Design of Food Freezing Equipments

Design of Food Freezing EquipmentsPresented by:madhuFreezing MethodsNatural Convection Freezing- the food product is the heat source and the fluid is usually cold air. slow, relatively uncontrollable.

Forced Convection Freezing-fans are used to increase the velocity of the air. more controllable than natural convection freezing.

Liquid Immersion Freezing-the food product is immersed in a cold liquid, generally brine, glycol, or a sugar solution, which is cooled by a refrigerant evaporator. results in considerably higher heat transfer coefficients - shorter freezing times. usually frozen in liquid-tight packaging to prevent cross-contamination.

Contact Freezing- achieved by placing food products between two metal plates filled with evaporating refrigerant. providing the high rate of heat transfer more compact than convection or immersion freezing systems.

Cryogenic freezing involves either spraying liquid nitrogen or carbon dioxide snow onto the food surface or immersing the food product directly into the liquid cryogen. shorter freezing times than air or liquid immersion freezing- high rate of surface heat transfer. does not require any refrigeration machinery However , the cost of the cryogen is relatively high in most locations - too costly for freezing large quantities of food.

The selection of freezing equipment should take the following factors into consideration:The rate of freezing required; The size, shape and packaging requirements of the food;Batch or continuous operation, The scale of production, Range of products to be processedThe capital and operating costs.

Equipment design criteria Product Suitability-Selection of a freezing process that is suitable for the product.Rapid surface freezing is probably only achievable in a cryogenic freezer. The internal part of the product may be frozen at a slower rate using a different type of freezer.

Predicting Food Freezing Times and Heat Loads Designers are using mathematical models.A range of mathematical models exists for calculating the temperature profile of a food product.Models are also available for predicting the heat load . Heat loads from equipment , people in the freezer, heat infiltrating into the freezer through the walls, ceiling, floor, and openings .EconomicsMust consider both capital and operating costs.Capital cost of freezers is generally high when compared with other processing equipment.It is also important to consider the space taken up by the freezer. Freezers often represent a small fraction of the operating cost.The operating costs of freezers vary with the product throughput and include electricity, labor, maintenance, and cleaning costs, mass and quality loss.Reliability And Maintenance No back-up system for the freezer in a food processing operation.Regular checking and maintenance are essential to minimize the chance of breakdowns.

Cleanability And Defrosting The low temperatures at which freezers operate prevent the growth of the bacteria and molds. It is therefore possible to clean freezers infrequently.Defrosting must be carried out periodically to prevent frost building up to an unacceptable level.

Types of freezerSelection of freezing equipment should take the following factors into consideration

the rate of freezing required the size, shape and packaging requirements of the foodbatch or continuous operationthe scale of productionrange of products to be processedcapital and operating costs

Freezers are broadly categorized into

mechanical refrigeratorscryogenic freezers

An alternative classification, based on the rate of movement of the ice front is

slow freezers and sharp freezersquick freezersrapid freezersUltra-rapid freezers

Quick freezersBatch Air Blast FreezerCommon form of forced convection freezing

Consist of insulated room containing fans which force air over refrigerant evaporator then circulate over the food product

Temperature: -30oC to -40oC

Air velocity: 1.5 to 6.0 m.s-1

Moderate rapid rate.

Drawback:

Bulging of product occurs

Continuous Air Blast Freezer These are modified air-blast freezers in which a continuous flexible mesh belt is formed into spiral tiers.

Advantages include:

Automatic loading and unloading

Low maintenance costs

Flexibility for different products

Batch plate freezerConsist of several layers of plates & are present in horizontal or vertical arrangements.

VPF are used for unpacked, where HPF are used for both.

Advantages:Rapid rate of coolingMinimize bulging of product

Continuous plate freezerTwo forms i.e. drum or belt.

Drum contain continuously rolling plates & product are placed in between them.

In belt consist of two long stationary plates, conveyed with plastic film.

Second belt consist of steel belts.

Used for thin & flat sided food products like fish fillets.

Rapid freezers Fluidized Bed freezerModification of air-blast freezing

High air flow velocities: 2-5 m/s & bed depth: 2-13 cm

Both are determined by food size and shape.

Advantages :

Higher heat transfer coefficients, shorter freezing times, higher production rates (l0,000kgh-1) and less dehydration of unpackaged food than blast freezing.

Limited to particulate foods such as peas, corn kernels & diced carrots.

Impingent freezer Numerous Jet nozzles are used for pass the air at very high velocity.

Process is continuous & air flow is perpendicular.

High rate of heat transfer coefficient.

Ultra-rapid freezersLiquid Immersion Freezer Direct immersion freezing

Packaged or unpackaged foods frozen by immersion in or by spraying with a freezant that remains liquid throughout the process.

Freezant : propylene, glycol, glycerol, sodium chloride, and mixtures of salt and sugars

Used for canned citrus juice concentrate poultry, fish and shrimp

Advantages:

Rapid freezing

Easily adapted to continuous operations

Liquid N2 freezerContains continuous belt that conveys the product through a tunnel.

The temp of N2 i.e -1960 C is used.

Other arrangements include :batch cabinet, full immersion baths & spiral belt system.

0.3-1.5kg of N2 is reqd. to cool the 1kg of product & it depends on the composition, initial temp of product.

Carbon Dioxide freezerAt atmospheric pressure CO2 sublimes instead of passing through the liquid phase.

Temperature of CO2 is -790 C.

The mix of cold vapors & solid removes the heat from the foods when in contact.

CO2 freezer has a slightly higher cryogen consumption than the N2.

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