COVER STORY SKILLS WORKSHOP HVAC&R SKILLS WORKSHOP MODULE 35 PULLOUT PULLOUT An introduction to commercial and industrial refrigeration systems Produce case Although refrigeration was invented for the preservation of food, there are many other commercial uses to which it has been applied. This article will highlight those applications and the equipment used. Freezing and chilling of products In order to achieve an acceptable freezing or chilling performance, the designer needs techniques to enable the accurate prediction of freezing and chilling rates and also product heat loads during these processes. Factors affecting freezing and chilling • the initial temperature of the product During freezing or chilling, the temperature of the product changes with time. The variation of temperature depends on a number of factors: • the surface heat transfer conditions between the product and the cooling medium • the size of the product • the shape of the product • the thermal properties of the product (a) density (b) thermal conductivity (c) specific heat (d) freezing temperature (e) latent heat content • the temperature of the cooling medium The exact physical analysis of freezing and chilling situations is a complex mathematical exercise, but approximate freezing and chilling methods that have been developed from research are available. Freezing or chilling of solids Freezers used are generally of two major types – air blast freezers and plate freezers. Cryogenic tunnels can be considered as a variation on an air blast freezer, however, these are only 15 SKILLS WORKSHOP of mobile cool and cold rooms transporting perishable goods throughout the world, not all of them necessarily refrigerated. In some instances the insulation and the frozen product may provide sufficient protection for the time required to effect delivery. Cool room A room refrigerated to chill and store a product without freezing it. Their size can vary from a small room to suit a retail butcher’s shop to quite large premises where bulk storage of product is required for extended periods, e.g. fruit. A common method of construction utilises prefabricated panels consisting of two sheets of painted steel or aluminium bonded to each side of a suitable, impermeable, self extinguishing, insulating material The meat preparation and packaging room in a supermarket is usually a ‘cool’ room with the operating temperature level a compromise between product and personnel requirements. Industrial blast chiller. appropriate for very short term solutions or very high value products due to their extreme carbon footprint when compared with vapour compression refrigeration systems. The techniques to predict freezing rates, freezing times and heat loads are similar for both types. Air blast chillers are widely used for chilling products. One method uses a modified Plank’s equation which takes account of the six factors listed in the previous sub-clause plus the thermal centre or mass average temperature at the end of the freezing process. Of the factors listed above all but the last two are generally fixed. The role of the designer is to manipulate these to optimise the design in terms of cost and energy efficiency. Liquid chilling There are many important situations requiring the chilling of liquids including milk cooling, production of chilled water used in processing, cooling of beer and cooling of secondary refrigerants (glycol or brine). The ways that cooling is achieved can be divided into two major types: • continuous use of in-line heat exchangers • batch use of external heat exchangers, jackets or cooling coils to chill a reservoir of liquid 16 As for solids chilling, in both cases the heat flow depends on the overall resistance to heat transfer, the area across which heat transfer can occur and the mean temperature difference between the liquid and the cooling medium Q = U A ∆ tav where: Q = heat flow (W) U = o verall heat transfer coefficient (W/m2°C) A = heat transfer area (m2) ean temperature difference Dtav = m between the cooling medium and the liquid (°C) Refrigeration plant A term used to describe collectively the items of equipment used to compress, condense, expand and evaporate the refrigerant medium used in the system. The following items relate specifically to refrigeration: Cold storage The trade or process of preserving perishables by refrigeration. Although this discussion concentrates on static installations, it should be remembered that there are many types Refrigerated enclosures and cabinets Amongst the smallest refrigerated cabinets is the domestic refrigerator. In commercial applications, the merchandising and display cabinets are usually purpose built for the specific task. Typical examples include: • Drink cabinet – usually vertical with glass doors, multi shelved and incorporating a self contained air cooled refrigeration system rejecting the heat into the immediate