Variable Flow Controller  
by T.B. Jekel  

	This controller determines either the optimum fraction of flow to 
be sent through the coil, or the fraction given a constant coil outlet 
temperature when subject to changing loads was developed.  A curve fit of 
the saturation line on the psychrometric chart (enthalpy and humidity 
ratio), the space set-point temperature and humidity, and the SHR are used 
with Newton's method to predict the saturation temperature at which the 
load line intersects the saturation line.  The optimum fraction of flow 
rate through the coil is calculated using the saturation temperature and 
humidity ratio (coil outlet state), set-point temperature and humidity 
ratio, and the circulation flow rate of air through the system.  The inputs 
to the controller are the space set-point temperature, humidity ratio, and 
enthalpy, the space load and sensible heat ratio, and the circulation flow 
rate.  Also, if the performance with a constant coil operating temperature 
is desired, the outlet coil temperature must be input, and if the optimum 
performance is desired, the convergence tolerance used in Newton's method 
must be input.  The coil outlet state was assumed to be saturated at the 
outlet temperature because in order to take into account the bypass 
fraction, the inlet state of the A/C coil is needed and this significantly 
complicates the calculations.  Output from the controller is the fraction 
of the total flow that should be sent through the coil and the temperature 
of the outlet air from the air-conditioner coil. 

REF: T.B. Jekel "Modeling of Ice-Storage Systems"
     Masters Thesis 1991 University of Wisconsin at Madison