Inconsistent humidity levels were wreaking havoc on a chemical process that relied on outside ambient air for its combustion airstream. A heat exchanging system solved the problem.

Two rental dehumidifiers were connected to the furnace inlet via the permanent heat exchange unit using flexible duct.

For years, a leading chemical manufacturer had trouble controlling the moisture level of the air entering a furnace used in one of its production processes. As a result, the manufacturer occasionally would have to scrap batches of chemical that did not meet quality standards. In addition, production had to be shut down frequently to drain water that had collected in the unit.

But, that changed when the company installed a dehumidification system designed to remove moisture from the combustion air. To minimize production downtime, rental dehumidification units were used while the permanent system was brought online. With the dehumidification system, the chemical supplier is achieving consistent product quality.

In the production process, a centrifugal fan brings outside air into the furnace for combustion, and a reaction occurs that produces a liquid chemical. Previously, the combustion air's temperature and humidity level varied according to ambient conditions, so the amount of water brought in varied. This variation affected the manufacturer's ability to maintain consistent chemical quality. The company needed a way to maintain moisture levels in a steady state -- not only to improve the quality of the chemical, but also to slow the onset of advanced corrosion in the furnace. The company decided to add a drying system to solve these problems.

After evaluating proposals from several drying equipment manufacturers, the company selected the Wringer dehumidification system from Des Champs Laboratories, Natural Bridge Station, VA. Des Champs designs and manufactures energy-saving equipment for conditioning outside air.

The dehumidification system's design best suited the chemical supplier's needs. A counterflow air-to-air plate heat exchanger is the heart of the dehumidifying system. The unit removes moisture from ambient air by cooling it with a mechanical cooling coil, much like an air conditioning unit. Inlet air flows through one side of the heat exchanger; cooled, dehumidified air flows through the other. The resulting heat transfer between the airstreams saves energy in two ways: First, it precools the inlet air before it enters the dehumidification coil, thereby reducing the cooling coil's load and minimizing the equipment size requirements. Second, it reheats the supply air to a temperature more usable for the process.

The chemical manufacturer worked with Ed Banko, staff consultant engineer from Des Champs, to optimize the system for drying combustion air. A complex yet complete piece of equipment resulted.

"The most unique aspect of the project was the customer's desire to control the incoming combustion air's dewpoint -- rather than its temperature and relative humidity," Banko said.

Knowing the dewpoint allows the chemical manufacturer to calculate how much refrigeration is needed for a given flow rate, and dewpoint serves as setpoint. Furnace operators were concerned with the amount of moisture coming in per pound of air, regardless of temperature.

According to Banko, relative humidity typically is used as criteria for heating, ventilation and air conditioning applications involving human comfort. But, for industrial processes, dewpoint rules.

"The moisture level is more critical in an industrial process, so dewpoint is used. With air conditioning in a building, most people hardly notice a five percent variation in relative humidity. But, in this application, a five percent change can jeopardize the production process," Banko said.

Adding another complicating factor, the chemical manufacturer elected to use an environmentally friendly refrigerant, R-134a, in the dehumidifier. The new refrigerant replaces the more conventional R-22, which has served as a transition refrigerant between older, chlorinated versions and new fluids that offer zero ozone-depletion potential. The R-134a has a lower pressure and capacity than R-22, so more compressors than normal were needed to drive the cooling. The company chose R-134a with an eye toward the future, knowing that HCFC refrigerants such as R-22 will be phased out within the next 20 to 30 years as a part of the Montreal Protocol.



Bring On the Rental Units

A general contractor installed the furnace's dehumidification drying system in early spring, and startup debugging followed. The situation became critical as the hot and humid summer season approached, so Des Champs offered the chemical supplier the use of its rental dehumidification units until the permanent unit came online. The company took them up on it.

Des Champs' rental dehumidifiers allow customers to continue normal operations while they renovate facilities or install permanent systems. The rental systems are designed to accommodate any dehumidification situation that might arise, and they are configured to ease setup and operation.

