A refrigeration system for freezing freshly-baked bread is under design. The system must be able to continuously cool N = 4,250 loaves of bread per 8-hour working day from T1 = 25°C (the ambient temperature) to T2 = -10°C. Tests show that the average bread loaf has a mass of m = 450 g and a specific heat capacity of c = 2.93 kJ/kg-K. In addition, when the bread is cooled below Tf = 0°C, the water in the bread freezes releasing an additional delta Hfs = 109.3 kJ per loaf. The proposed design circulates air past cooling coils that cools the air to Tair = -20°C. The cold air is then circulated past the bread and returned to the cooling coils.
a.) As the engineer in charge of the project, determine the required steady rate of cooling capacity for this process.
b.) To maintain consistent product, the change in temperature of the air blown over the bread must be less than delta Tair = 8°C. Determine the minimum required volumetric flow rate of air (assume that the air is at atmospheric pressure, Patm = 100 kPa).
c.) A refrigeration contractor has indicated that they can supply refrigeration equipment for the bread cooling project that has a COP = 2.1. Note that COP is defined as the ratio of the refrigeration provided to the power required. Determine the electrical service that will be needed for the motor used to operate the cycle.
d.) Compare the COP with the maximum possible COP for this application.