So we made some adjustments to the airflows but at this point the room was in balance and I really didn’t think there was anything else that could be done. Thankfully, the certification company (Air Source Technologies) and more importantly the Engineer owner, Bruce Ferguson, had more tricks up his sleeve to make more progress. 

What Bruce explained was that when you’re dealing with a negative pressure, the room will take air from wherever it feels; more to the point though, it’ll get air from the “path of least resistance.” So if that path is getting air from above the ceiling where it’s not clean, so be it. We needed to give the room the air it needed from a “path” of our choosing; where the air would be cleaner. 

As an experiment, we made a large opening above the doorway into our prep area, directly outside of our cleanroom. BUT, we put a HEPA filter inside this opening so that whatever air was coming through it the air would be clean. 

Bingo. The experiment worked. The particle counts dropped significantly and so did the viables. However, we had a large hole above our door to get into our prep area. We needed a more permanent solution. What we did next was no small undertaking (NOR INEXPENSIVE). It would involve shutting down our cleanroom for a period of time to change out the HVAC system so that we had one that could supply MUCH more air; and air that was cool and dry.

While that’s easily typed out, it’s actually not so easy to accomplish. Remember, we really should have air that’s cool and dry to keep our humidities below the 60%. While this isn’t a requirement it’s strongly suggested. High humidities can lead to mold growth, no bueno. The current air handling system just didn’t keep up, plain and simple.

We contacted a local, trusted HVAC contractor that had experience with cleanrooms. They had installed many systems in both hospitals and manufacturing buildings. This is an EXTREMELY important point not to miss. Not all cleanrooms are created equal. Not all builders are created equal. NOT ALL HVAC contractors are created equal. You need to do some homework when selecting these contractors. A lot of people take to social media for suggestions and that’s not a bad start for getting recommendations but the search shouldn’t just stop there. You need to do background checks on ALL vendors. Trust, but verify. 

So we were assigned an engineer to take a look at our space and design a system with the following requirements: A system that could supply in excess of 2200 CFM at 65 degrees and less than 45% relative humidity. What I learned here is that this is a very difficult and expensive ask of an HVAC system. A lot of systems are able to deliver chilled air but it comes at a price, the humidity may be higher. If you raise the temperature of the air to bring the RELATIVE humidity down, then you’re out of spec on temperature (and uncomfortable). 

Relative humidity. Let’s remember what that exactly means. The humidity or amount of water in the air is dependent upon the temperature. The more warm the air, the more water it’s able to hold. So the amount of water in the air is RELATIVE to the temperature. At low temperatures, the air just simply doesn’t hold a lot of water so it will appear that the relative humidity is really high, but that’s only because the denominator is a low number. Think about it like this:

Let’s say the air at 60 degrees can hold 10 grains (650 mg) of water, but at 90 degrees it can hold 100 grains. The amount of water per se didn’t change, just the amount of water the air is able to hold. So if we have 1 grain of water at 60 degrees and 1 grain of water at 90 degrees, the relative humidities will look VERY different even though the water content didn’t change.

These are just hypothetical numbers but let’s take a look:

1 grain / 10 grains (amount of moisture at 60 degrees): 10% relative humidity


1 grain / 100 grains (amount of moisture at 90 degrees): 1% relative humidity

So, playing that numbers game with temperature and humidity, that’s all it is; a numbers game. The REAL number that matters is the amount of moisture in the air as measured by dew point; the number of grains of water in the air. 

Again, this is no small task for an HVAC system. To get to where we were asking to be we needed a more specialized system. Let me just say, there’s always more than one way to skin a cat, this is what we did. 

We purchased what’s called a dessicant dehumidifier. Inside this monstrous piece of equipment is a spinning wheel with desiccant (dries out the air) inside. As the wheel turns it’s heating up and drying out the air. It then needs to be cooled back down and delivered into the space. Oh and by the way, it also has to be cool enough to overcome any of the latent heat from inside the space being generated by the other pieces of equipment (i.e. the hoods, lights etc.). AND it has to run through HEPA filters so it’s clean. ugh.

How much did this system cost and did it work? Next time.

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