Evaporator superheat control With one temperature sensor using qualitative system knowledge

Abstract

Abstract-This paper proposes a novel method for superheat control using only a single temperature sensor at the outlet of the evaporator, while eliminating the need for a pressure sensor. An inner loop controls the outlet temperature and an outer control loop provides a reference set point, which is based on estimation of the evaporation pressure and suitable reference logic. The pressure is approximated as being linear and proportional to the opening degree of the expansion valve. This gain and the reference logic is based on calculation of the variance in the outlet temperature, which have shown to increase at low superheat. The parameters in the proposed controller structure can automatically be chosen based on two open loop tests. Results from tests on two different refrigeration systems indicate that the proposed controller can control the evaporator superheat to a low level giving close to optimal filling of the evaporator, with only one temperature sensor. No a priori model knowledge was used and it is anticipated that the method is applicable on a wide variety of refrigeration systems.

Emulation of semi-active flow control for evaporator superheat regulation

“…Model predictive control (MPC) of evaporator superheat has been explored [11], as well as using an MPC style controller to generate setpoints for lower-level existing control loops [24]. Current explorations include auto-tuning the PID controllers [4] and controlling superheat using only one temperature sensor [22]. …”

Single temperature sensor superheat control using a novel maximum slope-seeking method

“…An alternative is to use qualitative knowledge about the system and the behavior of the evaporator outlet temperature. A variance based control method was investigated in Vinther et al (2012a), where it was discovered that the variance of the outlet temperature increased at low superheat, which can be used for feedback purposes. This is also closely related to the automatic variance control method (Moir, 2001), which in some cases has been used in the conventional two sensor superheat control to adjust the reference.…”

Single temperature sensor based evaporator filling control using excitation signal harmonics

“…We have called this method harmonic control and it is presented in [11]. This is another approach for single sensor superheat control than the variance based method we presented in [12] and the benefit of harmonic control is that it does not require a system model, it works for a wide operating range, and it does not require any reference set points, which together means that it has a high degree of Plug and Play.…”