Refrigeration Flashcards

Question

factors that influence freezing time

Answer 1

↑ ΔT, h, k ↓ freezing time [ ΔT= temp change h = convective heat transfer coefficient at air/ ice interface (W m^-2 K^-1) k= thermal conductivity of the frozen phase (W m^-1 K^-1)] ↑ ρ, L, a ↑ freezing time [ ρ = density (kg m^-3) L = latent heat of freezing of the liquid (J. kg^-1 ) a = dimension (normally thickness/diameter) ]

Answer 2

Defects in Assumptions: ~ Density independent of temperature ~ K independent of temperature [ K frozen food greater than unfrozen food] ~ Temperature of the freezing medium is constant ~ Temperature of unfrozen material is at Tf at the start of process ~ Freezing point is fixed ~Gives ball park figure ~ For more accuracy use different equation ~ Planks assumption not valid for food but valid for water

Answer 3

Direct Contact: ~ More efficient – no barrier ~ Rapid freezing – Individual Quick Freezing (IQF) Indirect Contact: (i.e. Food on plate placed on top of refrigerant)

Answer 4

Air blast freezing: ~ High air velocities -> high h value ~ Moisture loss Fluidised freezing: ~ High air velocities -> high h value ~ Good contact between refrigerant and product ~ Suitable for particulate products eg peas Immersion Freezing (with a pre-cooling step) ~ Liquefied gas sprayed directly onto product [ Change of phase – evaporation of gas eg CO2 or N2] ~ Very rapid freezing ~ Superior quality product

Answer 5

Plate freezer: ~ Used for block low volume products ~ Pressure applied to lower freezing time Air blast (Packaged food): ~ Increased freezing time but decreased moisture loss ~ For unusual shaped objects Scraped Surface Heat Exchanger: ~ Same as used for high viscosity fluids (difficult process applications) ~ Removed crystals formed on outside ~ Partial freezing of liquid (60-80%) [Objective to get a frozen ice slurry of ice crystals]

Answer 6

~ Natural sources (ice, snow, climatic conditions) ~ Cryogenic agents ~ Mechanical refrigeration (Air blast freezers/chillers; Plate freezers)

Answer 7

allow to transfer of heat from the cooling chamber to a location where the heat can be easily discarded Refrigeration Cycle ``` transfer heat by refrigerant suh as: ~ Change of state (liquid to vapor) ~ Boiling point e.g. -33.3°C ~ Latent heat ~ Pressure – boiling point ```

