A refrigeration system is to cool bread loaves with an average mass of 350 g from 30 to −10°C at a rate of 1200 loaves per hour with refrigerated air at −30°C. Taking the average specific and latent heats of bread to be 2.93 kJ/kg·°C and 109.3 kJ/kg, respectively, determine (a) the rate of heat removal from the breads, in kJ/h; (b) the required volume flow rate of air, in m3/h, if the temperature rise of air is not to exceed 8°C; and (c) the size of the compressor of the refrigeration system, in kW, for a COP of 1.2 for the refrigeration system.
(a)
The rate of heat removal from the breads.
Answer to Problem 130RP
The rate of heat removal from the breads is
Explanation of Solution
Determine the mass flow rate of the bread.
Determine the average temperature of bread.
Here, the initial temperature of the bread is
Determine the rate of removal of heat from the breads.
Here, the mass flow rate of breads is
Determine the rate of cooling of bread.
Here, the latent enthalpy of the bread is
Determine the total rate of heat removal from bread.
Conclusion:
Substitute
Substitute
From the Table A-1, “Molar mass, gas constant, and critical-point properties” to obtain value of gas constant of air as
Substitute
Substitute
Substitute
Thus, the rate of heat removal from the breads is
(b)
The required volume flow rate of air.
Answer to Problem 130RP
The required volume flow rate of air is
Explanation of Solution
Determine the mass flow rate of air.
Here, the rate of heat removal from air is
Determine the density of the air.
Here, the pressure in the air is
Determine the volume flow rate of air.
Conclusion:
From the Table A-2, “Ideal-gas specific heats of various common gases” to obtain value of specific heat of pressure of air at approx.
Substitute
Substitute 101.3 kPa for
Substitute
Thus, the required volume flow rate of air is
(c)
The size of the compressor of the refrigeration system.
Answer to Problem 130RP
The size of the compressor of the refrigeration system is
Explanation of Solution
Determine the size of the compressor of the refrigeration system.
Conclusion:
Substitute
Thus, the size of the compressor of the refrigeration system is
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Chapter 6 Solutions
Thermodynamics: An Engineering Approach
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