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Pressure relief valves are the primary component in an engineered safety system intended to prevent the catastrophic failure of refrigeration equipment due to overpressure conditions that can occur during abnormal standby or operating excursions. ASHRAE Standard 15 (ASHRAE 2007) prescribes methods for determining the mass flow rate (capacity) requirements for pressure relief devices aimed at protecting the pressure vessels and positive displacement compressors used in refrigeration systems. With the good intention of enhancing system safety, designers, contractors, or owners often install relief devices on other types of refrigeration equipment that may include: heat exchangers, piping and pumps. Unfortunately, the sizing and selection of pressure relief devices in these situations often occurs without the use of a clear and consistent basis; thereby, decreasing rather than increasing system safety. This paper reviews the pressure relief valve sizing methodology for vessels and presents approaches for relief capacity determination applicable for other types of refrigeration equipment not explicitly covered in ASHRAE 15 including oil separators, shell-and-tube heat exchangers, plate-and-frame heat exchangers, oil cooling heat exchangers, and product storage tanks. One of the principle aims of this paper is to document a basis for relief device capacity determination to ensure these types of protected components remain safe during abnormal excursions that can lead to high pressures. Although the methods presented in this paper are intended to apply across a wide range of refrigeration equipment and operating conditions, it is not possible to neatly prescribe relief device sizing and selection criteria to cover all situations. As such, the use of sound engineering principles and the application of engineering judgment should be always expected. [ABSTRACT FROM AUTHOR]

Simple analysis of the thermodynamics of the refrigeration cycle suggests that lowering the condenser pressure should result in decreased compressor energy and that the lowest possible pressure is constrained by the temperature of the available heat sink (usually the ambient condition). Thus optimization of the condenser pressure seems straight-forward. However, in practice there is a more complex trade-off, as lowering condenser pressure increases the condenser fan energy. The trade-off is particularly acute when variable speed drives are used to control the condenser fans. In addition, there can be other constraints such as expansion valve performance. This paper demonstrates how the theoretical optimal condensing temperature can be defined using a mathematical model that takes into account the characteristics of the condenser, the condenser controls, the refrigeration load, and the ambient temperature. The model is used to explore practical condenser pressure control strategies for a year of operation of a case study refrigerated warehouse facility. The strategies were the optimum defined by the model and alternative control strategies including fixed head pressure set-point, minimum approach to the ambient temperature, and fixed condenser fan speed. The alternative controls are much simpler to implement and often the penalty relative to the optimum strategy is small. [ABSTRACT FROM AUTHOR]

Air cooling coils operating with a refrigerant temperature below 0oC require regular defrosting to restore cooling performance. Defrosting creates parasitic load on the refrigeration system and can result in short term temperature fluctuations in the refrigerated space. The relationships between defrost heat load, latent loads in the refrigerated facility, defrost melt water flows, and defrost efficiency are quantified. Practical suggestions to minimize the impact of defrost include: minimizing latent heat loads on the facility, establishing a defrost termination method that avoids unproductive heating to the space, varying the frequency of defrost to match the latent load the facility experiences, and monitoring defrost by measuring the melt water flow. While on-demand defrost systems should vary both the frequency and duration of defrost near optimally, they are not always reliable and can schedule defrost at inopportune times. Timed (but manually adjusted occasionally) initiation and temperature termination often provide the best combination of cost, effectiveness and robustness for industrial systems. [ABSTRACT FROM AUTHOR]