Calculate Condensate From Cooling Coil: A Practical Guide

The condensate produced by a cooling coil is a key factor in HVAC system design, drainage planning, and indoor humidity control. This article explains how to calculate condensate generation using common inputs such as air flow, humidity ratio changes, and entering conditions. Practitioners can apply the method to estimate drain requirements, verify coil performance, and troubleshoot condensate issues.

Understanding Condensate Basics

Condensate forms when moist air is cooled below its dew point, causing water vapor to condense on the coil surface. The amount of condensate depends primarily on three factors: the amount of dry air flowing across the coil, the change in humidity ratio of the air (before and after cooling), and the properties of the air being treated. In HVAC terms, the condensate rate is the mass of water removed per unit time, typically measured in pounds per hour (lb/hr) or gallons per hour (gal/hr).

Key Equations for Condensate Calculation

The standard approach uses the humidity ratio, W, which is the mass of water per mass of dry air. The condensate rate, Q_cond (in lb/hr), is calculated as:

Q_cond = m_dot_dry × ΔW

Where:

• m_dot_dry = mass flow rate of dry air (lb dry air per hour). In the U.S. customary system, m_dot_dry can be estimated from airflow (CFM) using m_dot_dry ≈ CFM × 0.075 lbm/ft³.
• ΔW = W_in − W_out, the change in humidity ratio across the coil (lb water per lb dry air).

To express condensate in gallons per hour (gal/hr), use:

Q_cond (gal/hr) = [m_dot_dry × ΔW] / 8.34

Notes:

• W_in is the humidity ratio entering the coil; W_out is the humidity ratio leaving the coil.
• All humidity ratios should be based on consistent conditions, typically at standard pressure (about 14.7 psi) unless a different ambient pressure is provided.
• For precise results, psychrometric data or a psychrometric chart can determine W_in and W_out from temperature and relative humidity inputs.

Step-by-Step Calculation Guide

1. Obtain air flow and conditions: Record the coil’s supply air CFM and the entering air conditions (temperature and relative humidity) as well as the leaving air conditions after the coil (typically at the evaporator coil exit).
2. Determine humidity ratios: Use psychrometric charts or calculators to find W_in and W_out from the recorded conditions.
3. Compute ΔW: Subtract W_out from W_in to obtain the humidity ratio change across the coil.
4. Calculate mass flow of dry air: Convert CFM to m_dot_dry with m_dot_dry ≈ CFM × 0.075 lbm/ft³.
5. <strongFind condensate rate: Multiply m_dot_dry by ΔW to get Q_cond in lb/hr; convert to gal/hr by dividing by 8.34.

Worked Example (U.S. Customary Units)

Scenario: A cooling coil handles 1,200 CFM of supply air. Inlet air has W_in = 0.018 lb water per lb dry air; outlet air after the coil has W_out = 0.009 lb water per lb dry air. Assume standard pressure conditions for psychrometric data.

Calculation:

• m_dot_dry ≈ 1,200 × 0.075 = 90 lb dry air per hour.
• ΔW = 0.018 − 0.009 = 0.009 lb water per lb dry air.
• Q_cond = 90 × 0.009 = 0.81 lb of water per hour.
• Q_cond (gal/hr) = 0.81 / 8.34 ≈ 0.097 gal/hr.

Interpretation: With these inputs, the coil would generate roughly 0.1 gallons of condensate per hour under steady operation. If the coil handles larger air flow or a greater humidity ratio drop, condensate production will increase proportionally. For design and drainage planning, this method provides a baseline estimate that can be scaled to real-world variability.

Practical Considerations for Accurate Results

• <strongAccurate inputs matter: Use measured or verified readings for CFM, inlet and outlet temperatures, and humidity to improve accuracy.
• <strongHumidity ratio data: When precise W_in and W_out aren’t available, use psychrometric charts or software to estimate them from temperature and relative humidity readings.
• <strongEffect of coil high-rise conditions: Different coil configurations, airflow distribution, and coil surface roughness can affect condensate drainage and surface condensation patterns, potentially altering ΔW.
• <strongUnit consistency: Ensure all inputs are in consistent units (imperial for U.S. calculations) to avoid conversion errors.
• <strongReal-world factors: Some condensate may be recaptured or re-evaporated in humid climates, which can affect net condensate collection at the drain pan. Include these considerations in system design.

Common Applications and Implications

Accurately estimating condensate helps in:

• Designing proper condensate drain piping and pan capacity to prevent overflow and water damage.
• Sizing condensate pumps and control strategies for uninterrupted operation.
• Predicting humidity control effectiveness and comfort levels in occupied spaces.
• Diagnosing abnormal condensate rates that could indicate coil fouling, air leakage, or sensor errors.

Tips for Field Verification

• Measure actual CFM with a flow hood or anemometer, then cross-check with airflow commissioning data.
• Take temperature and humidity measurements at supply and coil outlet to compute W_in and W_out more accurately.
• Inspect drain pans and lines for blockages or leaks that could skew perceived condensate generation or drainage performance.
• Use data logging over a typical weather day to capture variations in condensate production and drainage requirements.

Further Resources and Tools

Engineers can leverage:

• Psychrometric charts and calculators to derive humidity ratios from temperature and relative humidity data.
• HVAC software packages that model coil performance and moisture removal under dynamic conditions.
• Standards and guidelines from building codes and ASHRAE for condensate handling and drainage.