Refrigeration problems in practice
Refrigeration problems in practice
Saturday, 11 April 2009
Steve James discusses examples of refrigeration problems we have encountered over the years.
Surveys of ready meal plants performing industrial scale cook-chill operations have highlighted a number of factors that impede the effective chilling of cooked products. Any delay and extension to the optimum cooling time, after cooking to the critical control temperature, will lead to shorter shelf life of the product and greater risk of bacterial growth to unacceptable levels.
A key factor when considering the performance of a blast chiller is its ability to reduce the air temperature and maintain it at the control point. The longer the room takes to pull-down to the pre-set air temperature, the greater the delay to maximum cooling rate and hence the longer the product cooling time. The figure below shows clearly the effect of loading warm product into a blast chiller with insufficient refrigeration capacity. The temperature of the air returning to the cooling coil reached 8.5°C after the first batch of hot sauce was loaded and peaked at over 11°C after the final load. Seven and a half hours after loading the air temperature had still not recovered to its set point. During the survey of one facility in approximately 40% of the chilling runs monitored, the blast chiller never managed to reduce the air temperature to its control point, indicating overloading/under sizing of refrigeration load capacity.
Air on and air off coil temperatures during cooling in blast chiller
A practice that must be avoided is the removal of product from the blast chillers before the specified minimum temperatures have been reached. Loading of the blast chiller with hot product whilst partially chilled products are already being chilled has also been identified as causing problems. Monitored temperatures clearly show a rise in the air temperature as the hot product is introduced, which in turn causes a rise in product temperatures. Failure to ensure that evaporator coils are effectively defrosted and all fan units are operational before chilling can drastically reduce the effectiveness of blast chillers. These problems are associated with incorrect operation of blast chillers and are not caused by incorrect design of the refrigeration system. These may all be obvious but have been observed on several occasions in more than one factory. Other problems include; incorrect thermostat settings resulting in too warm or even frozen product; insufficient airflow around all (or any) loaded products limiting effective heat transfer; deterioration in refrigeration performance due to lack of maintenance; leaving products too long in blast chillers designed to use sub-zero temperatures, resulting in partial product freezing.