6. Heating and cooling liquid and solid food (DONE)

Batch heating in industry

• contains agitator that stirs liquid in vessel

• used to make soup for example

Sensible vs latent heat

• Sensible: heat you can feel (temperature)

• Latent: Heat you cannot feel (heat needed for phase transitions)

How fast heating takes place formula

Types of heat exchangers: plate system

• only works for low viscosity products.

Types of heat exchangers: tubular system

• for more viscous products

• usually pretty long for there to be enough residence time

Types of heat exchangers: tube in shell

Types of heat exchangers: scraped surface

• For cooling

• Can remove crystals

Temperature changes in streams for cross current

• E.g. heating with steam, steam stays constant temperature but heats up other product. 

Temperature changes in streams for co-current

Temperature changes in streams for counter current

• Most efficient

What does each part of the 1/h_ov equation represent?

• 1/h_h = resistance for heat transfer at the heating medium

• thickness/conductivity = resistance to heat transfer of the wall

• 1/h_c = resistance in cooling medium

Cross-current heat exchange

• Assumption: constant T for heating medium (steam)

Co-current heat exchange

Counter-current heat exchanger

Options for heating of semi-solid food

Options:

1. First package, then heat

2. First heat than use aseptic packaging

Process for first packaging and then heating (autoclave)

• Batch system: autoclave

• Solid products (cans/bottles): autoclave

• Steam injection

• High pressure at sterilization temperature

• Cooling by spraying water

• Residence time

Process for first packaging and then heating (continuous system)

Process for first heating and then packaging

• Steam injection (direct heating)

• continuous system + aseptic packaging

• Semi solid foods

Biot number

• A number to help you find out if resistance is inside limiting or outisde limiting

• Bi>10 → resistance inside limiting

• Bi<0.1 → resistance outside limiting

Resistance to heating of a solid product

• For a solid product, outside the product there is convection. 

  • Therefore, convection is limiting to the heating process

• Inside the product there is conduction

  • Therefore conduction is limiting

How to find if inside or outside is resistance limiting

• If inside is limiting, you want to make the can smaller

• If outside the can is limiting, you want to make the can larger 

• Bi > 10 → resistance inside is limiting

• Bi < 0.1 → resistance outside is limiting

Meaning of high and low Bi

• Bi large → h_ext high and/or gamma low and/or d_c large → high delta T

• Bi small → h_ext low and/or gamma high and/or d_c small → low delta T

What are the three “archetypal’’ forms of solid products?

• Infinitive cylinder (don’t count top and bottom because its infinitive)

• Infinitive slab

• sphere

Dimensionless temperature Y

Y is always between 0 and 1

What is dc in bi and fo

• One sided heating:

  • Dc is the whole thickness

• Two sided heating:

  • Dc is the half thickness

• All sided heating (sphere or cylinder)

  • Dc is the radius

How to find dimensionless temperature with two different shapes

• Find them for each shape and multiply the dimensionless temperatures.