The Relative Humidity Design Challenge

[RandomUserName]

Per ASHRAE 62.1 - 2013:

5.9.1 Relative Humidity

Occupied-space relative humidity shall be limited to 65% or less when system performance is analyzed with outdoor air at the dehumidification design condition (that is, design dew-point and mean coincident dry-bulb temperatures) and with the space interior loads (both sensible and latent) at cooling design values and space solar loads at zero.

Note: System configuration and/or climatic conditions may adequately limit space relative humidity at these conditions without any additional humidity-control devices. The specified conditions challenge the system dehumidification performance with high outdoor latent load and low space sensible heat ratio.

How many of you look at this performance metric when sizing and selecting systems?

I would love some help to see if I am doing this right; I have 2 years experience now and am trying to learn as much as possible.

I am trying to figure out a way to do this in Carrier HAP. I have already modeled my building in question, and as of right now I have the following performance (FYI system in question is small CAV DX RTU):

Total Coil Load: 6.2 Tons
Total Coil Load: 73.8 MBH
Sensible Coil Load: 61.2 MBH
Coil CFM at Aug 1600: 4069 CFM
SHR: 0.83
OA: 450 CFM
OA DB/WB: 93.4/76.9 °F
Entering DB/WB: 73.5/64.5 °F
Leaving DB/WB: 59.3/58.5 °F
Coil ADP: 57.8 °F
Bypass Factor: 0.1
Resulting RH: 63%
Design supply temp.: 58.0 °F

Now that I have run my design cooling day simulation, I can make the following modifications to the program to try the dehumidification challenge:

1. Change the design cooling conditions in my weather properties to 70.0°F DB / 69.5°F WB
2. Remove all wall and roof solar loads in each space

Thing is, when I do this, the program automatically lowers coil CFM (to 1131 CFM) to meet the sizing data cooling supply temperature requirement of 58°F:

Total coil load: 2.1 Tons
Total coil load: 25.1 MBH
Sensible coil load: 14.4 MBH
Coil CFM at Jul 1500: 1131 CFM
SHR: 0.571

OA: 450 CFM
OA DB/WB: 70.0/69.5 °F
Entering DB/WB: 70.8/65.6 °F
Leaving DB/WB: 58.9/58.4 °F
Coil ADP: 57.6 °F
Bypass Factor: 0.1
Resulting RH: 63%
Design supply temp.: 58.0 °F

If I now go back and force a user-generated supply airflow CFM of 4096 (from our design cooling day ... this is a CAV system) and force a dehumidification requirement of 65%:

Total coil load: 5.0 Tons
Total coil load: 59.7 MBH
Sensible coil load: 48.8 MBH
Coil CFM at Jul 1500: 4096 CFM
SHR: 0.818

OA: 450 CFM
OA DB/WB: 70.0/69.5 °F
Entering DB/WB: 70.3/63.3 °F
Leaving DB/WB: 59.1/58.5 °F
Coil ADP: 57.9 °F
Bypass Factor: 0.100
Resulting RH: 65%
Design supply temp.: 58.0°F


This challenge day requires 10.9 MBH of latent cooling, whereas my cooling design day requires 12.6 MBH. Does this mean I meet the challenge design day (EDIT) if I select a RTU to meet the design cooling day?

 

[317069]

why did you change OA from 93.4F to 70.8F

 

[RandomUserName]

To test the system during a dehumidification design condition. See the quoted text in my original post.

"analyzed with outdoor air at the dehumidification design condition (that is, design dew-point and mean coincident dry-bulb temperatures)"

That being said, it looks like I was incorrectly interpreting the design dew-point and mean coincident dry-bulb temperatures.

I looked these values up in my ASHRAE Fundamentals book: they are 76°F DP , 84°F MCDB.

Rerunning the program:

Central Cooling Coil Sizing Data

Total coil load: 6.2 Tons
Total coil load: 74.8 MBH
Sensible coil load: 55.4 MBH
Coil CFM at Jul 1500: 4096 CFM
Sensible heat ratio: 0.740

OA: 450 CFM
OA DB/WB: 84.0/78.0 °F
Entering DB/WB: 71.8/64.5 °F
Leaving DB/WB: 59.1/58.4 °F
Coil ADP: 57.7 °F
Bypass Factor: 0.100
Resulting RH: 65%
Design supply temp.: 58.0 °F

Now my latent load (19.4 MBH) is larger than my cooling design day latent load (12.6 MBH). What is the best selection options at this point now? Select a system that can meet our dehumidication design day latent load of 19.4 MBH and can meet our cooling design day sensible load of 61.2 MBH?

What are your thoughts?

 

[317069]

What is the dew point of 93.4DB/76.9WB, I doubt it is 75DB

 

[RandomUserName]

The DP at 93.4°F DB/76.9°F WB is 70.5°F.

The DP at the design challenge is 76°F with a DB of 84°F.

I do not understand where you are going with this; care to explain?

 

[317069]

I am wondering if the ASHRAE 62 standard calculates at the dew point of the outdoor air design parameters (93.4db/76.9wb) and as you say the dew point is 70.5 (I agree), why AHREA table uses 76F as a dew point, do we have two dew points for same parameters, I know one condition has one dew point, also from where ASHRAE considered 84F.
76 is not dew point of outdoor air in your case but why ASHRAE call it dew point?

 

[317069]

When you re-run your load at 84db/78wb, did you consider the solar load as a 0?

 

[RandomUserName]

Yes When rerun at 84DB/78WB/76DP, the solar loads on all exterior walls and roofs were removed.

 

[317069]

How did you remove the solar load, I mean did you remove it from data entry such as making windows area =0 or you removed manually from the load table after running the calculation, I am asking to see if the program would give same output in both cases.

 

[RandomUserName]

I removed walls and roof areas completely before it ran the model setting roof, wall, window, and door areas to zero. So in your terms I did what you are describing first.

 

[317069]

From your posts I see that you ran calculation in three outside db/wb conditionss, first at 93.4/76.9, second at 70/69.5, third at 84/78 and the sensible loads were 61.2, 14.4, and 55.4 MBH respectively, now if we compare between first and third case we find out that the difference is about 6MBH only, this gives an idea that the sensible load is not much related to the weather condition, but how this sensible load became 14.4 in second case, reduction from 93F to 84F gave 6MBH of reduction in sensible while the reduction from 93F to 70F gave 47MBH of reduction in sensible, I am just comparing calculations