BFRL: HVAC&R - Publications

Potential Benefits of Smart Refrigerant Distributors

Potential Benefits of Smart Refrigerant Distributors (7.21 MB)

Payne, W. V. and Domanski, P. A.
Keywords:
expansion device , superheat control , refrigerant distribution , non-uniform airflow , heat transfer , heat exchanger modeling , evaporator models , tube-by-tube model , fin types
Abstract:
The main goal of this study was to investigate the benefits possible for finned tube refrigerant evaporators when refrigerant distribution was precisely controlled to produce a desired equal superheat in each circuit. This goal was accomplished by examining three different finned tube evaporators; a wavy fin, wavy-lanced fin, and a wavy-lanced fin evaporator with tube sheets separated. The effects of non-uniform airflow on capacity were also examined while superheat was controlled in each evaporator circuit. In parallel with the experimental effort, a modeling program was implemented and validated with the experimental results and then used to determine the savings in evaporator core volume possible if refrigerant distribution was controlled by a smart distributor. In extreme cases, the savings in core volume could be as much as 40 %. Within the experimental part of this study, all three evaporators could avoid significant performance degradation using the ability to control superheat within each of the three finned tube circuits. As an example, with cross-counter flow configuration, uniform airflow, and exit manifold superheat fixed at 5.6 °C (10.0 °F), the wavy fin and wavy-lanced fin evaporator's capacity dropped by as much as 41 % and 32 %, respectively, as the superheat was allowed to vary between the circuits. Control of superheat was shown to be even more important during cross-parallel refrigerant flow due to the rapid pinching of the refrigerant and air temperatures. For the wavy and lanced finned evaporators in cross-parallel flow, capacity dropped by 85 % and 78 % as superheat changed from 5.6 °C (10.0 °F) to 16.7 °C (30.0 °F). As the coil faces were blocked to produce a non-uniform airflow, pressure drop through the coils increased substantially and control of superheat was shown to restore performance. The non-uniform airflow tests showed that when airflow rate was held constant, the losses in capacity due to low airflow over a portion of the coil could be recovered to within 2% of the original uniform airflow capacity by controlling superheat. The more non-uniform the airflow over the coil, the more capacity was improved by controlling superheat. A combination of results obtained from laboratory testing and simulations indicate the influence of tube-to-tube heat transfer on capacity degradation. The impact of tube-to-tube heat transfer was negligible in tests with a uniform 5.6 °C (10 °F) superheat, but it was significant in tests involving 16.7 °C (30 °F) superheat. Between the two possible conduction mechanisms of heat transfer that may occur, longitudinal fin conduction is responsible for degraded performance rather than longitudinal tube conduction, which has insignificant impact. The upgraded version of the EVAP5 evaporator model, which accounts for tube-to-tube heat transfer based on tube temperatures, was able to predict key return bend temperatures which indicated the occurrence of tube-to-tube heat transfer. However, the study also confirmed that longitudinal heat conduction is affected by the fin design, air-side heat transfer coefficient, and moisture removal process.

Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899

Disclaimer | FOIA | Information Quality | About NIST

NIST is an agency of the U.S. Department of Commerce

Last updated: 4/21/2005