Low-speed units are typically integral in design as shown in Figure 10-4. “Integral” means that the power cylinders that turn the crank shaft are in the same case (same housing) as the cylinders that do the compressing of the gas. There is one crank shaft. Typically, integrals are considered low-speed units. They tend to operate at 400-600 rpm, although some operate as low as 200 rpm.
Figure 10-5 shows a very large integral compressor. This would be typical of compressors in the 2,000 hp to 13,000 hp size. The size of this unit can be estimated by the height of the handrails above the compressor cylinder on the walkway that provides access to the power cylinders, This particular unit has sixteen power cylinders (eight on each side) and four compressor cylinders.
It should be obvious that one of these large integrals would require a very large and expensive foundation and would have to be field erected. Often, even the compressor cylinders must be shipped separate from the frame due to weight and size limitations. Large integrals are also much more expensive than either high-speeds or centrifugals.
For this reason, even though they are the most fuel efficient choice for large horsepower needs, large integrals are not often installed in oil and gas fields. They are more common in plants and pipeline booster service where their fuel efficiency, long life, and steady performance outweigh their much higher cost.
There are some low horsepower (140 to 360) integrals that are normally skid mounted as shown in Figure 10-6 and used extensively in small oil fields for flash gas or gas-lift compressor service. In these units the power cylinders and compressor cylinders are both mounted horizontally and opposed to each other. There may be one or two compressor cylinders and one to four power cylinders. They operate at very slow speed. Their cost and weight are more than similar sized high-speed separable units, but they have lower maintenance cost, greater fuel efficiency, and longer life than the high speeds.
The major characteristics of low-speed reciprocating compressors are:
• Some one and two power cylinder field gas compressors rated for
L40 hp to 360 hp.
• Numerous sizes from 2,000 hp to 4,000 hp.
• Large sizes 2,000 hp increments to 12,000 hp.
• 2 to 10 compressor cylinders common.
• High fuel efficiency (6-8,000 Btu/bhp-hr).
• High efficiency compression over a wide range of conditions.
• Long operating life.
• Low operation and maintenance cost when compared to high speeds.
• Usually must be field erected except for very small sizes.
• Requires heavy foundation.
• High installation cost.
• Slow speed requires high degree of vibration and pulsation suppression.