Abstract
This chapter presents an experimental study on the effects of some of the most important variables that affect the infiltration rate in open refrigerated vertical display cases at steady operation. The variables were defined in dimensionless forms and were categorized in two groups. The goal was to examine the cross-effects of the first group (primary variables) by performing tests on the permutation of the variables, and to obtain some correction functions that contain the effects of the (secondary) variables thought to have smaller impact on infiltration. Tracer gas method, a new means to measure infiltration rate, is described and compared with other conventional methods. Most variables showed noticeable impacts on infiltration either linearly or non-linearly, some of whom did not show very well-defined patterns with infiltration, which indicates there are significant cross-effects among the variables. Others such as temperature difference and relative humidity showed no or negligible effects on infiltration.
TopIntroduction
An air curtain is usually a straight rectangular or curved jet of air that is intended to separate two spaces that have different properties or characteristics such as different temperatures, relative humidity, and airborne particles such as dust, bacteria, flying insects, hazardous vapors. Air curtains have been widely used and incorporated as part of the HVAC system of buildings at the entrance of residential, commercial and industrial buildings. They have other applications in protecting artwork (de Santoli et al. (2005)), fire protection in tunnels (Juraeva et al., 2014; Felis et al., 2010; Guyonnaud et al., 2000), and incineration process (Schapiro, 2002).
Perhaps the second common application of air curtains is in open refrigerated display cases (also called open refrigerated display cabinets), which are normally used to store medium-temperature food products such as unfrozen dairies, meat and vegetables. These systems are manufactured in horizontal and vertical forms, but the focus of this work will be on the vertical type of the systems (Figure 1), which are hereafter called Open Refrigerated Vertical Display Cases (ORVDCs). The most commonly known reasons for having an open frontal area rather than closed are:
Figure 1. (Courtesy of Hill Phoenix Company) - 1.
Increasing the accessibility of the food on the shelves to customers
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Avoiding formation of a layer of mist on the glass doors after even short open duration, in the cases with doors
There are several heat sources that increase the cooling load in these systems:
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Radiation heat gain from the room and people, especially through the open front
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Radiation heat gain from lighting of the system
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Conduction heat transfer through the system walls
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Heat generated during the defrost cycles
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Heat of fans’ electromotors
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Heat of anti-sweat heaters
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Heat associated with the higher temperature of the food products during or after loading
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Heat generated by wall-flow friction in the ductwork and honeycombs
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Latent and sensible heat carried (infiltrated) in by outside warmer air
Among the above factors it has been shown (Gaspar et al., 2009; Howell & Adams, 1991) that infiltration, with more than 70% contribution to the total cooling load, is the most crucial factor. Simply, being open causes the outside air to penetrate onto the shelves of ORVDCs and result in an increase in the temperature of the food products. Upon sensing temperatures above the prescribed for stored food, the temperature sensors will more frequently trigger the refrigeration system in order to supply cold air. This significantly increases the energy consumption and costs; however, it seems that by making the food more easily reachable to the customers, sales growth exceeds the extra costs, convincing enough for the supermarkets to keep using the open display cases.
Key Terms in this Chapter
Tracer Gas: A gas other than the main working fluid in a system that is injected to the working fluid to visualize or identify some flow characteristics such as diffusion rate, dispersion rate, mean velocity, etc.
Entrainment: The action of engulfing surrounding fluid by a relatively faster stream (such as a jet). This typically occurs via vortices that are created due to velocity gradients between the surrounding fluid and the jet.
Jet: Stream of an external flow that has relatively greater mean velocity than its surrounding fluid. Jets can have different cross sections at their point of origination (nozzle), but common shapes are rectangular and circular.
Turbulence: Agitation, disorder and randomness in a flow. A turbulent flow is three dimensional in nature and is characterized by its mean velocity and amplitudes of fluctuating components of velocity. Typically turbulence is associated with greater momentum loss, less fouling effect, and enhanced heat transfer.
Refrigerated Display Case Cabinets: A system that is widely used in supermarkets and grocery stores to store variety of food products at prescribed temperatures. They are either closed type, or open (with no door), and have vertical or horizontal configurations.
Air Curtain: A jet of air that creates a virtual barrier between two environments that should maintain different properties such as temperatures; relative humidity; and physical, chemical or pathogenic species.
Infiltration: A process during which a fluid can penetrate into an environment, space, duct, etc. Since the properties of the infiltrated fluid can be different, the properties in the space may change.
Energy Efficiency: Efficiency of a system that consumes energy to operate.