environment. • Dairy cabinet – usually vertical, open and multi shelved. Refrigeration is usually provided by a remote (either water or air cooled) condensing unit. • Island case – normally open top, used for displaying packaged frozen food. In some applications sections of casing are brought together extending the cabinet to more than 15 metres long. Refrigeration is usually provided by a remote (either water or air cooled) condensing unit. • Closed display case – used for displaying fresh meat and fish, refrigeration is usually provided by a remote (either water or air cooled) condensing unit. • Fruit and vegetable case – usually single deck, sloping towards the front. Refrigeration is usually provided by a remote (either water or air cooled) condensing unit. • Cryogenic container – for the long term storage of medical and pharmaceutical products at very low temperature, e.g. animal and human semen. COVER STORY HVAC&R SKILLS WORKSHOP MODULE 35 PULLOUT Of the above, the drink, dairy and island styles are commonly referred to as ‘reach in merchandisers’. Plate freezer A contact freezer in which the refrigerated surface is a flat metal plate. Cold room and freezer room A room refrigerated to freeze and store a product. Commonly found in the meat and fish industries, especially for export. PULLOUT Construction may be as for cool rooms or from PIR insulation between two concrete skins which provides very good thermal stability. This alternate form of construction has found appeal in the wine industry to encase large fermentation and storage tanks. Chest freezer Access to this type is by raising a lid, also commonly referred to as a deep freeze. Well freezer This type of cabinet may be open or closed top depending on usage. It is commonly used in supermarkets, ice cream parlours and some restaurants for displaying and serving from large ice cream containers. Quick freezing The process of freezing a food or other product at a rapid rate of temperature reduction to produce a desired crystalline structure in the frozen product. This is generally appropriate for small products such as peas, corn or chips. Freezing tunnel An elongated enclosure provided with rapid cold air circulation for the freezing of foodstuffs. Used in the processed meat and vegetable industry. Product is usually automatically conveyed through the freezing tunnel. Horizontal plate freezer. evaporator coils of refrigerating units. Common defrost methods are electric element and reverse cycle. Intercooler An intercooler is the intermediate vessel between the high stage and low stage in a compound system. The purpose of the intercooler is to cool the discharge gas of the low-stage compressor, as well as subcooling the liquid before it is fed to the low stage evaporators. Baudelot cooler Forced draft cooler Used almost exclusively in applications where air is used as a secondary medium to maintain or cool the product. Frost heave The term used to describe the rising of a cold room floor due to the expansion of freezing water in the supporting soil. It can also occur in cold room walls that are poorly or inadequately vapour sealed. Defrosting This is the planned process of removing or preventing the accumulation of frost (ice) on Used in industrial applications where it is desired to cool the fluid near to freezing point e.g. a milk or beverage cooler. The fluid is circulated over the outside of the open heat exchanger thus facilitating the cleaning of surface. Liquid overfeed system (liquid recirculation) Cascade refrigerating system One having two or more separate refrigerant circuits, each with a compressor, condenser expansion device and evaporator, where the evaporator of one circuit cools the condenser of another (lower temperature) circuit. Cascade refrigerating systems have become popular in supermarket systems using CO2 for the lower temperature circuit and a HFC in the higher temperature circuit. This has meant that only CO2 is circulated to the evaporators in the cabinets (liquid recirculation to the dairy cases and direct expansion to the freezers) and the HFC circuit has a very small amount of refrigerant in it. The cost of lost refrigerant and damage to the environment are both dramatically reduced. n DA08 HVAC&R – An Introduction manual T H E A U S T R A L I A N I N S T I T U T E O F A I R C O N D I T I O N I N G A N D H E AT I N G A system feeding an evaporator with refrigerant at a rate greater than the amount evaporated which means there is a mixture of gas and liquid leaving the evaporator along the suction line. The gas returning from the evaporator will be saturated and not superheated. Next month’s workshop — Solar air conditioning DA08 HVAC&R AN INTRODUCTION APPLICATION MANUAL Australian Institute of Refrigeration, Air Conditioning and Heating Information in this month’s Skills Workshop has been compiled from AIRAH’s DA08 HVAC&R An Introduction manual. 17