Y.T. Ge

Simulation of the performance of single jet air curtains for vertical refrigerated display cabinets

Air curtains are widely used in refrigerated display cabinets as well as doorways of cold rooms and retail premises. The main purpose of the air curtain is to reduce the air exchange and hence heat and moisture transfer between the conditioned environment and the surrounding ambient. This paper presents a comprehensive model, based on the finite difference technique, which can be used to predict and optimise the performance of air curtains. Based on results obtained from the model, correlations for the heat transfer across refrigerated display cabinet air curtains have been developed to enable quick calculations and parametric analyses for design and refrigeration equipment sizing purposes. Both models have been validated against results from tests on a vertical refrigerated display cabinet air curtain.

Mathematical modelling of supermarket refrigeration systems for design, energy prediction and control

Abstract This paper describes a mathematical model developed to simulate the performance of supermarket refrigeration systems. Such a model can be used for the comparison of different systems and control strategies in terms of their energy and total equivalent warming impact. The model is based on a large number of component models which have been linked together within the TRNSYS environment. Major component models include the compressor, air-cooled condenser, thermostatic expansion valve, display cabinet and control. The overall system model has been validated against monitored data obtained from both a laboratory-based system and a full-scale system in a supermarket in Scotland. The value of the model is illustrated by determining and comparing the effectiveness of head pressure and variable-speed control against fixed head pressure and constant speed control. It is shown that even at summer ambient conditions the system can be operated without problems at much lower head pressures than is done in practice under fixed-pressure control strategies. The use of variable-speed control on one of the compressors can also provide better control of the suction pressure and a substantial (up to 23 per cent) energy savings compared to on-off control.

Air-cooled condensers in retail systems using R22 and R404A refrigerants

A steady-state simulation model for a finned-tube air-cooled condenser has been developed by using the distributed modelling method. The model can be used to predict 3-D variations of parameters for both air and refrigerant sides. The model has been validated by comparing outputs from the model with test data, given in reference papers, derived from air-cooled condensers with differing dimensions and refrigerants. Application of the model is concentrated on optimisation and exploration of existing and new control strategies for controlling refrigerant discharge-pressures in retail refrigeration systems, where air-cooled condensers are being utilised. Meaningful results are obtained for both the optimal design and development of the new control strategies and for performance comparisons when different refrigerants are employed in the condensers.

Reduction of refrigeration energy consumption and environmental impacts in food retailing

Publisher Summary The modern supermarket depends on electricity for lighting, ventilation and, above all, refrigeration to protect a vast selection of meats, dairy products, fruits, and vegetables. Supermarkets are among the greatest single end-users of electricity with typical annual electrical energy consumption in the region of 1000 kWh/m2 sales area. The refrigeration systems account for between 40 and 50 % of the electricity used, whereas lighting accounts for between 15 and 25 % with the Heating, Ventilation and Air Conditioning (HVAC) equipment and other utilities, such as bakery making up the remainder. All refrigeration systems have the potential to leak because pressures in the system are usually many times higher than atmospheric. It is estimated that between 10 and 30 % of the refrigerant charge in large systems is released to the atmosphere each year, contributing to the depletion of the ozone layer and to global warming. Refrigerant loss also contributes to the reduction of the operating efficiency of the system, leading to increased power consumption and greenhouse gas emissions, higher maintenance costs, and eventual system failure. This chapter discusses the types of refrigeration systems used in supermarkets and their environmental impacts due to energy consumption and refrigerant leakage and identifies the ways in which these impacts can be reduced.