Building a flexible system
Figure 3 shows a schematic representation of how the Elliott cycle refrigerators can be configured with a heat exchanger and trap to form a compressed air dryer. Unprocessed compressed air is introduced into the air input, where it flows through the heat exchanger tubes, shown as a single S-shaped tube.
The refrigerator produces a cooled vapor cloud that is saturated with normalized micro-droplets and discharged into the coolant cavity of the heat exchanger. The vapor cloud is forced to flow through the heat exchanger — where it cools the outside of the exchanger tubes — and out the coolant discharge port.
When the unprocessed air comes in contact with the inside of the cooled exchanger tubes, its temperature drops. As the air temperature drops, its affinity for water vapor also drops. As the water is shed from the compressed air, it condenses onto the inside of the exchanger tubes. The condensed water runs down the inside of the tubes along with the compressed air flow and is discharged through the heat exchanger output.
The air and liquid water are routed through a separation trap, which forces the dry air into a series of turns designed to separate the liquid water from the air stream. The separated water collects into a small reservoir and is periodically drained through the trap’s drain valve. The now dry compressed air flows through the output of the trap and is introduced to the distribution system.
The sheer simplicity of a system such as this allows flexibility when designing the balance of the compressed air system. In addition to having no moving parts and requiring no electrical components, the system’s entire exchanger and trap can be constructed of Schedule 40 welded piping components. Doing so produces a durable piece of equipment with a life expectancy that better matches the compressors it serves.
A sectional view of an Elliott cycle dryer designed for small, packaged compressors is shown in Figure 4. The body of the heat exchanger is constructed from a piece of Schedule 40 steel pipe with forged steel fittings and mild steel tube plates. The assembly features all-welded construction for longevity and reliability. The internal tubes of the exchanger are made of Type 122 copper and bonded to tube plates using a proprietary process.
The refrigerator mounts into the 12-in. female NPT port at the top of the heat exchanger shell, and the coolant discharge is located at the bottom of the shell. The trap assembly is of the same construction style as the heat exchanger — all-welded Schedule 40 pipe components with forged steel fittings. The input nipple of the trap is specifically matched to the reducing bushing that makes up the heat exchanger output.
Units in the 25 through 200 scfm range (5 to 40 hp) are generally mounted directly to the compressors or receivers that they service. Dryers 250 scfm and larger (50 hp and up) are generally mounted to an integral basetrap assembly and are stand-alone units, as shown in Figure 5.
The dryer in Figure 5 is placed between the output of the compressor and the input of the receiver. This arrangement produces a clean, simple installation and is the preferred arrangement for an Elliott cycle dryer for systems using a rotary screw compressor. For smaller, reciprocating compressors it is generally recommended that the dryer be located on the output of the receiver and after the master pressure regulator, if used.
Because the Elliott refrigerator produces a vapor cloud as the coolant, the effective cross-section of the heat exchanger must be limited to a section that the vapor discharge can effectively penetrate. This cross section limits the construction of the heat exchangers to units that have approximately 500 scfm drying capacity. To manufacture larger capacity dryers, 500 scfm heat exchangers are ganged together on common manifolds. Dryers are manufactured in this configuration for flows to 3000 scfm (six 500-scfm heat exchangers). For dryers more than 3000 scfm, a secondary, bolt-together manifold is manufactured for grouping smaller units together. As an example, a 5000-scfm dryer is assembled utilizing two 2500-scfm units on common input and output manifolds and with common controls.
Brian S. Elliott is chief of engineering at Air Options Inc., Houston, and author of Compressed Air Operations Manual, published by McGraw Hill. For more information call (713) 721-9619, e-mail email@example.com., or visit air-options.com.