Why is minimum pressure specified on a PSU water cooling circuit?

Question

I work in consultancy and I am designing a product for a customer. The customer has requested a specific PSU. I have contacted this PSU manufacturer and they request 6 L/min of coolant flow and minimum 3 Bar. I now have to specify a suitable pump to deliver these fluid characteristics to the PSU.

I have confirmed this is not a typo for "maximum pressure," but no reasoning for specifying a minimum pressure is forthcoming.

My confusion is why the flow rate AND minimum pressure are specified. The manufacturer does not know what pressure differential I require to generate the specified flow around my bespoke circuit - so why specify a minimum pressure AND a flow rate?

Answer 1

You did not specify what materials you are working with and neither the dimensions. I made some assumptions.

First of all, the coolant circuit is likely driven by a pump. Therefore you need to know the flow rate $\dot{Q}$ the pressure needed on the pressure side. You can get more information here.

For your specific setup to cool the PSU you have pressure drops along your tubings, curvatures and through the height differences.

To keep things simple I just assumed a height difference of 20 cm and no differences in pressure in different reservoirs where the coolant might be pumped. Then the only significant pressure drop is the result of friction pressure loss. I also did not account for any curvatures. For $\rho$ I assumed water.

$\Delta p_r = \lambda \cdot \dfrac{L}{d} \cdot \dfrac{\rho}{2} \cdot u^2$

I assumed rubber tubings with the darcy friction factor formulae $\lambda$ = 0.02

The plot below shows 3 different assumptions in lenghts of the tubing and inner diameter.

1. $L$ = 2 m and $d$ = 0.01 m
2. $L$ = 2 m and $d$ = 0.005 m
3. $L$ = 10 m and $d$ = 0.005 m

Pumps have characteristic curves as well but the pressure decreases with increasing $\dot{Q}$. The intersect of the two curves is the operating point.

Source: ctgclean

Now either the pump is fixed and you need to adjust your pressure drop along the tubing accordingly or vice versa. Therefore the specification of a minimum pressure instead of maximum.

I hope that shed some light on your question.

Answer 2

The pressure requirement could also be to raise the temperature at which the water boils. The boiling point of water depends on the pressure it's at; higher pressure raises the boiling point, as the following image shows.

As you can see, water at atmospheric pressure boils at 100 C, but water at 3 bars boils at about 130 C. When water starts to boil, it becomes less effective at removing heat. So, the PSU internals might operate hotter than 100 C, which requires that the water be under pressure to prevent boiling. See this Wikipedia article on critical heat flux for a more detailed explanation.

On a side note, coolant reaching its critical heat flux was a major factor that doomed reactor #4 at the Chernobyl nuclear power plant.