Psychrometric Chart

Understanding how a psychrometric chart works

Written by Patrick Chopson
Updated over a week ago

This article covers the interpretation of analysis.tool automated Psychrometric Chart as seen on the climate report.

What is a Psychrometric Chart?

A psychrometric chart is an industry-standard tool that follows ASHRAE Standard 55. With the diagram, readers can visualize the interrelationships between dry air, moisture, and energy over a typical year for their project location. It can be a useful diagram to find practical solutions to make the space thermally comfortable for the occupants.

Understanding the Psychrometric chart

Use the diagram and paragraphs below to understand the psychometric chart.

Key Definitions

Taken from ASHRAE Standard 55:

• Operative temperature: the uniform temperature of an imaginary black enclosure in which an occupant would exchange the same amount of heat by radiation plus convection as in the actual non-uniform environment.

• Humidity Ratio: the ratio of the mass of water vapor to the mass of dry air in a given volume.

• Thermal comfort: That condition of mind that expresses satisfaction with the thermal environment and is assessed by subjective evaluation.

Components of Chart

1. Axis - There are two main axes: the x-axis represents the temperature (operative), and the y-axis represents the humidity ratio.

2. Saturation/Relative Humidity Lines: The curves extending from lower left to upper right are known as Relative Humidity (RH) lines. Each curve indicates a different percentage of relative humidity. The top-left-most curve has 100% RH and is known as the Saturation curve because the air is fully saturated along this line no matter what the temperature is.

3. Enthalpy lines: The curves extending from the lower right to the upper left are known as constant enthalpy lines. Enthalpy is read using the wet-bulb temperature and the enthalpy scale usually present outside the body of the chart (not provided here).

4. Data Points: The data points indicate the properties of air for each location. They can be displayed in a number of styles depending on the choice of grid for the chart. Here, each data point or opaque block (blue to yellow to red) represents the number of hours where a specific condition occurs. The number of hours can be known using the legend. The accumulation of all hours on your psychometric chart makes the 8760, or the number of hours in a year.

5. Comfort Zone: Comfort zone is a combination of acceptable conditions that a specified percentage of occupants will find thermally comfortable. The vast majority of the available thermal comfort data pertains to sedentary or near sedentary physical activity levels typical of office work. The clothing insulation is between 0.5 clo and 1.0 clo for the zone showed in this diagram. Refer to ASHRAE Standard 55 for more information.

6. Polygons: polygons are the collection of data points that share common climate properties. There are 7 polygons in the pyschometric chart such as the comfort zone, moderate zone, warm dry zone, and so on. The 7 polygons will correlate to the 7 percentages seen in the "Impact of Design Strategies" legend, discussed next.

Interpreting the Psychrometric Chart

A number of inferences can be drawn by a quick look at the chart, such as determining your overarching climate profile, like whether it's hot dry, warm humid, moderate climate, or towards the cooler side. As well as what conditions your occupants experience most often.

The second is using the psychrometric chart to identify the most beneficial design strategies (passive and active) that can be used to improve thermal comfort for the occupants. By adopting strategies that counterbalance non-ideal outdoor climate conditions, we move indoor conditions towards the comfort zone.

Using the Polygons to select Design Strategies

Alongside the Psychrometric Chart, analysis.tool also generates a percentage breakdown on the Impact of Low Energy Strategies (shown below). Each colored percentage reflects a polygon in the psychrometric chart. In the example below, the area inside the red polygon represents the comfort zone. The other colored percentages represent times in a year in which a recommended strategy might negate the environmental discomfort and push it into the comfort zone. This list is therefore an effective indicator of top design strategies to integrate. Using the Atlanta Psychrometric chart, we see that 5.19% of the annual outdoor conditions will be within the comfort zone. If the designer implements strategies to increase internal heat gain this could extend the thermal comfort zone to 29.76% (5.19 + 24.57) of annual working hours in a year. With each strategy the percentage is added to the total percentage of 5.19%.

A number of ways in which the strategies can be implemented are as follows:

1. Evaporative cooling:

• Cooling air through the evaporation of water

• By using evaporative coolers

2. By increased natural ventilation

• placing doors and windows on opposite sides

• maximizing vertical height between air inlet and outlet to produce stack ventilation

• using open-plan interiors to promote natural cross ventilation

3. Thermal mass + Night ventilation:

• Providing mass to store heat, providing "inertia" against temperature fluctuations

4. Occupant use of fans:

• Circulating air throughout a room to accelerate the evaporation of perspiration

5. Internal heat gain:

• Sensible heat generated by internal heat sources (people, lights, and equipment) is a time-delayed cooling load.

• Internal heat gain through people, lights, and equipment

• To reduce winter night-time heat losses, heavy drapes, insulated blinds, or operable window shutters can be used

7. Desiccant dehumidification:

• Removing moisture from the air by using a desiccant, a material that easily attracts and holds water vapor.

• Using a desiccant dehumidifier

8. Dehumidification:

• The moisture or water vapor or humidity is removed from the air keeping its dry bulb (db) temperature constant.

Using chilled water or refrigerants

Happy Modeling!