Vari-Cool Evaporative - Page 6

Using the appropriate altitude psychrometric chart, chart the performance of the indirect and direct evaporative cycles.

(a) Plot the entering condition (DB & WB) to the indirect unit. Calculate the temperature drop (cooling effect) of the unit :

[Ent.Air - (WB depression)(5 efficiency) = DB Lvg. Air

Moving horizontally from the entering air point, plot the leaving air temperature on the chart. the indirect unit provides total sensible cooling.   this point represents the leaving air from the indirect unit and the entering air condition to the direct unit.

(b) Using the entering DB & WB conditions tot he direct unit calculate the temperature drop using the direct unit efficiency.

[Ent Air - (WB depression)(%efficiency) + DB Lvg. Air

Moving diagonally up the wet-bulb line (from the entering condition to the direct unit) plot the leaving air temperature on the chart.

This point is the leaving air temperature from the two stage evaporative system at the design entering air condition.

If this supply air temperature is satisfactory, third stage booster refrigeration may not be required.

h2 = leaving air temperature I/d evaporative unit

h1=leaving supply air temperature required

From this calculation the booster refrigeration capacity can be determined and cooling coil selection made.  The coil may be located either ahead of, or on the leaving air side of the direct unit.  Appropriate coil surface area and temperature must be determined based on the location of the coil and the entering air conditions.

Note that it may be advisable in some cases to increase the system supply air quantity which will reduce the refrigeration load and increase the effective capacity of the two evaporative stages.  Keep in mind that the booster refrigeration will only operate at high outdoor wet bulb conditions which comprise a small portion of the total system operating hours the two stage evaporative system will provide the total cooling effect during the majority of cooling hours.