Photo via : Michael Swan  In this "dissecting" series of posts we discuss every problem in the NCEES "Sample Questions + Solutions" book. The afternoon section problems in this book are numbered 501 through 540. This post is about problem 503 for the HVAC&R Depth (PM) exam. 
Description of Problem 503: The problem statement provides data (temperatures, cooling load, etc) for a simple vaporcompression refrigeration system. Additionally, a Ph 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 vaporcompression refrigeration cycle, and its graphical representation in a Ph 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 endofchapter homework problem found in the ASHRAE book “HVAC Simplified” by S. P. Kavanaugh). This problem requires knowledge of the vaporcompression 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 R22 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: (A) 39.1 (B) 42.3 (C) 50.1 (D) Cannot be determined with the information provided You may download the solution via the link below. Have any questions? Need help? Leave a comment!
