Am I estimating my airflow needs right?

[Gooserider]

Since87 quote: Originally posted by Gooserider "Water Cooling Flow Rate and Heat Transfer" Zhentar, see the bottom part of this one, it addresses your comment about pump heat. (sorry, you got it wrong...) Yes, all the power going into the pump in a closed loop system exits the pump as heat. (The proportion that exits through the pump casing and through the water will vary depending on the system.) Only in a system where water is pumped from one location to another, is power exiting 'the system' in the form of kinetic energy.

Off the immediate topic, but another way to look at it is from an 'overcoming friction' viewpoint. This has the advantage of applying to the power consumption of HDD's as well. In the ideal 'Newtonian perpetual motion universe' once we got our disk drive spinning, or our coolant circulating, they would continue to do so as long as we didn't do anything to stop them, and without further energy input.

However, we have the problem in this universe of friction, which will make our coolant stop flowing, and our drives stop spinning UNLESS we keep constantly putting energy into them. Friction expresses itself as heat.

Thus the power of the pump, or the HDD is really the amount of energy which must be constantly added to replace the heat losses caused by friction. This means that those power numbers show up as HEAT in the system, which must be dissipated. In a WC system, that heat ends up in the radiator...

Gooserider

[airspirit]

Okay, I'm going to break away from any science here: when planning for cooling, NEVER go for exact amounts. If you think you need 79 or so CFM, plan on using around 200 or so. Really: you can never have enough. You should be able to get one large blower or something that will provide that airflow in near silence if you look and tune hard enough.

NEVER try to skimp on airflow. There is more producing heat in your system than what you are measuring ... think bridges, mosfets, ram, IC's, Fred the Cockroach, etc ....

[CoolROD]

I updated the spreadsheet today to convert temps to farenheit and to post the temperature rise (dT)

edit: Whew...I haven't been able to post since the update last night. I was playing with my account when the update started and lost all privelidges to my account. I couldn't even open the members page. I tried resetting my password, reactivating my account, and emailing the staff several times. I even sent a screen shot of the flags I was getting. That really sucked b/c I was in the middle of discussions....and had to watch other people answer questions directed to me...

I thought I had been banned from here and didn't even know that I was "on the bubble".

edit: I also added a disclaimer about actual fan CFM.

[Gooserider]

airspirit: Okay, I'm going to break away from any science here: when planning for cooling, NEVER go for exact amounts. If you think you need 79 or so CFM, plan on using around 200 or so.

I'm doing just that, according to my estimates, I 'need' 72.8 CFM. Keep in mind that all the numbers that went into my heat budget were either MFG specs (pump label) or intentional overestimates (100W/CPU, 2X number of drives, etc) and then included an extra 100W as 'fudge factor'. If I was to be more 'realistic' on my heat budget, it would have been on the order of 160 Watts instead of 414

Then I used the '50% of rated CFM through a rad' figure that BillA and Joe mentioned in their articles. Since I'm going to be using two 120mm fans in parallel, I started looking for high Static Pressure (38mm) fans in the 80-100+ CFM (each) range. Currently, I'm looking at Sanyo-Denki 102.3 CFM fans for $5.00 each @ Electronic Goldmine.

In addition, while I want my case to have SLIGHT positive pressure to keep the dust bunnies out, I'm also putting two additional lower volume 120mm X 25mm fans on the case exhausts. For those I'm looking at a couple of Delta WFB1212M fans, (2.95 ea @ Electronic Goldmine) good for a nominal 72CFM each. Since I'm planning to pretty much seal the case (except for whatever leaks through drives and whatnot) the exhaust fans will be in series with the top intake fan, which in effect makes the two pressures add together.

I would hope I never need to run it this way, since I'm sure the noise would be tremendous, but I'm guessing that if I cranked all the fans to max, I'd be doing serious negative case pressure, and sucking about 100CFM+ through the case, and another 50 through the bottom fan / duct combo.

Really: you can never have enough. You should be able to get one large blower or something that will provide that airflow in near silence if you look and tune hard enough.

My plan is to get overkill grade fans, then seriously undervolt them w/ temp sensitive controls. I've had some trouble finding a good temp sensitive controller that I liked. My current plan is to make a hybrid unit of a DigiDoc front end feeding the fans through an NTC thermistor controlled FET power ckt.

NEVER try to skimp on airflow. There is more producing heat in your system than what you are measuring ... think bridges, mosfets, ram, IC's,

Agreed, part of why I did such an over estimate.

Fred the Cockroach, etc ....

'Fred's not here Man...' He prefers Windows systems... (more bugs!)

CoolROD: I updated the spreadsheet today to convert temps to farenheit and to post the temperature rise (dT)

I think I may have missed the link on this one, where is it, and what's the title? - I have a Linux box, but StarOffice opens .xls files pretty well. (albeit slowly - one of our target metrics will be if the girlfriend and I can both open and run SO at the same time)

Gooserider

[CoolROD]

Gooserider:

I think I may have missed the link on this one, where is it, and what's the title?

Earlier in this thread, but here it is again:

HTML

Spreadsheet

[Gooserider]

Found it, looks good overall, but I think I may have spotted a brain fart in the HTML version (haven't looked at the .xls yet), unless I'm missing something...

