basic principles and concepts of using the psychrometric chart. A psychrometric chart is an attempt to show the relationships in many of the properties of moist air. The chart shows all of the following properties: dry bulb temperature, wet bulb temperature, relative humidity, dew point temperature, humidity
How do you explain a psychrometric chart?
A psychrometric chart presents physical and thermal properties of moist air in a graphical form. It can be very helpful in troubleshooting and finding solutions to greenhouse or livestock building environmental problems.
How is psychrometric chart calculated?
Take a ruler and draw a vertical line at the dry bulb temperature of 70 and draw a horizontal line where the dew point is 50. The point where these two lines intersect is known as a 'state point'. Plot the state point and you can also read the rel- ative humidity, wet bulb, enthalpy, vapor pressure and humidity ratio.
What is the principle of a psychrometer?
A psychrometer measures humidity by taking both a wet-bulb and a dry-bulb temperature reading. With those two values known, the other properties of the air, including its moisture content, can be determined by computation or by reading a psychrometric chart.
Which are the 5 factors indicated on a psychrometric chart?
The psychrometric chart indicates the properties of this water vapor through the following parameters, each of which is explained in more detail below:dry bulb temperature.wet bulb temperature (also known as saturation temperature)dew point temperature.relative humidity.moisture content (also known as humidity ratio)More items...•
How do you make a psychrometric chart?
1:036:25Psychrometric Chart - How to draw - YouTubeYouTubeStart of suggested clipEnd of suggested clipSo first we draw a horizontal line. And then a vertical line on the right hand side. And extend itMoreSo first we draw a horizontal line. And then a vertical line on the right hand side. And extend it at the top with a horizontal. And then draw a curve on the left hand side.
How do you use a psychrometric chart example?
0:414:02Psychrometric Chart Examples - YouTubeYouTubeStart of suggested clipEnd of suggested clipSo our starting point we're at 10 degrees C and R in the saturated line and we see the enthalpy perMoreSo our starting point we're at 10 degrees C and R in the saturated line and we see the enthalpy per kilogram is 29. Now what we want to do is move horizontally. We want to keep the moisture.
What is the principle of humidity?
Humidity is defined as the amount of water present in the surrounding air. This water content in the air is a key factor in the wellness of mankind. For example, we will feel comfortable even if the temperature is 00C with less humidity i.e. the air is dry.
What is the principle operation of humidity?
Working Principle of Humidity Sensor Low resistivity materials are used for the construction of a Resistive sensor. This resistive material is placed on top of two electrodes. Change in the resistivity value of this material is used to measure the change in humidity.
Why is it called psychrometer?
psychrometer (n.) "instrument to measure moisture in the atmosphere," 1838, from Latinized form of Greek psykhros "cold" (see psychro-) + -meter. Earlier it was another word for "thermometer" (1749).
What are the 6 psychrometric processes?
PSYCHROMETRIC PROCESSESA view of psychrometric chart is shown:The different psychrometric processes are shown:Sensible Cooling.Sensible Heating.Cooling and Dehumidification.Sensible Heat Factor (SHF)Heating and Humidification.Cooling and Humidification.More items...•
What are the parts of psychrometric chart?
What Are the Parts of a Psychrometric Chart?Temperatures.Dry Bulb – This is the temperature reading found on a typical thermometer. ... Specific Volume & Density – Specific volume is measured in cubic feet per pound. ... Enthalpy – This is the measurement of heat energy.More items...•
Which of these is not included in the psychrometric chart?
Hence partial vapour pressure is not shown on psychrometric chart.
How do you solve a psychrometric chart problem?
4:3812:53psychrometry solved questions | psychrometric chart problemsYouTubeStart of suggested clipEnd of suggested clipSo you solve DV partial pressure it comes out to be 15 point zero 6 mmm HD. So and and saturationMoreSo you solve DV partial pressure it comes out to be 15 point zero 6 mmm HD. So and and saturation pressure you have to find the saturation pressure is always found out to be add dry bulb temperature.
How is ADP calculated from psychrometric chart?
When we know the psychrometric conditions of inlet & outlet air, we can use the psychrometric chart to define ADP. Locate the inlet & outlet points in the chart and draw a line through it which represents the SHF. Extend the line and locate ADP at the intersection on saturation line.
What is psychrometric calculator?
A psychrometric calculator is an engineering device used to calculate the properties of a gas-vapor mixture, and most often is employed to analyze water vapor in surrounding air.
How is humidity ratio calculated?
Humidity Ratio (W) Humidity ratio is measured in units of pound-mass of water per pound-mass of dry air. By multiplying by 7000 [grains per pound-mass], this value may be expressed in grains of water per pound-mass of dry air.
Heating, ventilation, and air conditioning
The psychrometric chart (Fig. 2) is a tool for understanding the relationships between the various parameters of supply air and the relative humidity. This template allows a designer or operator to “work backwards” from a desired room relative humidity to the desired condition of the air as it enters the supply duct.
