Feature . . . Export CA Conditions  

The calculated SA (CA) Tdb, RH and airflow can be fed (exported) to the main AHU module as precooled (treated) outdoor air. The current activity's state is retained when you recall back this module for input changes, etc.

Design Approach explained . . . 

Two methods of sizing are modelled:

(1) 'SA to AHU' method, i.e. 'cold' supply air delivered to AHU.

You generally start by setting the desired Off Coil temperature to be delivered to the AHU. To handle all the OA latent load, the Off Coil setpoint is usually just below the Room air dewpoint. If the PAU requires to handle some or all of the room latent (moisture) load, then the Off Coil setpoint needs to be much lower than the Room air dewpoint.

(2) 'SA to Room' method, i.e. 'space neutral' supply air delivered directly into the air-conditioned space.

You specify the desired space neutral air temperature in addition to the desired Off Coil temperature. In this method, the conditioned OA is reheated to the space neutral air temperature and supplied directly into the room. Note that all the sensible cooling that is done during the coil process is wasted in this method.

All airflows calculated are actual airflow, i.e. with air density variations. The governing equations used are:

sensible heat = m x Cp x dT

latent heat = m x hfg x dW

Psychro Chart . . .  

Psychro chart showing the precool coil process is plotted for each calculation. The psychro chart can be saved as an image file.

Calculation Examples . . .  

The following design calculations are done with aPsychroAC program.

Worked Example 1 - SA to AHU (SI units)

Given:

Summer design conditions: 34 ^oC DB, 40% RH

Room design conditions: 24 ^oC DB, 50% RH

OA ventilation airflow = 5535 CMH

OA is to be precooled to 10 ^oC DB, 87% RH

Ignore fan heat gain, if any.