Cooling-Coil Bypass and Latent Capacity at Two Apparatus Dew Points
A 10,000 cfm built-up AHU at sea level takes mixed air at 80°F dry-bulb and 67°F wet-bulb. The chilled-water coil has a manufacturer's bypass factor of 0.15 and an apparatus dew point of 50°F. The space sensible load is 300,000 Btu/h and latent load is 90,000 Btu/h. The owner wants to raise the chilled-water temperature to improve chiller efficiency, which would raise the ADP to 55°F. Coil airflow and bypass factor stay the same. The space is to be held near 75°F and 50% RH.
1. Estimate the leaving-air dry-bulb and humidity ratio at ADP 50°F. Does condensate form?
Read the worked answer
Use approximate sea-level psychrometrics. Entering air at 80°F DB/67°F WB is about W1 = 0.0116 lb/lb and h1 = 31.9 Btu/lb. Saturated air at 50°F is about W_adp = 0.00766 lb/lb and h_adp = 20.3 Btu/lb.
With bypass factor BF = 0.15:
W2 = W_adp + BF(W1 - W_adp) = 0.00766 + 0.15(0.0116 - 0.00766) = 0.00825 lb/lb
h2 = h_adp + BF(h1 - h_adp) = 20.3 + 0.15(31.9 - 20.3) = 22.0 Btu/lb
Solve h2 = 0.240T + W2(1061 + 0.444T):
22.0 = 0.240T + 0.00825(1061 + 0.444T)
T2 ≈ 54.5°F
Because W2 < W1, moisture is removed; condensate forms. The leaving air is about 54.5°F DB and 0.00825 lb/lb.
2. Compute total and latent capacity at ADP 50°F and compare them with the space loads.
Read the worked answer
Mass flow: 10,000 ft³/min × 0.075 lb/ft³ × 60 min/h = 45,000 lb/h.
Total coil capacity:
Q_total = 45,000(31.9 - 22.0) = 445,500 Btu/h ≈ 37.1 tons
Latent capacity, using about 1,076 Btu/lb of water vapor:
Q_latent = 45,000(0.0116 - 0.00825)(1,076) ≈ 162,000 Btu/h ≈ 13.5 tons
Space loads are 300,000 Btu/h sensible plus 90,000 Btu/h latent = 390,000 Btu/h total, or 32.5 tons, with 7.5 tons latent.
At full flow the coil has enough total and latent capacity. However, the coil capacity exceeds the space load, so at full flow the space will overcool unless the coil or airflow is modulated. At part load, the fixed bypass factor and coil ADP will change the sensible heat ratio, so humidity control must be checked at the minimum-load condition, not just design.
3. Evaluate the proposed ADP increase to 55°F. Will the coil still meet the latent load and space humidity?
Read the worked answer
At ADP 55°F, saturated air is about W_adp = 0.0092 lb/lb and h_adp = 23.2 Btu/lb.
W2 = 0.0092 + 0.15(0.0116 - 0.0092) = 0.00956 lb/lb
h2 = 23.2 + 0.15(31.9 - 23.2) = 24.5 Btu/lb
Total capacity becomes:
Q_total = 45,000(31.9 - 24.5) = 333,000 Btu/h ≈ 27.8 tons
Latent capacity becomes:
Q_latent = 45,000(0.0116 - 0.00956)(1,076) ≈ 98,800 Btu/h ≈ 8.2 tons
The total capacity is now below the 32.5-ton space load, so the space temperature cannot be held at design. The latent capacity is only slightly above the 7.5-ton latent load, leaving almost no part-load margin. The proposed ADP increase is not acceptable without adding coil surface/airflow or separate dehumidification. Chiller efficiency gains would be offset by lost space control.