Analysis of Packaged Air Conditioning System for High Temperature Climates

Ammar M Bahman, Purdue University

Abstract

Packaged air conditioning (AC) units, called Environmental Control Units (ECUs), are being increasingly used by the U.S. military, especially in hot ambient temperature climates. The compact packaging of ECUs resembles unitary-type rooftop or room AC systems, and they are used to cool personnel and equipment in enclosed spaces such as shelters, vehicles, and containers. Despite these similarities, ECUs have distinctive features that aren’t found in commercial packaged AC units. An ECU is designed to sustain harsh and extreme weather conditions up to 51.7 °C (125 °F) which is a design set-point by the military. As the outdoor temperature increases, both the cooling capacity and coefficient of performance (COP) of ECUs drop dramatically. In addition, the compact design degrades airflow uniformity due to air maldistribution across evaporator coil, which results in further performance degradation. Therefore, the goal of this study is to identify ways to improve the component as well as the system performance of the ECUs in the field at high ambient temperatures. A passive solution was evaluated to compensate for the degradation in performance of ECU evaporators, known as the interleaved circuitry method. The interleaved circuitry method, where the refrigerant from a circuit with high air flow is routed to a circuit with low air flow and vice-versa, has been investigated to determine its effectiveness in reducing the air maldistribution effect. Air velocity measurements in front of the ECU’s evaporator have been conducted in psychrometric chambers and the measurement locations have been defined by the log-Tchebycheff rule. The velocity profile was obtained by the Lagrange Interpolation method as percentage values. The system performance after interleaved circuitry implementation was compared to the baseline system at different operating conditions up to 51.7 °C (125 °F). The results showed that the interleaved circuitry method increased the superheat uniformity of the individual circuits and improved the cooling capacity and COP up to 16.6% and 12.4%, respectively. Furthermore, the tuned model predicted the evaporator cooling capacity within a mean absolute error of approximately ±10%. Moreover, vapor injection (VI) with economization, where cool gas is injected to the compressor at an intermediate stage to absorb the heat generated during the compression process, has been experimentally and numerically assessed to significantly improve system performance. The ECU has been retrofitted with an economized vapor injection (EVI) system and experimentally characterized in side-by-side psychrometric chambers. The performance of the EVI system for superheated and saturated injection conditions were compared to the case of without injection at different operating conditions. The results showed that the EVI system reduced the compressor discharge temperature by up to 5 °C, and improved the cooling capacity and COP by up to 12.7% and 3.1%, respectively. The experimental data have been used to develop, tune, and validate a detailed steady-state cycle model. The predictions of suction and injection mass flow rates, compressor power consumption, and system COP were within a mean absolute error of approximately ±5%. At last, the model has been employed to optimize the economizer geometry in order to maximize the system COP at designed ambient condition of 51.7 °C (125 °F). The optimization process resulted in maximum improvements in compressor discharge temperature, cooling capacity, and COP of 8.5 °C, 22.3%, and 17.3%, respectively.

Degree

Ph.D.

Advisors

Groll, Purdue University.

Subject Area

Thermodynamics|Mechanical engineering

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