You have a cooling load. Now what? The number on a calc sheet doesn’t size an AHU coil — the psychrometric process between the room and the cooling coil does. This article walks through the bridge: how room cooling load becomes supply air flow, supply air condition, coil entering condition, and final coil duty.
The five air conditions that matter
For any constant-volume AHU serving a single zone, identify five air states:
1. Room (R) — the design indoor condition. Office: 24°C / 50 % RH. Hotel guestroom: 22°C / 55 % RH. Hospital ward: 22°C / 50 %. ICU: 21°C / 50 %.
2. Supply (S) — the air condition leaving the diffuser. Typically 12-14°C / 90-95 % RH for a CAV system, or whatever VAV minimum reset target you’ve chosen.
3. Return (RA) — same as room (R) for fully-mixed terminal load.
4. Outside (O) — design outdoor condition for your climate (Mumbai 36°C / 80 %, Delhi 41°C / 25 %, Pune 38°C / 35 %).
5. Mixed (M) — the mix of return + outside that enters the coil. (mass-weighted: w_M = (m_RA × w_RA + m_O × w_O) / m_total).
The coil sees state M at the entering side and produces state S at the leaving side. Coil duty = m_total × (h_M − h_S). That’s your selection number, in kW.
Step-by-step calc walk-through
Inputs you already have:
- Q_sensible (sensible cooling load, kW) — from your cooling load calc
- Q_latent (latent cooling load, kW) — same source
- Q_total = Q_sensible + Q_latent
- SHR = Q_sensible / Q_total
- OA flow (L/s, by ASHRAE 62.1 VRP)
Step 1: Pick supply air condition.
Standard CAV: ΔT = 11°C between room and supply (24°C – 13°C). For high-latent (warm-humid) zones, you might choose 10°C supply for SHR matching.
Step 2: Compute supply air flow.
m_supply = Q_sensible / (1.006 × ΔT_sensible) [kg/s]
For 24°C room and 13°C supply, ΔT = 11°C, so m_supply ≈ Q_sensible / 11.07 [kg/s].
Step 3: Verify mass balance against latent.
Latent removal in supply air = m_supply × (w_R – w_S) × 2501 [kW]
This must equal Q_latent. If it doesn’t, either ΔT_sensible changes (push supply colder, supply less air) or you accept that part of the latent comes from OA pre-conditioning.
Step 4: Compute mixed air condition.
w_M = (m_RA × w_R + m_OA × w_O) / m_total
DBT_M = (m_RA × DBT_R + m_OA × DBT_O) / m_total
h_M ≈ 1.006 × DBT_M + w_M × (2501 + 1.86 × DBT_M)
Step 5: Compute coil duty.
Q_coil = m_total × (h_M – h_S) [kW]
This is your selection number. AHRI 410 ratings are at the leaving condition you specified; pick a coil rated at or below this duty.
Step 6: Coil entering and leaving water temperatures.
For 7°C/13°C chilled water: typical 4-row, 14 fpi coil delivers ~12.5°C leaving air with 16-18°C wet bulb at coil entering. Verify with the AHU vendor’s coil selection software.
Where engineers go wrong
Two failure modes dominate: over-supply and under-supply.
Over-supply happens when you size m_supply against Q_total instead of Q_sensible. The supply air flow ends up 30-40 % too high, the room over-cools at part-load, and you swing humidity to 65-70 %. Office IAQ complaints follow.
Under-supply happens when you ignore the latent contribution from OA. Your OA at 80 % RH carries 18-22 g/kg of moisture. The coil has to drop that to 9-10 g/kg. If the coil doesn’t have enough rows or chilled water flow, leaving air air ends up at 17-18 g/kg, the supply DBT looks fine, but the room creeps to 60-65 % RH within 2-3 hours of operation.
How MEPVAULT Calculators support this
Open Cooling Load Calculator → gives you Q_sensible, Q_latent, and SHR.
Open Psychrometric Analyzer → takes you the rest of the way: input your room + outdoor + mass flow ratios, pick “mixing” mode for state M, then “cool_dehumid” mode to compute coil duty between M and S.
From the Field — Engineer’s Notebook
On a 2024 data-centre office wing, the cooling load report came in showing SHR = 0.68 (latent-heavy because of 100 % OA make-up for the adjacent server farm). The AHU vendor offered a 4-row coil that delivered 13°C leaving DBT but only 11.5°C wet bulb — which means the coil was hitting condensation but not removing enough moisture. We pushed back to 6-row + 0.7 m/s face velocity, and the leaving wet bulb dropped to 10.0°C. The hard fact that drove the conversation was the SHR — 0.68 is too low for a 4-row coil and any decent psychrometric inspection would have flagged it. The vendor was sizing on total kW only.
5 common mistakes
1. Sizing supply flow on Q_total. Always Q_sensible. The latent comes off the same supply stream, but the flow rate is determined by the sensible side.
2. Picking a fixed ΔT regardless of climate. Hot-dry can use 12-14°C ΔT; warm-humid often needs 9-10°C ΔT to keep SHR matched.
3. Computing coil duty on (R – S) instead of (M – S). The coil sees mixed air, not return air. You’ll undersize by the OA latent every time.
4. Selecting AHRI ratings at the wrong leaving condition. AHRI 410 catalogue ratings are typically 27°C / 19°C WB entering at 7°C/13°C water — your job is to verify the actual entering condition against this and apply the catalogue’s correction factor.
5. Ignoring the post-coil reheat for humidity control. In warm-humid zones, you may need 1°C reheat to keep room RH under 60 % at part-load. Plan the coil duty + reheat together; don’t bolt reheat on later.
6. Forgetting fan + duct heat gain. The supply fan adds 0.5-1.0 kW per m³/s and ductwork picks up 1-3 % of room load. Add this to coil duty before final selection.
Designer’s checklist
- [ ] Cooling load calc complete (Q_sensible + Q_latent + SHR documented)
- [ ] Supply DBT chosen based on climate SHR profile (not a default 13°C)
- [ ] Mass flow computed on Q_sensible only
- [ ] Latent mass-balance verified at supply state
- [ ] OA flow per ASHRAE 62.1 (or NBC equivalent)
- [ ] Mixed air state computed (DBT, w, h)
- [ ] Coil duty in kW, not “TR” — AHU vendors quote kW for coil sizing
- [ ] Coil rows specified (4-row for hot-dry, 6-row for warm-humid baseline)
- [ ] Face velocity ≤ 2.5 m/s (catch-pan + drain pan checks)
- [ ] Drain pan slope + trap depth checked against IS 8543
Pairs with: Cooling Load Calculator, Psychrometric Analyzer, Cooling Load Methods Compared