Cooling Load Calculation using Psychrometric Chart

An advanced engineering tool for calculating total, sensible, and latent cooling loads based on air properties.

What is Cooling Load Calculation using a Psychrometric Chart?

A cooling load calculation is the process of determining the amount of heat energy that needs to be removed from a space to maintain a desired temperature and humidity. Using a psychrometric chart, or the formulas derived from it, allows HVAC engineers to analyze the properties of moist air and precisely calculate this load. Psychrometric charts graphically represent the thermodynamic properties of air, such as dry bulb temperature, wet bulb temperature, relative humidity, and enthalpy. By plotting the conditions of the air entering and leaving a cooling system on this chart, one can determine the change in heat content (enthalpy) and moisture content (humidity ratio). This is crucial for accurately sizing HVAC equipment.

The total cooling load is composed of two parts: the sensible load (related to the change in temperature) and the latent load (related to the change in moisture). A proper cooling load calculation ensures that an HVAC system is not oversized (leading to inefficiency and poor humidity control) or undersized (failing to cool the space adequately). This calculator automates the complex formulas behind the psychrometric chart to provide accurate load values for system design and analysis. For more complex building envelopes, consider a full HVAC design guide.

The Formulas Behind Psychrometric Cooling Load Calculation

The calculations performed by this tool are based on fundamental psychrometric equations. The core principle is to find the change in the air’s total heat content (enthalpy) as it passes through the cooling coil.

Total Cooling Load (Q_total): This is the total heat removed from the air. It’s calculated based on the difference in enthalpy between the entering and leaving air.

Q_total = ṁ × (h_in – h_out)

Sensible Cooling Load (Q_sensible): This is the heat removed to lower the air’s temperature.

Q_sensible = ṁ × c_p × (T_in – T_out)

Latent Cooling Load (Q_latent): This is the energy removed to condense moisture out of the air.

Q_latent = ṁ × h_fg × (W_in – W_out)

Description of variables used in the calculations. All units are SI for calculation consistency.

Variable	Meaning	Typical Unit (SI)	Typical Range
ṁ	Mass flow rate of air	kg/s	0.1 – 10
h	Enthalpy of moist air	kJ/kg	20 – 100
cp	Specific heat of moist air	kJ/kg·°C	~1.02
T	Dry Bulb Temperature	°C	0 – 50
hfg	Latent heat of vaporization of water	kJ/kg	~2501
W	Humidity Ratio	kgwater/kgdry air	0.005 – 0.025

This calculator also determines the Sensible Heat Ratio (SHR), which is Q_sensible / Q_total. This value is critical for selecting equipment that matches the specific sensible and latent load profile of a space. Explore our Duct Sizing Calculator for the next step in system design.

Practical Examples

Example 1: Standard Office Cooling

An office space requires cooling of 2000 CFM of air from 85°F and 50% RH down to a supply air condition of 55°F and 95% RH. This is a common scenario for a commercial building in summer.

• Inputs:
  • Entering Air: 85°F, 50% RH
  • Leaving Air: 55°F, 95% RH
  • Airflow: 2000 CFM
• Results (Approximate):
  • Total Cooling Load: ~69,000 BTU/hr (5.75 Tons)
  • Sensible Load: ~48,000 BTU/hr
  • Latent Load: ~21,000 BTU/hr
  • SHR: ~0.70

Example 2: Humid Climate Dehumidification

A building in a tropical climate needs to treat 5 m³/s of outside air at 32°C and 80% RH. The goal is to supply air at 18°C and 90% RH to maintain comfort and prevent mold.

• Inputs (Metric):
  • Entering Air: 32°C, 80% RH
  • Leaving Air: 18°C, 90% RH
  • Airflow: 5 m³/s
• Results (Approximate):
  • Total Cooling Load: ~275 kW
  • Sensible Load: ~85 kW
  • Latent Load: ~190 kW
  • SHR: ~0.31

Notice the very low SHR in the second example, indicating a high moisture removal requirement, which would influence equipment selection. You might need a system with enhanced dehumidification capabilities, as discussed in our guide on Advanced HVAC Systems.

