Open display cases at low temperatures are more sensitive to the store's environment. Their evaporator coils tend to freeze up more quickly, requiring longer and more frequent defrosting. Because of this, reach-in glass door freezers are more often chosen to display frozen food products since they are more reliable and less troublesome at low temperatures.
Although these cases are less prone to problems with fluctuations in the store's environment, they are still designed to work in a conditioned environment. The store's environment should not exceed a dry-bulb temperature of 75 degrees F and a relative humidity of 55 percent. Cases subjected to higher conditions may develop frosting and icing problems in the case, on its doors, and on the product.
It is important that any supermarket have an adequate HVAC system to keep the store cool and dry. It is much less expensive for a supermarket to dehumidify with an air conditioning or desiccant system, than having it done by the display cases.
Air PatternsEven though these cases have glass doors to seal the case from the store's environment, there is still a definite air pattern set up within them. The evaporator coil and its fans are located at the bottom of the cabinet with the fans located in front of the coil.
The fans force air through the evaporator coil, up a channel at the rear and top of the case, through a sets of honeycombs located on the top front of the case, and down the front of the case's shelving.
The honeycombs are designed to even out the airflow and develop a uniform air pattern down the front of the case. The velocity of this air curtain is approximately 350 to 450 feet per minute. Always refer to the manufacturer's literature for the design velocities, as velocities may differ from manufacturer to manufacturer, and from case to case.
Supermarket engineers typically control the case temperature by monitoring the evaporator's discharge air temperature. The discharge temperature usually operates in a range of 0 degrees to -10 degrees (-5 degrees to -15 degrees for ice cream applications). Engineers will use either mechanical or electronic temperature controls to regulate the case temperatures, but electronic controllers are more often used today. Temperature controls are wired to control either a liquid line or suction line solenoid, depending on the engineer's design.
Design engineers will also monitor the discharge air temperature to set off an alarm if the temperature exceeds a predetermined temperature for a specific delay period. The alarm can be configured as either a local or remote alarm. A local alarm sets off some type of light or buzzer in the store to notify personnel of a problem. A remote alarm notifies someone - such as a facility manager or refrigeration contractor - of problems.
Joe Marchese is owner of Coldtronics, Pittsburgh. He can be reached at 412-734-4433, www.coldtronics.com, or firstname.lastname@example.org.
Publication date: 09/06/2004