Section 2 of 6 Applied Psychrometrics

Transcription

1 Section 2 of 6 Applied Psychrometrics Psychrometric chart - your energy map Key psychrometric variables for humidity Predicting building condensation Hot weather Cold weather Predicting moisture sorption and mold growth 1

2 Maps help us understand where we are and how far we have to go to our destination Distance quantified by the map s scale Long distance... lot of effort 2

3 Psychrometric chart helps us understand the current energy state of air, and how much work we ll have to do to change it Long distance... lot of effort Required work is quantified by the chart s (many) scales 3

4 Each axis quantifies a different property of the air Wet bulb temperature Dew point temperature Humidity ratio Relative humidity Dry bulb temperature 4

5 Consider typical indoor conditions 75 F 50% Relative Humidity 5

6 75 F Dry bulb temperature The temperature of the air as measured by a thermometer that has a dry sensing bulb 6

7 50% Relative humidity The amount of water vapor in the air... compared to its maximum capacity at that temperature. A relative (not absolute) measurement of humidity 7

8 Grab your chart and plot the point - 75 F, 50% RH 8

9 Humidity ratio = lb/lb = 65 gr/lb The weight of water vapor in the air... compared to the weight of dry air alone (lbw lba) An absolute measurement of humidity ( lbw 1 lba) x 7000 gr/lb = 65 9

10 Dew point temperature = 55 F The temperature at which the vapor contained in the air will begin to condense. The dew point is another absolute measurement of humidity 10

11 Predicting indoor condensation (hot weather) At design conditions in Hampton, VA, any outdoor air sucked into the cavities above ceilings is 83 F and 151 gr/lb. How cold does the ceiling surface have to be to produce condensation? In other words... What is the dew point of air at 83 F, 151 gr/lb =? ) 11

12 Assessing mold risk - School summer vacation In Hampton, VA, outdoor air infiltrating into an empty school during the summer time has a dew point of 79 F. Thermostat is reset to save energy. If the AC system keeps the building at 85 F, what is the probable Relative Humidity indoors? In other words - What is the RH of air when the dew point is 79 F and the dry bulb temperature is 85 F? RH =? 12

13 Mold risk on building surfaces - Hotel room 13

14 Using the chart to find RH at the SURFACE Dew Point =19.6 C Surface = 20.7 C 14

15 Helps explain common mold growth in hotels (and underlines the importance of air-tight duct connections) 15

16 Predicting cold weather condensation in walls In Calgary, Alberta, small amounts of air are leaking outward from the swimming pool 85 F, 60% RH. That air will condense moisture when it contacts a surface with a temperature of?. In other words, what is the dew point of air at 85 F, 60% RH? Dew point =? 16

17 Wet bulb temperature = 62.5 F The temperature of the air as measured by a thermometer with a wet sensing bulb. The temperature at which water will begin to freely evaporate into the air. 17

18 Finding humidity from DB + WB bulb temperature In Phoenix, AZ, the design dry bulb temperature is 110 F. During hours when the outdoor temperature is that hot in Phoenix, the average wet bulb temperature is 70 F How much humidity is in that outdoor air at peak dry bulb conditions? In other words, what is the dew point, humidity ratio and relative humidity of air at 110 F, 70 F WB? Dew point =? Humidity ratio =? RH =? 18

19 Psychrometric summary for air at 75 F, 50% RH Wet bulb: 62.5 F Dew point: 55 F RH: 50% Humidity ratio: 65 gr/lb Dry bulb: 75 19