Two rental units are available. The first is the indoor air quality (IAQ) model, which cools 95oF (35oC) inlet air to 75oF (24oC) and 45% relative humidity, removing up to 230 lb of moisture an hour. The second unit is Des Champs' deflooding design, which is built for heavy duty dehumidification. This unit discharges large quantities of hot dry air, resulting in low relative humidity and rapid moisture removal. It produces air at 135oF (57oC) and 5% relative humidity from 95oF inlet air. Both designs have a rated capacity of 10,000 cfm.

Des Champs sent two units -- one IAQ and one deflooding unit -- to satisfy the 14,000 cfm capacity the company required. Sent from the factory mounted to flatbed trailers, the units remained on their flatbeds during operation. This capability minimized setup and connection requirements.

Users cite portability and independence as advantages to using rental units -- they do not require plant resources. Normally, an onboard diesel engine and electric generator power the units, but they also can be directly powered with 460 V electrical service.

To ease installation, 30" flexible ducting was used to connect the rental dehumidifiers to the furnace. Made of woven polyethylene, the material is shipped folded flat, so it takes up little space, and the ducting's light weight and flexibility allow it to be maneuvered around obstacles. A single duct from each rental unit was connected to a makeshift port on the permanent dehumidification unit, allowing air to bypass directly to the permanent unit's outlet. Using flexible ducting in this manner allowed for fast deployment and eliminated the need to modify any structure.



Rental dehumidification units from Des Champs typically are powered by an onboard diesel generator.

A Permanent Solution

The chemical manufacturer used the rental units for about six weeks in production. While the rental units were running, Des Champs provided a service person onsite to get the permanent dehumidification system debugged and operating efficiently. After some tweaking, the unit worked well enough that the chemical supplier reported an unprecedented consistency in the product quality.

For cooling, the dehumidification system uses an air-cooled condensing unit with 12 compressors feeding four direct expansion valves and two cooling coils. Four interlaced cooling circuits operate in 12 stages. The system has a refrigeration capacity of 160 tons, based on the compressor manufacturer's rating for R-22, which translates to 100 tons for R-134a.

An onboard direct digital control (DDC) energizes the compressors when the dewpoint of the air exiting the cooling coil rises above 45oF (7oC). A malfunction signal is sent to a remote indicator in the furnace's control room when the dewpoint exceeds 47oF (7oC). Sensors measure temperature and relative humidity, and the controller calculates dewpoint from these parameters.

In specifying the dehumidification system, engineers used design conditions of 100oF (38oC) inlet temperature for summer and -10oF (-23.3oC) for winter. When operating in its summer mode, the dehumidifier's heat exchanger extracts excess moisture and precools incoming air to 68oF (20oC); the cooling coil then reduces the temperature further to 45oF (7oC). Once dehumidified, the air passes through the other side of the heat exchanger, where it is reheated to 89oF (32oC) before entering the furnace. The heat exchanger transfers 650,706 BTUs per hour at rated capacity and has an efficiency of 79.5%. In addition, the system has a rated flow rate capacity of 14,000 cfm and dehumidification capacity of 706 lb of water per hour.

Most Des Champs dehumidification systems are designed with a supply fan. In this case, however, the pre-existing centrifugal fan was used to bring air into the furnace. Connected to the unit's supply outlet, the fan pulls air through the system.

To protect against chemical fume exposure, Des Champs applied special coatings to the system. The heat exchanger and the inner and outer surfaces of the housing were covered with a corrosion-resistant coating while the cooling coil was purchased with a baked phenolic coating.

With development of the new dryer behind them, chemical company officials now say the dehumidification system has become a fundamental piece of their process equipment. It keeps the furnace in a steady-state operation, and they can produce high quality chemical with a minimum expenditure of energy.



Des Champs Laboratories, formerly of Natural Bridge Station, Va., designs and manufactures energy-saving equipment for conditioning outside air. Since publication of this article, Des Champs was acquired by Munters Corp.  The industrial dehumidification systems are now marketed by Munters Corp., Buena Vista, Va., as Des Champs brand products. For more information, call (540) 291-1111 or visit www.munters.us or www.deschamps.com.

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