The Energy Efficiency of Dehumidified Two-Stage Air Compression

Abstract

Compressed air systems (CAS) are an integral component of most manufacturing facilities yet converting electricity to useful pneumatic energy may be 10-15% efficient while the remaining 85-90% of energy is lost to the surroundings as thermal energy. Although best practices to reduce operating cost focus on eliminating inappropriate uses and system losses, none suggest improving the thermodynamic process of air compression, which is the focus of this study. For example, a two-stage air compressor (TSC) with intercooling can reduce the energy consumption by up to 18% when compared to a single-stage model. The humidity of inlet air however, can limit allowable intercooling and increase the demand of a TSC by as much as 7%. This investigation parametrically studies the dehumidification of air prior to its compression by a TSC to evaluate the net change of energy demand using a TSC equipped with a dehumidifier when compared to the standalone TSC. The equations used to model the TSC and dehumidifiers are derived by using thermodynamic principles and a fractional factorial experiment. The dehumidifiers will be evaluated with makeup air from outdoors, which is normally the inlet air for the CAS. The makeup air temperature ranges from 70 to 110 °F and its relative humidity ranges from 30 to 90% and it is either cooled to a temperature of 45 °F by using a vapor compression refrigeration cycle (air conditioner) or dehumidified by a rotary solid desiccant dehumidifier (desiccant wheel). After dehumidification, the air is compressed by a TSC to a discharge pressure ranging from 75 to 150 psig. The results show that an air conditioner installed at the inlet of a TSC can reduce the energy demand of the system by as much as 11%. In contrast, a desiccant wheel fitted onto a TSC can cause the TSC to consume as much as 5% more specific work. Additionally, the total demand of the desiccant wheel dehumidified TSC increases as much as 80% in the form of thermal energy necessary to regenerate the desiccant wheel.