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

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?

• I assume that 3 bar is 3 bar pump differential (or pump outlet to atmosphere = 4 bar absolute for a useful gain on @Carlton's curves. – Russell McMahon Jun 19 '15 at 10:58

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.

• I think that's the reason, 3 bars for just the static pressure losses is a really big number. – Algo Jun 17 '15 at 22:59
• Arguably it's 4 bar on that curve = 150 C. If the pump has effectively zero head pressure we'd tend to say it was making ~0 pSi and ~ 0 bar. A pump with 14.5 psi pumping head would tend to be seen as 1 bar. ie its gauge (= differential) pressure and not absolute pressure that is intended when people fail to spell it out. So 3 bar differential = 4 bar absolute giving a tad under 150 C BP. That's a useful gain over 100 C or over 130 C. – Russell McMahon Jun 19 '15 at 10:56
• Funny, my original post said "150 C" because I was looking at the wrong line on the graph. I see now that I was right, for the wrong reason. You are correct, the graph is in terms of absolute pressure whereas the PSU spec is likely in gauge pressure, meaning that we should be at 4 bars on the graph, not 3. – Carlton Jun 19 '15 at 12:34

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.

• Hi @idkfa, Thankyou for taking the time to offer this detailed response. I understand flow is driven by a pressure differential. 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? – James Izzard Jun 17 '15 at 9:20
• I think it will help us if we knew who actually delivers what. Who does deliver the pump and who does design the circuit? A little bit of elaboration on that would be great. – idkfa Jun 17 '15 at 9:29
• Yes - apologies for not clarifying, I work in consultancy and I am designing a product for a customer. The customer has requested a specific PSU (this company reo.co.uk/c/variable_power_supplies). I have contacted this PSU manufacturer and they request 6L/min of coolant flow and minimum 3 Bar. I now have to specify a suitable pump to deliver these fluid characteristics to the PSU. This is no problem, but I was curious to understand where this 3 bar had come from. Thanks – James Izzard Jun 17 '15 at 9:35
• Then the PSU manufacturere designed the circuit and knows the pressure drop. The pump you have to deliver has to deliver a minimum of 3 bar at 6 L / min. Any less and the flowrate can't be achieved. If you deliver more they might be able to regulate the pump or it does not matter and you'll achieve higher flow rates. – idkfa Jun 17 '15 at 9:40
• Sorry, again I am not being clear - I am designing the circuit - I only have an coolant in/out port on the PSU. – James Izzard Jun 17 '15 at 9:41