# Help understanding heat pipes parameters

#### monz

Joined Mar 12, 2023
2
I need help understanding heat pipes parameters.

Let's say I need to transfer heat from the heating component on the PCB to the heat sink. For this I want to use a heat pipe. To calculate the final temperature on the PCB component, I use thermal resistances. But there is no such parameter for heat pipes. They have Qmax.

For example: https://www.mouser.com/pdfdocs/Heat-Pipes.pdf

I don't understand what Qmax is and how to go from it to thermal resistance? It cannot be that the heat pipe transfers 60W without loss (as if the radiator is attached directly and not through the heat pipe).

Help please me sort out this question.

#### crutschow

Joined Mar 14, 2008
32,088
It cannot be that the heat pipe transfers 60W without loss (as if the radiator is attached directly and not through the heat pipe).
But they essentially do.
It vaporizes a low boiling-point liquid at the hot end of the pipe, which absorbs heat, and condense the vapor at the other end to give up the heat, with essentially no thermal resistance, other than provided by the metal mounting plates at each end of the pipe.;

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#### monz

Joined Mar 12, 2023
2
But they essentially do.
It vaporizes a low boiling-point liquid at the hot end of the pipe...
Do I understand correctly that the boiling point at low temperature is possible due to low pressure (close to vacuum) in the pipe?

#### Janis59

Joined Aug 21, 2017
1,639
Bit story about heatpipes. We got the order to create the few Watts cooling system giving minus 40C at remote desert having no water. Normal would be to use the small water circulator from tap and average strong Peltier element. Thus, the water is inaccessible, I got the strongest available Peltier to minus 120 C and add the huge air fan radiator to 0.1 C/W. Result: at first the Peltier wools into snow layer, then, after minute snow dies, and after more minute that place becomes hot, and after few minutes reddish warm. What hapens? The power is only 11W, thus the temperature difference anywhere just CANNOT be more than 1.1 C, But it is at least 400-500C. Investigation shown the POWER at that 50x50 cm ribbed radiator may be dissipated up to several kilowatts, but the power DENSITY (W/mm2) is clearly insufficient. Then I take the best heatpipe construction was available at computer gamer shop. Added the Peltier and voila, device was satisfactory. Then after a year we got the letter from Peltier company. They claimed that our customer wanted to multiplicate our design around our back and asked to them what type of heatpipe they may reccomend. And informed me that their Peltier elemnt is not designed to use with any air cooling and is not immaginable let it would work with. I still count that the greatest compliment to our skills. So, the gross outcome: C/W just shows the cooling effect, whilst max Watts shows the max W/mm2 at coolable patch.

#### WBahn

Joined Mar 31, 2012
28,514
Do I understand correctly that the boiling point at low temperature is possible due to low pressure (close to vacuum) in the pipe?
The pipe only has some mass of the working fluid in it that is saturated with that fluid's gas. For distilled water at room temperature, that's a pressure of only about 3% of atmospheric. But above 100°C, the vapor pressure is above standard atmospheric pressure. Water heat pipes are often rated to work up to about 200°C, at which point the vapor pressure is getting into the vicinity of 20 atmospheres (~250 psi).

The key is that, over the entire operating temperature range, there must be both liquid and gas in saturated equilibrium.

#### Janis59

Joined Aug 21, 2017
1,639
For instance, the water enthalpy is 4.19 kJ/kg at liquid phase, less than 1.5 kJ/kg at vapour phase, but over the 2 MJ/kg for providing the phase change (boiling). That is the hearth of heat pipe effectivity. Spiritus simply is lowering the boiling temperature making it more handy. https://en.wikipedia.org/wiki/Heat_pipe