What is a dry bulb?
Dry-bulb temperature is located on the horizontal, or x-axis, of the psychrometric chart and lines of constant temperature are represented by vertical chart lines.
How to determine dewpoint temperature?
When air is cooled, the relative humidity increases until saturation is reached and condensation occurs. Condensation occurs on surfaces which are at or below the dewpoint temperature. Dewpoint temperature is determined by moving from a state point horizontally to the left along lines of constant humidity ratio until the upper, curved, saturation temperature boundary is reached.
What is the temperature of a sling psychrometer?
A sling psychrometer* gives a dry-bulb temperature of 78°F and a wet-bulb temperature of 65°F. Determine other moist air properties from this information. Two useful air properties for environmental analysis in agricultural buildings would be relative humidity and dewpoint temperature. Relative humidity is an indicator of how much moisture is in the air compared to desirable moisture conditions, and dewpoint temperature indicates when condensation problems would occur should the (dry-bulb) temperature drop.
How to find the dry bulb temperature?
Find the intersection of the two known properties, dry-bulb and wet-bulb temperatures, on the psychrometric chart, Figure 1. The dry-bulb temperature is located along the bottom horizontal axis. Find the line for 78°F, which runs vertically through the chart. Wet-bulb temperature is located along diagonal dotted lines leading to scale readings at the upper, curved boundary marked "saturation temperature." The intersection of the vertical 78°F dry-bulb line and the diagonal 65°F wet-bulb line has now established a state point for the measured air. Now read relative humidity as 50 percent (curving line running from left to right up through the chart) and dewpoint temperature as 58°F (follow horizontal line, moving left, toward the curved upper boundary of saturation temperatures). This example is shown in Figure 3 so you may check your work.
Why do we use psychrometric charts?
Understanding psychrometric charts can help you visualize environmental control concepts, such as why heated air can hold more moisture or, conversely, how allowing moist air to cool will result in condensation. This fact sheet explains how characteristics of moist air are used in a psychrometric chart.
How does evaporative cooling work?
Evaporative cooling uses heat contained in the air to evaporate water. Air temperature (dry-bulb) drops while water content (humidity) rises to the saturation point. Evaporation is often used in hot weather to cool ventilation air. The process moves upward along the line of constant enthalpy or constant wet-bulb temperature, for example, from point D to point E in Figure 5. Notice that hot dry air (points D to E with a 24° F temperature drop) has more capacity for evaporative cooling than hot humid air (points F to G with only a 12° F temperature decrease).
How much humidity can a greenhouse have in winter?
A rule of thumb for inside typical greenhouses or animal buildings during winter conditions is that a 10°F rise in air temperature can decrease relative humidity 20 percent. Use of a psychrometric chart will show that this is roughly true.
How to Read The Psychrometric Chart?
- The Psychrometric Chart has 4 important elements to master. They are the dry bulb temperature, wet bulb temperature, relative humidity and saturation line.
How to Use Psychrometric Chart?
- The Psychrometric Chart can be used to design and troubleshoot HVAC systems. Nowadays, manufacturers incorporate data from the chart into their equipment selection software. In the field, design and commissioning engineers use the chart to troubleshoot humidity problems.
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Psychrometric Chart and Air Characteristics
Use of Psychrometric Chart in Greenhouse and Barn
- Example 1 Find air properties
A sling psychrometer* gives a dry-bulb temperature of 78°F and a wet-bulb temperature of 65°F. Determine other moist air properties from this information. Two useful air properties for environmental analysis in agricultural buildings would be relative humidity and dewpoint temper… - Example 2 Winter ventilation
Often air is heated before it is introduced into greenhouse or young-livestock building environments. Consider an application where outdoor air at 40°F (dry-bulb) temperature and 80 percent relative humidity is heated to 65°F (dry-bulb) before it is distributed throughout the buildi…
Definitions
- The air surrounding us is a mixture of dry air and moisture and it contains a certain amount of heat. We are used to hearing about air temperature, relative humidity, and the dewpoint in discussions of weather conditions. All these properties and more are contained in a psychrometric chart. Chart shape and complexity take some getting used to. Refer to Figures 1 …
Example 3 Evaporative Cooling
- Evaporative cooling uses heat contained in the air to evaporate water. Air temperature (dry-bulb) drops while water content (humidity) rises to the saturation point. Evaporation is often used in hot weather to cool ventilation air. The process moves upward along the line of constant enthalpy or constant wet-bulb temperature, for example, from point D to point E in Figure 5. Notice that hot d…