Analysis of Problem 503: This is a "slam dunk" problem as long as you are aware that the speed of sound is the square root of the product of the particular gas constant, the specific heat ratio, and the absolute temperature. This relationship between speed of sound in an ideal gas and the temperature of the gas is in every Fluid Mechanics and Thermodynamics textbook.

To test your understanding of this subject, consider the following sample problem, which is just slightly more challenging than problem 503. This sample problem is a variant of an end-of-chapter homework problem from  "Fundamentals of Fluid Mechanics" 6th edition, by Munson, et al.

A military jet flies at a speed of 495 miles/hour at an altitude of 49,000 feet. A commercial airplane is flying at the same Mach number as the military jet but at an altitude of 27,000 feet. A plot showing the variation of temperature with altitude in the atmosphere is provided for your potential use.

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 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.

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 problems for the afternoon section in the NCEES "Sample Questions + Solutions" book are numbered 501 through 540. In this post we provide an analysis of Problem 502 of the Thermal and Fluid Systems depth (PM) section.

Description: The statement for Problem 502 provides a mass flow rate in a pipe and asks for a cross sectional area required to keep the flow  velocity within a given value.

Analysis: Problem 502  is not  complicated if you understand the relationship between mass flow rate and flow velocity. This relationship involves the fluid's density. The only  complication in this particular problem is possibly not being versed well enough in the use of  steam tables to obtain the density.

Here's a similar problem that tests this concept, to give you  additional practice:

A steam condenser operating at steady state admits steam at 5.0 psia with 50°F of superheat. At the discharge, the flow rate of saturated liquid is 400 gpm. The flow velocity at the inlet is 50 times greater than at the outlet...