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Lecture 11: Heat Transport

Thermal Gradients:

· heat transport by mass flow / convection:
air vs. liquid vs. phase change

· heat transport by conduction:

System Design

Example: maximum allowable gradient across the temperature controlled experiment chamber of DT=1°C, the minimum coolant flow rate for water as the heat transport media can be estimated as:

Forced Liquid Circulation System

Thermal Transients:

heating / cooling:

· thermal mass, heat capacity

· conduction / convection throughout controlled environment

simplified: Q = dU/dt = m * Cp * dT/dt

example: how long to cool 1 can of water (333 ml = 0.333 kg) with a 10 Watt heat pump from 25°C to 4°C?
dt = [ 0.333 kg * 4183 J/kg/K * (25-4)K ] / 10 Watt = 2925 J/Watt = 2925 seconds = 48 minutes
(note: assumes infinite thermal conductivity within water, actual time longer)

Water-Cooled Heat Exchanger

· Water  - Water Heat Exchanger: parallel, counter, cross flow

· Water - Air Heat Exchanger (radiator)

· Water-Cooled Cold Plate

Water-Water                            Radiator                                  

Cold Plates

`

Cold Plate Detail (from: http://www.aavidthermalloy.com/products/liquid/contact.shtml)

Design Considerations:

· Pressure drop at design flow rate

· Thermal resistance

· Materials compatibility / corrosion / deposits / biocide (silver, alcohol, pH)
(deionized water – difficult !)

· Accumulator / reservoir / thermal expansion:
losses (evaporation, diffusion, leaks), pressure fluctuations / dynamics, cavitation, thermal expansion (operational, storage)

· Freezing / Boiling of water / over- / under-temperature of samples (science safety):

o active (sensor, computer, switch)

o passive (bimetallic, precision 1-5°C, typical 10°C, tolerance, hysteresis / differential; life cycles 5,000 to 100,000)

· Blocked flow (deposits, air bubbles / vapor lock)

Water Heat Exchanger References:

Aavid: http://www.aavid.com/
R-Theta: http://www.r-theta.com/indexf.html (FabFin, Aquafin)
Design Guide: http://www.r-theta.com/products/aquasink/aquasink.pdf (also on ASEN5519 web page - PDF)
Lytron: http://www.lytron.com/

Safety Features:

Bi-metallic Safety Switches:

The principle behind a bimetallic strip thermometer relies on the fact that different metals expand at different rates as they warm up. By bonding two different metals together, you can make a simple electric controller that can withstand fairly high temperatures. This sort of controller is often found in ovens. Here is the general layout:

Two metals make up the bimetallic strip (hence the name). In this diagram, the bottom metal would be chosen to expand faster than the blue metal if the device were being used in an oven. In a refrigerator, you would use the opposite setup, so that as the temperature rises, the upper metal expands faster than the green metal. This causes the strip to bend upward, making contact so that current can flow. By adjusting the size of the gap between the strip and the contact, you control the temperature.

Spaceflight: precision thermostats, such as http://www.ti.com/snc/products/controls/ptherm.htm (Texas Instruments Klixon)

Heat Pipes:

Passive heat pump, phase change to increase heat transport.

· Thermacore:  http://www.thermacore.com/

· Swales and Associated: http://www.swales.com/products/index.html

· http://www.cheresources.com/htpipes.shtml Also at ASEN5519 website.

· http://www.benchtest.com/heat_pipe1.html

The Heat Pipe - How it works (from Thermacore)

MEDIUM	MELTING PT (° C )	BOILING PT. AT ATM. PRESSURE (° C)	USEFUL RANGE (° C)
Helium Nitrogen Ammonia Acetone Methanol Flutec PP2 Ethanol Water Toluene Mercury Sodium Lithium Silver	- 271 - 210 - 78 - 95 - 98 - 50 - 112 0 - 95 - 39 98 179 960	- 261 - 196 - 33 57 64 76 78 100 110 361 892 1340 2212	-271 to -269 -203 to -160 -60 to 100 0 to 120 10 to 130 10 to 160 0 to 130 30 to 200 50 to 200 250 to 650 600 to 1200 1000 to 1800 1800 to 2300

Heat pipes remove heat from the source in a two-phase process. As heat is generated, a liquid at one end of the pipe evaporates and releases the heat to a heat sink by condensation at the other end. The liquid is returned to start the process over through a wick structure on the inside of the heat pipe.

