Reprinted with Permission ©2003 Process Heating www.process-heating.com SEPTEMBER 2003 2 1

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SNow that we are some-what familiar with all ofthe data ranges andcurves on the psychro-metric chart, let’s figure

how to use it. I will jump straight in.Suppose you are at sea level. Taking

a thermometer and measuring theambient air temperature, you get 82°F(28°C). This is the dry bulb tempera-ture. You take a reading with yourhygrometer that tells us that the rela-tive humidity is 60 percent. Looking atthe chart in figure 1, you plot this con-dition at point A. From the chart, youcan read off the remaining propertiesof this air as follows:

• The wet bulb temperature is 72°F(22°C).

• The dewpoint, or saturation, tem-perature is 68°F (20°C). That is, ifyou cool the air, the water will startto condense out at 68°F.

• The humidity ratio is 0.014 poundsof moisture per pound of air.

• The specific volume is 14 ft3 perpound of dry air.

• The enthalpy is 36 BTU per poundof dry air.

Next, imagine that you take thissame air and heat it up indirectly to200°F (93°C). In terms of the physicalconstituents of the air, you are chang-ing nothing. The amount of water inthe air remains the same, so you canfollow the humidity ratio line until youreach the dry bulb temperature line of200°F. This is point B. Again, from thecurve, you read off the correspondingproperties of the air as follows:

•The wet bulb temperature hasincreased to 96°F (36°C).

•The dewpoint temperature has not

changed.•The humidity ratio has not changed.•The relative humidity has dropped

from 60 percent to 3 percent.•The air has expanded so that 17 ft3 of

this air now weighs one pound.•The enthalpy, or energy of the air, has

increased to 64 BTU per pound ofdry air.

Suppose you now pass this heated airthrough a moist product. For thisexample, we’ll make a huge assump-tion that all the energy in the air is usedfor evaporation. It never is, but I can

take the liberty under guise of illustra-tion. If this were the case, and wechose that the air, after passingthrough the product, was at 110°F(43°C), the air would follow the con-stant wet bulb temperature line (adia-batic because no heat is entering orleaving the system) until it reached thedry bulb temperature of 110°F, whichis point C. At this point, you obtainthe following data:

• The wet bulb temperature hasremained the same.

• The dewpoint temperature has risen

Darren A. Traub

HOW TO USE IT

The Psychrometric Chart

Figure 1. Using psychrometric charts and empirical data obtained from the fieldwill allow for a reasonably accurate model of a dryer to be built.

A B

D

C

to 94°F (34°C).• The humidity ratio has increased to

0.035 pounds of moisture per poundof dry air. It has picked up 0.021pounds of moisture per pound ofdry air from the product.

• The relative humidity is back to 60percent. Coincidence? Yes!

• The air has densified so that 15.15ft3 of air now weighs one pound.

• The energy in the air has remained

relatively constant. This is part ofour huge assumption that the sensi-ble heat in the air has been replacedby latent heat in the moisture.

Let’s not make any more assump-tions; instead, we will replace themwith presumptions. Presume you takeyour dry bulb thermometer and meas-ure the temperature of 110°F above theproduct bed. And, by placing some wetgauze on the bulb in the same location,you measure a wet bulb temperature of94°F. This would be point D on thecurve. For the air to follow path B toD, you obtain the following data:

• The dew point temperature has risento 88°F (31°C).

• The humidity ratio has increased to0.029 pounds of moisture perpound of dry air. It has picked up0.015 pounds of moisture perpound of dry air from the product.

• The relative humidity is at about 52percent.

• The air has densified so that 15.0 ft3of air now weighs one pound.

The energy in the air has reduced to58 BTUs per pound of dry air. This is areduction of 6 BTUs per pound of dryair. Where has it gone? Losses! Thereare losses from radiation, conductionand convection. The product hasincreased in temperature. The skin ofthe dryer is hotter than ambient steel.The system is constantly losing heat.

Next month, I’ll take this examplefurther by putting that air inside a pieceof process equipment such as a dryeror oven. PH

Darren A. Traub is executive vice president

of Drytech Engineering, Irvine, Calif., a

company specializing in thermal drying

and related system technologies. He has

engineered and managed the project exe-

cution of numerous drying and bulk mate-

rials-handling systems in the United

States, Africa, the Middle East and China.

He can be reached at (949) 262-1222 or

e-mail darren@drytecheng.com.

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THE AMOUNT OF WATER

IN THE AIR REMAINS THE

SAME, SO YOU CAN FOL-

LOW THE HUMIDITY

RATIO LINE UNTIL YOU

REACH THE DRY BULBTEMPERATURE LINE.