Many professionals estimate that about 95% of all compressed air systems can be classified as utility in nature. Utility air applications include common compressed air tools like blow-off guns, wrenches, grinders, hammers, spraying equipment, cylinders, etc. Utility air is well suited for these industries, and they most often use a refrigerated dryer to maintain recommended dew point of the air. Output from the compressor is typically processed through a refrigerated dryer that will produce a dew point ranging from 38° to 50° F.
Traditionally, utility air is processed through CFC-based refrigerated dryers before it is released to the plant's distribution system. These dryers require regular maintenance and have a life span of less than 10 years. Therefore, as a plant manager years ago, I decided to develop an alternative to CFC-based dryers.
The search for a better solution
I studied all the traditional refrigeration systems and found nothing I thought was applicable. I did, however consider water evaporation within a vacuum, which would dramatically improve the temperature shift of a typical evaporative system. However, all the systems I designed turned out to be nearly as complicated as a regular CFC-based system.
Therefore, I decided to use compressed air as the power source and ordinary tap water as the refrigerant. This permits an open-loop system — so instead of having to capture, compress, and recirculate a refrigerant, the open-loop system simply releases moisture and air into the atmosphere.
Using compressed air and tap water allows production of an extremely simple refrigeration cell with no moving parts and no electrical components. Coupling these Elliott Cycle refrigerators with an ordinary tubein- shell heat exchanger produces an effective refrigerated compressed air dryer that is environmentally friendly, has virtually no moving parts, and a rated life of 25 to 30 years.
The Elliott refrigeration cycle, Figure 1, is based on fluid evaporation. Compressed gas is injected through an injector gas input nozzle which, in turn, drives a two-stage venturi. The first-stage venturi pulls a partial vacuum within the fluid chamber. A volatile fluid bleeds into the fluid chamber, drawn in by the partial vacuum. The rate at which fluid bleeds into the chamber is controlled by a fluid control — which is an adjustable needle valve.
Fluid atomizes into micro-droplets as it enters the chamber. This atomized fluid is then forced out through a fluid nozzle into the second-stage venturi. A dry gas is drawn into the cover gas chamber via the partial vacuum formed by the second-stage venturi. The cover gas and the atomized fluid are forced together as they pass through the mixing throat of the second-stage venturi. As the mixture enters the expansion nozzle, the atomized fluid rapidly evaporates, which, consequently, produces a low discharge temperature.
Keeping construction simple
Elliott Cycle refrigerated dryers operate in the same fashion as any other refrigerated dryer: compressed air is cooled in a controlled environment, and condensed water is trapped and periodically drained out. The principal difference is the simplicity of the Elliott refrigeration cycle and the durability of the equipment.
In practice, the refrigerator body, shown in sectional view in Figure 2, is made of machined PVC plastic with a plastic water vapor nozzle pressed into the core. The air nozzle, also made of PVC, is bonded into the core of the body. The space formed between the OD of the air nozzle and the core of the body is the water vapor chamber. The space between the OD of the water vapor nozzle and the body forms the cover gas chamber.
The water vapor chamber and the air nozzle have 18-in. NPT ports. A length of 18-in. plastic tube connects the cover gas port to the cover gas orifice. A compressed air inlet line mounted to a standard 14-in. compression street tee supplies compressed air to the refrigerator. The water inlet port fitting contains a small plastic orifice that meters the proper amount of water into the water vapor chamber. The outlet nose of the body is equipped with a 12-in. male NPT thread, which allows it to be screwed directly into the shell of a heat exchanger.
This refrigeration cycle can be used with different gases and fluids. The design presented here is specifically intended for compressed air-drying applications and, as such, can only be used with compressed air and water. Other gases and aromatic fluids can be selected to lower the discharge temperature of the refrigerator. However, components must be constructed of materials that are chemically compatible with the chosen fluids.
The discharge temperature is controlled primarily by the amount of water metered into the refrigeration unit. Too little water creates a sub-freezing condition in the discharge. Therefore, the rate of water introduced must exceed that which can be completely evaporated. Doing so prevents any chance of freezing within the heat exchanger and also improves thermal transfer.
The excess water forms micro-droplets that will normalize to the same temperature as the gas within the discharge. These cold micro-droplets then come in contact with the OD of the heat exchanger’s tubes and provide for high-efficiency thermal transfer.