What goes into a cooling coil selection

A chilled water coil has to do two jobs at once: lower the air temperature (sensible cooling) and remove moisture (latent cooling). Manufacturers select the final coil in their software, but the designer has to give them the right inputs and check that the result makes sense. This guide focuses on those inputs. For the psychrometric theory behind them, see Psychrometric Processes in HVAC.

Step 1: Calculate the coil load

Coil load comes from the airflow and the change in air condition across the coil, not from the room load alone, because it also includes outdoor air and fan heat:

Total load (Btu/h) = 4.5 × CFM × (h_entering − h_leaving), with enthalpy h in Btu/lb

Sensible load (Btu/h) = 1.08 × CFM × (T_entering − T_leaving)

Sensible heat ratio (SHR) = Sensible load / Total load

In SI: Total load (kW) = 1.2 × airflow (m³/s) × Δh (kJ/kg), and Sensible load (kW) = 1.21 × airflow (m³/s) × ΔT (K). Read enthalpies from a psychrometric chart or the Psychrometric Calculator. The entering condition is usually the mixed air (return plus outdoor air) at the coil face.

Step 2: Set the leaving air condition and find the ADP

The leaving air condition must satisfy both the room sensible load and the room latent load. For comfort cooling, 55°F DB / 54°F WB (12.8°C / 12.2°C) is a common starting point; spaces with high latent loads may need lower.

On the psychrometric chart, the coil process is a straight line from the entering state through the leaving state. Extend it until it meets the saturation curve: that point is the apparatus dew point (ADP), the effective surface temperature of the coil.

If the line never reaches the saturation curve, or the ADP is below about 45°F (7°C), the leaving condition is not achievable with normal chilled water and needs another look. A printable chart is available in the Psychrometric Chart PDF.

Step 3: Calculate the bypass factor and choose the rows

Not all air touches the cold coil surface. The bypass factor (BF) describes the share that effectively passes through untreated:

BF = (T_leaving − ADP) / (T_entering − ADP)

Contact factor = 1 − BF

A lower bypass factor needs a deeper coil (more rows), more fins per inch, or a lower face velocity. Typical values for plate-fin coils at about 500 fpm:

These are approximate figures for preliminary selection. Chilled water comfort coils are usually 4 to 8 rows; more rows add air pressure drop, so check the fan’s static pressure too.

Step 4: Size the face area

Face area (ft²) = CFM / Face velocity (fpm)

Keep face velocity at or below about 500 fpm (2.5 m/s). Above that, condensate can be blown off the fins and carried into the ductwork. Many designers use 400 to 500 fpm (2.0 to 2.5 m/s), which also keeps the coil’s air pressure drop reasonable.

Step 5: Set chilled water flow and temperature difference

Water flow (GPM) = Total load (Btu/h) / (500 × Water ΔT (°F))

Chilled water is commonly supplied at 42 to 45°F (5.5 to 7°C) with a 10 to 16°F (5.5 to 9 K) temperature rise. A larger ΔT cuts pumping energy but needs a deeper coil. Keep tube water velocity at roughly 2 to 6 fps (0.6 to 1.8 m/s), and give the coil’s water pressure drop to the pump designer; see Chilled Water Pump Head Calculation. You can check flows with the Chilled Water Flow Calculator.

Step 6: Estimate condensate

Condensate (lb/h) = 4.5 × CFM × (W_entering − W_leaving) / 7,000, with W in grains/lb

Divide by 8.34 for US gallons per hour. Use this to size the drain pan connection and condensate piping; see the AC Condensate Drain Calculator or the AC Condensate Drain Calculator Excel.

Cooling coil selection: worked example for a 10,000 CFM AHU

An AHU handles 10,000 CFM (4,720 L/s) of mixed air entering the coil at 80°F DB / 67°F WB. The required leaving condition is 55°F DB / 54°F WB. Chilled water is supplied at 44°F with a 12°F rise.

These figures go to the manufacturer as the selection basis: 10,000 CFM, 80/67°F entering, 55/54°F leaving, 44°F CHW with a 12°F rise, maximum 500 fpm face velocity. Check their selection against the rows, water pressure drop and air pressure drop above. Try your own numbers in the Cooling Coil Calculator.

Common mistakes

• Selecting the coil on room load instead of the full coil load including outdoor air.
• Specifying only total capacity, so the coil meets tons but misses the latent load.
• Asking for a leaving condition whose ADP is lower than the chilled water can reach.
• Pushing face velocity above 500 fpm and getting water carryover into the ductwork.
• Forgetting that extra rows add air pressure drop, which the fan must overcome.

For AHU coil and drain pan installation details, see the Air Handling Unit Installation Detail DWG.

Frequently asked questions

What is apparatus dew point (ADP)?

It is the effective surface temperature of the cooling coil, found where the coil process line meets the saturation curve on the psychrometric chart.

What is a good bypass factor for a cooling coil?

For comfort cooling with chilled water, about 0.05 to 0.15. Lower values need more rows or more fins.

What face velocity should a cooling coil have?

At or below about 500 fpm (2.5 m/s) to prevent condensate carryover. 400 to 500 fpm is common.

