Presentation on theme: "HVACR416 - Design Psychometrics Part 2. Temperature The temperature is the measurement of molecular activity in a substance. In the HVAC industry we are."— Presentation transcript:
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 bulb o 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 Air o 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)