Posted by Abby Normal on March 6, 2005, 3:46 pm
For Mobile Alabama, ASHRAE says a 2% design humidity level of 82F @ 130
grains, a 1% level of 83F @ 139 grains and at a 0.4% design level 83F
@ 142 grains.
Designing using averages gets one in trouble all the time. Good for
calculating energy consumption over a period of time, useless for
designing a working system.
Humidity design levels are 24 to 32 per cent higher than using 0.0154.
With 30 cfm
Posted by nicksanspam on March 6, 2005, 6:08 pm
ASHRAE percentiles are averages :-) To use the method below to estimate
air infiltration, replace the average numbers with actual measurements.
Many HVAC criminals ignore energy consumption...
Posted by Abby Normal on March 6, 2005, 6:30 pm
Systems designed on averages are condusive to growing mushrooms
Posted by Noon-Air on March 6, 2005, 7:01 pm
I'm confused... maybe I am just not thinking in the same terms or using the
same measurements or something....
My design parameters for Mobile and are almost identical to Hattiesburg, MS
area where I am.... this what I use for design criteria
95 degrees ODB
77 degrees OWB
To *maintain* 75 degrees @ 50% RH inside
28 degrees ODB
To *maintain* 72 degrees inside
At the OD design temps, the system should be running continuously to
maintain the ID temps
or am I missing something???
Posted by Abby Normal on March 6, 2005, 10:02 pm
What happens Steve, is you use a dry bulb temperature to design the air
conditioning for, and the wet bulb temperature they give you coincides
as what is typical when it is that hot out.
Sometimes it can be much more humid when it is not so hot out.
The 95/77 you mention for Hattisburg is about 111 grains of humidity or
0.0159 lb W/lb da similar to the number Nick was using.
On commercial buildings when everything was designed for these DB/WB
temperatures, especially when there was a lot of ventilation air, what
was happening that at times where it was not as hot out, but more
humid, the systems MAY not remove enough humidity.
The worst case or 0.4% humidty design level for Jackson is 84F @ 142
grains or 84F db 79 wb. A 2.5% design dry bulb there maybe 95F and the
coinciding wet buld is 76F, about 105 grains.
An air conditioning system can work great when it is 95 degrees out,
but sometimes when it is not so hot but there is more moisture in the
air, the systems will not be able to dehumidify enough.
Using Jackson for an example, the outdoor temp can be 11 degrees below
the design temperature the system was designed around, and therefore
the run time will be reduced and it will not be removing as much
moisture as it is not running as long. In addition to reduced run time,
there is 35% more moisture in the outside air than perhaps the system
was designed to deal with.