RH Control using Duct Heaters
When duct heaters are installed in the ducts and used to raise the temperature of the air passing through, the relative humidity (RH) decreases because the warmer air can hold more moisture, making the same absolute amount of water vapor present in the air a smaller percentage of the total moisture capacity.
Let’s break down the statement further to make it clearer:
Understanding Relative Humidity (RH)
Relative humidity is a measure of the amount of water vapor present in the air compared to the maximum amount of moisture the air can hold at a given temperature. It is expressed as a percentage. Warm air can hold more water vapor than cold air at the same time, so if you have a specific amount of water vapor in warm air, it will have a lower relative humidity compared to the same amount of water vapor in cold air.
Duct Heaters and Temperature Increase
Duct heaters are devices installed in ducts of HVAC systems to raise the temperature of the air passing through. When the air passes over these duct heaters, it gains heat and becomes warmer
Effect on Relative Humidity
As the air gets warmer due to the duct heaters, its capacity to hold water vapor increases. However, the amount of water vapor in the air remains the same (assuming no additional moisture is added or removed). This means that the absolute amount of water vapor in the air doesn’t change, but the air can now hold more moisture due to the increased temperature.
Percentage of Total Moisture Capacity
With the increased temperature, the same amount of water vapor now represents a smaller percentage of the air’s total moisture-holding capacity. In other words, the air is not as “saturated” as it was at a lower temperature, even though the actual amount of water vapor hasn’t changed.
Example: RH Variation with Temperature
Let’s say we have two air samples, one at a colder temperature of 10°C and another at a warmer temperature of 30°C. Both samples contain 10 grams of water vapor. At 10°C, the air is close to its saturation point, so the RH would be close to 100%. However, when the same 10 grams of water vapor is in air at 30°C, the air can hold much more moisture, and the RH would be much lower, let’s say around 30%. So, even though the absolute amount of water vapor (10 grams) is the same in both cases, the RH is different due to the temperature difference.
In summary, by using duct heaters to raise the air temperature, the RH decreases because the warmer air can hold more moisture, making the same absolute amount of water vapor in the air a smaller percentage of the air’s total moisture capacity, resulting in a lower relative humidity.