Design and operational factors of the heat exchanger which affect thermal resistances of: a) water;b) water/blood barrier; c) blood

 

Heat exchangers are designed to add or remove heat from the blood

 

Principles of heat transfer

•     Heat transfer represents the transfer of kinetic energy from molecules with a higher energy (higher temperature) to molecules with a lower energy (lower temperature)

•     This transfer represents the transfer of kinetic energy from the higher energy source to the lower

•     Heat transfer is possible in one of three forms: conduction (through solids), convection (from solids to liquids with motion of liquid carrying fluid away from the solid-liquid interface), radiation (electromagnetic mechanism)

•     Within  the CPB heat exchanger, the major forms of heat transfer are forced convection (the water & blood of the heat exchanger are actively pumped past the stainless steel interface; hence, the term forced), and conduction within the stainless steel

 

•     Determination of rate of heat transfer

      Amount of heat transferred to blood = thermal energy in blood leaving oxygenator minus thermal energy in blood entering heat exchanger

      Thermal energy = specific heat of blood C (0.9 kcal•kg-1•°C) times absolute temperature

      Therefore: Heat transfer: H = C • F • (Ti-To)  [kcal/min]

            C = specific heat; F = blood flow rate; Ti & To = inlet & outlet temp

 

•     As blood flows past the heat exchanger surface, a thermal boundary layer is generated in which the temperature varies from the temperature of the wall to that of the free stream yet unaffected by the heat exchanger

 

•     Effect on heat exchange by variables:

      i) Temperature differential between water & blood

            • The greater the differential to more profound the boundary layers

      ii) Flow rate of blood past heat transfer surface

            • The greater the flow the less time for equilibration across boundary levels

      iii) Flow rate of water past heat transfer surface

            • See above

      iv) Conductivity of heat transfer material

            • If conductivity is poor, will be a limiting factor in heat exchange

      v) Surface area of heat transfer material

            • To maximise heat efficiency of heat transfer, the surface for heat exchange is maximised by:

                         i) larger heat exchanger (increased priming volume)

                        ii) Use of fins extending into blood

 

 

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