Description of Problem 503: The problem statement provides data (temperatures, cooling load, etc) for a simple vapor-compression refrigeration system. Additionally, a P-h diagram for the refrigerant is given. You are asked to calculate the power requirement for the compressor.
Analysis of Problem 503: This problem is very straightforward – as long as you are intimately familiar with the thermodynamics of the vapor-compression refrigeration cycle, and its graphical representation in a P-h diagram. You have to be able to quickly draw the compression process in the diagram, and then read the associated enthalpy change.
Here is a problem we have crafted (inspired in part by an end-of-chapter homework problem found in the ASHRAE book “HVAC Simplified” by S. P. Kavanaugh). This problem requires knowledge of the vapor-compression refrigeration cycle.
HVAC&R Depth (PM) Exam Sample Problem #3
The manufacturer table shown below present gross compressor refrigeration capacity (qr) in thousands of Btu/hr (MBtu/hr), the required input power (kW), and the refrigerant mass flow rate (mr) for a hermetic scroll compressor with R-22 refrigerant.
A refrigeration system with an evaporating temperature of 40°F and condensing temperature of 120°F uses this compressor. Additionally, there is a 20°F compressor suction superheat and a 15°F liquid subcooling at the condenser discharge...
The heat rejected at the condenser (MBtu/hr) is most nearly:
(D) Cannot be determined with the information provided
You may download the solution via the link below.
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