Chilled Water AHU Cooling Coil Sizing Calculator
Size commercial air handling unit (AHU) chilled water cooling coils by calculating total, sensible, and latent cooling capacities, moisture condensate removal rates, apparatus dew point (ADP), bypass factor, water GPM, and waterside hydraulic pressure drop per ASHRAE and AHRI 410.
AHU Cooling Coil Thermal Capacity & Hydronic Ratings
Psychrometric Process Vector & Coil ADP Diagram
Cooling & Dehumidification LinePsychrometric Heat Balance & Chilled Water Coil Sizing Mechanics
When moist air enters a chilled water cooling coil with a tube surface temperature below the air's dew point, sensible cooling and latent dehumidification occur simultaneously. The overall cooling process is dictated by enthalpy differences between entering and leaving air states, while hydronic flow rate is governed by the water-side sensible energy absorption.
| Parameter | Governing Standard Formula | Physical Significance | Design Target / Rule of Thumb |
|---|---|---|---|
| Total Heat Duty (Qtotal) | 4.5 × CFM × Δh | Enthalpy energy extraction rate | 1 Ton = 12,000 Btu/hr = 3.517 kW |
| Sensible Capacity (Qsens) | 1.08 × CFM × ΔTdb | Dry-bulb air temperature reduction | SHR = Qsens / Qtotal (typically 0.65 to 0.85) |
| Chilled Water GPM | Qtotal / (500 × ΔTw) | Hydronic coolant demand | 1.5 to 2.4 GPM/Ton (ΔTw = 10°F to 16°F) |
| Air Face Velocity (Vface) | CFM / Aface | Speed of air across finned face | 400 – 500 FPM (max 500 without eliminators) |
The Apparatus Dew Point (ADP) and Bypass Factor (BF)
Air flowing across a cooling coil does not achieve 100% thermal equilibrium with the cold fin surface. A portion of the air comes into direct contact with the fins and cools down to the Apparatus Dew Point (ADP)—the effective surface temperature of the wetted coil. The remaining fraction bypasses contact unaffected. The Bypass Factor (BF) is defined geometrically on the psychrometric chart:
Deeper coils (6 to 8 rows) provide higher contact factors ((CF approx 0.88 - 0.95), (BF approx 0.05 - 0.12)), pulling leaving air closer to the ADP and maximizing latent moisture extraction.
Moisture Condensation Drainage Rate
The rate of water vapor stripped from the entering airstream dictates the required condensate drain pan and trap piping diameter:
Worked Engineering Example: Sizing a 10,000 CFM VAV Cooling Coil
Design Objective: Size a 6-row chilled water coil for a 10,000 CFM commercial AHU receiving mixed air at 80°F dry-bulb / 67°F wet-bulb ((h_{in} = 31.62) Btu/lb, (W_{in} = 78.4) grains/lb) and supplying conditioned air at 55°F dry-bulb / 54°F wet-bulb ((h_{out} = 22.62) Btu/lb, (W_{out} = 60.1) grains/lb). Chilled water is supplied at 44°F and returned at 56°F ((Delta T_w = 12^circ ext{F})) across a 21.0 sq ft face area.
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Calculate Cooling Capacities:
(Q_{total} = 4.5 imes 10,000 imes (31.62 - 22.62) = mathbf{405,000 ext{ Btu/hr}} (33.75 ext{ Tons})).
(Q_{sensible} = 1.08 imes 10,000 imes (80.0 - 55.0) = mathbf{270,000 ext{ Btu/hr}} (22.50 ext{ Tons})).
(Q_{latent} = 405,000 - 270,000 = mathbf{135,000 ext{ Btu/hr}} (11.25 ext{ Tons})).
Sensible Heat Ratio: (SHR = rac{270,000}{405,000} = mathbf{0.667}). -
Determine Chilled Water Flow Rate (GPM):
(GPM = rac{405,000}{500 imes (56.0 - 44.0)} = rac{405,000}{6,000} = mathbf{67.5 ext{ GPM}}).
Specific hydronic flow index = ( rac{67.5}{33.75} = mathbf{2.00 ext{ GPM/Ton}}). -
Evaluate Air Face Velocity & Moisture Carryover:
(V_{face} = rac{10,000 ext{ CFM}}{21.0 ext{ sq ft}} = mathbf{476 ext{ FPM}}).
Operating below 500 FPM ensures surface tension holds condensate on fin surfaces, draining smoothly into the pan without droplet carryover into downstream filters. -
Condensate Removal Rate:
Moisture stripped = (Delta W = 78.4 - 60.1 = 18.3) grains/lb dry air.
(dot{m}_{cond} = rac{10,000 imes 60 imes 18.3}{7000} = 156.9 ext{ lb/hr} = mathbf{18.8 ext{ GPH}}). -
Apparatus Dew Point (ADP) & Bypass Factor:
From the psychrometric condition line connecting (80°F, 67°F) through (55°F, 54°F) to saturation:
(ADP = 50.8^circ ext{F}).
(BF = rac{55.0 - 50.8}{80.0 - 50.8} = rac{4.2}{29.2} = mathbf{0.144}) (Contact Factor (CF = 0.856)).
5 Fatal Traps in Chilled Water AHU Coil Sizing & Operation
1. Low ΔT Syndrome & Central Chiller Plant Starvation
Operating with oversized coils, dirty air filters, or hunting 2-way control valves causes chilled water to return at 48°F to 50°F instead of the design 56°F–58°F. This drops temperature difference from (Delta T = 12^circ ext{F}) to (6^circ ext{F}), demanding double the pumping GPM for the same cooling tonnage. Central plant distribution pumps run at 100% capacity, starving distant buildings while chillers sit lightly loaded at low COP.
2. Moisture Droplet Carryover Above 500 FPM
When air face velocity exceeds 500 to 525 FPM (2.5 to 2.7 m/s) on wet cooling coils, aerodynamic shear overcomes water droplet surface tension on fin edges. Droplets blow off the coil face into the fan plenum and downstream supply ductwork, soaking final HEPA filters, rotting internal acoustic insulation, and spawning toxic black mold (Stachybotrys). Always design coil face area for (le 475) FPM or specify stainless moisture eliminator blades.
3. Laminar Flow Transition at Part-Load (Re < 2,100)
When a modulating 2-way control valve throttles water velocity below 2.0 ft/s (0.6 m/s), tube water flow transitions from turbulent to laminar ((Re < 2,100)). In laminar flow, the waterside convective heat transfer coefficient ((h_i)) collapses by 60% to 75%, destroying coil capacity and causing leaving supply air humidity to spike uncontrollably despite low space sensible loads.
4. Freezing Air Stratification Coil Freeze-Burst
In economizer or 100% outdoor air units during winter, outdoor air at 10°F and return air at 70°F do not naturally mix in rectangular plenums. Cold air stratifies along the floor, hitting lower coil circuits. Even with warm water circulating, boundary layer freezing expands and bursts copper return bends, resulting in tens of thousands of gallons of water damage. Static air blenders and low-limit freeze-stats are strictly mandatory.
5. Control Valve Authority Hunting (N < 0.50)
Installing an oversized modulating control valve where valve pressure drop is less than 50% of total branch pressure drop (Valve Authority (N < 0.50)) turns an equal-percentage valve into an aggressive quick-opening switch. The valve continuously hunts between 10% and 40% stroke, creating massive temperature swings, actuator motor burnout, and degraded humidity control.