Welcome. Vaisala Industrial Measure Webinar Series - Humidity Theory, Terms & Definitions. Yumi Alanoly Vaisala Application Engineer

Transcription

1 Welcome Vaisala Industrial Measure Webinar Series - Humidity Theory, Terms & Definitions Yumi Alanoly Vaisala Application Engineer

2 Agenda 1. Why do we measure humidity? 2. Dalton s Law 3. Relative humidity and temperature 4. How is relative humidity and temperature affected? 5. Calculated parameters: Td, x, ppm, Tw, h Page 2 / YSH / Humidity Webinar Series Vaisala

3 Water Vapor Theory - H 2 O - Exists in the three phases - Which phase depends on the amount of thermal energy that is present Page 3 / YSH / Humidity Webinar Series Vaisala

4 American Meteorological Society Glossary Hu-mid-i-ty Humidity 1. Generally, some measure of the water vapor content of air. Page 4 / YSH / Humidity Webinar Series Vaisala

5 Dalton s Law The total pressure of a gas is equal to the sum of the different gases partial pressures P t = P 1 + P 2 + P n air around us P t = P N2 + P O2 + P w + P misc. English chemist, meteorologist, physicist ( ) Page 5 / YSH / Humidity Webinar Series Vaisala

6 Practical Example of Dalton s Law Nitrogen... 77% Oxygen... 21% Water vapor... 1% Other gases... 1% 1000 mbar = 770mbar + 210mbar + 10mbar + 10mbar How does this change in Denver? Pt = 840 mbar 840 x 77% 840 x 21% 840 x 1% 840 x 1% N2 + O2 + Pw + Other Pt = Pw + Pdry Page 6 / YSH / Humidity Webinar Series Vaisala

7 Definitions Psychrometry

8 Partial Pressure of Water Vapor (psi,mbar,hpa,inhg ) P w The key parameter that affects all other humidity parameters Note: The only two properties that can affect a change in P w adding or removing water vapor changes in system pressure Page 8 / YSH / Humidity Webinar Series Vaisala

9 Saturation Vapor Pressure (psi,mbar,hpa,in hg ) P ws Pw (mbar) Saturation Curve Temperature ( C) On the saturation curve evaporation and condensation are in equilibrium and occur at the same rate P w = P ws dewpoint = temperature wet bulb = dry bulb RH = 100% Note: The only property that affects P ws is temperature Page 9 / YSH / Humidity Webinar Series Vaisala

10 Relative Humidity (%) RH Relative humidity is the ratio of water vapor partial pressure present in a gas (P w ) to the saturation vapor pressure of water at that temperature [P ws (t)] or The amount of water vapor present in air (gas) expressed as a percentage of the amount needed for saturation at the same temperature. Page 10 / YSH / Humidity Webinar Series Vaisala

11 Relative humidity %RH = 100 x P w P ws (t) %RH = 100 x = (t=80) 42% Pw (mbar) Temperature ( C) P w =200 P ws =480 Note: Relative humidity is strongly proportional to temperature and its measurement is very sensitive to temperature differences. Page 11 / YSH / Humidity Webinar Series Vaisala

12 Temperature and Relative Humidity %RH = 100 x P w P ws (t) Pw (mbar) Temperature ( C) T a = 60 C P ws = 200 mbar P w = 70 mbar T a = 90 C P ws = 700 mbar P w = 70 mbar RH = 100 x 70/200 = 35 %RH RH = 100 x 70/ 700 = 10 %RH Page 12 / YSH / Humidity Webinar Series Vaisala

13 Temperature and Relative Humidity %RH = 100 x P w P ws (t) Pw (mbar) Temperature ( C) T a = 60 C P ws = 200 mbar P w = 70 mbar RH = 100 x 70/200 = 35 %RH T a = 40 C P ws = 70 mbar P w = 70mbar RH = 100 x 70/ 70 = 100 %RH Page 13 / YSH / Humidity Webinar Series Vaisala

