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Written by Chuck Henderson, Vice President Henderson Engineering Co., Inc. Closed System The closed system dryer is typically used for gases other than air or when extremely low dewpoints are required. See Illustration 7.
Closed System dryers are usually considered specialty dryers. Process gas is not vented to atmosphere nor is it contaminated with ambient air. This dryer is a good choice for drying natural gas or other product gases. The Closed System dryer regenerates by recirculating a captive volume of gas through a blower, heater, the regenerating tower and a heat exchanger. Given the right operating conditions, the Closed System dryer is capable of producing constant dewpoint of -100°F or lower. There are several things to look for when specifying a closed system dryer. The most important item is the method used to separate the regeneration and process air flows. You can use either a booster blower and increase the pressure of the regeneration air or a differential pressure valve and drop the pressure of the process gas. We recommend the booster blower for several reasons. First, with a differential pressure valve, you are reducing the pressure of your air by 2-3 PSI. Any pressure drop is expensive; purposely dropping your line pressure is just plain foolish. Second, the valve and controller are very sensitive instruments which are prone to mechanical failure. One of two things can happen. Either the valve opens more, causing an even higher pressure drop, or it closes, which reduces the regeneration system and does not regenerate the dryer. Either way, the dryer is not working as it should. The operating cost of the closed system is determined by the HP of the booster blower, heater KW, and GPM of cooling water. The blower on this size dryer would be 3 HP. The heater is 24 KW; however, we don't use all of the heater capacity. The actual KW requirement is 19.4 KW. The cooler has to cool 200 SCFM down from 375°F to 100°F. It will require 3.3 GPM of 85°F water to accomplish this. Our power requirements are: blower 1 KW, heater 19.4 KW, cooler 3.3 GPM. The annual operating cost will be $8,545.38.
C = ([HPx8760] + [KWx6570]) x E + ([GPMx525600]/1000) x WC
Where: C = annual operating cost HP = blower KW KW = heater KW E = cost of electricity GPM = water rate WC = water cost/thousand gallons (typically $1/1000 gallons)
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