Combining refrigeration and desiccant dryers generates low dew points and saves energy.
Edited by K. Korane
|Hybritec dryers combine the energy savings of a refrigerated dryer with the low dew points of a desiccant dryer. Flow capacities range from about 700 to 5300 scfm. |
Most industrial pneumatic applications require dry compressed air. Water vapor in the air that condenses and accumulates in the air distribution network can damage components, degrade efficiency, ruin manufactured products, and result in costly system failures.
Dryers remove water vapor from the air, which lowers its pressure dew point (PDP) — the temperature to which compressed air can be cooled before reaching its humidity saturation point. Water begins to condense and accumulate once the PDP is reached. Thus, depending on operating conditions, drying the air is often essential. But air dryers consume energy, and there’s more attention than ever on systems designed to maintain the required PDP as efficiently as possible.
Two common types of dryers are refrigeration and desiccant. Refrigeration dryers condense moisture from compressed air by cooling the air in heat exchangers chilled by refrigerants. Refrigeration drying is the preferred method of compressed air treatment for pressure dew points down to about 38°F.
Desiccant dryers remove water vapor from air by adsorbing it on the surface of a microscopically porous desiccant. The desiccant does not react chemically with the water, so it need not be replenished. However, it must be dried (regenerated) periodically. Desiccant dryers are used for PDPs below 38°F and typically reach –40°F, although these systems have higher specific power requirements compared to refrigerant dryers.
Kaeser Compressors, Fredericksburg, Va., has introduced its new Hybritec dryer series that reportedly combines the energy savings of a refrigerated dryer with the low dew points of a desiccant dryer.
Until now, running refrigeration and desiccant dryers as a system was technically cumbersome and only cost-effective for applications requiring large to very large volumes of compressed air. “These dryers are more expensive, so the payback must be high enough to warrant the extra initial cost,” explains Michael Camber, Marketing Services Manager with Kaeser. “The savings on small systems would take longer to pay back.”
|Hybritec units first treat air with a refrigerated dryer to remove the majority of the air’s water vapor. This reduces pressure dew point to 38°F. Compressed air then flows to the desiccant dryer, which has a much easier task to perform than if the air had come directly from the compressor. Less energy is therefore required in this second stage to dry the compressed air to a pressure dew point of –40°F. Finally, air returns to the refrigerated dryer to be reheated and delivered to the plant air system. |
And the payback in energy savings depends on the cost of power, which varies widely by region, the number of hours of operation, and the volume of air being used, which varies by application, says Camber. “Further, it depends on the operator’s outlook on what is a good investment. For one user a two-year payback may be all they’ll accept; another might be satisfied with a five-year payback.”
Now, however, Kaeser’s design is economical for a wider range of industrial systems, according to company officials.
Hybritec units first treat air with a refrigerated dryer to remove the majority of the air’s water vapor. This minimizes energy consumption and ensures a consistent pressure dew point of 38°F. The compressed air then flows to a desiccant dryer, which has a much easier task to perform than if the air had come directly from the compressor. Less energy is therefore required in this second stage to dry the compressed air to a pressure dew point of –40°F. (Such low PDPs are necessary, for example, to keep compressed air networks that are installed outdoors free from condensate, even in winter.) Finally, air returns to the refrigerated dryer to be reheated and delivered to the plant air system.
As mentioned above, a major benefit of hybrid dryers is lower power consumption. Hybritec dryers can provide significant savings compared to single-stage, heat-regenerated desiccant dryers. With an assumed frost period of four months per year, for example, it is possible to reduce energy costs by up to 67% by bypassing the desiccant dryer when ambient temperatures are higher, so that only the refrigeration dryer treats compressed air to produce 38°F dew points. Even if the system is required to provide a pressure dew point of –40°F year-round, the energy saving advantage over conventional desiccant dryers can still be as high as 50%.
|Because of the relatively low power requirements for refrigeration dryers, hybrid dryers are worth considering if applications need pressure dew points below 38°F at flow rates of 700 scfm or greater. Split dew points shown for the Hybritec are for units producing –40°F PDP for one-third of the operating cycle. Power requirements are calculated per ISO 7153 Option A. |
Automatic temperature sensing aids this process. Equipped with a thermostat control system, Hybritec dryers automatically switch from frost protection operation at colder times of the year to pure refrigeration dryer mode during the warmer months. The summer/winter combination approach for frost-protection applications has clear energy advantages over stand-alone desiccant dryers and separate components installed in series. And, according to Kaeser, these advantages become even more pronounced with rising energy prices.
The unit’s control system also adds to energy savings. For instance, digital scroll control is available for refrigeration dryers with flow rates to 2355 scfm. This lets the scroll-type refrigerant compressor run either fully loaded or completely unloaded, explains Camber, which reduces power demands.
Cylinder unloading is another energy-saving feature, he notes. “The larger Hybritec dryers are equipped with multi-cylinder reciprocating compressors,” says Camber. When the system runs at somewhat less than full capacity, unloading one or more cylinders — meaning no compression occurs as the piston moves up and down — saves energy. In addition, speed-controlled fan motors are available on air-cooled refrigeration dryers and the desiccant dryer features a PDP sensor and corresponding regulator to adjust cycle times to match load.
Plus, the Hybritec’s drying cycle time is 16 hr (at PDP = –40° F). This is typically two times longer than that for conventional heated desiccant dryers. This means less-frequent switching between the two desiccant chambers, thereby reducing the thermal cycling of the desiccant and minimizing the volume of dried compressed air that is purged into the ambient surroundings each time the regeneration process takes place.
Another benefit, according to Kaeser officials, is the lower maintenance costs from longer desiccant service life. The reduced thermal stress on the desiccant significantly extends its life.
|Compared with conventional desiccant compressed-air dryers, hybrid dryers offer lower total lifetime costs. Calculations are based on 13 summer and 23 winter operation, 8760 operating hours per year, and electricity priced at $0.19/kWh. |
And due to the lower temperature and significantly reduced moisture content of the air coming from the refrigeration dryer, Hybritec systems require far less desiccant per scfm than conventional desiccant dryers. These two factors greatly reduce the maintenance cost of the system.
Other features, such as a diagnostics mode for the valve-switching sequence, electronic filter monitoring, cold coalescing filter, particulate filters, and automatic drain valves complete the air treatment system to help ensure clean and efficient operation.
According to the company, these factors add up to lower lifetime operating costs, compared to conventional desiccant compressed-air dryers. Application engineers can provide a detailed cost analysis comparison for any application and recommend the best method to meet specific needs.
The hybrid units are available across a wide power spectrum. Maximum pressure rating is about 150 psig with dryers sized for flows from 706 to 5295 scfm.
For additional information on Kaeser dryers, visit us.kaeser.com.