AAON RL-240 - Manual (Page 37)

37
Draw-Through Coils
Figure 15 - Draw-Through Drain Trap
The X dimension on the draw-through trap
should be at least equal to the absolute value
of the negative static pressure in the drain
pan plus one inch. To calculate the static
pressure at the drain pan add the pressure
drops of all components upstream of the
drain pan, including the cooling coil, and
add the return duct static pressure. Include
the dirt allowance pressure drop for the
filters to account for the worst-case scenario.
The height from top of the bottom bend of
the trap to the bottom of the leaving pipe
must be at least equal to one half of the X
dimension. This ensures that enough water
is stored in the trap to prevent losing the
drain seal during unit startup
Note: The absolute value of the fan inlet
pressure will always be greater than or equal
to the absolute value of the static pressure in
the drain pan on draw-through units, so the
fan inlet pressure is a safe value to use for
the drain pan static pressure.
Table 6 - Draw-Through Drain Trap Dimensions
Draw-Through
Drain Pan Pressure Trap Dimensions
Negative Static X X/2
(inches of water) (inch) (inch)
-0.50 1.50 0.75
-1.00 2.00 1.00
-1.50 2.50 1.25
-2.00 3.00 1.50
-2.50 3.50 1.75
-3.00 4.00 2.00
-3.50 4.50 2.25
-4.00 5.00 2.50
-4.50 5.50 2.75
-5.00 6.00 3.00
-5.50 6.50 3.25
-6.00 7.00 3.50
-6.50 7.50 3.75
-7.00 8.00 4.00
-7.50 8.50 4.25
-8.00 9.00 4.50
Blow-Through Coils
Figure 16 - Blow-Through Drain Trap
The Y dimension of blow-through traps
should be at least equal to the value of the
positive pressure in the drain pan plus one
inch. This ensures that there will be enough
water stored in the trap to counter the static
pressure in the drain pan. To find the
pressure subtract any pressure drops
between the drain pan and the supply fan
from the fan discharge pressure. The worst-
case scenario for blow-through coils is the