Heating, Ventilation, and Air Conditioning
The psychrometric chart (Figure 108-2) is a tool for understanding the relationships between the various parameters of supply air and the relative humidity. This template allows a designer or operator to “work backwards” from a desired room relative humidity to the desired condition of the air as it enters the supply duct.
Properties of Humid Air
The psychrometric chart is a most useful design tool for air conditioning engineers. A typical chart is shown in Fig. 1.15 [ 4 ]. The air properties on the chart are:
Sensors and Auxiliary Devices
The reading text for this course was originally written by, ... Robert McDowall P. Eng., in Fundamentals of HVAC Control Systems, 2008
All-air Systems
The operation at the winter design conditions is now considered, using the system designed for summer operation. The air conditioned space now has a sensible heat loss, q S ′, and a latent heat gain, ql.
Packed Towers
A. Kayode Coker, in Ludwig's Applied Process Design for Chemical and Petrochemical Plants (Fourth Edition), Volume 2, 2010
Fundamental Relationships of Heat and Mass Transfer in Solar Seawater Desalination Systems
For convenient calculation, the coordinate graphs with state parameters for moist air are applied to determine the states and parameters of moist air and analyze its thermodynamic process. The most common one is the psychrometric chart (h-d diagram).
What Is a Psychrometric Chart?
A psychrometric chart represents the psychrometric properties of air. With this chart, engineers can better assess psychrometric processes and find practical solutions. While this chart looks complicated and even intimidating, it’s actually quite helpful and simple to understand once you grasp the basic properties of air. If you know two parameters of air where the lines will cross each other, the psychrometric chart can do the rest of the work for you.
What temperature do you read on a thermometer?
Dry Bulb – This is the temperature reading found on a typical thermometer. You can find a psychrometric chart that offers these temperature ranges:#N#Low temperatures that range from -20 degrees FDB to 50 degrees FDB#N#Normal temperatures that range from 20 degrees FDB to 100 degrees FDB#N#High temperatures that range from 60 degrees FDB to 250 degrees FDB 1 Low temperatures that range from -20 degrees FDB to 50 degrees FDB 2 Normal temperatures that range from 20 degrees FDB to 100 degrees FDB 3 High temperatures that range from 60 degrees FDB to 250 degrees FDB
How to measure humidity in a bulb?
Step 1: Locate the dry bulb temperature. This will be measured in degrees Fahrenheit or Celsius and will be along the bottom axis. Also identify the vertical line for each temperature. Step 2: Locate the humidity ratio, sometimes labeled as a mixing ratio. This will be along the right vertical axis.
Why do engineers use psychrometric charts?
A psychrometric chart prevents engineers from spending time on tedious mathematical formulas. While there are online calculators and applications to help make calculations, using the chart correctly provides engineers with a more accurate reading as long as you know two parameters of air. Knowing how to read a psychrometric chart is a wise skill for engineers to have in the event that technology fails or isn’t available.
What is a wet bulb thermometer?
Wet Bulb – This is a typical thermometer’s standard reading if the sensing bulb is covered with a wet wick or sock and exposed to air flow.
What is sensible heat ratio?
Sensible Heat Ratio – This is the total sensible heat flow divided by the total heat flow.
What is the standard air dot?
Standard Air Dot – This dot marks the measurement for standard air. Standard air is typically 70 degrees Fahrenheit with a relative humidity of 54% and 60 gr/lb of specific humidity.
What is a wet bulb temperature line?
The wet bulb temperature lines are the inclined straight lines (diagonal lines) and nonuniformly spaced. At any point on the saturation curve, both dry bulb and wet bulb temperature are equal.
What is enthalpy line?
Enthalpy (total heat) lines are inclined straight lines (diagonal lines). These lines are uniformly spaced, parallel and also coinciding with the wet bulb temperature lines. The values of enthalpy are given on the scale provided above the saturation curve.
What is a horizontal line parallel to the X axis called?
Horizontal lines parallel to X-axis, but non-uniformly spaced are called as dew point temperature lines. Dew point temperature is read on the saturation curve. At any point on the saturation curve, both dry bulb temperature and dew point temperature are equal.
What is the most valuable tool available to the air conditioning engineer?
Psychrometric chart is probably the most valuable single tool available to the air conditioning engineer or design technician.
Why is a psychrometric chart useful?
Psychrometric chart is very useful for finding out the properties of air, which are required in the field of air conditioning.
What are vertical lines on a temperature chart called?
Vertical lines uniformly spaced and drawn parallel to Y-axis on the chart are called as dry bulb temperature lines.
What is a 100% R.H. curve?
They are curved lines parallel to saturation curve. Generally, these lines are drawn with values 10%, 20%, 30%, up to 100%. The curve showing 100% R.H. is known as saturation curve.
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.
What is Psychrometric Chart?
Psychrometric charts are graphic representations of the psychrometric properties of air. By using psychrometric charts HVAC engineers can graphically analyze different types of psychrometric processes and find solution to many practical problems without having to carry out long and tedious mathematical calculations.