14 Temperature and Relative Humidity Rule of Thumb #1 Rule of Thumb #1* -As temperature increases, air becomes drier (RH decreases) -As temperature decreases, air becomes wetter (RH increases) drier and wetter are relative terms; applies to a closed system where pressure and water vapor content do not change Page 14 / YSH / Humidity Webinar Series Vaisala

15 What about pressure and Relative Humidity? Recall Dalton s Law of Partial Pressures P t = P w + P dry If double total pressure; then 2(Pt) = 2(Pw + Pdry) = 2Pw + 2Pdry so Pw changes proportionately to overall pressure changes remember that Pws remains unchanged because T is unchanged Page 15 / YSH / Humidity Webinar Series Vaisala

16 Pressure and Relative Humidity P t = 1000 mbar We double the total pressure so P t = 2000 mbar What happens to Pw? Pws? Pw (mbar) Temperature ( C) T a = 80 C P ws = 500 mbar P w = 200 mbar RH = 100 x 200/500 = 40 %RH T a = 80 C P ws = 500 mbar P w = 400mbar RH = 100 x 400 / 500 = 80 %RH Page 16 / YSH / Humidity Webinar Series Vaisala

17 Pressure and Relative Humidity Rule of Thumb #2 Rule of Thumb #2* -As pressure decreases, air becomes drier (RH goes down) -As pressure increases, air becomes wetter (RH goes up) * drier and wetter are relative terms; applies to a closed system where temperature and water vapor content do not change Page 17 / YSH / Humidity Webinar Series Vaisala

18 Relative Humidity Application Example Relative humidity is the common parameter in HVAC applications where comfort balanced with efficiency is the main concern. Page 18 / YSH / Humidity Webinar Series Vaisala

19 T d Dewpoint (ºC,ºF) T d The temperature at which the water vapor pressure in a gas mixture equals the saturation water vapor pressure The temperature at which a moist gas is saturated with respect to a plane surface of pure liquid Page 19 / YSH / Humidity Webinar Series Vaisala

20 Beer temperature = 38F Td = 20F Glass temperature above the dewpoint no condensation Page 20 / YSH / Humidity Webinar Series Vaisala

21 Beer temperature = 38F Td = 45F Glass temperature below the dewpoint condensation appears Page 21 / YSH / Humidity Webinar Series Vaisala

22 Dewpoint T d Tn m Pw log A 1 changes with water vapor changes with pressure Note: Dewpoint is not a temperature dependent parameter Page 22 / YSH / Humidity Webinar Series Vaisala

23 Dewpoint & Pressure - Rule of Thumb Pressure Effects refers to the phenomenon that when the gas system is compressed or expanded the dew or frost point temperature will change proportionally to the change in pressure. Rule of Thumb -As pressure increases, dewpoint temperature rises, air becomes more moist (RH increases) -As pressure decreases, dewpoint temperature goes lower, air becomes drier (RH decreases) drier and wetter are relative terms; applies to a closed system where water vapor content does not change Page 23 / YSH / Humidity Webinar Series Vaisala

24 T d/f Frostpoint (ºC,ºF,ºK) T d/f The temperature to which a given portion of air must be cooled at constant pressure and constant water vapor content in order for saturation to occur The temperature at which a moist gas is saturated with respect to a plane surface of pure ice Note: T d/f is a Vaisala term which means dewpoint above 32ºF and frostpoint 32ºF and below Page 24 / YSH / Humidity Webinar Series Vaisala

25 Dewpoint versus Frostpoint Frostpoint Dewpoint C C C C C C C C C C C C C C C C C C C C C C Page 25 / YSH / Humidity Webinar Series Vaisala

26 T d/f Frostpoint T d or T d/f Td/f gives you dewpoint at 32 degrees (F) and above and frostpoint below 32 degrees (F) Td gives you dewpoint across the entire range of temperatures and assumes supercooled water below 32 degrees (F) Page 26 / YSH / Humidity Webinar Series Vaisala

