1 HBD Air Cooling System TK Hemmick for the HBD group 8/25/06

2 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 C.  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.

3 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?

4 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.

5 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.

6 Air requirements (theory)  One board…  15 Watts = 15 Joules/sec.  Air can be approximated well as an ideal diatomic gas (C P =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 NOTE: Upper limit (ignores natural convective cooling)

7 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.

8 Pressure and Flow (theory)  P in final tube = 2.5” water P tot <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 5/6 F 4/6 F 3/6 F 2/6 F 1/6 F

9 Pressure & Flow (measurement-1)  Assemble prototype manifold.  All tubing dimensions correct.  Measure air flow parameters.

10 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.

11 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:

12 Example Commercial Blower…  The above small (2” x 2”) commercial blower meets all the specifications for our needs.  This will be evaluated and compared to other similar units.  Blower selection will be discussed separately.

13 Proposed System  Manifold: PVC pipe, ¾” ID, running underneath HBD cable tray. Nylon NPT  3/8”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.