FOOD ENGINEERING: PSYCHROMETRICS AND DRYING
Diagram
Two thermometers side by side in the same air
Dry bulb thermometer
An ordinary thermometer, open to the air. It reads the true temperature of the air.
↓
Wet bulb thermometer
A thermometer with its bulb wrapped in a wet cloth. Evaporation cools the bulb, so it reads lower than the dry bulb.
↓
The difference
The gap between the two readings is the wet bulb depression. A large gap means dry air, and a small gap means humid air.
Figure 1: A dry bulb and a wet bulb thermometer reading the same air. Drawn in our own style from Brooker, Bakker-Arkema and Hall, Drying and Storage of Grains and Oilseeds, Springer, 1992, and Fellows, Food Processing Technology: Principles and Practice, Woodhead Publishing.
Introduction
Dry bulb temperature and wet bulb temperature are two temperatures read from two thermometers standing side by side in the same air, and together they tell the dryer operator how humid the air is. The dry bulb temperature is the ordinary air temperature, read from a plain thermometer. The wet bulb temperature is read from a thermometer whose bulb is wrapped in a wet cloth, and evaporation from the cloth cools the bulb, so it reads lower.
The core difference is evaporation: the dry bulb is untouched by it, while the wet bulb is cooled by it. When the air is dry, water evaporates fast and the wet bulb reads much lower. When the air is already humid, little water evaporates, and the two readings are close together.
Quick comparison
Table 1: Dry bulb temperature compared with wet bulb temperature
| Aspect | Dry bulb temperature | Wet bulb temperature |
|---|---|---|
| What it is | The true temperature of the air | The temperature of a wet surface cooled by evaporation |
| Thermometer | Plain bulb, open to the air | Bulb wrapped in a wet cloth or wick |
| Reading | Higher, or equal in saturated air | Lower than, or equal to, the dry bulb reading |
| Effect of humidity | Not affected by humidity | Rises as humidity rises, and falls as air dries |
| In saturated air | Equals the wet bulb temperature | Equals the dry bulb temperature |
| In dry air | Unchanged by the dryness | Much lower than the dry bulb reading |
| What the gap means | The gap is the wet bulb depression | A large gap means dry air, a small gap means humid air |
| Use | The air temperature for drying and storage | With the dry bulb, it gives humidity from the psychrometric chart |
| Air movement | Reads the same in still or moving air | Needs moving air past the wet bulb for a true reading |
| Food example | The drying air temperature in a grain dryer | The wet bulb reading that sets the drying rate of the grain |
Scientific explanation
Definition
Dry bulb temperature is the temperature of air measured by an ordinary thermometer with a dry bulb. Wet bulb temperature is the temperature measured by a thermometer whose bulb is kept wet, so evaporation cools it to the lowest temperature that evaporation can reach in that air.
Working principle
Water evaporates from the wet cloth, and evaporation takes heat from the bulb, so the bulb cools. It cools until the heat taken by evaporation is balanced by the heat arriving from the surrounding air. Dry air takes up water fast, so the bulb cools a long way. Humid air takes up little water, so the bulb cools only a little. In air that is already saturated, no water can evaporate, so the two thermometers read the same.
Apparatus
The instrument is the psychrometer, also called the sling psychrometer, which carries a dry bulb thermometer and a wet bulb thermometer side by side. Brooker, Bakker-Arkema and Hall describe the psychrometric chart that is used with the two readings.
Measurement procedure
The wick of the wet bulb is wetted with clean water, and air is moved past both bulbs, by whirling a sling psychrometer or by a fan, until the wet bulb reading is steady. Both temperatures are read at the same time, and the pair of readings is then used on the psychrometric chart to find relative humidity and other air properties.
Factors affecting readings
The wet bulb reading depends on the humidity of the air, on the air moving fast enough past the bulb, and on the wick being clean and fully wet. Still air gives a wet bulb reading that is too high. A dirty wick, or water that is not clean, also changes the reading. The dry bulb reading is affected by direct sun or by heat from nearby surfaces, so both bulbs must be shaded.
Formulas and calculations
The books read give no formula that turns the two readings into humidity by arithmetic alone. The working relation is the wet bulb depression: wet bulb depression equals dry bulb temperature minus wet bulb temperature.
Worked example: a dry bulb reads 30 degrees C and a wet bulb reads 20 degrees C. The wet bulb depression is 30 minus 20, which is 10 degrees C. A depression of 10 degrees C is large, so the air is dry, and the relative humidity is then read from the psychrometric chart for the pair 30 degrees C dry bulb and 20 degrees C wet bulb. If the wet bulb read 29 degrees C instead, the depression would be 1 degree C, so the air would be nearly saturated.
Industrial applications
- Dry bulb temperature: setting and controlling the air temperature in grain dryers and food dryers.
- Dry bulb temperature: storage rooms for grain and dried foods, where air temperature is monitored.
- Wet bulb temperature: finding the humidity of drying air from the psychrometric chart, so the dryer can be set correctly.
- Wet bulb temperature: checking whether drying air is dry enough to take up moisture from the grain.
- Both readings together: working out the drying rate and the safe storage conditions for grains and oilseeds.
Advantages and limitations
- Dry bulb temperature, advantage: simple to measure with an ordinary thermometer, and it is the temperature that controls drying heat.
- Dry bulb temperature, limitation: on its own it says nothing about humidity, so it cannot tell whether the air will dry the food.
- Wet bulb temperature, advantage: together with the dry bulb it gives humidity and the drying power of the air without an electronic sensor.
- Wet bulb temperature, limitation: it needs moving air, a clean wet wick and a steady reading, and it is less handy than a modern humidity sensor.
FAQ
Why does the wet bulb read lower?
Because water evaporating from the wet cloth takes heat from the bulb and cools it.
When do the two thermometers read the same?
In saturated air, at 100 percent relative humidity, when no more water can evaporate.
What is wet bulb depression?
The dry bulb reading minus the wet bulb reading. A large depression means dry air.
How is humidity found from the two readings?
The pair of readings is used on the psychrometric chart, which gives relative humidity and other air properties.
Does the wet bulb need moving air?
Yes. Air must move past the wet bulb, or the reading comes out too high.
References and further reading
- Fellows, P., Food Processing Technology: Principles and Practice, Woodhead Publishing (FTN book on disk).
- Brooker, D. B., Bakker-Arkema, F. W. and Hall, C. W., Drying and Storage of Grains and Oilseeds, Springer, 1992 (Google Books page read: Psychrometric Chart, p. 41, and Maps of Wet-Bulb Temperatures and Depressions, p. 412).
