WA-2199 (RP-51) -- Dynamic Response of a Finned-Coil Refrigerant Evaporator to Step Changes in Refrigerant Flow Rate PDF

The dynamic response of the evaporator-expansion valve control loop has been under study at the University of lllinois for several years. ASHRAE-sponsored RP-51 has concentrated on the response characteristics of the evaporator under the direction of Technical Committee 10.2 as a part of the larger program of determining the response characteristics of the feed-back control loop consisting of a refrigerant-in-tube evaporator and a thermostatic or superheat-controlled expansion valve. The motivation for the program is to be able to determine what combinations of coil and expansion valve will result in sensitive, yet stable control.

A previous ASHRAE paper (l) discussed how the combined valve and evaporator characteristics influence stability in this loop. To analyze the loop, the response characteristics or transfer functions of both evaporator and valve must be known, and the objective of RP-51 is to determine the transfer function of the evaporator. The first phase of the evaporator study (2) reported the transient response of the mixture-vapor transition point to step changes in flow rate for a horizontal glass-tube evaporator. The results reported in this present paper are from tests and analysis on a single circuit, copper tube, aluminum-fin coil used to cool air.

Several other researchers have also attacked the control problem of refrigerant flow to an evaporator. Particularly noteworthy are the early studies of Danig (3) who was one of the first to represent the problem in the terminology of the control engineer, and Huelle who has developed the "minimum stable signal" concept. The work reported in this current paper helps provide needed data for application of the Danig and Huelle approaches.