What are the properties of air?
The series of articles on properties of air discussed important properties of air like relative humidity, dry bulb temperature, wet bulb temperature, dew point temperature, sensible heat and latent heat. We shall now see how the air behaves when it is subjected to changes in temperature and humidity to suit the various applications for which the air conditioning is meant. The behavior of the air can be studied very conveniently and accurately by using a psychrometric chart.
What does the outer curve on the WB mean?
The outermost curve along the left side indicates the Wet Bulb (WB) temperature scale. The constant WB temperature lines are the diagonal lines extending from WB temperature curved scale downwards towards the right hand side of the chart. All the points located along the constant WB temperature line have the same temperature.
How to understand psychrometric chart?
The psychrometric chart looks complicated with vast numbers of lines and curves in it, but is very easy to understand if you know the basic properties of air. You will also understand its worth when you actually use it considering the fact that you won’t have to use any formulae to find the properties of air in different conditions, all you will have to know is two parameters of air and the rest are easily found on the chart.
Where is the dry bulb temperature scale?
The dry bulb temperature scale is shown along the base of the shoe shaped psychrometric chart forming the sole. The DB temperature increases from the left to the right. The vertical lines shown in the chart are the constant DB temperature lines and all the points located along a particular vertical line have same DB temperature.
Is a constant WB temperature line the same as a constant DP temperature line?
All the points located along the constant WB temperature line have the same temperature. Since the dew point temperature of the air depends on the moisture content of the air, constant moisture lines are also constant DP temperature lines.
How to find the constant wet bulb temperature?
To locate lines of constant wet-bulb temperature, at saturation, dry-bulb temperature = dew point temperature = wet bulb temperature, one point on each wet bulb line is already located, that is, the point of intersection of each dry-bulb line with the saturation line. Consider, for example, point A, which represents saturated air at 70°F (DB = WB = DP). Now determine another point on the 70°F wet bulb line, for example, the point where it intersects the 80°F DB line. To locate this point refer to property of moist air and noted that at 80°F dry bulb a 10 °F (80 – 70) wet-bulb depression gives an RH of 61%. The point B is plotted on the 80°F dry bulb line at the point of intersection with the 61% RH line. Another point on the 70°F wet bulb line may be determined in a similar way. At 92°F dry bulb, a 22°F (92 – 70) wet bulb depression gives a relative humidity of 33%. This point is labeled C, and it is noted that points A, B, and C lie on a straight line ie the 70°F wet bulb line. Other wet-bulb lines can be plotted similarly.
What is a wet bulb?
Wet-bulb temperature (WB) is the temperature of the air as read from a thermometer whose bulb is covered by a wet wick and whirled in the air. Wet-bulb temperatures are plotted as lines which originate on the saturation line and slope downward to the right. Wet-bulb temperature is an indicator of total heat (enthalpy), that is sensible heat plus latent heat.
How does conditioned air remove heat?
This heat removal is accomplished by introducing the conditioned air supply at a dry bulb temperature and specific humidity below those of the space condition. As the supply air warms to space conditions, it picks up sensible heat, and as it mixes with room air and warms up, it takes on moisture (latent heat). The process line from the supply-air condition to the space-air condition is represented by the sensible heat factor (SHF) line between the air-off coil condition point and the space condition point.
How to understand a psychometric chart?
The best way to understand the psychometric chart is to observe how such a chart is plotted. A brief description of the essential lines on the chart and how they are plotted will be given. Psychometric charts from different sources are not exactly alike, but all of them attempt to show the relationships among dry-bulb temperature, wet-bulb temperature, dew-point temperature, relative humidity, total heat (enthalpy), specific volume, specific humidity, and sensible heat. The several manufacturers of air conditioning equipment provide psychrometric charts in a range of sizes, up to large wall charts for office use, psychrometric charting softwares are also available.
What is dry bulb temperature?
Dry bulb temperature is a measure of sensible heat, and a change in dry bulb temperature represents a sensible heat change.
How does air conditioning work?
The usual summer cooling or refrigerated air conditioning is an example of this process. Theoretically, the air-water vapor mix ture is first cooled (sensible heat removed) along a constant-moisture line until the saturation line is reached. Further heat removal results in condensation of the water vapor, and moisture is thus extracted from the air at the same time as the dry-bulb temperature is being further reduced, and the process moves down the saturation line in its second phase. The cooling can be accomplished by passing the air through finned coils containing either chilled water or a direct-expansion refrigerant or by pulling the air through a water spray whose temperature is well below the dew-point temperature of the air to be conditioned.
What is relative humidity?
Relative-humidity (RH) is the ratio of the actual water vapor pressure in the air to the pressure of saturated vapor in air at the same temperature. With only slight error it can be said that relative humidity equals the percent saturation of the air, or the ratio of the actual specific humidity W to the specific humidity at saturation, Wsat, that is, RH ~ W/Wsat. Relative-humidity lines begin at the lower left and curve upward to the right with approximately the curvature of the saturation (100 percent RH) line.
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…