The Heat Pipe - How it works

Thermacore's Heat Pipe technology provides a cost effective solution for laptop cooling applications or for any other electronic cooling where minimal space and low maintenance requirements are involved.

Heat pipes are relatively simple devices. They passively transfer heat from the heat source to a heat sink where the heat is dissipated. The heat pipe itself is a vacuum-tight vessel that is evacuated and partially filled with a minute amount of water or other working fluid. As heat is directed into the device, the fluid is vaporized creating a pressure gradient in the pipe. This forces the vapor to flow along the pipe to the cooler section where it condenses, giving up its latent heat of vaporization. The working fluid is then returned to the evaporator by capillary forces developed in the heat pipe's porous wick structure, or by gravity. (See illustration)

The Thermacore Laptop Heat Pipe Solution

Thermacore has taken heat pipe technology originally used for space applications and applied it to laptop computer cooling. It is an ideal, cost effective solution. Its light weight (generally less than 40 grams), small, compact profile, and its passive operation, allow it to meet the demanding requirements of laptops.

Thermacore's HS-NB series of heat pipes is specifically designed for P-5 notebooks, but can also be adapted for other processors and component cooling. For an 8 watt CPU with an environmental temperature no greater than 40°C it provides a 6.25°C/watt thermal resistance, allowing the processor to run at full speed under any environmental condition by keeping the case temperature at 90°C or less.

One end of the heat pipe is attached to the processor with a thin, clip-on mounting plate. The other is attached to the heat sink, in this case, a specially designed keyboard RF shield. This approach uses existing parts to minimize weight and complexity. The heat pipe could also be attached to other physical components suitable as a heat sink to dissipate heat. (See photo of inside of laptop computer)

Thermacore's heat pipes provide a small profile without any interference to existing components and can be easily adapted to an existing package design. Because there are no moving parts, there is no maintenance and nothing to break. Some are concerned about the possibility of the fluid leaking from the heat pipe into the electronics. The amount of fluid in a heat pipe of this diameter is less than 1cc. In a properly designed heat pipe, the water is totally contained within the capillary wick structure and is at less than 1 atmosphere of pressure. If the integrity of the heat pipe vessel were ever compromised, air would leak into the heat pipe instead of the water leaking out. Then the fluid would slowly vaporize as it reaches its atmospheric boiling point. A heat pipe's MTTF is estimated to be over 100,000 hours of use.

The Cost Effectiveness of Heat Pipes

The flexibility of the Thermacore heat pipe solution provides an effective method for cooling processors in laptops. The cost of heat pipes designed for laptop use is very competitive compared to other alternatives. Cost is partially offset and justified by improved system reliability and the increased life of cooler running electronics. Heat pipes, in quantity, cost a few dollars each while an entire cooling system will cost between $5 - $10 in production quantities, depending on the final design. Standard design products are available to reduce cost even further.

check here for information:  http://www.thermacore.com/

Swales Company Information: What's the most cost effective way to dissipate heat?

The Swales Heat Pipe

Our heat pipes improve performance and extend equipment life, while they dissipate heat. The pipe can be made of flexible material or in a variety of shapes and sizes. This allows it to accommodate design constraints, yet still efficiently transport unwanted heat to an available heat sink.

The Swales heat pipe features:

· High thermal conductivity

· Lightweight compact construction

· Temperature uniformity

· No need for external power

· Reliable, no-maintenance design

· Silent, vibration free operation

· Quick thermal response

· Ability to operate in extreme environments

· Variety of shapes and sizes

· Allows heat sink to be located away from heat source

How a Heat Pipe Works

A heat pipe is a passive device that transports heat efficiently from one point to another. It is made of a sealed container with a working fluid and wick, or grooves, inside. As heat is applied to the evaporator section, the working fluid vaporizes and flows to the cool or condenser section of the heat pipe. There the working fluid condenses, releasing the heat of vaporization.
Heat Pipe Applications

The uses of heat pipes are virtually unlimited. Here are a few examples:

· Electronic enclosures

· Motor controllers

· Heat pipe heat exchangers

· Industrial drives

· Manufacturing processes

· Traction drives

· De-icing

· RF modules

· Computers

Swales can design and manufacture heat pipes in the size, shape and power rating you require. Our applications engineers are experienced, knowledgeable and available to assist you in optimizing a heat pipe system that will satisfy your needs.

This info from Swales (http://www.swales.com/products/heatpipe.html)

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