HVAC / cooling coil

Cooling coil selection and part-load curve

A coil is not a fixed capacity: it is a surface temperature plus a bypass factor. This selector places the apparatus dew point, finds the leaving state on the coil line, converts it into sensible and latent capacity, and traces what happens to that capacity as the airflow falls.

CoilWetContact factor74.3%Condensate11.2 L/h

Total capacity

23.29 kW

15.47 sensible · 7.82 latent

Sensible heat ratio

0.664

Wet coil, latent load present

Apparatus dew point

8.5 °C

Water 7.0 °C + 1.5 K approach

Coil rows

NTU 1.36 · BF 0.257

Psychrometric state points

The coil line runs from the entering air to the apparatus dew point; the leaving state sits on it at the bypass factor.

23.3 kW

Psychrometric state points
State	Dry bulb (°C)	RH (%)	W (g/kg)	h (kJ/kg)
Entering air	26.0	50	10.47	52.9
Apparatus dew point	8.5	100	6.88	25.9
Leaving air	13.0	84	7.80	32.8

Dry-air mass flow 1.160 kg/s · chilled water 1.01 L/s at 5.5 K rise.

Part-load capacity curve

Capacity falls more slowly than airflow because the bypass factor drops as NTU rises.

Sensible and latent split at part load

The leaving air moves toward the ADP, so the latent share grows as the load falls.

Canonical equations

The exact relationships implemented by this engine.

ADP = t_water,in + approach · The effective surface temperature of the coil.

BF = (t_leaving − ADP) / (t_entering − ADP) · Fraction of air that misses the surface.

Q = ṁ_da · (h₁ − h₂) · Sensible plus latent, from the enthalpy difference.

NTU = −ln BF, rows = ⌈NTU / NTU_row⌉ · Each row adds a fixed amount of NTU.

Show model notation and computationOpen the symbolic form, the variable definitions, and the step-by-step derivation when you need the detail.

A clear path from inputs to results.

A cooling coil is a surface temperature plus a bypass factor, not a fixed capacity. This selector places the apparatus dew point, finds the leaving state on the coil line, converts it into sensible and latent capacity, sizes the rows and the water flow, and traces what happens to all of it as the airflow falls.

Step-by-step methodWorked exampleAssumptions explained

How to use

Enter the airflow and the entering dry-bulb and relative humidity.

Set the leaving dry-bulb the space needs.

Enter the chilled-water temperature and the approach to the apparatus dew point.

Set the NTU per row and the water temperature rise.

Read the capacity, the SHR, the rows and the part-load curve.

The formula

\mathrm{ADP} = t_w + \Delta t_{app} \;;\; \mathrm{BF} = \frac{t_2 - \mathrm{ADP}}{t_1 - \mathrm{ADP}} \;;\; Q = \dot m_{da}(h_1 - h_2)

Reference: ASHRAE Handbook — Fundamentals, psychrometrics and cooling-coil apparatus dew point; Carrier psychrometric chart construction.

Worked example

1 m³/s from 26 °C / 50% to 13 °C on 7 °C water

Inputs

Airflow 1 m³/s at 26 °C dry bulb, 50% relative humidity.
Leaving dry bulb 13 °C.
Water in at 7 °C, approach 1.5 K, 0.35 NTU per row, 5.5 K water rise.

Calculation path

1
ADP = 7 + 1.5 = 8.5 °C, below the entering dew point, so the coil runs wet.
2
BF = (13 − 8.5) / (26 − 8.5) = 0.257.
3
W₂ = 0.00688 + 0.257 × (0.01047 − 0.00688) = 0.00780 kg/kg, about 7.8 g/kg.
4
Enthalpy falls from 52.8 to 32.8 kJ/kg, so Q = 1.188 × 20.0 = 23.3 kW.
5
NTU = −ln 0.257 = 1.36, so four rows at 0.35 NTU each.
6
Condensate 11.2 L/h; water flow 23.3 / (4.186 × 5.5) = 1.01 L/s.

Result

23.3 kW · SHR 0.664 · ADP 8.5 °C · 4 rows · 1.01 L/s.

Interpreting the result

The sensible heat ratio is set by the coil line, so it moves with the ADP and the entering air, not with the airflow. That is why a coil cannot be re-rated for a different entering condition by scaling capacity: a colder surface dehumidifies more, a warmer one less. The part-load curve is concave because NTU rises as the airflow falls, so capacity drops more slowly than flow.

Assumptions & limitations

Chilled-water coil with a constant ADP at part load; a DX coil would drop its ADP and remove relatively more moisture.

Bypass factor follows BF = e^−NTU with a fixed NTU per row.

One-dimensional coil line from the entering state to the ADP; no carry-over or reheat.

Standard atmospheric pressure, dry-air mass flow from the entering density.

Frequently asked questions

Why is my coil dry?

The apparatus dew point is above the entering dew point, so the surface never reaches saturation and no moisture condenses. Lower the water temperature or the approach.

Why does capacity fall more slowly than airflow?

At lower airflow the same coil surface has more NTU per unit of air, so the bypass factor drops and each kilogram of air leaves closer to the ADP.

What does the bypass factor mean physically?

It is the share of the air that passes through the coil without touching the surface. A BF of 0.257 means about a quarter of the air leaves essentially untreated.

How many rows should I choose?

Rows follow from the bypass factor the duty needs. More rows buy a lower BF, which buys deeper dehumidification at the cost of fan and water pressure.