Drying Mixology

September 15, 2008
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Photos courtesy of Begal Enterprises


Drying strategies, equipment and techniques have evolved dramatically over the years. Unfortunately there still is not a perfect drying system for all conditions. Of course, there are perfect conditions for some drying systems, but in a successful drying scenario, the conditions are constantly changing and the perfect conditions are lost.

The restoration industry is constantly searching for better methods and faster results. One way to enhance performance in less-than-perfect conditions is to mix systems. Some of the combinations described here have been around for many years, others are fairly new. There is no doubt there are some combinations that have been missed and some yet to be conceived.

LGR Dehumidifiers: Mix with Performance Amplifier, Air Conditioners or HEPA Air Scrubbers

The LGR dehumidifier is the workhorse of the restoration industry. It is easy to transport and set-up and has no special power requirements. The LGR’s ability to remove water between 70°F and 90° cannot be matched for efficiency and effectiveness. Unfortunately, LGRs have their limitations. Because an LGR dehumidifies by cooling water vapor below its dew point, they are unable to achieve specific humidity levels much below the freezing point, 32°F or 28gpp (grains per pound). Even the best LGRs cannot achieve dew points lower than about 45° F below the intake air temperature.

When operating in conditions of 95° F, the lowest attainable dew point will be about 50 degrees, which is a specific humidity of 55 gpp. This one of the reasons many drying jobs stall at the end of days two and three. There is little grain depression because the ambient conditions are nearly the same specific humidity as the dehumidifier’s maximum cooling capacity. A common strategy when a drying job stalls due to high ambient temperatures is to cool the area down so the dehumidifier can produce lower specific humidity levels. At 70° F an LGR can dehumidify air to about 30 gpp.

An LGR performance amplifier is a device that attaches to the top of a down-flow LGR. It contains an air-to-air heat exchanger to pre-cool the air entering the dehumidifier. The unit is ducted from and back to a cool air source. Since the heat exchanger does not transfer moisture, the specific humidity of the cool air source is irrelevant.

The amplifier will pre-cool the air presented to the dehumidifier to approximately the average temperature of the two air streams. For example, if your drying chamber is 100° F and the cool air source is 70° F, the air presented to the dehumidifier will be about 85° F. This will allow the dehumidifier to produce air with a specific humidity approaching 38 gpp rather than the 66 gpp produced from the 100° F air. Since the draw of the performance amplifier is less than 2 amps, the energy penalty is minimal for the increase in performance. Some innovative restorers have attached water to air heat exchangers (radiators) to their LGRs and utilized tap water to pre-cool the ambient air.

It is common for the central air conditioning system of a water-damaged structure to be operated during a dry down. The exception to this is when there is a risk of contaminating the system or when there is no need for cooling. The air conditioning system has the capacity to cool incoming air approximately 10 to 15 degrees.

Improved LGR performance can be achieved when the air from an air conditioning supply vent is ducted directly into the LGR. Just as with the performance amplifier, the cooler the air entering the LGR, the lower the specific humidity produced. The drawback of this strategy is that the overall amount of cooling provided to the space does not change, so if the cooled air is directed into the dehumidifier, it is unable to cool the space as effectively, so the ambient temperature of the drying chamber will rise. This means the temperature of the air treated by the A/C system is warmer and, therefore the supply air ducted into the dehumidifier will be warmer. In the end, the advantages of using the central A/C system can be marginal at best.

A different result is gained by adding a portable air conditioner because the overall cooling capacity in the structure has been increased by the capacity of the portable. The air conditioner’s cooling capacity is measured in BTUs (British Thermal Units); 12,000 BTUs are referred to as a “ton.” If a home has a 4-ton A/C system (48,000 BTUs) and a 1-ton portable A/C is added, you have increased the total air-conditioning capacity by 25 percent.

A 1-ton portable AC will provide approximately 400 cfm of cooled air with a 12- to 18-degree temperature drop. The airflow of the condenser or heated air stream will be equal to or greater than the cooled air stream. When combined with an LGR operating in ambient conditions of 100°F, the air entering the LGR will be lowered to 88°F to 82°F. This will allow the LGR to produce air with a specific humidity around 36 gpp.

Since the airflow of the portable A/C is probably greater than the LGR dehumidifier, you cannot simply duct all of the A/C airflow into the dehumidifier. The LGR’s performance is based on the design airflow of the system; increasing or decreasing the design airflow will degrade performance. An open ducting method – where ducting between two devices contains open areas to allow each device to process different airflow volumes – should be employed.

