Answers to those Doggone Thermal Design Questions
By Tony Kordyban
Copyright by Tony Kordyban 2004
Dear Tony,
We have had to bite the bullet sooner than most, and switched to liquid cooling. Well, not exactly. We don’t have liquid touching the electronics. But we were stuck trying to make some indoor-rated electronics work outdoors, and the only way we could make them work in the extreme heat of summer on the rooftop of a building was to chill the inlet air with some cold water pipes. The outside air can get up to 50°C, and the electronics is only able to handle 40°. There is a water chiller nearby for another industrial process, that we are allowed to tap into.
Outside air passes over these cold water pipes and then enters the box containing our video surveillance equipment (oops, wasn’t supposed to say that!) A fan pushes the heated air out through an exhaust vent. It seems to do a good job, at least most of the time.
The problem is that it doesn’t seem to work sometimes. The water chiller maintains a constant water temperature, and the water flow rate is constant, controlled by a pumping system. The air flow rate through the box should be constant, because the fan speed is regulated. But the air temperature drop across the water pipes seems to vary with the phases of the moon. Some days I get a 20° drop, and other days only about 7°. And the hotter the weather, the worse the effect seems to be. What’s going on?
Mr. B, which rhymes with T, which stands for Trouble in River City
Dear Mr. B,
Sounds to me like you are running into the Dew Point.
This is not a commercial for that strange green soft drink that causes anxious young men to perform dangerous stunts. It is a commercial for my favorite liquid — water.
You said that the thermal performance of your air chilling system varies with the phases of the moon. I suspect that it actually varies with changes in the weather, specifically the Relative Humidity (RH).
In past columns I have preached that humidity can be ignored in electronics cooling. Water vapor normally makes up a small percent by weight of air, and does not change the ability of air to carry heat away from hot electronics. So you can safely do your thermal tests at 10%RH and know that your electronics will still have the same thermal performance even if the RH changes to 90%.
I was safe in making that general claim, because in most situations the electronics are hotter than the surrounding air, and so the air only gets hotter as it passes through. There are no more pesky phase changes to worry about until the temperature gets so high that the air starts to ionize and change to a plasma.
But now, Mr. B, you have entered a new world, called Sub-Ambient.
Once you start chilling the air, you can’t ignore humidity anymore. Because if you chill it too much, you run into the wall called the Dew Point.
Dew Point is nothing new to you. You hear about it in the weather report every night. It is one of those weather buzz words, like barometer and Doppler radar, that you have learned to ignore while waiting for the Weather Model to say if it is going to rain or not. They usually give the Dew Point as a temperature, which it is. But they don’t tell you what it means or why you want to know it. Because most of the time, you don’t care.
Dew Point is another way of measuring the Relative Humidity. I know that you remember why it’s called “Relative” Humidity. The hotter air gets, the more water vapor it can hold. At 10°C the air can hold about 8 grams of water vapor per kilogram of air. At 50°C it can hold more than 80 grams, or ten times as much water. Both conditions are 100% Relative Humidity.
How do we get Dew Point out of that? Let’s say right now outside the temperature is 20°C and the humidty is 80%RH. The sun then goes down and the air cools. The amount of water vapor in the air doesn’t change. But as the air cools, the Relative Humidity goes up (because at cooler temperature, it is capable of holding LESS vapor.) By the time the temperature gets down to 16°C, the RH is 100%. That is the Dew Point. It is called Dew Point because that is the temperature at which droplets of dew will begin to form as water vapor condenses out of the air. But even when the temperature is 20°C and RH is 80%, we say the Dew Point is 16°C.
I said that the Dew Point was something like a wall.
If you are trying to cool air below the Dew Point, you will hit a wall. The wall of phase change.
When a material goes through a phase change, such as evaporation or melting, it absorbs a lot of heat without changing temperature. Heat a pot of water on the stove. The temperature goes up steadily until the water starts to boil. At boil it hits a wall — the temperature won’t go up until all the water has changed phase from liquid to vapor.
You won’t be able to chill the air below the Dew Point until you condense enough water out of the air stream to lower its Relative Humidity below 100%. While your chilling system is working to condense water out of the air, it is taking 2300 Joules per gram of water away from your ability to get the air temperature down.
That would explain why your chilled water pipes seem to work worse at high air temperatures. The worst weather seems to combine high temperature and high humidity — and at high temperature and high RH, there is a lot of water in the air. You use up all your cooling power making dew and hardly any cooling the inlet air to your electronics. Reducing humidity might make the enclosure more comfortable for you, but it does nothing to cool your spy cameras.
Have you noticed puddles forming under your chilled water pipes? Those are not leaks.
My recommendation is to change to a closed-loop air flow system. Instead of passing outside air over the cold water pipes and then dumping it back outside your box, keep circulating the same air over the pipes over and over. Seal up the inlet and exhaust vents. The only thing that enters and leaves your box should be the water in the pipes. That way you condense out the humidity once, and after that, all the cooling power of the cold water goes into keeping the air temperature low.
By the way, one fun and convenient way to calculate the Dew Point under various weather conditions, without pulling out the old psychrometric chart, is to use the web calculator at:
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Isn’t Everything He Knows Wrong, Too?
The straight dope on Tony Kordyban
Tony Kordyban has been an engineer in the field of electronics cooling for different telecom and power supply companies (who can keep track when they change names so frequently?) for the last twenty years. Maybe that doesn’t make him an expert in heat transfer theory, but it has certainly gained him a lot of experience in the ways NOT to cool electronics. He does have some book-learnin’, with a BS in Mechanical Engineering from the University of Detroit (motto:Detroit— no place for wimps) and a Masters in Mechanical Engineering from Stanford (motto: shouldn’t Nobels count more than Rose Bowls?)
In those twenty years Tony has come to the conclusion that a lot of the common practices of electronics cooling are full of baloney. He has run into so much nonsense in the field that he has found it easier to just assume “everything you know is wrong” (from the comedy album by Firesign Theatre), and to question everything against the basic principles of heat transfer theory.
Tony has been collecting case studies of the wrong way to cool electronics, using them to educate the cooling masses, applying humor as the sugar to help the medicine go down. These have been published recently by the ASME Press in a book called, “Hot Air Rises and Heat Sinks: Everything You Know About Cooling Electronics Is Wrong.” It is available direct from ASME Press at 1-800-843-2763 or at their web site at http://www.asme.org/pubs/asmepress, Order Number 800741.
