HVACR416 - Design Psychometrics Part 2. Temperature The temperature is the measurement of molecular activity in a substance. In the HVAC industry we are.

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