Deter Difficulties with Desiccant Dryers

Correct installation really matters in compressed air systems.

By Chris E. Beals, Air System Management, Inc.

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UNBALANCED FLOW
Another dryer issue we constantly find is unbalanced flow through paralleled desiccant dryers. One plant stands out because it had flow meters downstream of its two paralleled 2,000-scfm heated split-stream desiccant dryers, enabling us to see the unbalanced flow. The two dryer skids, which included pre-mounted filters, were installed next to one another with similar length piping. So, while they weren't specifically piped in using a balance header arrangement (Figure 2), the piping didn't significantly unbalance the flow. However, as the pressure drop across the dryer filters increased, the instrument air pressure in the plant dropped to an unacceptable level — therefore personnel changed out the filter elements of one dryer. Because that swap raised the downstream pressure sufficiently, they didn't bother to replace the filters in the other dryer. The higher pressure drop across the dryer with the dirty filters diverted most of the flow through the dryer with the clean filter elements until the pressure drop across the paralleled dryers equalized.

Returning to the plant a couple of years later, we found a third dryer paralleled with the original two. It was installed approximately 200 ft from the others using undersized (2-in.) pipe and, again due to pressure differential mismatch, was getting significantly less than one-third of the flow.

This example points out the problem of paralleling dryer skids with pre-mounted pre- and after-filters. If you intend to parallel dryers, purchase them without the filters pre-mounted and then install them as shown in Figure 2. That diagram illustrates a balance header arrangement that equalizes the pressure drop in the piping across the three dryers and their filters. To reduce maintenance and pressure drop, specify each filter at least one or two sizes larger than the combined rated flow of the three dryers. If you plan to remove the filters from existing dryer skids, then install a fourth filter to provide the required redundancy.

Typically, a dryer installation has two pre-filters and two after-filters, with one of each kept in reserve so it's available for use when changing the element in the filter that had been online. From time-to-time, to reduce the pressure drop, personnel may operate all the filters simultaneously. The problem with this approach is that when the pressure drop across the filters is 2 psi and personnel then isolate one filter to change its elements, the pressure drop across the online filter increases to 7 psi. The question is, "Will personnel actually change the filter elements when the pressure drop is only 2 psi? Aren't they more likely to wait until the pressure drop reaches 4 or 5 psi?" If so, when they isolate one of the filters, the pressure drop across the other filter will jump to 14 psi or more.

WORTHWHILE ADDITIONS
Whenever piping between the compressor and air dryer is exposed to ambient temperatures lower than the compressor discharge temperature, consider installing a pre-cooler directly after the compressor, especially for air-cooled units, or use chilled water in the after-cooler on water-cooled ones. Typically, this requires modifying the water piping on a lubricated rotary screw compressor to prevent the chilled water from going through the oil cooler.

If the ambient temperature is lower than the temperature of the air leaving the pre-cooler or after-cooler, then install a mist eliminator, wet air receiver or knockout tank just upstream of the dryer pre-filters and insulate and heat trace the headers between the discharge of the receiver/knockout tank and the inlet of each dryer. A mist eliminator is the preferred option in systems with lubricated compressors, while in systems with "oil free" compressors, the receiver or knockout tank works well.

Remember the performance and life of a desiccant dryer depends upon protecting it from the condensate that can form in the upstream piping. Taking appropriate steps may increase installation costs but the resulting air quality provides a payback in the form of reduced maintenance of compressed air components and, in the case of heated desiccant dryers, significant energy savings.

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