Originally Posted by
mlkwal1
The vfd compared to fan cycle controls on cond fan motors
head pressure controls turn the fan on and off based on a single pressure...in this case head pressure.
the vfd's turn the fans on and off based on the pressure differential between the saturated condensing pressure and the saturated evaporator pressure.
all txv's (mechanical or electronic) are rated by differential pressure (and a few other things). txv's move their rated tonnage based on this differential pressure. no differential pressure and there isn't any refrigerant movement.
if the saturated evaporator pressure drops and the saturated condensing pressure stays the same (because the pressure switches only switch based on a single pressure), then the txv tonnage rating goes up and you can move more refrigerant.
but what happens when the evaporator load goes up? the saturated evaporator pressure goes up, but the saturated condensing pressure stays the same (because of the pressure switches). due to this issue, the tonnage goes down and you can trip due to a heavy load.
by using the differential pressure, you have several advantages.
1. maintains txv tonnage at all evaporator loads and condenser loads.
2. when the saturated evaporator pressure is low AND the outdoor air temp is low (causing the pressure switch to turn OFF some fans), the vfd's would actually turn ON some fans in order to lower the head pressure and gain efficiency (assuming the differential is high enough)
"Right" is not the same as "Wise".
Don't step on my favorite part of the Constitution just to point out your favorite part.
Just because you can measure it, doesn't mean it is important. Just because you can't measure it, doesn't mean it isn't important.
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