What are the wet bulb temperature and dry bulb temperature in wood drying?
The commonly used hygrometer consists of two calibrated thermometers. One thermometer's mercury bulb is wrapped in gauze, with the gauze soaked in clean water underneath, while the other thermometer's mercury bulb is not wrapped in gauze. A thermometer with a mercury ball wrapped in wet gauze is called a wet bulb thermometer, and the temperature measured by it is called the wet bulb temperature, expressed as t wet; A mercury globe thermometer without gauze is called a dry bulb thermometer, and the temperature measured with it is called the dry bulb temperature.
When the air humidity is low, the moisture contained in the gauze wrapped around the wet bulb evaporates into the air. When water evaporates, it absorbs heat from the wet bulb, making the wet bulb temperature lower than the dry bulb temperature, that is, lower than the temperature of the air. The difference between the dry bulb temperature and the warm bulb temperature is called the hygrometer difference, also known as the difference between dry and wet bulb temperatures. As it is related to evaporation capacity, the numerical value of the hygrometer difference can also be expressed as the drying potential, which is:
ε=t dry - t wet
The drier the air and the greater the difference in humidity readings, the stronger the air's ability to hold water vapor, and the faster the moisture in wet wood evaporates. On the contrary, the wetter the air and the smaller the difference in humidity readings, the weaker the air's ability to hold water vapor, and the slower the evaporation of moisture in wet wood. When the air is completely saturated with water vapor and the humidity is 100%, the dry bulb temperature and wet bulb temperature are equal, and the difference between the hygrometers is zero. At this point, the moisture in the wet wood stops evaporating or the air and water vapor on the surface of the wood are in dynamic equilibrium.
In wood drying production, the humidity value of the air can be checked on a humidity meter based on the difference between the dry bulb temperature t and the humidity meter.
Example: In a forced circulation drying chamber, the two readings on the hygrometer are t-dry=76 ℃ and t-humidity=72 ℃, respectively, to determine the relative humidity of the air in this state.
Solution: First calculate the humidity meter difference:
ε=t dry - t wet=76-72=4 (℃)
According to the humidity meter difference of 4 ℃ and dry bulb temperature t=76 ℃, the air humidity at this time can be found in Appendix 3, that is:
φ=83%
