2001 NCEES Mech/HVAC problem 519

My Question is, what is the correct way to calculate outdoor air load from ventilation.

Problem 519 Givens: Qsens = 90,000 BTU/hr; Qlat = 40,000 BTU/hr; Supply air= 3,600 CFM @ 55 db. Room temp = 78degF db/ 45%rh; Outdoor air = 700cfm 92degF db/76degF wb.

The problem ask for total load. I know you need to find the load from ventilation and add it to the other given loads. However, is it 4.5XcfmX (houtdoor air -hsupply)? Or is it 4.5XcfmX(houtdoor air - hroom air)?

Problem 59 in the "Mechanical PE Sample Exam, 2nd Edition, by Michael Lindeburg says that OA from ventilation is equal to 4.5XcfmX (houtdoor air -hsupply). However problem 519 in the 2001 NCEES says it's the other.

NCEES is correct for this problem.  See MERM (version 13) equation 43.28.  I can't speak to what problem 59 from Lindeburg's sample exam is wanting, since you didn't post the problem, but my guess is they are different problems.  Does the Lindeburg problem include recirculating air, by chance?

Audi driver said:

NCEES is correct for this problem.  See MERM (version 13) equation 43.28.  I can't speak to what problem 59 from Lindeburg's sample exam is wanting, since you didn't post the problem, but my guess is they are different problems.  Does the Lindeburg problem include recirculating air, by chance?

Click to expand...

A STORE IN A SHOPPING MALL IS TO BE MAINTAINED AT 75DEGF/45% WITH SUPPLY AIR OF 55DEGF/30%. THE SPACE COOLING LOAD IS 73000BTUH SENS AND 26000 BTUH LATENT AT OUTDOOR DESIGN CONDITIONS OF 94DEGF/72DEGF. THE VENTILATION REQUIREMENT IS 850CFM. THE COIL LOAD DUE TO VENTILATION IS ???

They us the outdoor air enthalpy and the supply air enthalpy for the answer.

The difference is coil load vs. total load.  If you look at Figure 43.4 in the MERM, it illustrates this.  Coil load is heat required to cool outside air to supply air, the total load is what is required to maintain the room temperature.  See this for some, perhaps, additional clarification http://courses.washington.edu/me425/425%20Psychrometric%20Review%202007.pdf

Please note that HVAC is not my strong point, but this is my understanding of the difference between these two problems.

It's all in the wording of the question.  Lindburg's problem asks for ventilation load which is Q = CFMoa x 4.5 (hoa-hsa)

The NCEES question asks for total load on the coil and it is just a matter of how they set up the energy balance in their solution.

The solution they set up is Qt = Qs + Ql + Qoa   

Qoa is NOT the ventilation load but the load to get the mass of outdoor to return air conditions. 

Since the problem states the room loads and gives the SA to the room, if you you use the ventilation load given above, it would account for the load of the mass of outdoor air from return air conditions to supply air conditions twice.

In other words...regardless of outdoor air, the room needs 3600cfm of SA.  So if you ignore outdoor air for a moment, this load is then Q=3600 x 4.5 x (hra-hsa).  Now consider 700 cfm of outdoor air.  Well that 700 cfm load from RA conditions to SA conditions is accounted for with the previous calculations so the remaining load is what they call Qoa - 700 x 4.5 (hoa - hra).

Hope this wasn't too confusing.

With that said, I think this is the type of problem that would get thrown out.  I worked it the same way you did and had to think for a while to justify their solution with the above.  The reason I think it will get thrown out is that they only give the dry-bulb temperature of the supply air so a true energy balance isn't possible.  They make the assumption that the total coil load also removes all the latent load from the space which cannot always be assumed (at least without further information).  If this assumption held true for all cases there would be no such thing as dehumidification sequences of operation or the need to monitor room humidity levels.  Also, for this assumption to hold up, performing an energy balance at mixed air conditions would yield the same answer and it doesn't.  For their solution to hold up, return air could have to be 3600 cfm with addition of 700 cfm OA and require 700 cfm or relief/exhaust air prior to mixing that isn't stated in the problem.  Since the typical energy balance calculation yields 130,000 btuh and the addition of sensible and latent loads yields 130,000 I would assume that the latent load indicated includes the latent load of outdoor air.

So yeah I think this is one where 90% of test takers would say 130,000 btuh so it would get tossed.