FOOD PROCESSING AND PACKAGING PART 2

HEAT EXCHANGERS

INTRODUCTION

• In process industries, the transfer of heat between two fluids is generally accomplished using heat exchangers.

• Example of a heat exchanger: Hot and cold fluids are separated by a tube wall or tube surface. The process involves several steps:

  • Convection: Heat transfer from the hot fluid to the wall/tube surface.

  • Conduction: Heat transfer through the tube or plate material.

  • Convection: Heat transfer from the tube wall to the cold fluid.

TYPES OF HEAT EXCHANGERS

• DOUBLE PIPE HEAT EXCHANGER

  • Thin tubes circulate hot fluid and cold fluid:

  • Hot fluid enters from one side, while the cold fluid enters from the other.

  • Temperature at the outlet of cold fluid (Tc.out) and at the inlet of hot fluid (Th,in) are crucial parameters.

• CROSS FLOW HEAT EXCHANGER

  • Unmixed Context: Involves configurations where both fluids are unmixed or one is mixed while the other remains unmixed.

  • Different configurations can influence the efficiency of heat transfer between fluids.

• SHELL AND TUBE HEAT EXCHANGER

  • Consists of tubes (tube-side fluid) and a shell (shell-side fluid) where fluid can flow through the pipes without direct mixing.

FLUID FLOW CONSIDERATIONS

• Heat Transfer Assumptions:

  • It is assumed that fluid temperatures remain constant during the heat transfer process. However, in practical applications, the temperatures of both fluids typically change.

  • Heat transfer calculated across the barrier assumes two distinct heat reservoirs separated by a barrier (the tube wall).

FLUIDS FLOWING IN PARALLEL (CASE 1)

• Characteristics of fluid behavior:

  • The temperature difference between the condensing and cooling fluids is critical.

  • Length of the evaporator must accommodate the heat transfer process effectively.

FLUIDS FLOWING IN PARALLEL (CASE 2)

• Involves flowing both liquid and gas:

  • Both temperatures of fluids are varying and can be expressed through a temperature difference (s, t).

COUNTER FLOW ARRANGEMENT

• Description: Fluids flow in opposite directions, enhancing heat transfer efficiency.

• Benefits:

  • Greater mean temperature difference between streams, requiring less surface area for the same heat transfer rate.

LOGARITHMIC MEAN TEMPERATURE DIFFERENCE (LMTD)

• Heat exchange effectiveness is often represented by:

  • $rac{Q}{U} = rac{ ext{LMTD}}{ ext{Area}}$

  • An expression derived from energy balances which can apply across various conditions in heat exchangers.

• For counter flow, it's represented as:

  • $= rac{(T<em>{hot, in} - T</em>{cold, out}) - (T<em>{hot, out} - T</em>{cold, in})}{ ext{ln} rac{T<em>{hot, in} - T</em>{cold, out}}{T<em>{hot, out} - T</em>{cold, in}}}$

WORKED EXAMPLES

• Example 1:

  • Exhaust gases cooled from 450°C to 150°C; analyzed using values of specific heat.

  • Specific heat of gases: $1.13 kJ/kg.K$; Overall heat transfer coefficient: $140 W/K$. Tasks include calculating:

  • Heat transferred per hour,

  • Surface area needed for both parallel and counter flow setups.

• Example 2:

  • Counter-flow heat exchanger cooling 1400 kg/hr of oil from 100°C to 30°C while water initially at 20°C is involved.

  • Water flow rate: 1300 kg/hr; focus on calculating:

  • Heat transferred from the oil,

  • Water outlet temperature,

  • Heat transfer area,

  • Advantages of parallel vs. counter flow configurations.

ADDITIONAL RESOURCES

• Suggested videos for further studies include:

  • Types of Heat Exchangers

  • Classification of Heat Exchangers

  • Applications of Heat Exchangers

  • Links provided for direct access.

FINAL REMARKS

• Keep in mind critical elements such as the efficiency of heat transfer due to varying temperatures and fluid properties.

• Understanding the configurations of heat exchangers enables better design and implementation in industrial applications.