 Photo via : United States Dept. of Agriculture | 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 505 for the HVAC&R Depth (PM) exam. |
| Description of Problem 505: The problem gives some data for a liquid desiccant dehumidifier. (e.g., incoming air dry and wet bulb temperatures, CFMs, desiccant solution incoming temperature, etc). You are asked to determine the dry bulb temperature of the air leaving the dehumidifier. In case you didn't know, liquid-desiccant conditioners (absorbers) contact the air with a liquid desiccant, such as lithium chloride or glycol solution. Desiccant solutions work by moisture transfer caused by a difference between water vapor pressures at their surface and of the surrounding air. When the vapor pressure at the desiccant surface is lower than that of the air, the desiccant attracts moisture, so the air is dehumidified. Analysis of Problem 505: There are a few features that make this problem confusing, and quite frankly a "bad" problem. There is the usual superfluous information (this time in the form of a large chart giving you performance data for a liquid absorbent conditioner as well as the misleading statement that a "psychrometric chart is provided on page 83 for your possible use"). But the problem statement provides a value for a "bypass factor". This is a source of confusion because unlike the superfluous/unnecessary stuff previously mentioned, this "bypass factor" is crucial to arrive at NCEES' answer. The issue we have, is that for liquid-desiccant conditioners the concept of "bypass factor" is not typically used or even defined in the relevant literature. Chapter 24, DESICCANT DEHUMIDIFICATION AND PRESSURE-DRYING EQUIPMENT of ASHRAE's 2012 HVAC Systems and Equipment Handbook makes no mention of a "bypass factor" in the entire section dedicated to liquid desiccant equipment. This is also the case for other authoritative references (e.g, the textbook by Sauer et al., and the book by Kavanaugh). Once you see the solution to the problem, you realize that NCEES meant something analogous to the bypass factor used in cooling coil analysis (defined as the equivalent fraction of air that bypasses the cooling coil while the remaining portion of the air is completely cooled to the apparatus dew-point temperature) So, according to NCEES definition of "bypass factor" for a liquid desiccant conditioner, the incoming desiccant solution temperature is analogous to the apparatus dew point - the temperature that the air would reach if there were no bypass. Hence: BF = (Tair,out - Tsolution)/(Tair,in - Tsolution) Once you realize what they mean by "bypass factor" then the problem is straightforward. You use the equation above to solve for the temperature of the air leaving the unit, Tair,out. Customarily in these blog posts we would follow the analysis of NCEES' problem with a problem of our own to give you extra practice with the topic being evaluated. However, for this one, we're not quite sure what the topic is, or what the point of this problem is. Nevertheless, here is a problem dealing with a dehumidifier using a continuously rotating desiccant wheel: HVAC&R Depth (PM) Exam Sample Problem #5 The sketch shows a packaged unit dedicated to conditioning the air in a natatorium (an indoor swimming pool room).  The unit uses a silica gel desiccant dehumidification wheel. The unit's heating coil provides the reactivation energy for the desiccant dehumidification process. The unit's cooling coil cools and dehumidifies the air prior to entering the desiccant wheel. The table below provides the known information for the summer design condition:  Assume all the water removed by the wheel from the air at state 2 is transferred to the air exhausted to the atmosphere. |
| Under the conditions described above, the dew point temperature (°F) at state 6 is most nearly: (A) 51 (B) 67 (C) 72 (D) 75 Have questions? Need help? Then leave a comment below. You may download the solution here: 
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