How to Use This Cooling Load Calculator

1. Select Unit System: Choose between Imperial (°F, CFM) and Metric (°C, m³/s) units. The labels and default values will update automatically.
2. Enter Air Conditions: Input the dry bulb temperature and relative humidity for both the air entering the cooling coil and the desired air conditions leaving the coil.
3. Set Airflow and Pressure: Provide the volume of air to be cooled per unit of time (Airflow Rate) and the local atmospheric pressure (important for altitude corrections).
4. Analyze the Results: The calculator instantly provides the Total, Sensible, and Latent Cooling Loads, along with the Sensible Heat Ratio (SHR).
5. Visualize the Process: The simplified psychrometric chart plots the entering and leaving air states, drawing a line to represent the cooling and dehumidification process.
6. Copy or Reset: Use the ‘Copy Results’ button to save a summary to your clipboard or ‘Reset’ to return to default values.

Key Factors That Affect Cooling Load Calculation

The accuracy of a cooling load calculation depends on several key factors beyond just the air properties. While this tool focuses on the load across a coil, a whole-building load calculation must also consider:

• Solar Gain: Heat from sunlight passing through windows, walls, and roofs. South and west-facing windows are major contributors.
• Internal Heat Gains: Heat emitted by people, lights, computers, and other equipment within the space.
• Building Envelope: The insulating properties (R-value) of walls, roofs, and floors. Poor insulation increases the load.
• Ventilation and Infiltration: Outside air brought in for ventilation or leaking in through cracks and openings adds a significant thermal load.
• Occupancy: The number of people in a space contributes both sensible heat (body temperature) and latent heat (respiration/perspiration).
• Building Location and Orientation: The local climate (design temperatures) and the building’s orientation to the sun fundamentally define the external loads.

A comprehensive analysis might require tools like our Building Energy Modeling Tool.

Frequently Asked Questions (FAQ)

1. What is the difference between sensible and latent load?

Sensible load is the heat energy that changes the temperature of the air, which you can feel. Latent load is the “hidden” energy tied to the phase change of water (from vapor to liquid), which relates to removing moisture from the air. Both must be managed for comfort.

2. Why is Relative Humidity (RH) of the leaving air so high?

As air is cooled, its ability to hold moisture decreases. When air passes over a cold cooling coil, its temperature drops significantly, often below its dew point. This causes condensation, but the air leaving the coil is still very close to the saturation point (100% RH) at that new, lower temperature.

3. What is a typical Sensible Heat Ratio (SHR)?

A typical comfort application SHR is around 0.70-0.80. Dry climates may have a higher SHR (more sensible cooling needed), while humid climates will have a lower SHR (more latent/moisture removal needed). An SHR of 1.0 means there is no moisture removal at all.

4. How does altitude affect cooling load?

Altitude affects air density and pressure. At higher altitudes, the air is less dense. For the same volume of air (CFM), the mass is lower. Since load calculations are based on mass flow rate, this reduces the cooling capacity. You must input the correct atmospheric pressure for your altitude to get an accurate result.

5. Can I use Wet Bulb Temperature instead of Relative Humidity?

This calculator uses Relative Humidity as the primary moisture input. However, on a full psychrometric chart, any two properties (like dry bulb and wet bulb) can define a state point. This tool simplifies the inputs for web use. For field measurements, check our HVAC Field Measurement Guide.

6. What is a “Ton” of cooling?

A “ton of refrigeration” is an Imperial unit of cooling power. One ton is equal to 12,000 BTU/hr. It historically refers to the energy required to melt one short ton (2,000 lbs) of ice in 24 hours.

7. Why is my calculated load different from a rule-of-thumb?

Rules of thumb (e.g., 500 sq ft per ton) are extremely inaccurate because they don’t account for insulation, windows, climate, internal gains, or ventilation. A proper calculation using psychrometric principles, like this one, is essential for correct equipment sizing.

8. What happens if the SHR of my equipment doesn’t match the load SHR?

If the equipment’s SHR is too high, it will cool the air but may not remove enough moisture, leaving the space feeling cold and clammy. If the equipment’s SHR is too low, it may over-dehumidify the space, leading to dryness and inefficiency. Matching the SHR is key to comfort and performance.

Related Tools and Internal Resources

Continue your HVAC system design with these essential resources:

• Duct Sizing Calculator: Properly size your ductwork to ensure efficient air delivery.
• HVAC Design Guide: A comprehensive guide to the principles of heating, ventilation, and air conditioning system design.
• Building Energy Modeling: For complex projects, simulate the energy performance of your entire building.
• Advanced HVAC Systems: Learn about different types of systems, from VRF to chilled beams.
• Refrigerant Pipe Sizing: Calculate the correct pipe diameters for your refrigerant lines.
• HVAC Field Measurement Guide: A practical guide for technicians on taking accurate air property measurements.