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HBD Air Cooling System

TK Hemmick for the HBD group 8/25/06

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Why is Cooling Needed?

The noise debugging of the HBD showed that RF shield covers must be applied above the pre-amp cards.

These covers give excellent noise performance but trap heat.

The pre-amps & LDO regulators reach 50-60C.

These temperatures are not a fire hazard, but they would limit the lifetime of the pre-amps.

So…we designed a way to cool them.

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How can cooling be accomplished?

The covers: Block normal convention (the problem). Form a channel for air (the solution).

There are 6 covers around the azimuth and 6 gaps between the covers.

The gaps have sufficient room for a 3/8” tube from which to supply air. How much air? How to deliver air?

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Overview of Information

System requirements: Power load handling.

Amount of air required (theory) Amount of air required (measurement)

Air handling system. Pressure & Flow (theory) Pressure & flow (measurement)

System parts (Prototype & Proposal) Exploded view of parts. Materials list. MSDS links.

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Power load.

One preamp card requires: +6V at 1.8 A -6V at 1.2 A 18 Watts

LDO regulators on board can handle higher temp, preamps cannot. Pre-amps = +5V at 1.8 A & -5V at 1.2 A 15 Watts.

Air flow should remove 15 Watts from every board.

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Air requirements (theory)

One board… 15 Watts = 15 Joules/sec. Air can be approximated

well as an ideal diatomic gas (CP=7/2 R)

15 Joules in one second. Allow 20 degree rise. Requires 0.64 liters in one

second. Requires ~40 l/min~80 cfh

l

litermolesmoles

V

molesn

n

TnCQ P

64.0040.0

026.0

026.0

20315.82

715

NOTE: Upper limit (ignores natural convective cooling)

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Air requirements (measurement)

One circuit card was outfitted with a small tube to spread air flow over card.

Tube = 3/8” diameter, 2.5” water, 5 small holes (uniform flow w/ reasonable pressure & small holes)

Pre-amp temp as a function of flow is plotted. 50 cfh is good target for cooling each board. 6 board per side = 300 cfh = 5 cfm per side.

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Pressure and Flow (theory)

P in final tube = 2.5” water Ptot <0.2” water to assure uniform

air delivery. F = 5 cfm, L=15” (conservative) =1.8x10-5 Pascal x sec r = 0.26 inches (d=0.52 inches).

Flow is not purely laminar… Set ID of tubing to ¾”

F 5/6F 4/6F 3/6F 2/6F 1/6F

25.0

4

tot

6

)8(15

8

6

15P

P

LFr

r

LR

FRP

RFP

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http://www.google.com/search?hl=en&lr=&client=firefox-a&rls=org.mozilla%3Aen-US%3Aofficial&q=2*%28%2815*5+cubic+feet+per+minute+*+8+*+1.8E-5+pascal+seconds+*+15+inches%29%2F%286*pi*0.4+torr%29%29%5E0.25+in+inches&btnG=Search

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Pressure & Flow (measurement-1)

Assemble prototype manifold.

All tubing dimensions correct.

Measure air flow parameters.

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Pressure & Flow (measurement-2)

Measured parameters: 5 cfm flow. Pressure in tubes = 2.5”

water. 0.1” water difference

from first to last tube. 6” water is pressure at

supply. Design parameters met

exactly. 100% success.

Air flow uniform in all holes.

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Exploded (Prototype) Part List

Tubing ¾” ID Poly-flow tubing ½” ID Latex (surgical) tubing

Nylon plumbing TEE, coupling; hose barb; elbow; cap.

PTEG Plastic: 3/8” nominal tubing.

Link pointing to MSDS Files:

http://skipper.physics.sunysb.edu/HBD/MSDS/

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Example Commercial Blower…

The above small (2” x 2”) commercial blower meets all the specifications for our needs. http://www.acal-radiatron.com/download/micronel/u51dl-tec.pdf

This will be evaluated and compared to other similar units.

Blower selection will be discussed separately.

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Proposed System

Manifold: PVC pipe, ¾” ID, running underneath

HBD cable tray. Nylon NPT3/8”Hose Barb at 6 locations.

Jumper (manifold individual tube) 3/8” latex surgical tubing.

Flow tube: 3/8” ID mylar tube (0.010” thick wall) Nylon cap.