Economizer for System Energy Calculations
By S. Gabel and M. Brandemuehl for ASHRAE
Revised for TRNSYS by N. Blair and R. Schwarz

      	This model was adapted strictly for use with the TRNSYS Program
by the Solar Energy Laboratory.

	This subroutine represents a free-cooling outside-air economizer.
The economizer is controlled and operated much the same as a homeowner would 
open a window on a cool day when it is warm in the house.  It operates to 
mix cooler outside air with recirculated return air to reduce the need for
mechanical cooling.  The economizer algorithm assumes that the mixed airflow 
rate is known, corresponding to an energy analysis in which the supply airflow 
rate is determined by zone loads.  The mix between outside and return air is 
determined by the following control objectives:
     1. Maintain specified mixed air temperature setpoint.
     2. Maintain minimum ventilation even at very low outdoor temperatures.
     3. Reduce outside air to minimum when not beneficial for cooling.
	The third objective of outside air cut-off can be accomplished by 
several methods, including dry bulb temperature comparisons, wet bulb 
temperature comparisons, and enthalpy comparisons.  The algorithm has been 
designed to accomodate all such methods as well as provide an over-ride for 
heating applications.

REF: A Toolkit for Secondary HVAC System Energy Calculations  May 25,1992
     Joint Center for Energy Management  University of Colorado at Boulder