Secondary Loop Details
One reason for looking at alternatives to conventional direct expansion (DX) systems is to reduce refrigerant and leaks.
“There are approximately 35,000 supermarkets in the United States,” the report said. “A single store may contain up to 5,000 pounds of refrigerant (in a direct expansion system) to cool and freeze food and may experience leak rates of up to 30 percent.
“In addition to harming the environment, leaks negatively impact the profitability of supermarkets, as it is costly to clean systems and replace lost refrigerant. Environmental damage and high refrigerant costs are the primary drivers for the increased interest in alternative approaches to the traditional direct expansion system for supermarket refrigeration.”
The fact that store decision makers can consider secondary loop versus traditional DX is that the opportunities are there.
“The number of new and remodeled supermarkets in the United States is being driven by consumer trends toward increased freshness and convenience,” the report said.
“Various industry sources show a typical remodel cycle of seven to 10 years. Over one-half the store remodels were as a result of company policy.”
According to the report, key issues driving the upgrades include:
• Increasing concern about product safety;
• Increasing concern about environmental issues;
• Increasing interest in improved energy efficiency;
• Governmental regulations, both federal and state;
• Total cost of ownership/cost efficiency;
• Opportunities for strategic differentiation.
As the report noted, “All of these industry drivers are driving supermarkets toward new alternatives for refrigeration systems.”
The report said, “The ideal solution is an efficient design, which minimizes the amount of refrigerant required and, at the same time, protects the environment by reducing the number of refrigerant leaks.
“Of all the possible alternatives, a secondary loop system is deemed the optimal solution to address all of these issues.
BASIC APPROACHThe white paper provides details on both the basic secondary refrigeration system and a two-phase variation. Here’s the technical details given for the more basic approach:
“A secondary loop refrigeration system incorporates two different refrigerants to provide cooling. In all systems, the primary loop is a traditional direct expansion design that uses a phase change refrigerant such as R-404A and a compressor to circulate the refrigerant.
“This primary fluid is typically restricted to the machine room. A heat exchanger is used to transfer energy from the primary loop to the secondary loop. In most applications, the refrigerant in the secondary loop is a benign single-phase fluid that is circulated by a pump throughout the store to the individual evaporators.
“Propylene glycol has proven to be the most suitable secondary fluid as it is nontoxic, nonflammable, does not contribute to global warming, and provides optimal performance compared to other secondary fluid alternatives.
“The secondary fluid absorbs energy by increasing in temperature as it passes through refrigerated cases and unit coolers in the refrigerated spaces.
“One of the fastest growing applications of a secondary loop system is on the evaporator side of the refrigeration system. This design is often applied in a supermarket but could just as easily be applied to a storage warehouse.
“This application is nearly identical to a chilled-water loop used in comfort cooling, except the system is designed for lower evaporator temperatures and uses a water and glycol mixture instead of just water.”
TWO-PHASE SECONDARY LOOPThe white paper described a two-phase secondary loop as a “variant, which uses a two-phase mixture as the refrigerant.
“This variant takes advantage of the high latent heat required to change the phase of the refrigerant from liquid to solid. The higher the percentage of refrigerant undergoing a phase change, the higher the energy stored per pound of refrigerant.
“This reduces the flow rate required for the same cooling capacity. The concentrations of the fluids are closely regulated so as to operate at a point of minimum flow resistance. The combination of reduced flow and low flow resistance combine to reduce the power required for circulation.
“One example is an ice slurry, which consists of water and ice and is close to the consistency of slush. A type of antifreeze, such as glycol or ethanol, is included when the fluid temperature is lower than 32°F.
“In this design, the heat exchanger is modified so that it will charge a portion of the secondary coolant from liquid water to ice. This solution is either piped to storage tank for thermal storage, or pumped directly to the evaporators. This piping is also modified so the individual evaporators can be piped in series or parallel.
“A unique advantage of ice slurry systems is their surprisingly low pumping requirement. The flow resistance occurs at the boundary of the ice slurry and the inside wall of the pipe.
“At this interface, the pipe temperature is slightly warmer than the fluid, but sufficiently warm to melt a thin layer of ice adjacent to the pipe. This thin film of fluid acts as a lubricant, allowing the ice slurry to flow with lower power and a smaller pump.”
For more information, visit www.thecoldstandard.com/bohnwhitepaper.
Publication date: 03/10/2008