Potential benefits of smart refrigerant distributors, final report

Abstract

Prepared for the Under ART1 2 1 -CR Program Contract Number 605-20050 AIR-CONDITIONING AND REFRIGERATION TECHNOLOGY INSTITUTE Use of Non-SI Units in a Non-NIST Publication It is the policy of the National Institute of Standards and Technology to use the International System of Units (metric units) in all of its publications. However, in North America in the HVAC&R industry, certain non-SI units are so widely used instead of SI units that it is more practical and less confusing to use measurement values for customary units only in figures and tables describing system performance. EXECUTIVE SUMMARYThe 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 YO.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 OF), the wavy fin and wavy-lanced fin evaporator's capacity dropped by as much as 41 YO 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 YO and 78 YO as superheat changed from 5.6 "C (10.0 O F ) to 16.7 "C (30.0 OF). As the coil faces were blocked to produce a non-uniform airflow, pressure drop through the coils increased 1 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.

Procedimiento para el cálculo de circuitos en el modelado del flujo en intercambiadores de calor de aletas y tubos

“…(a) Wang(A) [5] (b) Wang(C) [5] (c) Wang(E) [5] (d) Wang(F) [5] (e) Wang(G) [5] (f ) Ge(A) [6] (g) Ge(B) [6] (h) Kuo(A) [4] (i) Kuo(B) [4] (j) Ge [7] (k) Liu [2] (l) Domanski [8] Figura 15: Ejemplos de circuitos validados.…”

Comparison of fin-and-tube interlaced and face split evaporators with flow maldistribution and compensation

“…Studies regarding the benefits by control of individual superheat have also been conducted. Payne and Domanski (2003) showed experimentally that the performance degradation due to a non-uniform airflow could be recovered to within 2% of the original cooling capacity at uniform airflow conditions. Kim et al (2009a) studied benefits of upstream vs. downstream control of individual channel superheat on a fin-and-tube five channel R410A heat pump numerically.…”

“…Flow maldistribution in fin-and-tube evaporators has been shown to decrease the performance of the evaporator and the system both experimentally (Payne and Domanski, 2003) and numerically (Kaern et al, 2011b;Kim et al, 2009b). Both air side and refrigerant side effects may cause flow non-uniformities, e.g.…”

Performance of residential air-conditioning systems with flow maldistribution in fin-and-tube evaporators

“…Payne and Domanski (2003) showed that the capacity dropped as much as 41% and 32% for two different fin-and-tube evaporators due to variable superheat values between the circuits when circuit pressure drops were induced and the overall superheat was fixed at 5.6…”