Talking points

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Psychrometry

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Talking points

  1. 1. Talking Points: Psychrometry
  2. 2. Definition and Key Elements• Psychrometry is the study of the physicaland thermal properties of air and watervapor mixtures.• The seven key physical and thermalcharacteristics used to describepsychrometry are: dry bulb temperature,humidity ratio, relative humidity, enthalpy,dew-point temperature, wet bulbtemperature, and specific volume.
  3. 3. Common Examples• When the body sweats to cool itself, it isusing psychrometry.
  4. 4. Definitions• Dry Bulb Thermometer-regularthermometer.• Humidity ratio. Saturated air is air holding100% of the water possible.– For every 20 degrees Fahrenheit temperatureincrease, the holding capacity of the airdoubles. (The inside of a hot car is often 20degrees more than the outside air.)
  5. 5. Definitions• Relative humidity is a percentage of thesaturated air content.• Enthalpy: Heat energy content of theair/water vapor mixture.– The air’s energy content changes if either orboth the dry bulb and humidity ratio changes.• Dew Point: The temperature at whichmoisture starts to condense from the air.
  6. 6. Definitions• Wet-bulb temperature: measured using athermometer whose bulb or sensor is coveredwith a moistened wick in a moving air stream.– The difference between wet and dry bulbtemperatures is important in evaporative cooling.• Specific volume: the volume in cubic feetoccupied by a pound of dry ear at a specific drybulb temperature and pressure.
  7. 7. Keys to defined variables• Knowing only two of the seven outlineddefinitions will lead to knowing all sevenvariables using the psychrometric chart.– A typical local weather forecast usuallyincludes the dry bulb temperature in degreesFahrenheit, dew point in degrees Fahrenheit,and the humidity measured in inches ofmercury.
  8. 8. Common Sense Answers• Evaporative cooling works even in humid areas,particularly in the afternoon.– As the temperature rises, so too does the amount ofwater that can be carried.• Water temperature has little effect onevaporative cooling.– Placing a gallon of 50 degree water on 90 degreecement produces 9000 Btu’s of cooling– Placing a gallon of 90 degree water on 90 degreecement produces 8700 Btu’s of cooling• A difference of only three percent.
  9. 9. Common Sense Answers• The key to getting the most out of anyevaporative cooling system is to maximizethe amount of air that comes into contactwith the moisture added.
  10. 10. Psychrometry• Psychrometry is the study of the physicaland thermal properties of air and watervapor mixtures.– “The key to getting the most out of anyevaporative cooling system is to maximize theamount of air that comes into contact with themoisture added.”– “Evaporative cooling works even in humidareas, particularly in the afternoon.”• As the temperature rises, so too does the amount of water thatcan be carried.Source: University of Kentucky

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