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HVAC design for high altitude with standard psych chart

aumechengr (Mechanical) (OP)
3 Sep 08 17:02

I am trying to determine how to use the standard psych chart in calculating the sensible and latent loads for a space. It is located in Salt Lake City, UT and will be a once through system.

I know that there will be a factor applied for the difference in air density ratios. If so I am assuming you read the numbers from the chart as if you were at standard conditions and apply it only to the following equations:

RSH = 1.08*Factor*CFM*Delta T
RLH = .68*Factor*CFM*Humidity Ratio

I would appreciate a little help with this.

Thanks in advance,


MintJulep (Mechanical)
3 Sep 08 17:20
Easier yet, there are many freeware psychrometric property programs, most of which allow you to specify elevation.
RossABQ (Mechanical)
4 Sep 08 9:14
Prior to availability of computerized selection programs, many fan and coil vendors used to select for sea level and try to correct the result to altitude.  For processes involving dehumidification or humidification, (not pure sensible) the results were not always accurate.  ASHRAE, Trane, and Carrier all have psych charts for 2500 and 5000 ft, I would use them from the get-go.  For sensible work at 5000' the first equation is approx. 0.90 x cfm x dT.

In the '90's I found many installations with incorrectly sized fan motors and coils from the old approach used in the '70's.  
quark (Mechanical)
5 Sep 08 4:09
ASHRAE HOF suggsts the following equations.

SH = 1.1 x cfm x dT
LH = 4840 x cfm x dW

Now the correction for altitude is the constant x (P1/Pa)
(Page 30.13)

P1/Pa can be calculated as (1-((6.8754x10^-6)*Z))^5.2559
Z is altitude in ft.

This correction is only for altitude. If you use the sensible heat corresponding to the temperature, you can have precise factors.

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