Humidifiers Many of us get sick with colds during winter in inois. One factor th

Question

Humidifiers Many of us get sick with colds during winter in inois. One factor that makes these illnesses worse is the very dry air we breathe indoors during the winter. This low humidity air wil dry out our throats and nasal passages leaving them irritated and prone to infection. Examine the situation below and answer the following questions It is a cold winter day and the outside air temperature is -10°C with a dewpoint temperature of -20°. Your apartment's furnace draws in this cooler air from outside and heats t to 20°C, which is approximately roonm temperature (68°F), as it blows the air through the vents in your apartment. While the furnace heats the air, no water vapor is added or taken from the air before it enters your apartment. Answer these questions What is the relative humidity of the air outside? 223% 2. What is the dewpoint temperature ofthe air inside your apartment? 3. What is the relative humidity of the air inside your apartment? You end up catching a cold and your doctor recommends getting a humidifier. So, you buy one and begin using it After 1 hour of using the humidifier, the relative humidity in your apartment increased to 60%. The air temperature stayed the same at 20°C, but what is the dewpoint temperature in your apartment now that you are using the humidifier? This figure wil help with solving this problem it is created by graphing the equations given above. Hint: the red line represents the value of the saturation vapor pressure when plotted against temperature. It also represents the value of the vapor pressure when plotted against dewpoint temperature 80 T C) SVP (mb) 40 30 20 10 70 73.8 42.4 23.4 12.3 50 40 30 20- L30 -10 2.9 20 0.5 0 30-20-10 0 10 20 30 40 50 Temperature (C) ©2005 Kendall/Hunt Publishing

Explanation / Answer

1. WKT, relative humidity is,

RH = (Vapour pressure(vp)/ saturation vapour pressure(svp))*100

Use the plot to identify the vp and svp values.

To = Temperature outside = -10oC

Corresponding svp = 2.9mb

Tdo = Dew Point Temperature outside = -20oC

Corresponding vp = 1.3mb

Therefore, the relative humidity outside is

RH = (1.3/2.9) * 100 = 44.83%

2. Now, the temperature inside is, Ti = 20oC

Corresponding svp = 23.4 mb

Since no water vapor is added or removed, the RH remains the same. Therefore,

RH = (vp/svp) * 100 = 44.83

Therefore, vp = (44.83*23.4)/100 = 10.5 mb

As it can be seen in the graph, the 10.5 mb vp corresponds to approximately, 7oC of dew point temperature.

3. The relative humidity inside also remains at 44.83% as there is now water vapor added or removed.

4. After using the humidifier, the RH = 60%. Now,

the temperature inside is still, Ti = 20oC

Corresponding svp = 23.4 mb

RH = (vp/svp) * 100 = 60

Therefore, vp = (60*23.4)/100 = 14.04 mb

As it can be seen in the graph, the 14 mb vp corresponds to approximately, 14oC of dew point temperature.

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