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HVACR416 - Design
PsychometricsPart 2
Temperature
• The temperature is the measurement of molecular activity in a substance.
• In the HVAC industry we are concerned with the amount of molecular activity in the air.
• Heated molecules move faster than chilled molecules, this hot air is easier to move.
• There are two types of temperature readings.o Dry bulbo Wet bulb
Dry Bulb
• Dry bulb is the measurement of the sensible heat level of the air.
• Sensible heat is the heat you can feel by touch and is the only heat that is measurable.
• Ex: Heating a 55 degree room to 75 degrees is a 20 degree sensible heat change.
Wet Bulb
• Wet bulb is measuring the moisture content of the air.
• To take a wet bulb reading put a moist towel or a shoelace over the end of a thermometer. Wait about 5 minutes, this is the wet bulb reading.
• Wet bulb temperature reading is compared to the dry-bulb reading to get the Relative Humidity level.
• The evaporation of the water on the wick (shoelace or paper towel) will leave the thermometer cooler, and show a lower temperature.
Humidity
• All air has moisture in it and this is called humidity.
• Large amounts of humidity can be seen in fog and clouds.
• Humidity is shown in a % and is called Relative Humidity.
Humidity Guidelines
• There are several guidelines used when talking about humidity.
• In the winter:
o Lower temperature can be offset by raising the humidity of the air. Higher humidity will make you feel warmer.
o Winter humidity is best when between 35 - 45 %.
Humidity Guidelines
Summer
o High humidity and high temperatures make the human body feel warm.
o Low humidity and high temperatures make the human body feel cooler.
o It is a requirement to keep the humidity in buildings at or under 50%, preferably under 45%.
Humidity
• Relative humidity is expressed as a percentage, however humidity is measured by weight.
• Humidity is expressed in pounds or grains. There are 7000 grains to a pound of water.
Humidity
• 100% humid air at 29.2 in. HG at 70 degrees can hold 110.5 grains of moisture.
• This same humidity weighs 0.01578 pounds per pound of air.
Relative Humidity
• Measurement used to calculate the percentage of moisture content in the air.
• Relative humidity is based on the weight of water vapor in a pound of air compared to the weight of the water that a pound of air can hold at 100% saturation.
Dehumidification
• As discussed before dehumidification is removing this moisture.
• It is based on the evaporator design, air speed across the coil, and the amount of water vapor in the air.
The Psychometric Chart
Airo Air has weight.
o Air occupies space.
o 1 cubic foot of air weighs .075 pounds at atmospheric pressure (sea level).
o Air offers resistance to movement (think about a car in a wind tunnel).
o It is difficult to wave a large piece of cardboard in the air.
The psychometric chart
• The psychometric chart allows you to calculate everything having to do with air and humidity on one chart.
• By plotting any two intersecting lines you can find:
o Dry and Wet bulb temperatures
o Relative Humidity
o Grains of moisture
The psychometric chart
• By plotting any two points you can find:
o Moisture volume to remove.
o BTU’s of cooling.
o Mixed air temperatures.
o Humidity to add.
o Weight of water to remove which can convert to gallons.
The psychometric chart
• The chart is made up of six sets of lines:
o Dry bulb temperature
o Wet bulb temperature
o Relative humidity
o Cubic Feet of Air
o Grains of moisture
o Total heat content (btu / lb of air)
The psychometric chart
Dry Bulb Temperature
Wet Bulb Temperature
Relative Humidity
Grains of Moisture
Total Heat Content
Volume of Air
