Cooling and Heating Coil Load Calculator: Sensible, Latent and Condensate

The load on an air cooling coil split into sensible and latent parts, with the condensate and the sensible heat ratio, or the load on a heating coil.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

A cooling coil takes heat out of air. If the coil surface is colder than the dew point of the air, moisture also condenses out. The total load is the change in enthalpy of the air times its mass flow. Part of it is sensible (the cooling of the air itself) and part is latent (the heat given up by the water vapour that condenses).

The mass flow of dry air comes from the volume flow and the specific volume of the air at the coil entry. The humidity ratio and enthalpy at each state come from the temperature and relative humidity with the ASHRAE psychrometric equations. The condensate is the air mass flow times the fall in humidity ratio.

The sensible heat ratio is the sensible load divided by the total load. A low ratio means the coil is doing a lot of dehumidification, which is typical of humid climates and of spaces with many people. A heating coil only warms the air, so the humidity ratio does not change and the relative humidity falls.

Formulas used

  • \dot m_a = Q / v_1
  • Q_t = \dot m_a (h_1 - h_2)
  • Q_s = \dot m_a (1.006 + 1.86 W_2)(T_1 - T_2)
  • Q_l = Q_t - Q_s
  • \dot m_c = \dot m_a (W_1 - W_2)

Worked example

With the default numbers in the calculator above:

  1. Entering air: humidity ratio, enthalpy, specific volume: 61.45 kJ/kg
  2. Mass flow of dry air: 1.148 kg/s
  3. Leaving air: humidity ratio and enthalpy: 34.73 kJ/kg
  4. Total cooling load: 30.68 kW
  5. Sensible load (the cooling of the air): 17.6 kW
  6. Latent load (the moisture removed): 13.08 kW
  7. Sensible heat ratio: 0.5737
  8. Condensate removed: 18.44 kg/h

Change the numbers in the calculator to see the full working, every step, for your own case.

Common questions

What is the sensible heat ratio?

It is the sensible load divided by the total load. A ratio of 1 means no dehumidification; air-conditioning of a humid space is often 0.6 to 0.8.

What humidity does a cooling coil leave?

Air leaves a cooling coil that is dehumidifying at a high relative humidity, typically 90 to 95 percent.

How much condensate will there be?

The calculator gives it in kilograms (litres) per hour. The coil needs a drain pan and a trapped drain sized for that flow.

Related calculators

Further reading: ASHRAE Handbook: Fundamentals (psychrometrics) and Systems and Equipment; Cengel and Boles, Thermodynamics: An Engineering Approach; Moran and Shapiro, Fundamentals of Engineering Thermodynamics.

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