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Commonly Used Equations
for Normal Coil Conditions and Approximations:
Fluid Heat Load, Mass Flow Rate and Temperature
Density of Air = ra = (PSIA x 144) / ((460 + EAT oF)(53.3)) = Lb./Ft3
where EAT is Entering Air Temperature
SCFM = ACFM (ra/.075)
where SCFM is air at Standard conditions (14.7 PSIA and 70 oF) and ACFM is measured at the actual temperature and pressure
Lb./Hr. Air = CFM x 60 Min/Hr. x Density of air
Dry Air (Sensible) Heat Load = (1.085 BTUH/SCFM-oF)(SCFM)| EAT oF - LAT oF |=BTU/Hr.
where EAT/LAT are entering/leaving air temperatures, and | | is the absolute value
Lb. /Hr. Dry Air = (CFM x 60 Min/Hr. x Density of air)/(1 + W)
where W is the humidity ratio Lbwater/Lbdry air
Wet Air (Sensible+Latent) Heat Load = (4.5 x SCFM x Dh)
where Dh= change in enthalpy of entering and leaving air BTU/Lbda
Water Heat Load = (500 BTUH/GPM-oF)(GPM)| EFT oF - LFT oF | = BTU/Hr.
where EFT/LFT are entering/leaving fluid temperatures, and | | is the absolute value
Liquid Other Than Water = (500 BTUH/GPM-oF)(Cp)(SG)(GPM)| EFT oF - LFT oF | = BTU/Hr.
where the liquid Specific Heat and Specific Gravity values are at average of the entering and leaving fluid temperatures.
Kilowatts = BTU/Hr. ¸ 3412
Refrigeration Tons = BTU/Hr ¸ 12,000
Condenser Tons vary as to the compressor kW, refrigerant type and temperature range. For traditional air conditioning use 1 Ton ≈ 15,000 BTU/Hr. as a rule of thumb.
Check Heat Balance between Dry Air (No condensation) and Water
(1.085 BTUH/SCFM-oF)(SCFM)| EAT oF - LAT oF |=BTU/Hr.= (500 BTUH/GPM-oF)(GPM)| EFT oF - LFT oF | where | | is the absolute value
Coil Face Area
Coil Fin Height = Number Tubes High x Tube Centers
Coil Fin Depth = Number Tube Rows Deep x Row Centers
Coil Face Area = FA = (Fin H" x Fin L")/144 = Ft2
Coil Face Velocity (a.k.a. Air Velocity) = SCFM / FA = Ft/Min.
Coil Casing and Overall Coil Size
Approximate Coil Casing Height = Fin H + (2 x Flange Width)
1 1/2" Standard Flange Width, 5/8" Minimum
Approximate Coil Casing Length = Finned L + (2 x 1 1/2")
Need Minimum 1 1/2" Flange* to protect tube return bends
Approximate Coil Casing Depth =(No. Rows -1) x Row Center + Actual Header Pipe OD + 1 1/2"
(See note if same end connection.)
Note: Must check if headers are hitting each other when side-by-side: (No. Rows-1) x Row Centers ³ Actual Header OD +1/2"
Approximate Overall Coil Length (Casing + Header but Excludes Connection.)
OAL = Finned L + 1 1/2"* + 3/8" + 2"+ Actual Header OD
Where 1 1/2" is Hairpin Casing Flange*, 3/8" is tube beyond casing endplate and 2" is Header Tube Stub
* Note: Reducing casing flange on coil hairpin end only exposes the tube bends to possible damage during installation and DOES NOT reduce the OA coil length