How many rows should a chilled water coil have?

Most comfort coils have 4 to 8 rows. The required number comes from the bypass factor needed to reach the leaving air condition.

What is external static pressure?

External static pressure (ESP) is the pressure the fan must develop to push air through everything outside the unit: supply ducts, return ducts, fittings, dampers, attenuators, diffusers and grilles. It is the number you put on the AHU or FCU schedule so the manufacturer can select the right fan.

ESP is not the same as total static pressure (TSP). TSP is ESP plus the internal losses of the unit itself (filters, coil, mixing box, heat recovery). Manufacturers add the internal losses; the designer provides the ESP.

Get ESP too low and the unit cannot deliver design airflow, so rooms stay warm. Get it too high and the fan runs louder, uses more power and the balancing dampers end up nearly closed.

Step 1: Find the index run

ESP is set by the single path with the highest resistance, called the index run or critical path. You do not add up every branch in the system. Usually the index run is the longest duct run with the most fittings, but a shorter run with a sound attenuator or extra dampers can sometimes be worse, so check two or three candidate runs.

Do this separately for the supply side and the return side.

Step 2: Calculate straight duct friction loss

Multiply the length of each section by its friction rate:

Straight duct loss = Length × Friction rate / 100

For low-pressure comfort systems sized by the equal friction method, the friction rate is typically 0.08 to 0.10 in.wg per 100 ft (0.65 to 0.8 Pa/m). If you sized ducts with a ductulator, use the same friction rate here. You can check any section with the Duct Friction and Velocity Calculator. For the difference between sizing methods, see Equal Friction vs Velocity Method.

Step 3: Add fitting losses

Every elbow, tee, transition and takeoff adds a loss. The most accurate method uses the fitting loss coefficient (C) from the ASHRAE Duct Fitting Database or SMACNA tables:

Fitting loss (in.wg) = C × Velocity pressure

Velocity pressure (in.wg) = (V / 4005)², with V in fpm

In SI: Velocity pressure (Pa) = 0.6 × v², with v in m/s (standard air).

Typical C values: smooth radius elbow 0.15 to 0.25, mitred elbow without vanes about 1.2, mitred elbow with turning vanes 0.2 to 0.35, branch takeoff 0.4 to 1.0 depending on flow ratio.

Step 4: Add terminal devices and accessories

Use manufacturer data at the actual airflow wherever you have it. When you don’t, these typical values are reasonable for preliminary design:

Step 5: Repeat for the return side, add a safety margin

Calculate the return index run the same way. Then:

ESP = Supply index run loss + Return index run loss + Safety margin

A 10 to 15% margin is normal. Avoid stacking margins (on friction rate, on fittings and again on the total), which is how ESP ends up double what the system needs.

External static pressure calculation: worked example for a 4,000 CFM AHU

An AHU supplies 4,000 CFM (1,888 L/s) to an office floor. Supply index run is 150 ft (46 m) with a friction rate of 0.10 in.wg/100 ft. Main duct velocity near the elbows is 1,200 fpm; branch velocity is 900 fpm. Return index run is 60 ft (18 m) at 0.08 in.wg/100 ft.

Velocity pressure at 1,200 fpm = (1200 / 4005)² = 0.090 in.wg

Velocity pressure at 900 fpm = (900 / 4005)² = 0.050 in.wg

With ESP fixed, the manufacturer adds internal losses to get TSP. You can then estimate fan shaft power with the AHU Fan Shaft Power Calculator, or run your own numbers in the Fan Static Pressure Calculator.

ESP for ducted fan coil units

Ducted FCUs are usually available in standard ESP ratings of roughly 0.1 to 0.3 in.wg (25 to 75 Pa). Keep FCU duct runs short and simple, calculate the index run the same way, and pick the next standard ESP rating above your result. A high-ESP FCU selected “to be safe” is a common cause of noisy ceilings in hotel rooms and apartments.

Common mistakes

• Including the coil and filter losses in ESP. Those are internal to the unit and already counted by the manufacturer.
• Adding up all branches instead of following only the index run.
• Ignoring fittings. On short, fitting-heavy runs, fittings can be more than half the total loss.
• Using catalogue losses at the wrong airflow. Diffuser loss roughly quadruples when airflow doubles.
• Stacking safety factors at every step.

For real-world plant room layouts showing AHU connections, attenuators and dampers, download the AHU Plant Room Plan and Sections Shop Drawing.

Frequently asked questions

What is a typical ESP for an AHU?

For low-pressure comfort systems, 0.5 to 1.5 in.wg (125 to 375 Pa) is common. Systems with VAV boxes, long runs or attenuators sit at the higher end.

What is the difference between ESP and TSP?

ESP covers losses outside the unit. TSP is ESP plus the unit’s internal losses such as filters, coils and mixing sections.

Should I include the filter in ESP?

Not if the filter is inside the AHU. Include it only when it is a separate filter section or grille installed in the ductwork.

How do I convert in.wg to Pa?

1 in.wg = 249 Pa. So 0.85 in.wg is about 212 Pa.