When to use Total Heat Equation vs. Sensible Heat Equation

[medieval_pancake]

Hi, guys just joined the form! I'm taking the test next week, and one thing I'm a little iffy on is when to use the total heat equation vs. sensible. Most the time, I can determine from the problem which one to use, but for example problem #42 on the engineer pro guides practice test is giving me a little trouble.

Problem: The AHU serving the classroom has a supply temperature of 55F and the space is to be maintained at 75db/50%RH, what CFM is required?

Given: 25 people @ 250btu/person (sensible) & 200btu/person (latent)

           Lighting = 4,000 btus; Computers =8,000 btus; Walls, Roofs, Windows = 22,000 btus; Ventilation = 7,500 btus (sensible) 7,500 (latent)

Answer Choices: (A) 2,210 CFM; (B) 2,675 CFM; (C) 2,790 CFM; (D) 3,865 CFM..............Correct Answer (A)

My Solution: Since there is a latent load from ventilation and people I assumed I would have to use Q(Total) = CFM*delta(H)*4.5 Btuh...so after totaling up the sensible & latent loads, getting the enthalpy values of the psych chart, and solving for CFM I got an answer of (B) 2,675 CFM...which is wrong. The solution to the problem only accounts for the sensible heat and used Q(sensible) =1.08*CFM*delta(T) to get answer of (A)2,210 CFM.

What key concept am I missing here? My though process so far has been if your sizing and kind of equipment/or coil you need to account for both sensible & latent loads. The only time you can use sensible only, is when there is no latent loads present? 

Any help will be greatly appreciated! Thanks.

 

[MikeGlass1969]

You are not given a Twb/RH/w of the supply air temperature.  The only thing you can give them is sensible for the room load.  I don't like the wording of this problem.

The term ventilation load is ambiguous.   It could be infiltration into the room or is this outside air?  Only one is figured for room load the other is figured into coil load.

 

[medieval_pancake]

ohhh, see I just made the assumption that the air was saturated and found the h(supply air) value at 55F ADP.

The problem doesn't say if the ventilation load is from outdoor air or infiltration. What do you mean "only one is figured for room load and the other is figured into coil load"? Are you saying that OA needs to be accounted for at the coil while infiltration air can be treated as sensible heat only? 

 

[MikeGlass1969]

Infiltration into the room would be part of the room load.  It would have both parts latent and sensible.    Outside air would be figured as part of the coil load.  The coil sees the Room load, the outside air load and the motor load from the fan.