10.6 Cooling & Heating Coil Selection: Face Velocity, Rows, Circuiting & Freeze Protection

10.6 Cooling & Heating Coil Selection: Face Velocity, Rows, Circuiting & Freeze Protection

Cooling and Heating Coils is its own NCEES sub-topic (3F), separate from the condensers and evaporators of 3D. Section 3.5 established the psychrometric behavior of a coil through the apparatus dew point and bypass factor. This section is about specifying the physical device: how large its face is, how deep it is, how the water is routed through it, and how it is kept from freezing.

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1. The Three Capacity Equations Must Agree

A coil selection is not finished until the air side and the water side reconcile.

All four constants are density constants. At 5,000 ft they scale by roughly 0.83 on the air side; the water-side 500 does not change.

Worked Example - Complete Coil Selection

Select a chilled water cooling coil for 12,000 cfm entering at 80 F dry-bulb / 67 F wet-bulb (h = 31.4 Btu/lb) and leaving at 55 F dry-bulb / 54 F wet-bulb (h = 22.6 Btu/lb), using 44 F entering water with a 12 F water temperature rise.

Total capacity:

• 4.5 x 12,000 x (31.4 - 22.6) = 4.5 x 12,000 x 8.8 = 475,200 Btu/hr = 39.6 tons

Sensible capacity:

• 1.08 x 12,000 x (80 - 55) = 324,000 Btu/hr

Latent capacity and sensible heat ratio:

• Latent = 475,200 - 324,000 = 151,200 Btu/hr
• SHR = 324,000 / 475,200 = 0.68

Required water flow:

• gpm = 475,200 / (500 x 12) = 79.2 gpm

Face area at a 500 fpm face velocity:

• A = 12,000 / 500 = 24 ft2, for example a coil 6 ft wide by 4 ft high

Cross-check the latent term independently: with entering W of about 0.0112 and leaving W of about 0.0086, 4,840 x 12,000 x 0.0026 gives 151,000 Btu/hr. The two agree, which confirms the chart readings are consistent.

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2. Face Velocity: The Governing Selection Parameter

$A_{face} = \frac{Q}{V_{face}}$

Carryover is the hard limit, and it is a wet-coil phenomenon. A heating coil, or a cooling coil operating dry above the dew point, can be pushed considerably faster. Air-side pressure drop grows roughly with the square of face velocity, so the fan energy penalty compounds quickly.

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3. Rows, Fins, and Diminishing Returns

Rows are the number of tube banks in the direction of airflow; fin spacing is typically 8 to 14 fins per inch.

Each additional row sees air that has already been cooled, so the driving temperature difference between the air and the coil surface is smaller and the row contributes less capacity than the one before it. Meanwhile every row adds essentially the same air pressure drop. The practical result:

Tighter fin spacing raises capacity per unit of face area but also raises pressure drop, worsens fouling, and makes cleaning harder - which is why hospital and laboratory coils are often specified at 8 to 10 fpi rather than 14.

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4. Circuiting and Water-Side Hydraulics

Circuiting determines how many parallel water paths run through the coil, and it is how the designer reconciles two competing requirements: enough water velocity for good heat transfer, and not so much pressure drop that pumping becomes expensive.

Design water velocity is normally held to 2 to 8 ft/s:

• Below 2 ft/s the Reynolds number can fall toward the laminar range, collapsing the tube-side heat transfer coefficient. The coil then fails to make capacity even though the flow rate looks correct on paper.
• Above 8 ft/s erosion of the copper tube wall and flow noise become the limits.

Coils are always piped counterflow - water enters at the air-leaving face - so the coldest water meets the coldest air. Reversing the connections can cost 15% or more of capacity, and it is a classic commissioning finding.

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5. Freeze Protection on Outdoor Air Coils

Any coil that can see outdoor air below 32 F is a freeze risk, and a burst coil floods a mechanical room. The protections are layered:

1. Low-limit ("freezestat") thermostat. A capillary averaging element senses the lowest temperature anywhere along its length, so a small cold stratification stripe trips it. The element must be long enough to cover the face: the standard rule is at least 1 linear foot of element per square foot of coil face area, serpentined across the face. For the 24 ft2 coil above, that is at least 24 ft of element.
2. Preheat coil upstream, holding mixed air above roughly 40 F before it reaches the chilled water coil.
3. Face-and-bypass dampers, which modulate airflow across the coil while keeping full flow through the tubes - historically the most robust arrangement for 100% outdoor air, because the coil never sees reduced water flow.
4. Glycol, typically 30% propylene glycol for burst protection, at the cost of about 15% reduced heat transfer and higher pumping power that must be included in the selection.
5. Constant full water flow with valve control on the coil bypass rather than throttling flow through the coil, since a throttled coil in cold air freezes fastest.

Steam preheat coils in outdoor air service must additionally use non-freeze (steam distributing) tube construction, be pitched to drain, be trapped with a properly sized float and thermostatic trap, and never be throttled by a modulating steam valve - throttling causes the condensate to stall and freeze inside the tube.