The requirements for air- and water-cooled chillers for medium and large process cooling applications have changed considerably over the last few years due to increasing environmental awareness and rising energy costs. While a system’s full-load value was once the primary consideration, partial-load efficiency is now equally important. After all, most systems run at a full load only a few months during the year. In addition, where reliability and investment costs were once the two main selection criteria for new systems, now reducing lifecycle costs and meeting new energy legislation requirements have become the main drivers.
Modern screw compressors are designed to meet these challenges. In fact, compact screw compressors (i.e., those with an internal oil separator) that are optimized for efficient operation can help industrial plants exceed current energy-efficiency requirements and add valuable operational savings to their bottom line. Following is an overview of some developments in screw compressor technology.
Air-Cooled ChillersIn air-cooled chillers, the condensing temperatures can vary considerably throughout a given day, month or year. This temperature variation can lead to sizeable differences in the cooling capacity that must be provided by the compressor under extreme conditions.
To handle this disparity, the compressor must have efficient capacity control. Modern screw compressors can be controlled from 25 to 100 percent through either a sliding four-step control or stepless infinite control. Some slider mechanisms use an automatic volume index adaptation to optimize performance based on partial-load operation. In these systems, the built-in motors provide a constant high-efficiency level over a large capacity range so that both low and high condensing temperatures as well as partial-load conditions can be handled effectively.
Applied costs also are reduced by eliminating the need for external oil separators. The use of internal oil separators permits screw compressors to be integrated easily into today’s systems and reduces preventive maintenance to checking the compressor’s oil quality annually. Sealed bearing chambers and pressure-unloaded bearings provide reliable bearing lubrication even with high condensing temperatures, which place a high load on the bearings.
Water-Cooled ChillersFor water-cooled applications with low condensing temperatures, compact screw compressors have been designed to operate efficiently under either full- or partial-load conditions. Liquid chillers incorporating the latest screw compressor designs can achieve high integrated part load values (IPLV) and seasonal energy efficiency ratios (SEER).
Three primary factors contribute to increased efficiency in the latest compressors:
- An ability to adapt the integrated volume ratios at a full or partial load to the low-pressure ratios.
- An optimized oil-management system.
- An ability to operate at lower condensing temperatures (figure 1).
Screw compressors also have been modified to increase cooling capacity and efficiency at moderate condensing temperatures. In the past, a limited motor selection restricted the use of the compressors to applications operating at full load. Now, motors can be matched precisely to operating conditions to provide maximum efficiency for both full and partial loads. The optimization of the motors also allows smaller contactors, cables and fuses to be used, thereby reducing the cost of these components.
As a result of these and other developments, the coefficient of performance (COP) for screw compressors has been improved for all load conditions. At lower condensing temperatures and with full use of the extended application range (i.e., a 10-degree lower saturated condensing temperature), modern screw compressors can provide a 20 to 30 percent increase in COP for full-load operation and up to a 60 percent increase for partial-load operation.
Selection SoftwareWhile the improved performance of screw compressors is welcome news for process cooling operations, the broader product range can make compressor selection more difficult. Fortunately, compressor manufacturers have also developed software that can help users choose a system based on specific operation variables. For example, users can specify the desired IPLV or SEER; the evaporation, ambient or water temperature for the load conditions; the desired time weighting; the number of circuits and compressors; and the compressor capacity based on load conditions. The software will determine the optimal compressor configuration and show all the data necessary to select the components. This process can simplify the configuration of the chiller or cooling system.
With the right equipment in place, plants with cooling operations will find it easier to keep operating costs in check while meeting increasingly stringent energy efficiency standards.