A refrigerated container filled with 10,000 lbs. of strawberries has been loaded on your ship. The set point of the box is 35°F and the interior box temperature is 90°F, and after 7 hrs. 37 minutes the box is lowered to 60°F. Based on the following information, which of the listed steps should be taken? Specific heat of cargo is 0.94 BTU/LB/°F, heat gain by container equals 4,000 BTU/hr., and refrigeration system capacity equals 3.42 tons.
• Specific heat calculation for cargo: Q = weight × specific heat × temperature change • Conversion of tons of refrigeration to BTU/hr (1 ton = 12,000 BTU/hr) • Total heat load vs. system capacity: cargo cooling load + container heat gain
• First, compute how many BTUs must be removed from 10,000 lbs of strawberries to lower their temperature from 90°F to 60°F using the specific heat given. Then divide by the elapsed time to find the required BTU/hr. • Convert the system capacity from tons of refrigeration to BTU/hr and compare it with the total load (cargo cooling + container heat gain). Is the unit keeping up with or falling behind the required rate? • Think about whether the measured temperature drop after 7 hours 37 minutes matches what a properly operating system of that capacity should be able to do.
• Verify your BTU removal calculation: Q = (10,000 lb) × (0.94 BTU/lb°F) × (temperature drop). • Check your time conversion to hours for 7 hours 37 minutes and use it correctly when finding BTU/hr. • Confirm the refrigeration capacity in BTU/hr (3.42 tons × 12,000 BTU/hr per ton) and whether this is sufficient given the cargo load plus 4,000 BTU/hr heat gain.
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