Part way down the sheet says:

Air Temperature Rise (dT) = 4.8119 Deg. C = 40.6614 Deg. F

:shrug:
Now I may not follow what you are trying to say there, but if you are saying the 2 temps are equivalent, I disagree...

I suspect that you did the conversion as if it was 4.8*C in actual Thermometer temperature (*F=(9/5 x *C)+32). If I'm reading things right, it should have been 4.8*C of DIFFERENCE, which would have been just 9/5 x *C... (did you by any chance just tell the spreadsheet to do a "*C > *F conversion?)

I know my usual 'Quick & Dirty in Head Conversion Formula' is that *F ~ 2 x *C (works for small values of C) which gives me 9*F, Which is a BIG difference...

Now according to your spreadsheet, you were estimating a 350W load. That's probably fairly close to what I'll actually be trying to cool, 414W calculation not withstanding. I said 2-3*C delta, you came up with 5*C, which isn't unreasonable, but that's only about a 10*F increase, not the 20*F increase you were talking about at the start of this thread. Is it possible that you made a similiar sort of error in your earlier math?

Gooserider

(edited for grammar fixes)

[CoolROD]

Damn -thanks for the response.

I had inadvertantly uploaded v.1.0 of the spreadsheet last night. It looks entirely different and didn't even match the html version...Fixed that.

I used F=(1.8*C)+32 (From the inside cover of my thermo book) -->is that not right?

I had your numbers (414 Watts, 72.4 CFM) plugged in and it gave 10.2 Deg C...

[Gooserider]

CoolROD: Damn -thanks for the response.

No problem, glad to be of help...

I had inadvertantly uploaded v.1.0 of the spreadsheet last night. It looks entirely different and didn't even match the html version...Fixed that.

OK, I guess I'll need to get the newer version then.... Let me know when it's up. (BTW, I find it useful whenever doing work that gets revised to use version #'s, both in the work itself, and in the filename).

I used F=(1.8*C)+32 (From the inside cover of my thermo book) -->is that not right?

NOPE! It is a subtle difference, but there are two different conversions... The first (and what most people think of) is 'Thermometer temperature' as in comparing the reading on a *F thermometer to a *C thermometer - 'Body temperature is ~37*C = (9/5 x 37)+32 = ~98.6*F'

That's the conversion you used from the thermo book. It compares thermometer readings....

But what you wanted to do was compare temperature CHANGE - "A is 5*C warmer than B". You don't know (or care) what A or B's actual temps are, you are only interested in the DIFFERENCE between them. The conversion is like that between inches and centimeters, just multiply *C x 1.8 = *F

The 1.8 converts the 'size' of a *C to a *F. The '+32 part comes in only as an 'offset' to allow for the fact that each scale has a different starting point.... Sort of like the difference between *C and *K - the size of a degree is the same, but the starting points are different so you need an offset to do the conversion.

I had your numbers (414 Watts, 72.4 CFM) plugged in and it gave 10.2 Deg C...

I could be wrong, but I think you got bit by the conversion bug... Check it out and let me know. Also the HTML, at least the version I saw, used 350W, not 414)

Incedentally, I'm still not sure of the source location, but I found my hard copy notes on the radiator heat loss numbers. It was a 'Bill A' test, dumping 240W into a 'Big Momma' radiator. His test results showed:

Change.........Items measured
3.8*F coolant in / coolant out.
2.0*F coolant out / exhaust air
11.8*F air in / coolant out.

Combining those numbers, I got 9.8*F for Air in / air out. Granted it was only 240W, but I'm going to be pushing twice as much air (I don't have the exact airflow #'s, not sure they existed) just by virtue of using two higher flow fans vs. one. My rad is also almost twice as big as the 'Big Momma' and FAR less restrictive.

Gooserider

[CoolROD]

Thanks for the response. Work has had me down in Atlanta, Ga. for 5 days....

The dT conversion for Farenheit was wrong. It used absolute vs. relative values...

I simply subtracted the Input Farenheit temp from the Farenheit result to get the dT_F to work. It won't work for the celsius people, but they won't want this conversion anyway.

sooo -->
v1.2 added a dT conversion and neatened up the format.
v1.3 sortof fixed the v1.2 conversion bug...

edit: oops -my server is having trouble and my disk quota is full. I can't upload a working update -should be up within a day...this is probably why the new versions didn't upload correctly last week before I left...

edit: Upgraded Disk Quota and uploaded files...also put in Gooserider's numbers from above...in v1.3

[Gooserider]

CoolROD Thanks for the response. Work has had me down in Atlanta, Ga. for 5 days....

Glad to see you back - I was afraid you'd gotten mad at me or something...

Glad you found the error and a fix for it. Let me know when the server issues are debugged and you have the updated version up...

BTW, I made a real crude copy of the data from BillA's corrected heater core charts (HERE, look @ chart #12) and according to his dissipation data, it looks like the 'Big Momma' core, can dump 1500 BTU / 500W at any reasonably good flow rate if I can get 0.15" pressure out of my fans.

According to the P-Q charts of both fans I'm looking at for my intakes, 12V has a max SP over .25", so that's not a problem at all, and shouldn't be until I get way undervolted. (BTW, @ .25", the BM chart shows >650W dissipation at flows >1gpm.

Considering that my rad is bigger, thicker, and will have two fans instead of one, I don't think dissipation will be a problem.

Gooserider