HVAC and Psychrometric Charts SI Transcript

8/9/2019 HVAC and Psychrometric Charts SI Transcript

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HVAC & Psychrometric ChartsSI Version

Energy University Course Transcript

Slide 1Welcome to HVAC and Psychrometric Charts. Today, we will introduce the concept of Psychrometrics, whilediscussing how Psychrometric Charts are utilized to drive HVAC sizing and evaluation. Please note: Thiscourse uses the international system of units (SI units). There is a separate course for US customary units.

This course is one in a series of HVAC courses offered by Energy University. Before taking this course, it isrecommended that you first participate in HVAC and the Characteristics of Air.

Slide 2For best viewing results, we recommend that you maximize your browser window now. The screen controls

allow you to navigate through the eLearning experience. Using your browser controls may disrupt thenormal play of the course. Click the paperclip icon to download supplemental information for this course.This course contains practice examples, and you will need to use the paperclip icon to download thepsychrometric chart to complete these exercises. Click the Notes tab to read a transcript of the narration.

Slide 3At the completion of the course, you will be able to:

Define psychrometrics

Read a Psychrometric Chart

State the benefits of using a Psychrometric Chart, and

Show various HVAC processes on a Psychrometric Chart

Slide 4Psychrometrics is the study of the thermodynamic properties of moist air and its effect on materials andhuman comfort. Psychrometrics applies the well understood relationships between humidity andtemperature in the air to practical problems. Commonly used psychrometric variables are temperature,relative humidity and dew point. There are less common variables that we will also discuss.

HVAC system designers use these factors to model the HVAC requirements depending on the location ofthe building and the needs of the occupants or processes within it. In the next section, well see in a simpleway how those factors are used to ensure an effective HVAC system.

Slide 5A psychrometric chart is a graph of the physical properties of moist air at a constant pressure. It is often

equated to an elevation relative to sea level. The chart we will be using in this course is for sea level. Thechart graphically expresses how various properties relate to each other, and is thus a graphical equation ofstate.

The thermophysical properties found on a psychrometric chart are:

Dry-Bulb Temperature

Humidity Ratio, also called Specific Humidity

Wet-Bulb Temperature

Dew Point


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Specific Volume

Relative HumidityEnthalpy, and

Vapor Pressure

Lets take a look at our chart.

Slide 6As you can see, the chart looks like a shoe.

The versatility of the psychrometric chart is that at a given pressure, by knowing two independent propertiesof the air, the other properties can be determined. Changes in state, such as when two air streams mix, canbe modeled easily and somewhat graphically using the correct psychrometric chart for the location's airpressure or elevation relative to sea level. For locations at or below 600 m, a common assumption is to use

the sea level psychrometric chart.

Lets take a look at all of the various thermophysical properties and how they are plotted on the chart.

Slide 7Dry-bulb temperature (DBT) is the temperature of an air sample, as determined by an ordinary thermometer.The thermometer's bulb is dry. It is typically the x-axis, or the horizontal axis, of the graph. The Metric Unitsare Celsius.


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Dry-Bulb temperatures are graduated from left to right along the sole.

Slide 8Dry-Bulb readings are plotted vertically.

Slide 9Humidity ratio (also known as moisture content or mixing ratio) is the proportion of mass of water vapor perunit mass of dry air at the given conditions (DBT, WBT, DPT, RH, etc.). It is typically the y-axis, the verticalaxis, of the graph. For a given DBT there will be a particular humidity ratio for which the air sample is at100% relative humidity: the relationship reflects the physics of water and air and must be measured.Humidity ratio is dimensionless, but is sometimes expressed as grams of water per kilogram of dry air.Specific humidity is closely related to humidity ratio but always lower in value as it expresses the proportionof the mass of water vapor per unit mass of the dry air and the water vapor.


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Humidity values are graduated, bottom to top, along the heel. This chart has grams of water per kilogram ofdry air.

The heel also shows vapor pressure. Vapor Pressure is the pressure exerted by the water vapor in air, andit varies with temperature since the water "holding" capacity of air changes with temperature. It is given inmillimetres of mercury.

Slide 10Humidity readings are plotted horizontally.


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Slide 11The values are graduated from the toe to the top. These values represent Dew Point and Wet-Bulb. This isalso known as the saturation line.

Slide 12Wet-Bulb readings are plotted from the inseam diagonally, down and right. They follow the roughly 45degree angle lines.


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Slide 13Dew Point readings are plotted horizontally. Its hard to read on the inseam so there is a handy easy to readscale on the outside of the heel. Here, we see it shown as 16 degrees Celsius.

Slide 14Specific Volume is in the body of the shoe. The lines are diagonaland steeper than the Wet-Bulb lines.


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Slide 15Relative Humidity (RH) is in the body of the shoe. The lines are swooped and range from 10% to 100% atthe inseam. Notice that 100% is the inseam.

Slide 16Enthalpy values are provided all around the outside of the shoe. Enthalpy readings are plotted diagonally,almost exactly at the same angle as the Wet-Bulb lines.

Slide 17A sample of air can be plotted if any two of the 7 variables are known. This is assuming the pressures arethe same.

A sling psychrometer is a measuring tool that can be used to determine Dry-Bulb and Wet-Bulb readings.This device contains two thermometers, and one has a wet sock on the tip. The person taking the readings


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would just sling it around for a minute and take the two readings. Electronic ones are available atreasonable prices.

