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ACRC Technical Report 138PDF


Title:Superheat Stability of an Evaporator and Thermostatic Expansion Valve
Author(s):Lenger, M.J.; Jacobi, A.M.; Hrnjak, P.S.
Subject(s):superheat stability
Abstract:In some refrigeration applications, difficulties arise in establishing stable evaporator operating conditions, especially when using a thermostatic expansion valve. The unstable superheat signal, sometimes called hunting, of an evaporator was investigated by developing a mathematical model of a thermostatic expansion valve and a two-passage concentric-tube evaporator. The model was then used to study the dynamic response of the evaporator and valve in response to changes in the system operating conditions. The evaporator model was based on a two-passage concentric-tube heat exchanger configuration. Equations for the conservation of mass, momentum, and energy were used to simulate the flow and heat transfer, where differential equations for the length of the two-phase region and mean void fraction allowed the dynamic behavior of the evaporator to be investigated. The model also has the capability to examine the effects of refrigerant and heat flux maldistribution among the passages. The thermostatic expansion valve model takes into account the pressure forces on the diaphragm as well as the pressure drop across the orifice when predicting the refrigerant mass flow rate. The geometrical parameters that were varied in this study included the orifice size, obstructing pin-tip angle, and diaphragm area. The model also includes the effects of the spring constant, bulb time constant, and offset temperature-as determined by the force applied by the obstructing pin when the valve is closed. Superheat response was investigated by imposing suction line pressure oscillations that varied over a range of frequencies. Large superheat fluctuations were found to exist in a given frequency band, where the period was found to be on the order of 50 to 100 seconds, and pressure oscillations in this range should be avoided in operation. Disturbances outside of this frequency band did not produce significant superheat responses. Factors influencing the magnitude of the superheat response depend on the frequency of the perturbations: at high frequencies the valve does not respond to superheatfluctuations (feedback), but is very sensitive to the slope of the flow rate versus superheat curve as detennined by valve geometry; on the other hand, at low frequencies the valve behavior is dominated by the superheat feedback, and the flow rate versus superheat curve is insignificant. The effect of the valve parameters was also investigated by imposing a step increase of the suction line pressure and simulating the response of· the evaporator superheat over time. This approach allowed comparison of the steady-state and transient behavior of superheat with different valve designs.
Issue Date:1998-07
Publisher:Air Conditioning and Refrigeration Center. College of Engineering. University of Illinois at Urbana-Champaign.
Series/Report:Air Conditioning and Refrigeration Center TR-138
Genre:Technical Report
Sponsor:Air Conditioning and Refrigeration Center Project 76
Date Available in IDEALS:2009-05-18

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