is that the bottom shelves are controlled to maintain the actual product temperature by cycling the shelf flow regulator in a traditional case the case enters defrost and stops defrosting as one unit all coils and refrigeration devices enter defrost at the same time when this happens the case temperature and product temperature rises until the defrost cycle has ended then the product temperature and case temperature is pulled down to the level of normal operation this momentary rise in product temperature two three or four times a day can directly affect the product life color and bacterial growth if this product rise happens to often it can cause a real food safety issue in the case with the design of the present invention one can defrost the top coil while still refrigerating the bottom pans next the pans can be defrosted will the top coil is still refrigerating by defrosting these separately in this fashion the product is always being cooled by one device while the frost level is being reduced on the other reducing the frost level is a must in all refrigerated applications in order to maintain case performance and cooling capacity since the product is always receiving cooling effect from one device the product temperature change during a defrost cycle is very minimal in addition to cycling defrost at different times the defrost times and duration can vary in this case the refrigerated shelves or pans are not as affected by frost as the top coil is therefore the top.