Dissecting the Mechanical PE - HVAC&R Depth (PM) Exam: Problem 504

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:

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HVAC&R Depth (PM) Exam Sample Problem #4

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...

...the relative humidity (%) of the air leaving the conditioned space is nearest:
(A) 45
(B) 55
(C) 94
(D) 100

You may download the solution via the link below.
Have any questions? Need help? Leave a comment!

hvacr_dissecting-04-solution.pdf
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