Through the years, all drying equipment, including instruments, has suffered from misinformation, or MUS (per Cliff Zlotnik, that’s all that “Made Up Stuff”). I like the equipment we employ, and even though I use all the tools of the trade, I have been labeled by more than a few as “the desiccant guy” which is as good a place to start as any.
Let’s look at the four most common myths of modern desiccants. They are:
- “Damnable to Duct”
- “Positive is the Pressure to Use”
- “Works Best when Cold” and
- “Too Expensive to Use”
Desiccants require ducting on every project. The water vapor in the air is adsorbed into the silica gel rotor on the process side (processing the air to be dried) and desorbed from the rotor on the reactivation side (where heated air reactivates the desiccant) thus forming a very humid “reactivation out” air stream. This water vapor must be ducted out of the affected area, usually to the outside atmosphere.
Over the last 20 years we have moved forward with flex duct, layflat and insulated HVAC ducting but clamps and connections have always proven tricky. The single best clamping method for securing duct to the unit or connections are ratcheting straps (a.k.a. cargo tie downs) from Wally World, Home Depot, auto stores, etc. They are inexpensive and much easier to acquire than the long stainless steel hose clamps we used to use.
For duct-to-duct connections, the best are either hog rings or industrial-strength zip ties. Never use duct collars going up the side of a building as wind may work the collars loose and a separation may occur, causing the duct to fall to the ground possibly injuring someone. To prevent this uncontrolled fall, one should always run cable or rope inside the duct from the top at the building to the ground. This way, if a failure occurs, the duct simply slides down the rope falling to the ground accordion style.
Duct work transitions through walls of a structure are much easier now than ever before. For example, Phoenix has its Easy Port, Kurt Bolden has his multi-transition Metal Duct Door. I go with these or plywood with metal collar transitions to provide a secure and weather-resistant transition in minutes.
Doorways are not always available for use, especially in occupied structures. The fire code may prevent you from blocking them, so get a commercial window installer on your subcontractor list. This is a quick and inexpensive way to make the transition if the windows do not open.
On smaller, portable desiccants, “reactivation out” can be ducted into HT LGRs for condensing, but remember to look for dryer, bath or stove vents (and ensure they exit the structure). Commercial structures may have fresh air make-up systems, which also evacuate indoor air from the building. They are usually found in or around the elevator shafts and mechanical areas; ask the building engineer for availability and CFM capability.
Ducting only the reactivation air out will generally result in a negative pressure environment. This will cause air outside the affected area to infiltrate. Be careful: if grains are high or the temperature is undesirable outside the affected area, the amount of this infiltration may make it really difficult to reach the really low grains desiccants are famous for, adversely affecting your drying time.
You may want to run a “reactivation air in” duct to bring the affected area to a neutral pressure (recirculation of reactivation air). Remember to always control your ingress and, on high-end homes or commercial projects, consult with an HVAC engineer to ensure no environmental or safety systems shall be adversely affected.
Recirculation or neutral pressure will generally produce the lowest grain level because you are removing moisture with every pass through the desiccant; as the building grains drop, so will your process-out grains. This occurs over and over until grains bottom out, usually in the teens or 20s with a purge or low reactivation temperature desiccant (which are difficult to over-dry with), and into the single digits with at standard or 4-hole desiccant.
Extra myth dispelled here: desiccants don’t split wood; people who do not monitor their drying progress split wood!
My personal favorite myth, and the most repeated, is that desiccants don’t like hot air, or that they don’t even dry when it is hot. Well this is BS, not just MUS. The proof is in the numbers in Table 1.
The table shows the Temperature/Relative Humidity/GPP in and then out, equaling pounds of moisture per hour removed. Notice that 400F/100 RH is the same water removal as 950F/30 RH. The air temperature is more than twice as hot and the unit is still drying well. Busted! I believe this myth started long ago with lithium chloride rotors that were far more temperature sensitive. The silica-gel units we use today should work well into the 110-1,200F range.
A great benefit of desiccants is that they produce heat as a by-product. If you want higher air temperature to raise the material temperature (increasing the material’s vapor pressure) while maintaining the lowest air vapor pressure possible, the desiccant achieves both. But keep in mind that contents (wine, candles. chocolates, etc) and people have a temperature limit (and sprinkler heads: never, never forget the sprinkler heads), so spot heating might be the best overall choice for heating stubborn materials.
Of course, the bottom line is always costs. In smaller units (300-900 CFM) desiccants are more expensive, but you charge more, resulting in the same 20-30 day rental ROI. In larger units, especially considering drying capability over the course of the job (total gallons removed), the costs are the same. Forty or so LGRs cost about the same as a 9,000 CFM desiccant (though you cannot cut up the 9000 and use it on 20 different jobs!). Justifying job costs to the adjuster is the important part, so know why you are using desiccants and educate the adjuster as to the reason. In a single-room loss you need hardwood flooring or plaster present to justify the additional rate and, with the drying power of newer HT LGRs, this type of project is open for either unit. Let’s look at the cost of a whole house loss:
- On a 3,000-5,000-square-foot loss, you would need either 3-to-5 LGRs or a 900 CFM desiccant.
- A 900 CFM desiccant at $550/day x 3 days = $1,650
- Five LGRs at $125/day x 3 days = $1,875
Commercial projects are where desiccants shine, as long as you have access to larger areas:
- One large 5,000 CFM at $1,500/day x 3 days = $4,500.
- 24 LGRs at $125/day x 3 days = $9,000.
- Even if you add in generator and fuel, the desiccant costs less.
I hope you have a better understanding of desiccants, have gained a few extra usage tips, and realize that no one knows everything about everything. I wish you profitable drying.