Humidification Basics Part 7: Isothermal Humidification

By Chad Edmondson (JMP) and Norman Hall (RLD)

In many ways, psychrometrics is like a Rubik's Cube. Change one thing, and you automatically change something else.  But instead of shifting colored cubes around, we’re shifting measures of air, moisture, and temperature.  And, like the Rubik’s cube, there is more than one way to arrive at our goal of a suitably conditioned space.

Specifically, there two basic strategies, isothermal, which we will discuss here, and adiabatic, which we will discuss in Part 8. Both strategies involve manipulating certain factors to achieve our goal -- but instead of twisting the sections of a Rubik’s cube, we’re going to slide things around on a psychrometric chart. 

Remember from our earlier blogs we said that it takes approximately 1000 BTU’s to convert one pound of water into one pound of water vapor. We can see exactly how this works by going back to the psychrometric chart and looking at a case where we have a steam humidifier in place that will add water vapor to the supply air.

What Isothermal Humidification Looks Like on a Psychrometric Chart

It takes energy to convert water to steam, so any type of steam humidification requires an outside energy source like electricity, natural gas or a steam boiler to transfer energy to water in order to change it from a liquid to a vapor, which is then put into supply air or space. How does this represent on a psychrometric chart?  The chart below makes it pretty clear.

At point #1 we have 42°F outside air that contains approximately 10 grains of water per pound of air entering our mechanical system.  That’s our starting point.  Using a steam humidifier to inject vaporized moisture directly into the supply air, we bring the RH up to 85% by adding 23 grains of moisture per pound of air (#2).  (The curved lines represent the RH in increments of 10%.) 

Notice that we haven’t changed the temperature of the supply air, we just added moisture, so we are moving vertically up the dry bulb line at 42°F.  In this example, given our existing space temperature, we can put this humidified air into the space which is maintained at 70°F. This shifts us directly to the right on the psychrometric chart since the supply air is now exposed to a different dry bulb temperature (70°F), which in turn drops the RH down to a more suitable 30% (#3).

This is what we call an isothermal humidification process, which technically means that we are increasing humidification without changing the air temperature. Rather, we are directly adding steam to the supply air, which raises the supply RH. We then put the humidified air it into a warmer space, which automatically causes the RH to drop. 

This process of humidification works very well in systems with:

- A low supply air temperature (because of absorption distances)

- Short but predictable absorption distances so that you don’t rain in the duct

- An onsite steam boiler

- A requirement for extremely accurate control

We use this method often in typical institutional systems.