Let's take a detailed look at what wet bulb temperature is in this video. Wet bulb temperature is a very important topic in the HVAC field. So, everyone should know about this.
There are two types of temperature. Dr bulb temperature and wet bulb temperature. The bulb is the place in the thermometer where the mercury is located.
When this bulb heats up, the mercury inside expands. We can measure the temperature value by how far the mercury expands. If you wrap a wet cloth around the bulb and measure the temperature that is the wet bulb temperature.
If you measure the temperature directly without anything on the bulb, that is the dry bulb temperature. The temperature that we normally measure is the dry bulb temperature. Dr bulb temperature is used in most common applications.
However, we use wet bulb temperature where evaporation takes place. Wet bulb temperature is the temperature measured by considering both air temperature and humidity. Let's learn more about this in detail.
Our body continuously generates heat and at the same time we also generate sweat. All of the sweat water takes away the heat generated by the body and evaporates. Because the sweat takes away the heat generated by the body and evaporates, our body is maintained at a certain temperature.
As we explained in our last video on humidity, water cannot evaporate properly when the humidity is high. So, when the humidity is high outside, the sweat on our body cannot evaporate properly. Because the sweat cannot evaporate, it cannot control our body temperature.
This makes us feel very hot. Since our body's cooling process involves evaporation, our body temperature is affected not just by air temperature but also by humidity. Therefore, measuring wet bulb temperature would be more suitable for determining human thermal comfort than measuring air temperature alone.
A wet bulb temperature between 15 degrees celsius and 25 degrees celsius is considered normal for the human body. Between 25 degrees celsius and 30 degrees celsius we start to feel uncomfortable. When the wet bulb temperature exceeds 30 degrees celsius there is a high risk of heat stress and heat stroke.
So it's best to stay indoors. In HVAC, wet bulb temperature is used in cooling towers because of the evaporation process that takes place there. The hot water from the supply line of the chiller's condenser enters the cooling tower flows down through the fills and then returns to the chiller through the return line.
The water temperature is high when it enters the cooling tower. As the water travels down through the fills, some of it evaporates During evaporation as we discussed earlier the water absorbs some heat and evaporates causing the remaining water temperature to decrease. The evaporated water vapor is expelled by a fan.
The remaining water now at a lower temperature returns to the chiller. However, when the humidity is high, the water cannot evaporate efficiently and the water temperature does not decrease much. As a result the water returns to the chiller at a higher temperature.
This reduces the chiller's efficiency. When the wet bulb temperature exceeds 25 degrees celsius, the cooling towers performance starts to decrease. The difference between the wet bulb temperature and the cooling towers return water temperature is the cooling tower's approach value.
Now, let's finally see how to measure wet bulb temperature using a psychrometer. Take a psychrometer that has two thermometers. Wrap a wet cloth around the bulb of one thermometer leave the other thermometer dry.
Now, when you rotate the psychrometer, all the heat in the air is transferred to the bulb. The mercury in the bulb expands according to the heat. We can measure the temperature by how far it expands.
In the wet bulb the heat from the air is also transferred to the bulb causing the mercury to expand. The mercury will expand to the same temperature as the air. However, as we discussed earlier when water evaporates it takes away some heat.
So, when the water evaporates, it removes some heat from the bulb. The removal of heat causes the bulb's temperature to decrease. This temperature reading is the wet bulb temperature.
When humidity is high, less water evaporates. So, less heat is removed from the bulb during evaporation resulting in a higher wet bulb temperature. When humidity is low, more water evaporates.
So, more heat is removed from the bulb during evaporation resulting in a lower wet bulb temperature. And the wet bulb temperature value will always be lower than the dry bulb temperature value. The heat absorbed when water evaporates is called latent heat.
We have already uploaded a video about latent heat on our channel. The link to that video is in the description and comments section for those who are interested. If you have any doubts about wet bulb temperature, please leave a comment below.
If you liked this video, please give it a thumbs up and share it with anyone who might find it useful. Don't forget to subscribe to our channel zebra learnings for more HVAC content.