Lets practice plotting points.

Slide 18Lets pretend we have a Dry-Bulb reading of 25 degrees Celsius. Find 25along the sole and draw a verticalline from that value. Take the DB line all the way to the saturation line.

Slide 19Lets pretend we have a Wet-Bulb reading of 18 degrees Celsius. Find 18 along the inseam and draw adiagonal line along the Wet-Bulb line. Draw a point where the two lines intersect.


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Now that we have plotted two of our values, lets find the remaining variables.

Slide 20Draw a horizontal line directly through the point.

What is the Dew Point?14.2C

What is the Humidity Ratio?10.1 grams moisture per kilogram dry air

What is the Vapor Pressure?12.0 mmHg

Slide 21Lets estimate the percentage of Relative Humidity (RH).It looks like its roughly 50%.


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Theres another way to calculate RH as well:

Take the Dry-Bulb to the saturation line

Go right to determine the grams moisture per kilogram dry air at saturation

Slide 22At saturation, Wet-Bulb and Dew Point are the same as the Dry-Bulb. At saturation, the humidity ratio of 25degree air is 20 g/kg.

Remember our sample humidity ratio was 10.1 g/kg. Divide that by 20 g/kg.Thats 10.1/20 = 50.5%

Slide 23Draw a line directly through the point and parallel to the Enthalpy lines. You may have to line up thereadings through the point.


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What is the Enthalpy? Looks like it is approximately 51 kJ per kg of dry air.

Slide 24Specific Volume (SV) can be estimated. It can be calculated, but it isnt really necessary. For this example,we estimate this to be 0.85 m3/kg dry air.

Slide 25Now take out a clean chart. You can download one right from this presentation by clicking on the paperclipicon. Plot the following air sample with a Dry-Bulb reading of 30C and a Relative Humidity reading of 60%.

Find the remaining values:What is the Humidity Ratio?

What is the Vapor Pressure?

What is the Wet-Bulb?

What is the Dew Point?

What is the Enthalpy?

What is the Specific Volume?


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Slide 26When you are done, your chart should look something like this.

Slide 27Here are your answers.

What is the Humidity Ratio?16.0 g/kg

What is the Vapor Pressure?18.8 mmHg

What is the Wet-Bulb?23.7C

What is the Dew Point?Just over 21C

What is the Enthalpy?71 kJ/kg

What is the Specific Volume?Approximately 0.87 m3/kg


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Slide 28Lets apply all that to an example.

A factory process requires air at 38 degrees C (Dry-Bulb) and 40% relative humidityThe intake air is 10 degrees C (Dry-Bulb) and 60% relative humidity.The process consumes 200 cubic meters of air per minute.

How many kW is required to supply this process with the air requirements?

Here we see our formula.

The amount of heat transferred can be expressed asQ = 1.201 * v * hwhereQ = total heat transferred (kW)1.201 = a constant for total heat equationsv = airflow (m3/s)h = enthalpy difference (kJ / kg)

Using the psychrometric chart, please calculate the answer.

Slide 29To solve the problem, first we need to determine the Enthalpy of the air samples. On the psychrometricchart, find 38 degrees C (Dry-Bulb) and take it up to the 40% RH line. Find the 10 degrees C (Dry-Bulb) andtake that up to 60% RH line. From the point where your Dry-Bulb lines meet the RH line, draw a line parallelto the Enthalpy lines.

For the 38 degrees C air sample, we find an Enthalpy of 83. For the 10 degrees C air sample, we find anEnthalpy of 23.

Here we see the correct answer.

Q = 1.201 * v * hQ = 1.201 * 200 m3/min * (83 -23)Q = 1.201 * 3.33 m3/s * 60Q = 240 kW


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Lets work through another exercise.

Slide 30How manykW of air conditioning are required to cool 30 cubic meters per minute of air that is 32 degrees Cwith a 60% relative humidity to 16 degrees C and a 60% relative humidity? To solve the problem, first weneed to determine the Enthalpy of the air samples. On the psychrometric chart, find 16 degrees (Dry-Bulb)and take it up to the 60% RH line. Find the 32 degrees (Dry-Bulb) and take that up to 60% RH line. From thepoint where your Dry-Bulb lines meet the RH line, draw a line parallel to the Enthalpy lines.

For the 16 degree air sample, we find an Enthalpy of 33 kJ/kg. For the 32 degree air sample, we find anEnthalpy of 78.


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Slide 31Next, we need to find kW required. Lets look at our formula.

Q = 1.201 * v * h

Here we see the required kW.

Q = 1.201 * v * hQ= 1.201 * 30 m3/min * (7833)Q= 1.201 * 0.5 m3/s * 45Q= 27 kW

Slide 32Lets summarize what we have learned in this course.

Psychrometrics is the study of the thermodynamic properties of moist air and its effect on materials andhuman comfort.

The thermophysical properties found on a psychrometric chart are:

Dry-bulb temperature

Humidityratio

Vapor pressure

Wet-bulb temperature

Dew Point

Specific Volume

Relative HumidityEnthalpy

The state of the air at a specified pressure is completely specified by two independent intensive properties.The other properties can be determined from the chart.

HVAC system designers use these factors to model the HVAC requirements depending on the location ofthe building and the needs of the occupants or processes within it.

Slide 33Thank you for participating in this course.

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HVAC and Psychrometric Charts SI Transcript