27 Application Example - compressed air compressor picture courtesy of Atlas Copco Dewpoint is the common parameter for measurement in compressed air systems and plastics production feed drying Page 27 / YSH / Humidity Webinar Series Vaisala

28 x Mixing Ratio or Humidity Ratio (g/kg, gr/lb) x the ratio of the mass of water vapor per unit mass of dry air to which it is associated Note: mixing ratio is an absolute measure, not affected by temperature or pressure Page 28 / YSH / Humidity Webinar Series Vaisala

29 Application Example drying process Mixing ratio can be used as a measure to help determine drying time where moisture content of a product is important like paper drying or dog biscuit drying. Page 29 / YSH / Humidity Webinar Series Vaisala

30 ppm v ppm w parts per million (volume/weight) ppm v - volume of water vapor per total volume of dry gas ppm w - mass of water vapor per total mass of dry gas Note: ppm is an absolute measure, not affected by temperature or pressure Mw is molecular mass of water ; Md is molecular mass of dry air Page 30 / YSH / Humidity Webinar Series Vaisala

31 ppm v and ppm w Td/f PPMw PPMv Mw/Md = Page 31 / YSH / Humidity Webinar Series Vaisala

32 Application Example glove box ppm is sometimes used in dry environments where very precise absolute measurement is required such as in a glove box or clean room Page 32 / YSH / Humidity Webinar Series Vaisala

33 a Absolute Humidity (g/m 3, gr/ft 3, lbs/mmcf) a - the mass of water vapor per unit volume of moist air - the density of the water vapor Page 33 / YSH / Humidity Webinar Series Vaisala

34 Application Example Precision Mass Labs where precise measurements of mass are recorded and the buoyancy caused by the air must be considered Page 34 / YSH / Humidity Webinar Series Vaisala

35 T w Wet bulb temperature (ºC,ºF) T w The temperature indicated by a thermometer sheathed in a wet cloth as air is passed over it Page 35 / YSH / Humidity Webinar Series Vaisala

36 Application Example evaporative cooler or swamp cooler By comparing the wet bulb temperature to the dry bulb temperature we can determine cooling capacity of an evaporative cooler. Page 36 / YSH / Humidity Webinar Series Vaisala

37 h Enthalpy (kj/kg; btu/lb) h Measure of the total energy in a moist gas Heat content Sum of the latent heat + sensible heat Page 37 / YSH / Humidity Webinar Series Vaisala

38 Application Example - HVAC Enthalpy is a useful measurement for determining HVAC equipment size and efficiency Page 38 / YSH / Humidity Webinar Series Vaisala

39 Psychrometric Terms relative humidity RH [%RH] partial pressure of water vapor P w [mbar; in.hg, etc.] saturation pressure P ws [mbar; in.hg, etc.] dewpoint/frostpoint T d/f [ C; F] absolute humidity a [g/m 3; gr/ft 3 ] mixing ratio/humidity ratio x [g/kg; gr/lb] wet bulb temperature T w [ C; F] ppmv ppmw enthalpy h [kj/kg; Btu/lb] Page 39 / YSH / Humidity Webinar Series Vaisala

40 Summary 1. Dalton s Law of Partial Pressures P w 2. Saturation Vapor Pressure P ws only changes with temperature 3. Other terms pressure and temperature effects 4. Rule of Thumb #1 temperature and RH 5. Rule of Thumb #2 pressure and RH 6. Dewpoint only pressure and water vapor content affect it Page 40 / YSH / Humidity Webinar Series Vaisala

41 Vaisala Humidity Resources On-line Humidity Calculator Slide Rule Calculator to order Psychrometric Chart - Humidity Conversion Formulas - For assistance with your humidity measurement Direct telephone: Website: instruments@vaisala.com Page 41 / YSH / Humidity Webinar Series Vaisala

42 Thank you! This concludes the webinar. Follow-up will arrive shortly with the resource links & further contact information. Page 42 / YSH / Humidity Webinar Series Vaisala