The Relative Humidity Design Challenge
The Relative Humidity Design Challenge
(OP)
Per ASHRAE 62.1 - 2013:
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?
Quote (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?





RE: The Relative Humidity Design Challenge
RE: The Relative Humidity Design Challenge
"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?
RE: The Relative Humidity Design Challenge
RE: The Relative Humidity Design Challenge
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?
RE: The Relative Humidity Design Challenge
76 is not dew point of outdoor air in your case but why ASHRAE call it dew point?
RE: The Relative Humidity Design Challenge
RE: The Relative Humidity Design Challenge
RE: The Relative Humidity Design Challenge
RE: The Relative Humidity Design Challenge
RE: The Relative Humidity Design Challenge