Let’s be pragmatic and just say that, for purposes of the PE exam, this process is a reduction in humidity ratio with simultaneous heating, following a line of constant enthalpy (or what is approximately the same, a line of constant wet-bulb temperature).
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Desiccant dehumidifiers remove moisture from air, but the operating principle differs significantly from cooling-based dehumidifiers. Instead of cooling the air to condense water, these dehumidifiers remove moisture from the air by exposing it to a medium known as a desiccant. The simultaneous heat and mass transfer that occurs during the process is complicated, and such an analysis is way beyond the scope of the PE exam. We just need to accept it as a fundamental principle that the air is heated while the humidity drops. 

One common way to perform this process in applications where there are large latent loads or there is a need for continuous humidity control of spaces, is to make the air stream flow across a slowly turning wheel comprised of many small channels coated with the desiccant.

The figure below shows an example of such a process represented on a psychrometric chart. The air enters the dryer at 70°F and 90% relative humidity. At these conditions, the humidity ratio is 100 grains per pound (0.0142 pounds of moisture per pound of dry air). What is the discharge temperature of the air if we wish to cut the humidity ratio by 50%?

To answer the question, we locate the inlet and draw the process line (following the line of constant wet-bulb temperature) until we hit the desired humidity ratio. From the graph, we see the air will be heated to approximately 102°F.
Now, here is a challenge for you:

​Give this problem a try and contact us ([email protected]) if you need help or to check your answer:
 
In an industrial facility, an air compressor runs four hours a day, seven days a week, drawing 5,000 CFM of air at 72°F, 60% relative humidity. The compressor manufacturer requires that the air into the compressor must have a dew point of 40°F or less, so the facility installed a desiccant dryer at the compressor air intake to achieve this condition. The minimum water removal rate (lb/hour) that the dryer must achieve is most nearly:

(A)     2.4
(B)    145
(C)    580
(D) 4,070

After having mentored hundreds of students to success with the HVAC PE exam, I have concluded that the students most likely to pass are those that understand the following fundamental topics: Energy Balances, The Vapor Compression Refrigeration Cycle, Psychrometrics, Load Estimating, and familiarity with the relevant tables and graphs in the reference handbook.

• The Energy Balance. This concept is key to just about every single thermodynamics and fluid mechanics problem you will encounter. Energy in must equal energy out. Seems simple enough, right? Well… depends. Are you dealing with ideal gases? Liquids? Heavy vapors such as refrigerants or steam? A mixture of vapor and liquid? There are nuances associated with each one and navigating them is crucial.
• The Vapor-Compression Refrigeration cycle (VCRC). The importance of this topic cannot be overemphasized. Judging from feedback from recent customers, the VCRC is the queen of the exam. You need to know your VCRC facts cold (pun intended). Look at this graph. Can you name all four states off the top of your head? Hint: Point 2 is “compressor discharge”
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Follow-up question: Do you know what each of the three horizontal double-arrow lines represent? Hint: one of them is the condenser duty.

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• Psychrometrics. Nobody should be surprised that the science of moist air is a heavily weighted topic in the HVAC exam. The lines on the chart associated with all psychrometric processes should be seared into your retinas.
• The old-school methods of load estimating: For this exam, you won’t be doing any Radiant Time Series or Heat Balance Method calculations, although you sure do have to know what those are. You will, however, use the methodology behind Manual-J type load estimating: the CLF/CLTD method. This method was removed from the ASHRAE Handbook of Fundamentals, and last appeared in the 1997 edition, so your PE exam study resources must cover this technique.
• All the tables and graphs in the reference handbook. Our philosophy is that if NCEES decided to include a table or graph in the reference handbook, then it’s “fair game” when it comes to the exam.
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  • Do you know how to use the tables for refrigerant pipe sizing?
  • Do you understand the purpose of the tables and graphs for circulation rate and refrigerant flow in liquid overfeed systems?
  • Do you know how to use the “Zone Air Distribution Effectiveness” table?