Mechanical MEP, Psychrometrics & Data-Center Cooling
HVAC Sensible Heat Ratio (SHR) & Coil Tons Formula
Calculate ASHRAE Sea-Level Air-Side Sensible Cooling Q_s = 1.08 * CFM * Delta_T_F, Latent Dehumidification Q_L = 0.68 * CFM * Delta_W_grains, Total Chiller Tons (/ 12,000), and Sensible Heat Ratio SHR = Q_s / Q_total.
Separate dry-bulb temperature drop from latent moisture condensation across cooling coils!
Sizing an air-handling cooling coil solely from dry-bulb thermostat drop ignores the massive latent heat of vaporization released when humid outdoor makeup air condenses on chilled fins! Under ASHRAE standard sea-level air density (0.075 lb/ft^3, 60 min/hr, c_p = 0.24 BTU/(lb*°F), and 7,000 grains/lb moisture latent heat), Air-Side Sensible Cooling Load is Q_s (BTU/h) = 1.08 * CFM * Delta_T_F and Latent Dehumidification Load is Q_L (BTU/h) = 0.68 * CFM * Delta_W_grains_lb! Dividing Total Coil Load Q_tot = Q_s + Q_L by 12,000 BTU/h gives Refrigeration Tons, while Sensible Heat Ratio SHR = Q_s / Q_tot distinguishes muggy natatorium/auditorium coils (SHR ~ 0.60–0.70) from dry sensible-only AI server halls (SHR ~ 0.95–1.00)!
Verify on a Commercial AHU Coil—Supply_Airflow = 5,000 CFM (B2), Dry_Bulb_Cooling_Drop Delta_T = 20 °F (75 °F mixed air cooled to 55 °F supply, B3), and Humidity_Ratio_Drop Delta_W = 15 grains/lb (80 gr/lb dried to 65 gr/lb, B4): Sensible Load Q_s is 1.08 * 5,000 * 20 = 108,000 BTU/h (9.00 Tons, E2); Latent Load Q_L is 0.68 * 5,000 * 15 = 51,000 BTU/h (4.25 Tons, E3); Total Cooling Capacity is 159,000 / 12,000 = 13.2500 Tons (E4); and Coil Sensible Heat Ratio SHR is 108,000 / 159,000 = 0.6792 (E5)!
| ASHRAE Air-Side Coil Metric (5,000 CFM, Delta_T=20 °F, Delta_W=15 gr/lb) | Formula | Verified Output |
|---|---|---|
| Sensible Cooling Load Q_s (BTU/h) | 1.08 * CFM * Delta_T_F | 108,000 BTU/h (9.00 Tons) |
| Latent Moisture Load Q_L (BTU/h) | 0.68 * CFM * Delta_W_grains | 51,000 BTU/h (0 BTU/h Dry Coil) |
| Total Coil Refrigeration Tons | (Q_s + Q_L) / 12000 | 13.2500 Tons (9.0000 Dry Coil) |
| Sensible Heat Ratio (SHR) | Q_s / (Q_s + Q_L) | 0.6792 (1.0000 Dry Server Hall) |
ASHRAE Sensible Heat Ratio (SHR) & Coil Tonnage Formula
Validate CFM > 0, non-negative Delta_T and Delta_W, and strictly positive total enthalpy drop (Delta_T + Delta_W > 0)!
=LET(cf_m, B2, dt_f, B3, dw_g, B4, IF(OR(cf_m <= 0, dt_f < 0, dw_g < 0, dt_f + dw_g <= 0), "Invalid Psychrometric Coil Load", LET(q_sn, 1.08 * cf_m * dt_f, q_lt, 0.68 * cf_m * dw_g, ROUND(q_sn / (q_sn + q_lt), 4))))Dry Data-Center CRAH Coil Check (Delta_W = 0 grains/lb)
Set Humidity_Drop B4 = 0 grains/lb (chilled water above dew point in a sealed server containment aisle): Latent Load E3 drops to 0, Total Tons E4 equals 9.0000, and SHR E5 reaches 1.0000!
=IF(ISTEXT(E5), E5, ROUND((E2 + E3) / 12000, 4))Try the example
Use the original values or change them to test your understanding.