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Dehumidification Demystified: Part 2

Photo courtesy of Polygon

Editor’s Note: To learn about the first three items you should be considering when looking into dehumidification options, check out the article from the November 2016 issue of CoatingsPro.

Dehumidification can be mystifying for coatings contractors. There are a variety of factors that should be considered before purchasing or renting equipment.


4. How Much Equipment Do You Really Need?

This has long been a subject of debate in the dehumidification industry. Picking a familiar air change rate is common among experts and those who think they are experts. Air changes are defined as the number of times the air in a tank or space is displaced by the ventilation equipment each hour. This may be a good start, and if enough information is known, this sizing method is often accurate.

Two air changes per hour has long been a baseline for desiccant dehumidification in tanks, for example. This convention cannot be assumed without knowing more about the containment or structure. The amount of air required varies greatly with the type of equipment used, size of the tank, integrity of the containment, number of openings, and other sources of ventilation. This design process is a tough one to simplify and, unless you have lots of experience, might be best left to the experts.

Sizing equipment is often looked at as a much more complicated process than it really is. Essentially, you want to look at two things: How much moisture is being introduced into the space and how much moisture are you able to remove with the equipment.

The primary source of moisture introduction into the space is air infiltration. There are some other more nominal factors, such as workers’ breathing and perspiring in the space; however, when blasting and coating, these factors tend to be negligible when compared to air infiltration. 

To handle air infiltration, you generally set up your equipment so that you have positive pressure. Positive pressure means that you are pushing more air into the space that you have conditioned to meet moisture needs than you are losing through natural losses and/or forced losses. By doing this, you are overcoming untreated moist air infiltrating the space. One of the biggest, and often overlooked, factors in determining how much air it will take to create this positive pressure is air loss, such as through dust collectors.

The good news is that it is pretty easy to make sure that the pressure is balanced. By simply comparing the CFM (or m³/min.) delivered by the dehumidification equipment to the CFM being pulled out with the other equipment, such as the dust collector and ventilators, you can see if it is about the same or if there is a difference. If the dehumidification equipment is supplying as much CFM or more when compared to the dust collector then it’s in good shape. As an example, it may only require 10,000 cfm (283 m³/min.) to dehumidify the space but there may be a 24,000 cfm (680 m³/min.) dust collector being used. This would result in 14,000 cfm (396 m³/min.) of infiltration, quickly overpowering the dehumidifier. Many times the dust collector is required to create a negative pressure in the tank or space. This creates additional infiltration that must be compensated for.

To determine how much moisture you need to remove, look at what conditions the surface needs. Then, size the dehumidification system to supply air that delivers moisture levels low enough to overcome the infiltration and provide the appropriate dew point temperature. The key here is not to overdo it.

It is not uncommon for dehumidification equipment to supply air with a dew point temperature that is way below what is needed for the required 50 percent surface relative humidity (RH). You can often blend in untreated air with the dehumidified air and still be providing the right conditions in the enclosure. By blending in this manner, you can substantially reduce the requirement for dehumidification equipment or compensate for infiltration when a negative air pressure is required.

If you choose the right equipment type and use the dehumidification equipment to create positive pressure in the space, you can control the environment and limit unwanted infiltration. If you set up your dehumidification system to include some blended untreated air, you can often still meet the requirements while reducing the volume of dehumidification equipment and accommodating other sources of ventilation.

5. How Can You Minimize the Cost of Climate Control?

It may seem obvious, but the primary cost drivers of climate control on a site tend to be equipment and energy, and if your job requires a generator, then energy is going to dominate your climate cost.

There are several ways in which these factors can be impacted. The biggest driver is to ensure that the equipment is properly sized. Clearly the more equipment you have, the more you are paying not only for additional equipment but also for additional energy to run that equipment. By walking through the process outlined in Step #4 for sizing equipment, you can help ensure you don’t have more equipment than you need.

A big way to help minimize the cost of your climate control equipment is to consider a blended system. You can use an air conditioner to improve the efficiency of a desiccant dehumidification system. Often, by feeding a desiccant with an air conditioner, you can do more work with less equipment as well as less treated CFM. If you are feeding less treated CFM into your space, you are also blowing less of this expensive treated air out the other side.

The other big factor is the cost to run your equipment. Energy costs will vary greatly depending on whether you are using customer-supplied electricity, a generator, or propane for your desiccant reheat. Simply picking the least expensive combination of energy sources can have a huge impact on the project cost.

Some other cost saving tips are to optimize the project’s sequence and work schedule. By holding blast, contractors are able to do things such as work during inclement weather and blast large areas at one time, the latter of which cuts down on setup and prep time. Savings from these methods can be substantial and in some cases can actually pay for the dehumidification.

There is a big difference between cost and price. The estimator should always weigh all of the costs and all of the savings to determine the best solution, the best production sequence, and the best vendor. To accomplish this, careful consideration should be given to optimizing equipment performance, determining how much dehumidified air is really needed, controlling energy costs by choosing equipment that is properly sized as well as configured to use the most cost effective energy available, and carefully planning project production sequences.

About the Author:

Nick Kline is a business development specialist for Polygon US, a temporary climate control provider. Polygon US services include humidity and temperature control for industrial coatings projects, commercial construction, water damage restoration, and many other commercial and industrial applications. Kline has more than 10 years of experience in the industrial and commercial arenas as well as a mechanical engineering degree. He uses this background to engineer and solve some of the most challenging temporary climate solutions for the Virginia and North Carolina markets. For more information, contact: Polygon US,

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