Fan law: Pulley size Formula
DPm X RPMm = DPf X RPMf
DP=Diameter of Pulley f=Fan or Driven
m=Motor or Driver
Actual Brake HP of a motor
BHP = HPnp( Amps/FLA)
np = Name plate
Synchronizes speed of a motor
7200 / # of poles
Convert Velocity pressure (Vp) into Feet Per Minute (FPM)
FPM = 4005(sqrt of Vp)
Convert V(velocity) into CFM
CFM = Asqft X Vfpm
Asqft = Area of the ductwork in square feet
Vfpm = Velocity in Feet Per Minute
Tp(total pressure) equals
Tp = Static (Sp) + Velocity (Vp)
The formula to figure the BTUH of an ac system using field measurements
BTUH = 4.5(CFM) x (Delta Enthalpy)
Fan law: Basic fan law formula
BHP2 = BHP1 X ((RPM2 / RPM1)cubed)
Shows that required power of a motor varies by the Cube of the speed increase of that motor.
The formula to figure the CFM of a Gas furnace using field measurements.
CFM = BTUH(output) / 1.08 X Delta T
1.08 = 60(.075 X 0.24)
60 = Adjust formula to one hour
Density of Dry air (.075)
Specific heat (0.24) of Dry Air
The formula to figure the CFM of an electric furnace
CFM = (E X I) X 3.414 / 1.08 X Delta T
E X I = watts of the heating elements
3.414 converts watts to BTU's
Fan law: The formula to determine BTUH of a Gas furnace using field measurements.
BTUH = 1.08 X CFM X Delta T
Chiller load formula
BTUH = GPM X 60 X 8.33 X Delta T
or
BTUH = GPM X 500 X Delta T
Convert Centigrade to Fahrenheit
C = (F - 32) / 1.8
Convert Fahrenheit to Centigrade
F = (1.8 X C) + 32
U value of insulation is equal to?
1/R Where R is the R value of the insulation.
R value of insulation is equal to?
1/U
Where U is the U value of the insulation.
Area of a circle
A = pi X Rsq
Formula for Power in Watts
W = I X E
ohms law
E = I X R
Formula for finding the resistance of a Parallel circuit
Rt = (R1 X R2) / (R1 + R2) or
1 / (((1/R1) + (1/R2) + (1/R3) + ....)))
If all resisters are the same value divide the common value by the # of resisters
Formula for finding the total caps in parallel
Ct = C1 + C2 + C3
Formula for capacitors in series
1 / (1/C1 + 1/C2 + 1/C3)
Find Watts when BTU's are known
W = BTU's X .293
.293 is the reciprocal of 3.414,
the number of BTU's in a watt.
Compression Ratio (CR)
CR = Head PSIA / Suction PSIA
Formula for PSIA when Inches of HG is known
PSIA = (30 - Inches HG) / 2
Figure the expected temperature drop across a chiller
Delta T = BTUH / (GPM X 500)
Circumference of a circle
C = pi X D
Fan law: The relationship of CFM to RPM change
CFM1/CFM2 = RPM1/RPM2
Doubling RPM doulbles CFM
Figure Approximate BHP when motor amps and applied voltage is known
Approx BHP = (E X I)/746
To many factors to make this a good formula
Formula to clock a furnace or boiler
BTUH = [(cuft X 3600)/sec.] x cf x 1000
cuft = amount of gas measured at meter; 3600 = secs in an hour; sec = time taken to measure gas used in secs. ; cf = correction factor (usually around 3.04)