During that general session, there were talks of refrigeration innovations and minimizing refrigerant charges in supermarkets.
INNOVATIONSScott Moore, senior engineer for PECI, noted some of the recent technologies included improved case efficiencies, a new generation of electronic expansion valves, and pre-cooling condensers.
With regards to case efficiency, Moore noted improved evaporator coils and air flow management. Coil design is resulting in increased heat transfer, integral sub-cooling passes, and the use of modular coils. For case air flow, there are improved air curtain design, lower back air plenums, and smoother air flow throughout. The net result, he said, is increased evaporator pressure requirements, reduced Btu requirements, and increased energy efficiency ratio.
Regarding EEVs he said, “The new generations are superior to original technology and they are less expensive, compact, more responsive, with lower superheat control, and they will optimize the total rack operation.”
He said they have benefits over thermostatic expansion valves, such as “larger modulation range, reduce time for superheat adjustment, optimization of rack suction pressure set point, and allowing lower operating head pressure.” EEV applications, he said, include, systems with a broad range of operating head pressures, dual temperature case systems, circuits with seasonal performance issues, mechanical sub-cooling circuits and circuits requiring precise temperature.
He described pre-cooling condenser innovations as, “Air cooled condensers that pre-cool ambient air via an adiabatic or evaporative process before entering the condenser. It performs like an air-cooling condenser during low ambient and performs like an evaporative condenser during high ambient. The water spray or flow strategy is based on ambient or condensing conditions.”
Benefits included, he said, energy savings and lower horsepower compressor requirements. Issues, he said, relate to water costs, media/nozzle/pump maintenance, possible corrosion of fins, bleeding/draining in winter, and airside pressure drop.
MINIMIZINGClay Rohrer, refrigeration system product manager for Hussmann Corp., looked at ways to minimize refrigerant charge in supermarkets.
One option was secondary glycol systems such as a central secondary system for medium temperature applications. In such situations, the secondary system uses two fluids - the chiller using a typical HFC refrigerant to cool the secondary fluid and the secondary fluid typically a 30 to 35 percent propylene glycol solution.
“This approach reduces refrigerant charge by 40 to 70 percent depending on engineering and layout. It is important to focus on the careful application of heat reclaim and minimizing condenser flooding charge to obtain the lowest charge,” he said.
He said such systems have leak rates typically below 10 percent, but that they can “consume 12 to 17 percent more energy than direct expansion.”
Another option, he said, is distributed secondary for medium temp. He said the set-up includes distributed systems plus a medium temp secondary chiller equaling distributed secondary.
“It reduces per unit charge as low as 40 pounds of refrigerant when water cooled and as low as 120 pounds of refrigerant when air cooled with advanced condensers,” he said. He said it is important to “optimize energy by matching the secondary fluid temperature to the display case requirements.” Such an approach “reduces installation costs by eliminating the need of a mechanical room.”
A third method is liquid secondary CO2 in low and medium usage. The primary (highside) system uses an HFC to cool the liquid CO2. The CO2 is pumped to the case similar to glycol. It reduces per unit refrigerant charge as low as 250 pounds when air cooled with advanced condensers, he said.
“CO2 does have a higher volumetric efficiency compared to HFC-404A which reduces pump power, refrigerant line sizes and CO2 required. Liquid CO2 typically operates at 2°F higher evaporator temperature while reducing parasitic losses - resulting in 5 percent more energy consumption than DX due to matching the lowest display case temperature requirements.”
A fourth approach involves a distributed system for medium and low temp applications. He said, “Distributed systems use typically HFC refrigerants for the entire application and reduces per unit refrigerant charge as low as 250 pounds when air cooled with advanced condensers.” He added, “Close coupling the system with the display cases reduce charge and leak potential. Leak rates are typically below 10 percent. Water-cooled applications further reduce charge by as low as 100 to 120 pounds per unit.”
A fifth way to minimize refrigerant charge is to use single compressor units for medium and low temperature applications. Here “single compressor units use typical HFC refrigerants at the display case with heat rejected to a water loop and fluid cooler.” Rohrer said the method typically reduces the per unit refrigerant charge as low as 4 pounds and “optimizes energy by specially matching load requirements and reducing parasitic losses.”
Further, he said, it “cuts installation time by more than half while being easily removable.”
A final method Rohrer described for reducing refrigerant charge is by employing micro-channel condenser technology which he said “reduces flooding refrigerant charge up to 55 to 75 percent typically. The all aluminum construction with micro-ports provides enhanced air side heat transfer. They are 40 percent lighter than typical coils. All joints are brazed simultaneously in a brazing oven.”