Problem 504 is virtually the same as Problem 502: You are given a flow velocity and mass flow rate, and are asked to calculate the flow cross-sectional area. In Problem 502 the substance was steam and you had to use the property tables to obtain the density. Here in Problem 504, however, the fluid is air so you use the ideal gas equation of state to find the density.

Here is a problem that can be found as a solved example in the Thermodynamics textbook by Cengel and Boles. It is slightly more challenging than problem 504 and gives you additional insight/practice with the ideal gas equation.

Air at 10°C and 80 kPa enters the diffuser of a jet engine steadily with a velocity of 200 m/s. The inlet area of the diffuser is 0.4 squared meters. The air leaves the diffuser with a velocity that is very small compared with the inlet velocity...

Analysis of Problem 504 This kind of problem is straightforward as long as you are aware of the relationship describing the variation of the tension in a cord or rope wrapped around a cylindrical surface due to friction. What makes this problem complicated is that, chances are, the last time you saw this relationship was in your sophomore year in engineering school when taking engineering statics. 

Here's a similar problem dealing with friction that is a little more challenging than Problem 504, yet it is not harder than anything you did in school:

The following experiment has been devised to determine the coefficient of friction between a cord and a steel cylinder of 0.2 m in diameter:

The steel cylinder is fixed and does not rotate. A small force is applied at end  of the cord. This force is slowly increased until the block is set in motion (sliding horizontally). The experiment was performed maintaining a constant angle of contact, ϕ=90°, between the cord and the cylinder. The force required to initiate movement of the block was measured to be 65 N. The mass of the block is 20 kg, and the static coefficient of friction between the block and the horizontal surface on which it slides is 0.25...

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 504 for the HVAC&R Depth (PM) exam.

Problem 504 tests knowledge of fundamental psychrometric calculations and also tests your understanding of basic air conditioning systems. After you read through all the superfluous information provided, you realize that you are given:

1. The indoor conditions (temperature and relative humidity) for a single zone system,
2. The conditions (dry, and wet bulb temperatures) of the supply air, and the flow rate of supply air in CFM,
3. The outside air conditions (dry, and wet bulb temperatures) and the flow rate of outside ventilation air in CFM.

You then are asked to calculate the total refrigeration load in tons – that is, to “size” the cooling coil. To solve this problem, you perform an energy balance on the air flowing through the cooling coil. The coil load is the air mass flow rate times the change in enthalpy of the moist air as it flows across the coil (i.e., as it goes from the “mixed” to the “supply” conditions). You can use a psychrometric chart to find the conditions at the “mixed” state because you have flow rates and conditions of the return air (the indoor conditions) and the outside air.

Here we present a problem that tests the same concepts as problem 504, but it is a little bit more challenging:

A quantity of 1400 cfm of air at 80°F/67°F enters an evaporator coil with a 0.12 bypass factor and a 45°F apparatus dew point. After leaving the coil, all the air is then supplied to a space with a total cooling load of 3 tons and a space sensible heating ratio (SHR) of 0.8. Under these conditions...