Heat exchangers: Types ( plate, coil, torpedo); Heat transfer efficiency; Priming volume; Optimal water flow & pressure; integral & external

1. Types

a) Torpedo

i) Hollow stainless steel tubes

a) Water runs within

b) Blood runs around tubes

b) Plate

i) Corrugated plates

a) Blood runs inside

b) Water runs between housing & steel

c) Coil

i) Corrugated coil

a) Blood runs over coil

b) Water runs within coil

2. Position in circuit

a) After pump but before membrane oxygenator

b) Before pump

i) Such designs must have a low resistance to flow

ii) Eg Terumo Capiox E

3. Integration within the membrane lung

a) Eg Bentley Univox

b) Benefit of reduced priming volume

4. Construction

a) The heat transfer surface may be organised into: spiral thin tubes, tubes, pleated sheets

b) Consideration of design to minimise “hot spots” which may damage blood components

c) Heat exchange may be enhanced by:

i) flowing the blood and water in opposite directions (countercurrent)

ii) flowing the blood and water perpendicularly (crosscurrent) to create secondary currents and minimise boundary layers

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

i) larger heat exchanger (associated with increased priming volume)

ii) Use of fins extending into blood (advantage of reduced number of water channels required with associated decrease number of sites for fluid leakage into blood)

5. _{Performance factor =} ^{Blood outlet temperature — Blood inlet temperature}

^{Water inlet temperature — Blood inlet temperature}

[Also known as Heat Transfer coefficient]

Scale from zero to 1