Answer 8

``` transfer of heat from cooling chamber to a discarded point (external) ``` ~ (Mechanical) vapour compression cycle ~ Absorption refrigeration cycle ~ Ejector refrigeration cycle

Answer 9

Compression (points 1–2) • refrigerant is in a gaseous state (point 1) • Work done by compressor • pressure and temperature elevated Condensation (points 2–3) • High pressure vapour enters heat exchanger (condenser) • Using air or water cooled atmospheres refrigerant gives up heat to surroundings • condenses to form a liquid • heat of condensation rejected to ambient Expansion (points 3–4) • Liquefied refrigerant enters expansion engine • Experiences pressure drop and drop in temperature • Mixture of liquid and gas leaves this process. Evaporation (points 4–1) • Energy received in heat exchanger (evaporator) • Refrigerant evaporates • Latent heat of evaporation needed is supplied by the cooling load => generating refrigeration • enters compressor and continues cycle

Answer 10

Ratio refrigeration effect obtained to the work done in order to achieve it COP = Refrigeration effect / work done = qe/ qw

Answer 11

Generator/ Condenser/ Evaporator/ Absorber / Pump/ Heat exchanger/ Two expansion valves / NH3–H2O absorption cycle/ Rectifier/ Dephlegmator

Answer 12

ammonia evaporated off the generator contains some water vapor ~ elevate evaporating temperature ~ water may also freeze along pipelines. Water must be removed Vapor driven off at generator flows countercurrently to incoming solution in rectifier Passes through dephlegmator and condenses some water-rich liquid, which drains back to the rectifier High-pressure refrigerant vapor 1 generated by generator--> condenses into liquid 2 in the condenser, --> heat of condensation rejected Condensed liquid passes through valve --> maintains pressure difference between condenser and evaporator Low pressure liquid enters evaporator 3 to evaporate --> heat required for evaporation provided by cooling load Vapor 4 absorbed by the liquid strong solution 10 coming from the generator in the absorber Heat of absorption rejected to environment The pump receives low-pressure liquid weak solution 5 from the absorber, elevates the pressure of the weak solution 6, and delivers 7 to the generator Generator: heat drives off refrigerant vapor 1 --> strong solution 8 returns to absorber 9 through throttling valve 10.

Answer 13

COP = Refrigeration effect / work done = qe/ (qg + Wm)

Answer 14

EU guidelines for cooling cooked meat ~ cooled within certain time limits post-cooking ~ Meat joints should not exceed 2.5 kg and 100 mm in thickness ~ should be chilled from 74ºC to 10ºC within 2.5 hours after the conclusion of the cooking process Conventional cooling methods ~ air blast, cold room and immersion cooling ~ depend on heat conduction for cooling inside of joints ~ relatively low thermal conductivity of meat ~ must maintain a temperature of the cooling fluid above 2ºC (to avoid surface freezing) ~ difficult to significantly increase the cooling rate.

Answer 15

~ Rapid evaporative cooling technique ~ Moisture in foods boiling under vacuum conditions ~ The products to be cooled are loaded into a closed chamber ~ Vacuum pumps are then used to evacuate air from the chamber ~ Pressure reduced and water starts to evaporate ~ Latent heat of evaporation supplied by product ~ Sensible heat reduced and cooling occurs

Answer 16

- vacuum pump | - condensation

Answer 17

~ free water ~ structure not damaged by water removal ~ porous structure e.g. Cooked meat/ Bakery/ Fishery/ Viscous food processing

Answer 18

leafy vegetables: field heat and prolong product shelf life

Answer 19

Speed and efficiency: * boxed or palletised products -> 30 min * 0.5°C/min * ↓ product hold up time ↑ production throughput * Tight delivery schedules * strict cooling requirements Reduce postharvest deterioration: * Prolonging storage life More uniform internal temperature distribution Rate not directly affected by sample size * large dimensions Precise product temperature control possible ↓ energy consumption

Answer 20

cannot replace established cooling techniques Weight: * In vegetables ~ 3–4% of original weight * Add water

Answer 21

Integration into baked bread lines modulated vacuum cooling For cooling baked products from oven * immediately after removal from the oven before packaging avoid vapour condensation in the wrapping * plastics bags Bread rolls, crusty breads, baked biscuits * Rack cooler/ In-line cooler Benefits: * precise control over cooling rates * increased product stability * humidity distribution * shape and texture * baking cycle reduced by 2hrs temperature range: 98 to 30°C weight loss: ~ 6.8% (conventional 3 and 5% * Compensate: reduce baking time, spray with sterile water

Answer 22

Temperature should be rapidly reduced retention of nutrients Conventional cooling * heat removed from core by conduction & to cooling medium by convection. * ↑surface heat transfer ↑ the velocity of the cooling medium * surface temperature approaches cooling medium temperature quickly but not so for the core * poor thermal conductivity and large dimensions To meet cooling guidelines the shortest dimension of the meat should not exceed a certain value Not feasible for cooked meat processors and caterers Weight loss * 72–75°C to 3–8°C results in 10–12% weight reduction * Conventional 6-7% Compensate weight loss * brine injection level Quality * Texture, Juiciness, Colour Immersion vacuum cooling

Answer 23

At sea: frozen in brine immediately Canning plants: * thawed * steam cooked to 65°C * Cooled 35 and 40°C * 3–4% weight loss