Mixed-Air Humidity Complaint on a 5,000 cfm Air Handler
A 5,000 cfm built-up air handler serves an open-plan office. On a design day the return air measures 75°F dry-bulb and 63°F wet-bulb, while outdoor air is 95°F dry-bulb and 78°F wet-bulb. The economizer is at minimum, so the unit draws 1,000 cfm outdoor and 4,000 cfm return. The cooling coil is selected to leave air at 55°F saturated. For two weeks the owner has logged 75°F space temperature with 60-63% relative humidity, and condensate is dripping from the unit casing instead of reaching the drain. The building operator reports that space temperature is acceptable, so the complaint is humidity, not cooling. A technician proposes lowering the supply-air temperature setpoint to 52°F. The existing coil is rated 15 tons total at 80°F entering dry-bulb/67°F entering wet-bulb, with 105,000 Btu/h sensible capacity at that rating point.
1. Estimate the mixed-air dry-bulb, humidity ratio, and enthalpy entering the coil. Is the entering condition inside or outside the coil's 80/67 rating point?
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Mix dry-bulb linearly by airflow: (0.80 × 75°F) + (0.20 × 95°F) = 79°F. Humidity ratio is also approximately linear by mass flow for these conditions: return at 75/63 is about 0.0092 lb/lb, outdoor at 95/78 is about 0.0164 lb/lb, so mixed W ≈ (0.80 × 0.0092) + (0.20 × 0.0164) = 0.0106 lb/lb. Enthalpy: return ≈ 28.2 Btu/lb, outdoor ≈ 41.3 Btu/lb, mixed ≈ (0.80 × 28.2) + (0.20 × 41.3) = 30.8 Btu/lb.
The entering dry-bulb of 79°F is close to the 80°F rating dry-bulb, but the entering wet-bulb is roughly 65°F, below the 67°F rating wet-bulb. The coil is therefore not outside its rating envelope; if anything, the lower wet-bulb means a slightly lower entering enthalpy and less latent load than the rating point. The problem is not that the coil is seeing an impossible entering condition.
2. Using the psychrometric values, estimate total coil load, sensible load, and sensible heat ratio (SHR). Can the 15-ton coil with 105,000 Btu/h sensible capacity hold 75°F at this design condition?
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Total load: 4.5 × cfm × Δh = 4.5 × 5,000 × (30.8 − 23.0) ≈ 175,500 Btu/h, or about 14.6 tons. Sensible load: 1.08 × 5,000 × (79 − 55) = 129,600 Btu/h. Latent load is the remainder, about 45,900 Btu/h, so SHR ≈ 129,600 ÷ 175,500 ≈ 0.74.
The proposed coil has only 105,000 Btu/h sensible capacity at its rating point. That is well below the estimated 129,600 Btu/h sensible load, so at the stated design condition this coil cannot hold 75°F even if it removes enough moisture. The unit may currently be holding temperature only because the actual load is below design, because airflow is higher than assumed, or because the space is being overcooled at times. Capacity must be checked against the actual entering conditions, not nominal tonnage alone.
3. The technician lowers the supply-air setpoint to 52°F. Explain why this may not fix the humidity and may create a new problem. What is the more likely root cause given the drain symptom?
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A direct-expansion coil selected for 55°F saturated leaving air has an apparatus dew point near 55°F. Lowering the setpoint does not guarantee a 52°F leaving condition unless the coil, expansion valve, and airflow can produce a lower ADP. It may simply drive the space colder, increase runtime and energy use, or freeze the coil if the refrigerant temperature falls too low. If humidity remains high, the added sensible cooling is not addressing the moisture source.
The outdoor-air latent load is large: roughly 4,840 × 1,000 × (0.0164 − 0.0092) ≈ 34,800 Btu/h, which is about three-quarters of the coil's estimated latent load. Excessive minimum outdoor air is therefore a prime suspect. The drain symptom points to a second problem: condensate is not being captured, possibly because of a plugged pan, missing or incorrectly sized trap, rusted-through pan, or biological growth blocking the coil and drain. Standing water and fouled fins reduce latent capacity and raise space relative humidity. The correct response is to verify and reduce outdoor air to the code minimum, inspect and repair the drain pan and trap, clean the coil, and then recheck capacity rather than lowering the setpoint alone.