2016 ASHRAE Annual Conference

This presentation provides an introduction and overview of the current standards for rating positive displacement compressors.  It reviews the industry standard polynomial equation used for presenting ratings and the basis for the reference rating conditions.  It also explains the uncertainty limits associated with the standards and what they mean as they apply to a single compressor versus a batch/rack of compressors.  It explains the limitation of most compressor ratings and provides suggestions on how these ratings should be applied with zeotropic refrigerants as well as how to perform superheat corrections.

Positive displacement compressors with vapor injection are commonly used in the vapor compression cycle to increase the refrigeration effect of evaporators or the heat rejection of a condenser. It is important to characterize injection flow just like the suction flow in order to allow system designer to size the components of HVAC equipment. This seminar presents a method to characterize compressors with vapor injection.  The method is based on the AHRI 10-coefficient model with the addition of another independent and dependent variable.  The accuracy of the method will be presented and its effect on the number of test points required.

This presentation demonstrates determining the optimal method to predict compressor performance over the application envelope maximizing accuracy for a given number of test points. The uncertainty in each method is estimated as a function of measurement reproducibility and/or product-to-product variation, especially at the typical rating points given in the performance rating standard. The AHRI standard 540 was evaluated using three sets of compressor test data, and showed small uncertainty within the operating envelope. The study included uncertainty analysis in power and mass flow rate extrapolation outside the envelope.  The presentation also covers a study on the effect of superheat.

Vapor injection in conjunction with economized cycle is becoming more wide-spread as the unit efficiency requirements become more stringent and more difficult to meet with the new low-GWP refrigerants.   Economized cycle substantially boosts both efficiency and cooling/heating capacity.  A two-stage design is used in reciprocating compressors with vapor injected between the low and high compression stages. One of the most difficult tasks is to properly size the displacement of low and high pressure stages. This paper examines what controls this sizing to optimize vapor injected compressor performance with respect to operating conditions, refrigerant properties, and unit capacity and efficiency goals.