Fan Characteristics

Axial, Centrifugal Fans:
- axial: lower backpressure, lower noise, straight air flow
- centrifugal: higher backpressure, higher noise, 90-degree turn in flow (can be advantages for flow routing)

Flow Rate vs. Pressure:
- ‘stall’: no flow, highest pressure;
- ‘free-flow’: highest flow rate, no backpressure
- need to know backpressure in installed system (ducting, air inlet / outlet screens, heat exchanger / fins / turbulence)
- ducts: too small / too many turns = high backpressure
- filters: too small = too high backpressure; too small: particle ingestation / ejection (safety)

Fan Power:
- electric power of fan added to heat to be transported; ‘push’ vs. ‘pull’ air

Speed Control:
- may help reduce noise / power draw during low heat

Environmental Compatibility:
- Acoustic Noise (axial lower than centrifugal; function of speed / number of blades);
some noise from fan / blades, other noise from flow; move fan away from lien of sight
- Moisture / High Humidity,
- Temperature,
- Vibration

Fan Selection:

· Q = mass flow rate * heat capacity * (outlet – inlet temperature) [ watt ]

· Be conservative (use highest heat, highest inlet temperature, lowest allowable outlet temperature, highest backpressure)

1. How much heat has to be rejected (assume at minimum: heat = dissipated electric energy) ?

2. What is the temperature environment (max. inlet temperature, min outlet temperature) ?

3. What is the heat transport media (air / moist air / water / water/alcohol, what pressure / reduced cabin pressure, high altitude)

4. => calculate what flow is necessary to transport heat

5. What is the backpressure as this flow rate (inlet / outlet screens, filters, ducting, obstructions,…)
- may require wind tunnel testing, simulation, prototyping
- biggest source for error / typically final design has too low flow to perform as desired.

6. =>choose available fan satisfying your requirements:
- use closest fan that has higher flow at higher backpressure
- consider other requirements: noise, environment, features (speed indication / control, stall protection, material, ….)

Fan References and Design Guides:

EBM-Papst: http://www.ebm.com/home.asp
http://www.ebm.com/pdfs/ebmselect.pdf wide selection / axial and centrifugal, application notes

Micronel: http://www.micronel.com/: miniature fans

Comair Rotron: http://www.comairrotron.com/
http://www.comairrotron.com/notes.html great technical reference section, environmentally shielded fans (Enviro-Shield Silicone)

Air Heat Exchangers

·        Finned Heat Exchangers

·        Pin-Fin

·        'Heat-Pipe' Based Finned Heat Exchangers

Extrusion Folded fins

Bonded Fin (higher density than extruded)

Pin Fin

Some novel integrated designs (5 mm /0.2” tall) (10 Watt Peltier cooler)
http://www.novelconceptsinc.com/indexsink.htm

Water Heat Exchanger

Heat Sink - Basics (from Marlow Industries):

Design or selection of the heat sink is crucial to the overall thermoelectric system operation and cooler selection. All thermoelectric coolers require heat sinks and will be destroyed if operated without one. The system temperature difference is typically quite different from the cooler temperature gradient. A typical design parameter might be to limit the heat sink temperature rise above ambient to 10 - 20C. The heat sink temperature leads to the cooler hot side temperature, which affects the cold side temperature that can be achieved with a TEC. Heat sink resistance is the measure of the ability of the sink to dissipate the applied heat and is given by:

HSR = (T₁ -T₂)/Q

where:

HSR = Thermal resistance in C/w

T₁ = Heat sink temperature in C

T₂ = Ambient or coolant input temperature in C

Q = Heat load into heat sink in watts (includes absorbed + TEC power)

The goal in the heat sink design is to minimize thermal resistance. This is achieved through exposed surface area and may require forced air or liquid.

The following schematic shows how the heat sink resistance can be determined. Ambient temperature is 27 C, the desired rise across the heat sink is 10C, or heat sink temperature at 37C. The load that must be dissipated is 10w. This gives a resistance of 10C / 10w or 1C / w.

The three basic types of heat sinks are: natural convective, forced convective and liquid cooled, with liquid cooled being the most effective. Typical values of HSR for natural convective range from .5C/w to 5C/w, forced convective from 0.02C/w to 0.5C/w and liquid cooled from 0.005C/w to 0.15C/w. In general, most applications involving thermoelectric cooling require forced convective or liquid cooled heat sinks.

Heat Exchanger References:

Aavid: http://www.aavid.com/

R-Theta: http://www.rtheta.com/indexf.html

Melcor: http://www.melcor.com/

Assembly with Thermoelectric Coolers: