Airflow Question

[JohnnyChrist]

I've resurrected an old Lian Li PC 6070 for a modding project. So far, in terms of airflow, I've cut a whole in the top for my CoolIT ECO ALC, added a second 80mm exhause in the back, and changed the two 80mm fans in front into a 120mm. The intake is about 70cfm (Box rating), each of the 80mm is about 25cfm (50 in total) and the ALC's stock fan is unknown, but I have added a 40cfm fan pushing into the radiator. So in total, its 70cfm intake, and anywhere from 50 to 90 cfm exhaust.

Heres a quick diagram:

(I got rid of the bottom HDD rack, so that won't obstruct airflow at all)

My question is, first of all, if this would work well. Second, what would be my approximate airflow of the ALC with the push-pull? Third, should I add a vent on the side (most likely near the GPU) to counter any negative or positive pressure that would build up? - Right now, the only form of intake is the 70cfm front fan.

 

[jsc]

Looks pretty good.

The basic rule for fans is:
front and side - intake, and
back and top - exhaust.

 

[cloudNINE]

Actually,

Ideally you want a positive air flow. Meaning, more air being pushed into your case than is being exhausted. But not too much. A little in-flow pressure keeps air cooled systems much cooler then a negative pressure set-up.

With a positive air flow, your graphic cards will operate cooler since their fan(s) will not need to work as hard.

By the looks of your diagram, you have a massive negative air-flow, which is counter productive for air cooling.

A good positive system might have 2x 120mm fans bringing air into the case and 1x 120-140mm blowing out - as an example.

 

[JohnnyChrist]

Actually,

Ideally you want a positive air flow. Meaning, more air being pushed into your case than is being exhausted. But not too much. A little in-flow pressure keeps air cooled systems much cooler then a negative pressure set-up.

With a positive air flow, your graphic cards will operate cooler since their fan(s) will not need to work as hard.

By the looks of your diagram, you have a massive negative air-flow, which is counter productive for air cooling.

A good positive system might have 2x 120mm fans bringing air into the case and 1x 120-140mm blowing out - as an example.

CPU is being liquid cooled.

As I said, I can only approximate the amount of exhaust from the cpu cooler's fans because of the radiator and the unknown airflow of the stock-fan, but i'm guessing it will be somewhere between 20-30. Creating very balanced or slightly negative pressures. Thanks for the comment though, on my next rig I will know.

 

[Paperdoc]

Looks OK to me as a decent starting point. I'll agree with CloudNINE on one thing - I prefer to have a slightly positive pressure inside (more intake capacity than exhaust), but ONLY because that means there is leakage air blowing OUT of the cracks in the case, and all the INflow is through dust filters. You ARE planning on a dust filter on that front intake fan, right? Then I will disagree with CloudNINE - you do not appear to have a massive imbalance.

However, all those calculations have a HUGE error margin. Fan Capacities always are done with unrestricted air flow. In practice the flow is always much less than the specs. A foam dust filter on the intake reduces it a lot. A CPU cooler heat exchanger the air must blow though is a big restriction. Even just the air flow path through the case, which is NOT a straight line and must swirl around things, is a flow restrictor. Bottom line is you CANNOT get the right answer by calculation - only a decent estimate to start with.

So, what to do? Build it and test it, adjust if necessary. Some time ago a post here suggested use a lit smoldering incense stick as a smoke generator. With the comp running normally, move the smoke source slowly around the case, especially near small openings and cracks (e.g the front of the optical drive or floppy drive, the PCI slot covers at the back, the open cover for the front-panel audio and USB ports). Which way does the smoke travel, and how fast? I prefer to have a gentle flow of air (smoke) OUT from the case at those leakage points. If you don't get what you want, figure out a change from there.

 

[JohnnyChrist]

Thanks for the input.

I've gotten the case together, and it flows great, I'm running 15C cooler under load! (Something must have been wrong with my Cosmos, LOL)

Anyway, I've got some grills, but they're very restrictive (It felt as though less than half of the air was getting in, so I took them off. Does anyone know of any good fan grill that doesn't restrict airflow much? Standard Chrome ones don't count.

EDIT: Just did a test, without screws, if I place the grill on the intake fan... it stays... because of the suction created.