Door-opening task shows humidity issue is relative

July 19, 2000
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WASHINGTON, DC — Here’s a job for you: Open a reach-in case door for 16 seconds, every 4 minutes, for 8 hours.

Why, you may ask, would one want to do that? The answer: To build a better mousetrap ... er ... reach-in.

That was one of the jobs in a research project designed to test refrigerated display cases at various ambient relative humidity levels. Two of the researchers presented their findings to an audience of supermarket engineers and technicians at the Food Marketing Institute Energy & Technical Services Conference, held here recently.

Richard Sweetser of Exergy Partners Corp. noted testing was done on a five-deck dairy case, a three-deck meat case, a five-deck reach-in freezer, and a narrow island frozen-food unit. According to Sweetser, critical test points included indoor rh, indoor drybulb temperature, and suction pressure.

Even though the units were modern models, researcher Ramin Faramarzi of Southern California Edison’s Refrigeration Technology & Test Center noted problems, including:

  • “Frost formation on the evaporator adversely affected the cooling capacity of the fixture due to increased pressure drop across the coil, which reduces airflow and reduces heat exchange effectiveness of the coil, which increases the coil TD.”

  • “Additional refrigeration energy is required to offset the effects of frost.”

  • “Frost growth is directly proportional to indoor air rh.”

A key solution suggested was the lowering of store relative humidity down to 35% from current 45 to 55% levels.

Using a simplified utility pricing structure for statistical purposes, Sweetser said, “Maintaining 35% rh in case aisles will result in between $900 and $5,000 in annual refrigeration savings due to restricted open-case coil loading.

“Further, controlling anti-condensate heaters could yield between $7,300 and $11,700 in additional annual savings, or shutting down anti-condensate heaters could yield between $8,250 and $15,000 in annual savings.”

He noted side benefits of reducing defrost cycles are reduced thermal shock on products and longer shelf life.

One other benefit mentioned throughout the presentation was a reduction in the time it takes for fog to clear out of a display case after a door has been open. The quicker it clears, the quicker products will come into the view of the next wave of shoppers down the aisle and the quicker those customers will pick a product to buy.

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