The ambient conditions were 95°F and 60% relative humidity, while the compressors' discharge temperature was 117°F and the dryer inlet temperature was 107°F. It was apparent that the coolers on the compressors needed to be cleaned. However, the temperature at the dryer's inlet was less than 115°F — so why wasn't the dryer meeting its -40°F pressure dew point target?
At a discharge temperature of 117°F, almost all the atmospheric moisture taken in by each compressor was leaving it in vapor form, along with oil vapor picked up as the air flowed through the unit. In other words, the moisture separator wasn't removing much, if any, condensate. The "no air loss" drains installed on the filters showed us that condensate was forming in the piping upstream of both the mist eliminator and the dryer pre-filter. Opening a drain valve between the pre-filter and the dryer proved that condensate was either condensing downstream of the pre-filter or passing through it and then flowing into the dryer and contaminating the desiccant.
After reviewing the options, personnel moved the air-to-water heat exchanger from downstream of the dryer to upstream of the mist eliminator and installed a moisture separator with a "no air loss" drain. The 87°F cooling tower water lowered the air's temperature to below ambient, which stopped condensate from forming in the piping; after personnel replaced the desiccant in the dryer, it began drying the air to a -40°F pressure dew point. Installing the heat exchanger upstream of the dryer also reduced the dryer's operating cost because the energy controls cut back the dryer heater energy and decreased the average purge air flow rate.
Installing a pre-cooler upstream of a heated desiccant dryer equipped with energy saving controls will reduce the energy required to operate the dryer in proportion to the amount of moisture the pre-cooler removes. It also extends the life of the desiccant. Types of pre-coolers include air-to-air or air-to-water heat exchangers as well as refrigerated air dryers. Refrigerated dryers, although not suited for all environments, also can serve in some cases as a temporary backup to the desiccant dryer. Note that installing a pre-cooler upstream of a "heatless" desiccant dryer (Figure 1) equipped with energy savings controls may improve its performance but doesn't provide significant energy savings.
A paper mill in the Carolinas had a similar problem with two of its "heatless" desiccant dryers — but this time the problem occurred during the winter. The plant had water-cooled compressors along with a wet receiver and two dryers that were installed outside, where the ambient temperature was 31°F. The discharge temperature of the compressors feeding the first dryer was 88°F but the dryer inlet temperature was only 65°F. As we measured the dryer inlet temperature, the problem became obvious once we spotted a moisture separator installed between the pre-filters and dryer. Excessive condensate collected in the pre-filters because they were installed near the floor; when we removed the pre-filter elements they were swollen from being soaked with condensate. Insulating and heat tracing the piping between the receiver and the inlet of the dryer resolved the problem and allowed removal of the moisture separator. We also recommended insulating the dryer towers to prevent losing the heat of adsorption.
The pre-filter for the second dryer was installed approximately 50 feet ahead of the dryer, providing plenty of distance for condensate to form between it and the dryer. Mounting the pre-filter directly on the inlet of the dryer and installing a wet air receiver just upstream to prevent condensate from slugging it banished that problem.
We see similar problems with desiccant dryers installed outside in plants all along the Gulf Coast where the nighttime temperatures often drop to the 40s or below during the winter months.