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Psychrometrics – Lecture 1

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- Reading:
- Chapter 9 in Henderson/Perry
- Handouts

Dr. C. L. Jones

Biosystems and Ag. Engineering

- Study of physical and thermal properties of air and water vapor mixtures.
- Simply put: the science of drying

Dr. C. L. Jones

Biosystems and Ag. Engineering

- Where:
- p = absolute pressure
- V = volume
- N = number of kg-moles
- R = universal gas constant
- T = absolute temperature (K)

Dr. C. L. Jones

Biosystems and Ag. Engineering

- For a fixed number of moles of an ideal gas:
- p1V1/T1=p2V2/T2

Dr. C. L. Jones

Biosystems and Ag. Engineering

- Atmospheric air is always a mixture of dry air and water vapor. Physical and thermal quantities describe the state of the mixture…
- Dry-bulb temperature: true temp. of the mixture
- Saturation pressure: pressure where liquid and vapor forms of water are in equilibrium… evaporation rate = condensation rate
- Use standard steam table or calculate (9.18)
ln psat = 31.9602 – 6270.3605/T – 0.46057lnT (for 255.38 to 273.16 K)

ln(psat/R’) = (A + BT + CT2 + DT3 + ET4)/(FT – GT2) (for 273.16 to 2533.16 K)

K = C + 274.15

- Use standard steam table or calculate (9.18)

Dr. C. L. Jones

Biosystems and Ag. Engineering

- Humidity ratio: mass of water vapor per unit mass of dry air
- Relative humidity: measure of amount of water air can hold at a certain temperature
- Specific Volume: vol. per unit mass of dry air at the state of the mixture…iow…space occupied by air…greater volume at warmer temperatures

Dr. C. L. Jones

Biosystems and Ag. Engineering

- Dew-Point Temperature: temperature at which water will begin to condense out of moist air. (constant humidity ratio and pressure)
- Wet-bulb Temperature: determined when air is circulated past a wet sensor tip. Temp. at which water evaporates and brings the air to saturation. (constant enthalpy) (adiabatic process)
- Enthalpy: heat energy content of moist air

Dr. C. L. Jones

Biosystems and Ag. Engineering

- Mollier vs. Traditional charts

Dr. C. L. Jones

Biosystems and Ag. Engineering

- Mollier vs. Traditional charts pg 252-253

Dr. C. L. Jones

Biosystems and Ag. Engineering

- Find a point using two properties…
- Can find all of the other 5 properties
- This point is called the “state point”
- Example: 25°C dry bulb and 18° C wet bulb
- Find enthalpy, sp. vol., humidity ratio, rh, dewpoint

Dr. C. L. Jones

Biosystems and Ag. Engineering

- Mollier vs. Traditional charts

Dr. C. L. Jones

Biosystems and Ag. Engineering

A carton of eggs was removed from the refrigerator and left on the kitchen counter. The refrigerator was set at a temperature of 5oC and the kitchen at 24oC, 30% relative humidity.

Will condensation occur (i.e., "sweat" appear on eggs)?

(def. of dew point: temperature at which water will begin to condense out of moist air)

SOLUTION

The corresponding dew-point temperature at 24oC and 30% relative humidity is 4.5oC. Since 4.5oC < 5oC, condensation will not occur.

Dr. C. L. Jones

Biosystems and Ag. Engineering

- Example 9.10 pg. 258
- Mass flow rate m = Q/v,
- Q=vol. flow rate,
- v=spec. vol. of air
q = m(Δh) where q = rate of heat transfer

So: q = (Q/v) Δh

- Mass flow rate m = Q/v,

Dr. C. L. Jones

Biosystems and Ag. Engineering

- Application example: power requirement for heaters
- Example 9.10 pg. 258
- v = 0.8724 m^3/kg,
- h1 = 55.8 kJ/kg,
- h2 = 76.3 kJ/kg

Dr. C. L. Jones

Biosystems and Ag. Engineering