Description of Problem 505: You are given the flow rate through an orifice plate flow meter and are asked to calculate the corresponding pressure drop.

Analysis of Problem 505: The relationship between pressure drop and flow rate for an orifice meter is a simple equation available in any Fluid  Mechanics textbook. There is a solved example in the ME Review Manual by Lindeburg that is virtually the same as this problem.

Here is a problem we've crafted dealing with a venturi flow meter to test your understanding of the equations that apply to these flow metering devices.

A venturi meter is installed in a 2-in diameter, schedule 40 steel pipe (ID = 2.067 in). The fluid in the pipe is ambient temperature No. 4 fuel oil with a specific gravity SG=0.85. The meter is provided with a U-tube manometer. From past experience with this meter, it is known that for a manometer fluid height, h, of 8 inches, the oil flow rate is 400 gallons per minute.

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:

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.