Screw Compressors - Emissions

Screw chiller emissions fall into two major categories direct (or on-site) and indirect (emissions resulting from the production of the energy used to operate the equipment).

Direct on-site emissions of an electric screw chiller are confined to the release of refrigerant due to leaks or servicing. Federal law now mandates no intentional release of CFCs. The user and the servicing agency are required minimize leaks and service release. Good preventive maintenance practices are essential. Other factors include the chiller age, application (single package or split system), compressor type (open compressors with their shaft seals leak more often than hermetic designs).

Today's typical semi-hermetic type chiller might lose about 3 to 5 percent of its charge annually. With the refrigerant charge running about 3 pounds per ton, the emission of refrigerant could total 0.12 pounds per ton per year. An open chiller might lose 5 to 7 percent, and thus emit about 0.18 pounds per ton per year. To allow for something less than ideal, a conservative emission estimate might total about 0.25 pounds per ton per year.

Gas-engine driven screw chillers must use open-type compressors. In addition to the same refrigerant emissions as an electric chiller, they also emit the products of combustion of the natural gas on-site. Also, the leakage of natural gas into the atmosphere, although small, is also believed to contribute global warming.

These emissions can be projected using the estimated annual gas consumption. The typical energy input (on an HHV basis) is about 8,600 Btu per ton-hour for a screw chiller. Using the annual ton- hours of cooling, the emissions of CO2 and the criteria gases can be estimated using these relative values of pounds per million Btu of fuel burned. The emissions of all gases other than NOx are relatively constant throughout the loading range. It can be expected the NOX emissions will vary a little, depending on the annual load profile.

While so-called "lean-burn engines" emit less NOX than conventional engines at full load, they emit much more at part load conditions. Since chillers most often operate at part load, the added expense of a lean-burn engine is not considered, at least not with current technology.

Indirect emissions occur at the power plants generating the electricity used to power all chillers. Comparison of alternative chiller designs (for example, electric versus gas), must include both the chiller itself and the system's auxiliary energy consumption. The power plant emissions can be estimated from the annual power consumption and serving utility's power plant emission data.

Utilities know their typical emissions on a "per kWh" basis.