When using a portable A/C, the condenser coil (heated) air stream should be ducted to and from outside the area to be conditioned. If only the exhaust air stream is ducted outside, the conditioned space will be depressurized and the amount of infiltration will equal the volume of air exhausted outside. This corrupts the integrity of the drying chamber by drawing in unconditioned air.

When contaminated areas require LGR dehumidification, the traditional procedure has been to operate the LGR inside the containment. Unfortunately this will require extensive cleaning to decontaminate the dehumidifier. Another strategy has been to locate the LGRs in the unaffected area and allow the negative air created by the HEPA to draw the dehumidified air into the containment. This eliminates the need for decontamination, but the rate of drying can be painfully slow.

A third alternative is to duct air directly from the HEPA air scrubber into the dehumidifier. This allows dehumidified air to be supplied inside the containment, but eliminates the need for decontamination of the internal LGR components. If combined with a variable-speed air scrubber, the airflow can be reduced to match the LGR and a closed duct system – ducting is attached directly from one device to another without any openings; the processed airflow volume of both devices is the same – can be utilized. If the HEPA airflow is greater than the dehumidifier, an open duct system must be used.

The combination of an LGR and a HEPA air scrubber can be a solution to the dilemma when a contaminated crawl space needs to be dried, but is too short for an LGR. The HEPA-scrubbed negative air from the crawl space is connected with an open duct system to the LGR, and the dehumidified air is ducted back into the crawl space. Caution should be taken to insure that the HEPA air volume exceeds the LGR’s airflow enough to properly depressurize the contaminated crawl space.

Desiccant Dehumidifiers: Mix with LGR Dehumidifiers, Air Conditioners and HEPA Air Scrubbers

Desiccant dehumidification is a common choice for commercial and specialty drying applications. One difficulty that can arise when operating small or medium sized desiccants inside a commercial structure is exhausting the hot and humid reactivation air stream outside of the structure.

A remedy to the reactivation air dilemma is to duct the reactivation air stream to an LGR dehumidifier. The LGR will condense the moisture out of the reactivation air inside the drying chamber. The best results will be obtained when the volume of reactivation air ducted to the LGR is less than or equal to half of the design airflow of the LGR dehumidifier. An open duct system should be used. This prevents the dehumidifier from being overwhelmed by the hot and humid reactivation air. This also allows the LGR to supply specific humidity nearly the same as the ambient conditions.

The performance of the desiccant can often be improved by ducting the air from the LGR back to the reactivation inlet. This is because the warmer and drier the reactivation air, the more effective the reactivation process.

Air conditioning can be used with desiccant dehumidifiers to pre-condition the process air prior to the desiccant dehumidification. This is a popular choice, especially when humidity levels are high enough that the AC can accomplish some water removal. Even if the AC does not accomplish any water removal, the performance of the desiccant will be improved by the cooler process air.

If it is important to control the temperature in the drying chamber, the process air stream from the desiccant can be ducted to an air conditioner, or portable AC units can be placed in the structure.

The same situations where an LGR dehumidifier could be combined with a HEPA air scrubber also apply to desiccant dehumidifiers. An open duct system should be used unless the air scrubber has a variable-airflow feature.

Heat-Based Systems: Mix with High-Temperature LGR plus Performance Amplifier and HEPA Air Scrubbers

Heat-based drying systems have become very popular in the last few years. The strategy of these systems is to provide the energy necessary to cause evaporation regardless of the specific humidity of the air. However, faster drying occurs when the specific humidity levels are lower. The high temperatures present in a heat-based drying site prevent the use of LGR dehumidifiers, but a high-temperature LGR with a performance amplifier can reduce the specific humidity in applications with ambient temperatures up to and over 125°F. The high-temperature LGR employs a bypass to boost airflow through the condenser coil while it slows the airflow through the evaporator or cooling coil. This longer dwell time allows the air to cool to lower temperatures and provide a lower specific humidity. As described earlier the performance amplifier should be ducted from and to a cooler air source. If the outside conditions are 80°F and the inside conditions are 125°F, the air presented to the LGR will be about 102°F. In a heat-based application the ambient specific humidity levels are much higher than an LGR-based system. The LGR plus performance amplifier significantly reduces this humidity. The combination of the higher temperatures and the lower humidity produces some extremely rapid drying results.

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