Many applications require cooling in the form of refrigeration or chilled water to meet customer temperature and humidity specifications for the end product or the space. Some applications with lower humidity and more stringent environment requirements lend themselves to a combined approach of mechanical cooling and desiccant dehumidification. When the moisture control level is below the practical limits of a system based only on cooling dehumidification, for example less than 40 DP, then a combined approach is preferred. This combined approach of mechanical cooling and desiccants is prevalent in many industries such as food, pharmaceuticals and the plastic industry.
A review the basic concept of a dehumidifier is in order before going further. While there are many dehumidification methods, this article focuses on the “rotary wheel” configuration (figure 1).
Desiccant dehumidification systems present a regenerative process that removes moisture from the air. To do this, desiccant (silica gel) is impregnated into a porous corrugated material that is then fabricated into a round wheel that rotates via a belt or chain drive motor assembly. As the wheel rotates, the inlet airstream is sent through the desiccant wheel. The desiccants in the wheel adsorb the moisture based on the vapor pressure differential between the airstream and the desiccant itself. Efficiency of moisture removal through the wheel improves as the difference in vapor pressure increases. In other words, the greater the difference in vapor pressure, the more effective the desiccant becomes. As the airstream passes through the desiccant wheel, it becomes significantly drier without the use of additional devices such as cooling.
The desiccant is regenerated by means of a heat source applied to the reactivation sector of the wheel, which is approximately 25 percent of the surface face. As hot air is forced through the wheel, the surface vapor pressure increases, causing the desiccant wheel to release its moisture. In most cases, this moisture then is transferred to an outside system of duct work. This process removes moisture from the wheel on a continuous basis.
Typically, heat sources that remove the wheel’s moisture are resistive electrical heaters, natural gas burners (direct and indirect) or steam.
The Role of Process CoolingProcess cooling is required mainly in two areas - pre-cooling and post-cooling - when combined with a dehumidifier.
- Pre-Cooling. To reduce the load on the system, the process inlet air is cooled before heading into the dehumidifier wheel. This reduced dehumidification load equates to a smaller unit, thus saving the customer money and space.
- Post-Cooling. Due to the heat of adsorption, the process air temperature increases once the air leaves the desiccant wheel. To prevent this, the outlet air will need to be cooled in most applications. The post-cooling coil will decrease the temperature of the air. It is regulated by output devices from the control system.
- Combined systems allow for lower moisture level dehumidification vs. by means of cooling only.
- The energy consumed to remove moisture at low levels of moisture content is less with a combined system.
- Combined systems can be used to dry air below 40°F dewpoint. At those temperatures, cooling-based only dehumidification systems exhibit condensate coil freezing on coil surfaces, lowering their efficiency.
- Combined systems are optimized so that each approach’s strengths and weaknesses are used to complement system operation.
- Combined systems allow end users greater flexibility with environmental parameters.
Two ExamplesAs mentioned earlier, there are many applications for dehumidifiers combined with process cooling. Here are two in more detail.
Lithium Batteries. Over the years, the manufacturing of lithium batteries has gone from relatively small sample batches to large, mass production operations. These high-energy batteries are used in a range of applications from laptops to cell phones and even some hybrid cars.
In the early years, moisture-free (inert gas) glove boxes were used to produce the batteries in small quantities. However, as the demand for batteries grew, so did the demand for larger-capacity production. At the same time, significant developments occurred in the production of low-humidity dehumidification equipment. This equipment allowed for up-scaling of the production process and allowed substantially increases in the production of lithium batteries.
The most important single factor governing the manufacture of lithium batteries is that they must be produced in a very low humidity environment with tight temperature control, thus requiring a solution such as a combined cooling and dehumidification system. Lithium reacts with water (vapor) to form lithium hydroxide, hydrogen and heat, so a low humidity production environment is important. Water vapor acts as a catalyst, thus the rate at which these reactions occur depends upon both the moisture level in the atmosphere and the time that the lithium metal is exposed to that moisture. The more exposure, the poorer the quality, performance and shelf life of the batteries.
Preventing Condensation in Meat Processing. Unwanted moisture is a problem in nearly all meat processing plants. Dangerous conditions such as fogging are the results of unwanted moisture forming on equipment, floors, ceiling and walls. Condensation happens in meat and poultry processing plants when warm humid air comes in contact with cooler surfaces such as those found in cut rooms, chiller rooms and packaging rooms.
Increased scrutiny by the USDA has resulted in companies actively searching for cost-effective solutions for fogging, icing, dripping and other moisture-related problems in order to comply with the USDA zero tolerance guidelines and avoid the dreaded non-compliance report (figure 2).
Condensation is the problem. Desiccant dehumidification combined with process cooling is the solution. Desiccant dehumidifiers precisely control the humidity and dewpoint while the process cooling regulates the temperature in the plant for a combined approach to prevent damaging condensation.
In conclusion, for those applications that require it, an approach combining mechanical cooling and desiccant dehumidification can deliver a low humidity, low temperature environment.