Calculated Industries 4090 - Manual (Page 77)

76 — ITI S
HEET
M
ETAL
/HVAC P
RO
Fan Law 2
The formula for Fan Law 2 (built into this calculator) is:
Fan laws use the temporary storage registers a, b, a-new, and b-new.
Fan Law 2 calculates using the entry of the three known variables
and
Ç r to calculate the unknown fourth value.
Example 1:
A fan is producing 15,300 CFM at 3.2” SP. If the fan is adjusted to
14,000 CFM, what will the new SP be?
KEYSTROKE DISPLAY
1. Enter current CFM into “a” old:
o o 0.
1 5 3 0 0 Ç 4 A 15300.
2. Enter new CFM into “a-new”:
1 4 0 0 0 Ç 7 A
n
14000.
3. Enter current SP into “b” old:
3 • 2 Ç 5 B 3.2
4. Calculate new SP or “b-new”:
Ç r SP
n
FAN LAW2 2.679311
Example 2:
After doing a traverse, you find that the duct has 1850 CFM and a
SP of 1.2”. You adjust the zone dampers and take a new SP that is
0.83. What is the new CFM?
KEYSTROKE DISPLAY
1. Enter current CFM into “a” old:
o o 0.
1 8 5 0 Ç 4 A 1850.
2. Enter current SP into “b” old:
1 • 2 Ç 5 B 1.2
3. Enter new SP into “b-new”:
• 8 3 Ç 8 B
n
0.83
4. Calculate “a-new” or new CFM:
Ç r CFM
n
FAN LAW2 1538.58
STORED
STORED
STORED
STORED
STORED
STORED
S
S
where,
SP = Static Pressure
CFM = Feet
3 per Minute