DONALD ESPLUND
JOHN SCHILDWACHTER
WASH AWAY YOUR COMPRESSOR PROBLEMS Injecting water or other liquid into the internal passages of +namic compressors is often a simple, inexpensive answer to many air- or gashandling problems. accomplish?
What does liquid injection
It reduces temperatures and horse-
power requirements, helps to prevent polymerization, and cuts the buildup of dirt particles. iquid injection is by no means a cure all.
Often, L because of its simplicity, it is misapplied. If care is not taken in application, liquid injection sometimes actually accelerates wear of the compressor or, at best, serves no purpose. When considering the use of a compressor with liquid injection the engineer must first answer these questions. They are: What is to be accomplished? What liquids are compatible with the gas? Where should the liquid be injected? With answers to these questions in hand, he can then intelligently design the liquid injection system, establishing nozzle sizes, rate of flow, entry angle of the liquid, construction materials, and other engineering incidentals. When properly designed, water injection can offer a low cost answer-at between 3 and 10% of compressor cost. Maximum results can be expected only when liquid injection is included in the original design. However, it is often possible to add facilities to an existing compressor. You should consider this technique when modifying equipment to meet changes in service, or to remove process bottlenecks. AUTHORS Donald Esplund is Abblication Engineer, Fluid Dynamics Debt., Allis-Chalmers Manufacturing Co., Milwaukee, Wis. John Schildwachter holds the position of Subervisory Engincer for the same department.
How Horsepower Can Be Reduced
The horsepower required to compress a gas is a function of the temperature rise of the gas during compression. Consequently, the smaller the temperature rise, the lower the horsepower requirements. In some applications, liquid injection can be used economically to reduce the temperature in each stage of compression and the over-all horsepower requirements can be reduced. The amount of temperature reduction that can be realized from liquid injection depends upon the heat absorbing capabilities of the liquid and upon the heat releasing capabilities of the gas. A small reduction in horsepower can often allow the use of the motor with the next lower frame size. With this you would not only realize a lower initial investment in a piece of equipment, but also a lower operating cost. In some air applications with water injection, horsepower requirements have been reduced as much as 8%. When used for horsepower reduction, the liquid is usually injected into the interstage passages in sufficient quantity to saturate the gas. Injection can be applied in one or more stages depending on the amount of horsepower reduction required and the pressure ratio of the unit. In some cases injection into the last stages will not do enough to justify the added investment. In our air applications where liquid injection is used for horsepower reduction, water is the liquid employed. In these cases the portion of the rotating element where the water is injected is made of a chrome steel alloy. Temperature Reduction
There are applications where a gas will explode or decompose rapidly at higher temperatures. If a liquid compatible with the gas is available, liquid injection can keep the temperature within safe limits. Figure 1 shows a unit on test equipment for liquid injection to prevent acetylene from reaching high temperatures. The liquid is usually injected into the interstage passages and into the eye of the impeller of centrifugal machines. It may also be necessary to apply the liquid to close-clearance areas, such as the shaft seals, to avoid possible hot spots. The heat absorbed by the evaporation of the liquid cools the gas enough to hold temperature to the desired value. Fluid properties determine whether special construction materials must be used. liquid Injection Helps Prevent Polymerization
Hydrocarbon gases have a tendency to polymerize as the temperature is raised. Sticky polymers build up on VOL. 5 4
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the stationary and rotating parts of the compressor. Buildup of these substances on the rotating element causes unbalance. Vibration occurs, leadiig to eventual failure of the bearings. Injecting a liquid into the compressor keeps the temperature low enough to retard the polymer formation. At the same time, the gas-exposed parts of the compressor are wetted, and buildup of the polymers that are formed is prevented. Because liquid injection reduces polymerization in two wayetemperature reduction and wetting-the liquid injection system must be designed accordingly. Figure 2 shows such a unit for butadiene production. Usually, the liquid is injected ‘into the interstage passages as well as to various sections of the impeller where polymerization is most likely to occur. In some cases liquid buffering of the shaft seals is recommended to prevent polymerization there and thereby to offer positive sealing. Sufficient excess liquid must be injected to wet suitably the gas-exposed parts. Liquids used successfully include emulsions of oil and water, solvents, and certain hydrocarbons. Flushing or Claoning Function Is Most Common
Another frequent application of liquid injection is in cases where the gas being handled is “dirty.” In these uses (examples are kiln gas and coke oven gases) the particles being carried by the gas stream enter the compressor and tend to coat all the parts exposed to gas. This coating or buildup can impair performance if clearances and blade angles are changed. Another potential danger of buildup is unbalance resulting in damage to the compressor. Normally, buildup will progress in a uniform fashion so that the weight added to the rotor will not cause unbalance. However, should some of the buildup break free, unbalance can occur and, with a high speed compressor, destroy the bearings in a matter of seconds. T o prevent this buildup of solids in the internal area of the compressor, liquid injection can be used for washing or cleaning. Figure 3 shows a typical single stage centrifugal compressor designed for water washing. Warhing Con Be Dona in Two Ways
First, the compressor can be continuously washed with In this case the liquid would be applied in the interstage passages of each stage, in the eye of the impeller, and at various places on the front and back of the impeller. Liquid buffering of the shah seals is also used to prevent buildup at the shaft ends. A second way is to reduce the unit’s speed periodically and flush a considerable amount of liquid into the inlet nozzle of the compressor to cleanse the internal parts. Need for flushing is determined by a decrease in volume capability or by the gradual increase in vibration amplitudes which indicate a deposit. After flushing, the unit is drained and put back on the line. Whether washing is done continuously under full load or intermittently at full or part load depends, of course, on the rate of buildup within the compressor and the need for continuous operation in a particular process.
a liquid while in operation.
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SPRAY ASSEMBLY (BEHIND IMPELLER) Fi&m 3
Liquid injection systems can be designed to solve several different problems: Figure 1. Water injection keeps acetylene safely cooled Figure 2. Injection prevents polymerization of butadiene Figure 3. Water wash stops solids buildu) from db6y gases 3b
INDUSTRIAL AND ENGINEERING C H E M I S T R Y
