Psychrometrics – Lecture 1. Reading: Chapter 9 in Henderson/Perry Handouts. Dr. C. L. Jones Biosystems and Ag. Engineering. Psychrometry. Study of physical and thermal properties of air and water vapor mixtures. Simply put: the science of drying. Dr. C. L. Jones

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Psychrometrics – Lecture 1

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

K = C + 274.15

Dr. C. L. Jones

Biosystems and Ag. Engineering

Psychrometrics – more terms

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

Psychrometrics – yet more terms

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)

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

Psychrometric Processes Application: power requirement for heaters

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

Dr. C. L. Jones

Biosystems and Ag. Engineering

Psychrometric Processes

Application example: power requirement for heaters