Calculating the change in relative humidity

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SUMMARY

The discussion focuses on calculating the change in relative humidity when air is heated from -15 degrees Celsius to 20 degrees Celsius without adding moisture. The initial relative humidity is 70 percent, and the participant attempted to use a saturation vapor pressure chart to find the new relative humidity. They calculated a new relative humidity of 57 percent, while the textbook states it should be 6 percent. The correct approach involves using the psychometric chart, which indicates that the humidity ratio remains constant when no moisture is added.

PREREQUISITES
  • Understanding of relative humidity calculations
  • Familiarity with psychometric charts
  • Knowledge of saturation vapor pressure at different temperatures
  • Basic thermodynamics principles related to air properties
NEXT STEPS
  • Study the psychometric chart for practical applications in HVAC systems
  • Learn how to calculate saturation vapor pressure using temperature
  • Explore the concept of humidity ratio and its implications in air conditioning
  • Review thermodynamic principles related to heating and cooling air
USEFUL FOR

Students in meteorology, HVAC engineers, and anyone involved in environmental science or building management who needs to understand the effects of temperature changes on relative humidity.

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Homework Statement


the air temperature is -15 degrees Celsius and the relative humidity is 70 per cent. If the air is brought indoors and heated to 20 degrees Celsius with no added moisture, what is the new relative humidity?


Homework Equations


RH=e/es x 100% or ws=epsilon*es/p


The Attempt at a Solution


could not do it mathematically -- looked up on a chart and found that sat. vp is about 1.9 mb at 70 per cent RH, so multiplied 1.9 x .70 and got 13.3 mb. Then, divided that by 23.4 mb, from the 20 degree value, and got 57 per cent RH. Book says it's 6 per cent. Any ideas?
 
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Which chart did you use? If you used the psychometric chart, then you can easily find the new relative humidity. No moisture added means that the humidity ratio is the same.
 

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