Psychrometric Chart
Many of the air-conditioning processes involve air that's experiencing energy changes. These changes arise from changes within the air’s temperature and its moisture content. The relationships between temperature, moisture content, and energy are most easily understood employing a visual aid called the “psychrometric chart.”
The psychrometric chart is an industry-standard tool that's wont to visualize the interrelationships between dry air, moisture and energy. If you're liable for the design or maintenance of any aspect of air con in buildings, a transparent and cozy understanding of the chart will make your job easier.
Initially, the chart are often intimidating, but as you're employed with it you'll discover that the relationships that it illustrates are relatively easy to know .
Once you're comfortable with it, you'll discover that it's a tool which will make it easier to troubleshoot air-conditioning problems in buildings. The ASHRAE course, Fundamentals of Thermodynamics and Psychrometrics1 goes into great detail about the utilization of the chart. That course also provides calculations and discussion about how the chart are often used as a design and troubleshooting tool.
we will only introduce the psychrometric chart, and supply a really brief overview of its structure.
The Design of the Psychrometric Chart
The psychrometric chart is made upon two simple concepts.
1. Indoor air may be a mixture of dry air and water vapour .
2. there's a selected amount of energy within the mixture at a selected temperature and pressure.
Psychrometric Chart Concept 1: Indoor Air may be a Mixture of Dry Air and water vapour .
The air we live in may be a mixture of both dry air and water vapour . Both are invisible gases. The water vapour in air is additionally called moisture or humidity. the quantity of water vapour in air is expressed as “pounds of water vapour per pound of air.” This ratio is named the “humidity ratio,” abbreviation W and therefore the units are pounds of water/pound of dry air, lbw/lbda, often abbreviated to lb/lb.
The exact properties of moist air vary with pressure. Because pressure reduces as altitude increases, the properties of moist air change with altitude. Typically, psychrometric charts are printed supported standard pressure at sea level. For the remainder of this we'll consider pressure as constant.
To understand the connection between water vapour , air and temperature, we'll consider two conditions:
First Condition: The temperature is constant, but the quantity of water vapour is increasing.
If the temperature remains constant, then, because the quantity of water vapour within the air increases, the humidity increases. However, at every temperature point, there's a maximum amount of water vapour which will co-exist with the air. the purpose at which this maximum is reached is named the saturation point. If more water vapour is added after the saturation is reached, then an equal amount of water vapour condenses, and takes the shape of either water droplets or ice crystals.
Outdoors, we see water droplets within the air as fog, clouds or rain and that we see ice crystals within the air as snow or hail. The psychrometric chart only considers the conditions up to the saturation point; therefore, it only considers the consequences of water within the vapor phase, and doesn't affect water droplets or ice crystals.
Second Condition: The temperature is dropping, but the quantity of water vapour is constant.
If the air is cooled sufficiently, it reaches the saturation line. If it's cooled even more, moisture will condense out and dew forms.
For example, if chilled canned drink is taken out of the refrigerator and left for a couple of minutes, the container gets damp. this is often because the moist air is in touch with the chilled container. The container cools the air that it contacts to a temperature that's below saturation, and dew forms. This temperature, at which the air starts to supply condensation, is named the Dew Point Temperature.
Relative Humidity
When an equivalent volume of air contains only half the load of water vapour that it's the capacity to carry at that temperature, we call it 50% relative humidity or 50% rh. this is often shown in Figure. Air at any point on the five hundred rh line has half the water vapour that an equivalent volume of air could have at that temperature.
As you'll see on the chart, the utmost amount of water vapour that moist air can contain increases rapidly with increasing temperature. for instance , moist air at the freezing point, 32°F, can contain only 0.4% of its weight as water vapour . However, indoors, at a temperature of 72°F the moist air can contain nearly 1.7% of its weight as water vapor—over fourfold the maximum amount . Consider Below Figure, and this example:
On a miserable wet day it'd be 36°F outside, with the air rather humid, at 70% relative humidity. Bring that air into your building. Heat it to 70°F. This brings the relative humidity right down to about 20%. this alteration in relative humidity is shown in Figure 2.3, from Point 1 : 2. A cool damp day outside provides air for a dry day indoors! Note that absolutely the amount of water vapour within the air has remained an equivalent , at 0.003 pounds of water vapour per pound of dry air; but because the temperature rises, the relative humidity falls.
Here is an example for you to undertake , using the Above Figure
Suppose it's a warm day with an outdoor temperature of 90°F and relative humidity at 40%. we've an air-conditioned space that's at 73°F. a number of the outdoor air leaks into our air-conditioned space. This leakage is named infiltration.
Plot the method on above Figure.
Find the beginning condition, 90°F and 40% rh, moisture content 0.012 lb/lb.
Then cool this air: move left, at constant moisture content to 73°F.
Notice that the cooled air now features a relative humidity of about 70%.
Relative humidity of 70% is high enough to cause mold problems in buildings. Therefore in hot moist climates, to stop infiltration and mold generation, it's valuable to take care of alittle positive pressure in buildings.
Psychrometric Chart Concept 2: there's a particular amount of energy within the air mixture
at a specific temperature and pressure.
This brings us to the second concept that the psychrometric chart illustrates. there's a particular amount of energy within the air water-vapor mixture at a particular temperature. The energy of this mixture depends on two measures:
1. The temperature of the air.
2. The proportion of water vapour within the air.
There is more energy in air at higher temperatures. The addition of heat to boost the temperature is named adding “sensible heat.” there's also more energy when there's more water vapour within the air. The energy that the water vapour contains is mentioned as its “latent heat.”
The measure of the entire energy of both the sensible heat within the air and therefore the heat of transformation within the water vapour is usually called “enthalpy.” Enthalpy are often raised by adding energy to the mixture of dry air and water vapour . this will be accomplished by adding either or both
_ Sensible heat to the air
_ More water vapour , which increases the heat of transformation of the mixture
On the psychrometric chart, lines of constant enthalpy slope down from left to right as shown in Figure and are labeled “Enthalpy.”
The zero is arbitrarily chosen as zero at 0°F and 0 moisture content. The unit measure for enthalpy is British Thermal Units per pound of dry air, abbreviated as Btu/lb.
Hope from this you got the basics of Psychrometric chart, Saturation point, Dew Point Temperature, Relative Humidity and Enthalpy. Please comment for any queries
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