Using Screw Compressors with Economizers to Improve Efficiency

Fitting screw compressors with economizers and side-loads provides a significant advantage over reciprocating compressors by improving energy efficiency and reducing the number of compressors needed. “Economizers are widely used, as a relatively low-cost way to provide an improvement in overall energy efficiency in many refrigeration plants,” said Joe Pillis, director of engineering, industrial refrigeration, at Johnson Controls.

Any pressure between suction and discharge can be located by drilling holes through the casing of a screw compressor. An economizer is simply a special port that is located at an optimized location as a secondary suction port, into which additional gas can be drawn from higher pressure sources.

Economizers provide sub-cooling of the condensed liquid going to the evaporator, resulting in an increase in capacity and efficiency. Some of the liquid from the condenser is evaporated to the lower pressure at the economizer port in order to refrigerate the remainder of the condensed liquid before it goes to the evaporator. The colder liquid provides an increase in refrigeration tonnage at the evaporator and an overall increase in system efficiency.

Compression efficiency tends to be higher with economizers because the overhead frictional losses to run the compressor do not increase with additional gas flow. “So it’s almost like the additional gas that you can shove into the economizer port is being compressed in a friction-less compressor,” Pillis said. “You don’t have to pay for the losses a second time. That’s partly where the energy efficiency comes from.”

The other advantage is that the added gas flow only has to be compressed from the higher economizer pressure to discharge, instead of all the way from suction to discharge, he said. Economizers can add 10 percent to 30 percent to the capacity of a single compressor, with a five-to-15 percent improvement in efficiency, depending on the evaporating temperature.

By contrast, a side-load uses the same economizer port on the compressor but instead of providing sub-cooled liquid to the primary evaporator, it provides the suction for a higher temperature evaporator. Side-loading can eliminate the need for a dedicated compressor on a higher temperature evaporator load, while efficiently adding that load to the economizer port on a compressor that is already running.

Many cold-storage warehouses have loading docks with high temperature evaporators running directly off the economizer port. This avoids the energy waste of using a backpressure regulator to service the dock with the primary low temperature suction, saves the cost of a dedicated “dock compressor,” and provides the same “frictionless” compression advantage enjoyed with the use of the economizer.

As an example, a screw compressor sized for 100 TR for a minus-10 degree cold-storage room can easily provide the suction for a 65 TR dock load at 35-degree room temperature, while providing efficient compression for the added capacity.

More energy savings can be produced when using economizers with variable speed drives (VSD). Variable speed drives boost the part-load efficiency of screw compressors compared with those using slide valves for unloading. With economizers and side-loads, the slide valve remains in the fully loaded position while capacity is regulated by speed. The economizer port maintains the same in relation to suction pressure while the compressor unloads. Therefore, the economizer can be used at all part-load conditions and the efficiency improvement is always available.

One of the newer efficiency boosting systems available today is the combination of variable-volume ratio compressors with adjustment of the volume ratio depending on the loading at the economizer port. Some of these compressors have a significant increase in internal thread pressure from the economizer or side-load gas flow. Adjusting the volume ratio down to account for the added gas flowing to the economizer port reduces the power significantly and avoids over compression.

The economizer might also lead facility owners to consider installing a less expensive single-stage compression system. Although a two-stage compression system remains more efficient, especially at low evaporator temperatures, the difference in savings becomes much less significant when using an economizer on a single-stage compression system. At minus-15 degrees evaporator temperature to 95 degrees condensing, the gain in HP/TR between a single-stage and two-stage compression system is about 17 percent. However, the difference in efficiency between an economized single-stage compared to a two-stage system drops from 17 percent to 5.4 percent.

“With the amount of money needed to install a two-stage compression system, a lot of people might look at only a five-percent difference and go with the economized single-stage design,” Pillis said.