Airflow direction in rack-mount cases

It's called a "hand warmer."

Standards are for sheep. If a rack is internally plenum-fed with cool (and sometimes super-temperature-controlled) air, it makes sense to intake from inside the rack and dump the warm air into the room. We do a bunch of electro-optical stuff that way, with the most temp-critical stuff in the back of the box. The fan's in front, so its own heat heads straight out.

Left-right will just spin hot air inside a rack. And probably restrict flow badly. Ditto bottom-top.

Small benchtop instruments usually blow out the back, so as not to annoy users with the air and noise. Whatever works best.

John

John Larkin wrote in news: snipped-for-privacy@4ax.com:

The standards exist so that designers and users can have a better chance of getting their equipment to play well with others. I've seen a lot of rack mount equipment, and the back to front airflow is quite rare. Brocade are doing something that is different.

Bottom to top may be installed with 1 or 2U high angled panels between the equipment to make the airflow in at the front/bottom and out and the top/back. Bottom to top is quite commonly used with vertical "line cards" in 4-6 U chassis.

The north American CO style with open backs will have farms of racks organised so that "cold aisles" and "hot aisles" alternate, with the equipment drawing air from the cold aisle, and blowing hot air into a hot aisle.

The ETSI ones are designed to have closed backs (which allows two racks to be placed back to back or against a wall). Typically, air will be exit out the top, and enter at the bottom front of the rack.

Thanks for your input.

Regards, Allan

On Jun 24, 12:02=EF=BF=BDpm, Allan Herriman wro= te:

mentioned

there be

I've seen many systems that blow from bottom to top - especially for racks that sit on a raised "computer-type" floor where the space underneath the floor acts as an air-conditioned plenum.

I've also seen (and unfortunately, had to work on) systems where the boards would overheat and shutdown when placed on extender cards. The use of extender cards made it possible to access the electronics, but simultaneously removed the board from the forced air - causing the regulators to shut down. I can still recall the occasional burn of the inside of my forearms when not paying really close attention to where the heat sinks were..... Ouch.!!

(Don't ever design something this way if you can help it.) -mpm

Actually, now that I think about it, the Larcan 40KW solid state VHF TV Transmitter blows from the top down. (But really, it's just blowing on the finals, which happen to be in the upper part of the cabinet. I don't think its intended to cool anything else in the cabinet - so that might not be a fair comparison??.)

Thinking a bit, bottom to top is helped by convection flows. Front panel versus back panel requires consideration of who and why some is at the front panel or back panel. The front panel is usually for meter readers and such who are very intolerant of warm air in their faces, back panels are usually full of connectors and such and represent a restricted flow. Sideways flows in racks are useful only in carefully managed situations. In all cases consider various users and maintainers and total heat transport.

The idea is to create a positive pressure inside the rack module being designed. One draws air in directly through the fans, and creates said positive pressure. The fans should never be driving the exiting air, they should always drive the incoming air.

So front of rack module inlet and rear out, the fans go in the front panel. For rear of rack module inlet, and front panel outlet, the fans go in the rear of the module.

This is particularly the case if you are filtering the incoming air.

Not if there are other modules positioned directly above and below the module in question.

One rarely sees rack modules designed with floor and ceiling ventilation, and the benefit is not as great as you might like to think.

Cool air intake does the most good.

Liquid systems are moving into the fray.

The big advantage of negative pressure is that it lets you locate and size various inlet holes to blow just the right amount of air on specific hot spots. You can't get that sort of tunability with a fan blowing in.

Plus, the heat from the fan motor heads right out, not in.

John

The problem with negative pressure systems is dust. Every little crack gets filled with it, and that proves that it does indeed get in, so a filtered system would lose cleanliness in a negative pressure system that doesn't sport a fully welded chassis and gasket sealed top and bottom lids. Which are nice anyway. Fan temps are negligible in systems where fans are needed. Otherwise straight convection would work. Since fans work the heat they add to systems is negligible in either flow case or they wouldn't be in business.

"Allan Herriman"

In the world of professional audio power amplifiers (where dissipations can exceed 1kW per amp), the front panels are mostly used for user controls and displays which leaves little room for any fans - so they wind up at fitted back along with the in /out and AC supply connectors.

The result is that " back to front cooling" is the pretty much the norm - though examples of the reverse exist and also back to dual side outlet cooling too.

The worst idea out is " bottom to top cooling" - cos although this the way air likes to flow and seems intuitively correct what you inevitably get with a rack full of such amplifiers is the internal temperatures rising alarming from bottom to top as well - since each amp is supplied with the heated air from the ones below !!!!

..... Phil

Here's a power amp with front intake, rear exhaust, and fans in the middle. Something to offend everybody.

ftp://66.117.156.8/L500_Front.jpg

ftp://66.117.156.8/L500_Side.jpg

John

With the difference being that there are no heat producing elements in the front section of the module.

PVC wiring, And it actually has side exhausts.

The blue heatsink in the front has 32 300-watt power fets on it, clamp mounted to copper heat spreaders. This is an MRI gradient amplifier, about 17 KW peak power output.

More on the back.

John

Well then it has non-standard power requirements that are outside normal rack module applications anyway.

"Phil Allison"

When two such amps are stacked - the air leaves from the top of one and the bottom of the other, providing there is some space above and below them.

But when three such amps are stacked ???

I know two Australian manufacturers who built their amps like this - Linear Transfer and PTM.

..... Phil

I see your point.

And i see your point as well.