the schedule for full development, bringing all stake holders together, and utilizing refrigeration consultants as a hub that receives, distributes, and coordinates with all stakeholders.”
Also, he said, consideration is given to equipment selection. “It is best to know whose equipment you will be designing around from the beginning” and that means factoring in “space planning, electrical loads, Btu loads, and component selection.” One additional note: “Have refrigerated case specifications figured out. If equipment is bid after design, expect some redesign.”
What he called “inputs” can include typical refrigeration equipment costs, typical maintenance costs, general agreed upon energy differences between system types, industry average refrigerant costs, and industry average leak rates, as well as typical refrigerant charge amounts.
And a word of caution: “Don’t believe advanced systems of the future will look like current advanced refrigeration systems of today.”
Commissioning Refrigeration Systems
The topic of commissioning refrigeration systems from design through operations was discussed by Paul Torcellini of the National Renewable Energy Laboratory, Caleb Nelson of CTA Architects Engineers, and Neil Monson of Target Corp.
Said Torcellini, “Commissioning in existing grocery stores could result in seven to 25 percent energy savings per year. Commissioning is crucial to proper refrigeration system design, installation, and operation.”
Commissioning is needed, he said, “to mitigate change and management risk; document, measure, manage and adjust; give quick visibility to issues; define owner’s project requirements; define basis of system design; and reconcile construction documents to project requirements.”
Nelson asked, “Has the system been designed as required?” He dwelt on the Cx (commissioning) plan and team. “There are a variety of ways to execute projects. There’s flexibility, but there must be an expectation set.”
Monson looked at the process from the perspective of training in such areas as condenser and rooftop piping, refrigeration systems and mechanical rooms, hangers and piping, walk-ins, cases, controls, and startup. He also touched on the topic of retro-commissioning for stores that might be operating poorly or have never been commissioned.